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Fingertip Representation in the Human Somatosensory Cortex: An fMRI Study
Abstract
Eight right-handed adult humans underwent functional magnetic resonance imaging (fMRI) of their brain while a vibratory stimulus was applied to an individual digit tip (digit 1, 2, or 5) on the right hand. Multislice echoplanar imaging techniques were utilized during digit stimulation to investigate the organization of the human primary somatosensory (SI) cortex, cortical regions located on the upper bank of the Sylvian fissure (SII region), insula, and posterior parietal cortices. The t test and cluster size analyses were performed to produce cortical activation maps, which exhibited significant regions of interest (ROIs) in all four cortical regions investigated. The frequency of significant ROIs was much higher in SI and the SII region than in the insula and posterior parietal region. Multiple digit representations were observed in the primary somatosensory cortex, corresponding to the four anatomic subdivisions of this cortex (areas 3a, 3b, 1, and 2), suggesting that the organization of the human somatosensory cortex resembles that described in other primates. Overall, there was no simple medial to lateral somatotopic representation in individual subject activity maps. However, the spatial distance between digit 1 and digit 5 cortical representations was the greatest in both SI and the SII region within the group. Statistical analyses of multiple activity parameters showed significant differences between cortical regions and between digits, indicating that vibrotactile activations of the cortex are dependent on both the stimulated digit and cortical region investigated.
... While Gelnar et al. (1998) failed to find a simple, medial-tolateral arrangement of digits 1, 2, and 5, this study demonstrated the greatest distance between digits 1 and 5. Statistically significant differences between each stimulated digit were seen, suggesting that the response observed in cortex to the same stimulus may vary with the area of cortex being studied and the digit stimulated (Gelnar et al., 1998). Hlustík et al. (2001) found an orderly somatotopy in both primary motor cortex and primary somatosensory cortex. ...
... While Gelnar et al. (1998) failed to find a simple, medial-tolateral arrangement of digits 1, 2, and 5, this study demonstrated the greatest distance between digits 1 and 5. Statistically significant differences between each stimulated digit were seen, suggesting that the response observed in cortex to the same stimulus may vary with the area of cortex being studied and the digit stimulated (Gelnar et al., 1998). Hlustík et al. (2001) found an orderly somatotopy in both primary motor cortex and primary somatosensory cortex. ...
... Despite the different methods used in the reviewed literature, finger maps were predominantly located in BA3b, BA2, and BA1 showing a lateral-to-medial and inferior-to-posterior organization from digit 1 to digit 5 in the cortex, in all studies besides Gelnar et al. (1998) who did not find simple lateralto-medial organization. This finding has been consistent across many studies conducted over more than 2 decades (Maldjian et al., 1999;Kurth et al., 2000;Hlustík et al., 2001;Overduin and Servos, 2004;Nelson and Chen, 2008;Schweizer et al., 2008;Sanchez-Panchuelo et al., 2010;Stringer et al., 2011;Besle et al., 2013;Martuzzi et al., 2014;Sánchez-Panchuelo et al., 2014;Schweisfurth et al., 2014Schweisfurth et al., , 2018Kolasinski et al., 2016;Sanchez Panchuelo et al., 2018;Schellekens et al., 2018Schellekens et al., , 2021Puckett et al., 2020;Arbuckle et al., 2021;Wang et al., 2021) and, thus, the authors feel that fMRI can be considered a reliable tool for somatosensory mapping research of the fingers at the group level. ...
Multiple studies have demonstrated finger somatotopy in humans and other primates using a variety of brain mapping techniques including functional magnetic resonance imaging (fMRI). Here, we review the literature to better understand the reliability of fMRI for mapping the somatosensory cortex. We have chosen to focus on the hand and fingers as these areas have the largest representation and have been the subject of the largest number of somatotopic mapping experiments. Regardless of the methods used, individual finger somatosensory maps were found to be organized across Brodmann areas (BAs) 3b, 1, and 2 in lateral-to-medial and inferior-to-superior fashion moving from the thumb to the pinky. However, some consistent discrepancies are found that depend principally on the method used to stimulate the hand and fingers. Therefore, we suggest that a comparative analysis of different types of stimulation be performed to address the differences described in this review.
... Nevertheless, the neural underpinnings for the dependence of somatosensory SD on the detection process remain elusive. Interestingly, during tactile detection, a stimulus produces activation in topographically corresponding parietal regions, including the primary somatosensory cortex (SI) (Gelnar et al. 1998), secondary somatosensory cortex (SII) (Ruben et al. 2001), and posterior parietal cortex (Huang et al. 2012). This observation raises the possibility that the neural substrates predicting an individual's tactile spatial discriminability (i.e., SD ability) may already become activated during somatosensory detection. ...
... These observations, in conjunction with abovedescribed persistent IPL and SPL activation during the entire SD process as well as prediction of IPL-SPL connectivity for SD performance, lead us to propose that an accurate assessment of somatosensory spatial properties arises from the dynamic interaction between top-down and bottom-up control of spatial attention in the posterior parietal cortex. Given we did not find a role of the SI and SII, whose activities also reflect spatial features during somatic detection (Gelnar et al. 1998;Ruben et al. 2001), our proposed mechanism supports the concept that the assessment of somatosensory spatial properties requires higher-order cognitive processes (i.e., the determination process in the current study), rather than simple somatotopic representation in the somatosensory cortex (Medina and Coslett 2016;Azanon and Longo 2019). ...
While detecting somatic stimuli from the external environment, an accurate determination of their spatial and temporal properties is essential for human behavior. Whether and how detection relates to human capacity for somatosensory spatial discrimination (SD) and temporal discrimination (TD) remains unclear. Here, participants underwent functional magnetic resonance imaging scanning when simply detecting vibrotactile stimuli of the leg, judging their location (SD), or deciding their number in time (TD). By conceptualizing tactile discrimination as consisting of detection and determination processes, we found that tactile detection elicited activation specifically involved in SD within the right inferior and superior parietal lobules, 2 regions previously implicated in the control of spatial attention. These 2 regions remained activated in the determination process, during which functional connectivity between these 2 regions predicted individual SD ability. In contrast, tactile detection produced little activation specifically related to TD. Participants’ TD ability was implemented in brain regions implicated in coding temporal structures of somatic stimuli (primary somatosensory cortex) and time estimation (anterior cingulate, pre-supplementary motor area, and putamen). Together, our findings indicate a close link between somatosensory detection and SD (but not TD) at the neural level, which aids in explaining why we can promptly respond toward detected somatic stimuli.
... A considerable degree of activation has been noted in these areas after stimulation with low-frequency vibration (<80 Hz), particularly in Meissner's corpuscle. Some studies have also compared the degree of activation on applying low-frequency vibration stimulation to the first phalange of a given finger [1][2][3][4][5][6][7][8]. It was reported that the activation area of the little finger was larger than that of other fingers, and that the activation area of the index finger was larger than that of the middle and ring fingers. ...
... Several studies have reported the brain activation associated with low-frequency vibration stimulation of each finger; the highest levels of activation have frequently been found in the S1 and BA3 regions on stimulation of the index finger [2,6,9,12,16], and a few studies have reported similar results after stimulation of the little finger [1,7]. Generally, the greatest level of activation has been reported to occur for stimulation of the index finger, which has a large distribution of sensory receptors. ...
In this study, we measured neuronal activation in the primary somatosensory area (S1) and Brodmann area 3 (BA3) using 3T functional magnetic resonance imaging (fMRI) while presenting a 250-Hz high-frequency vibrational stimulus to each of three phalanges (distal, intermediate, and proximal) of four fingers of the right hand (index, middle, ring, and little). We compared the nerve activation area between each finger and each phalange. Ten healthy male college students (26.6 ± 2.5 years old) participated in this study. One session consisted of three blocks: a rest (30 s), stimulation (30 s), and response phase (9 s). In the rest phase, the vibrational stimulus was not presented. In the stimulation phase, the vibrational stimulation was presented at any one of the three phalanges of the selected finger. In the response phase, subjects were instructed to press a button corresponding to the phalange that they thought had received the vibration. The subtraction method was used to extract the activation area. The activation area in the S1 was the largest when the little finger was stimulated (for the finger comparison), and largest when the second phalange was stimulated (for the phalange comparison). The BA3 showed similar trends, and there was no statistically significant difference.
... The human somatosensory cortex has been studied using a variety of noninvasive imaging techniques. In particular, a number of studies have focused on the somatopical organisation of SI (along the postcentral gyrus): in electroencephalography (EEG) and magnetoencephalography (MEG;Suk et al, 1991;Baumgartner et al, 1991;Hari et al, 1993;Nakamura et al., 1998), positron émission tomography (PET; Fox et al., 1987), and in addition fMRI (Gelnar et al, 1998;Sakai et al, 1995;Puce et al, 1995;Kurth et al., 1998;Disbrow et a/., 1998;Maldjan et al., 1999;Francis et al., 2000;Kurth et al., 2000). While the majority of MEG studies have been able to show the 'somatopical' mapping of the body in SI as predicted from invasive mapping procedure (e.g. ...
... At the time of writing, two papers (Maldjian et al., 1999 [vibrotactile]; Kurth et al., 2000 [electrical]) have shown a somatopical pattern of activation for all five digits within SI using fMRI. An equal number of papers have reported difficulties in detecting somatopy within SI (Kurth et al., 1997;Gelnar et al., 1998). Thus this issue is still contentious. ...
All organisms must possess the ability to detect environmental stimuli and transform them into a form of information that can be utilised to guide behaviour. As the primate sensory systems consist of multiple interconnected cortical areas, it is important to know where areas processing different aspects of a sensory stimulus are located, and also which dimensions of the stimuli are being processed in each area. The use of functional neuroimaging allows one to address both of these problems. Although much progress has been made regarding the functional and anatomical organisation of higher order visual areas such as IT (e.g. Milner and Goodale, 1996), there has been comparatively little headway in understanding the functional organisation of somatosensory processing in humans. One problem in particular, the delivery robust somatosensory stimulation in the neuroimaging environment, is not a trivial one. In summary, the field of somatosensory neuroimaging has not received as much interest as other sensory modalities. In this thesis, I will present the results of my studies, which can be divided into three sections. I) The design and implementation of stimulation within the scanning environment; II) examinations of the topography of digit representations within primary and tertiary somatosensory areas using fMRI, and; III) examinations of sensorimotor transformations and somatoform illusions. My results are discussed with reference to similar studies in other sensory systems, and are placed in the context of investigations using other non-invasive scanning technologies.
... It records the magnetic fields generated by brain activity using super-conducting interference devices, which allow neuronal activity to be explored completely non-invasively [1]. In contrast, although methods based on hemodynamic responses, such as functional magnetic resonance imaging (fMRI) indirectly measure the neuronal activity, they have relatively poor temporal resolution because blood oxygenation level only sluggishly tracks neuronal activity [6][7][8]; ...
... All participants gave written informed consent. The study was approved by the medical ethics committee (JEPeM), School of Medical Sciences, Universiti Sains Malaysia Kubang Kerian, with EPeM-USM Code: USMKK/PPP/JEPeM 275.3 (7). All participants were right handed. ...
PurposeTo localize sensorimotor cortical activation in 10 patients with frontoparietal tumors using quantitative magnetoencephalography (MEG) with noise-normalized approaches. Material and Methods
Somatosensory evoked magnetic fields (SEFs) were elicited in 10 patients with somatosensory tumors and in 10 control participants using electrical stimulation of the median nerve via the right and left wrists. We localized the N20m component of the SEFs using dynamic statistical parametric mapping (dSPM) and standardized low-resolution brain electromagnetic tomography (sLORETA) combined with 3D magnetic resonance imaging (MRI). The obtained coordinates were compared between groups. Finally, we statistically evaluated the N20m parameters across hemispheres using non-parametric statistical tests. ResultsThe N20m sources were accurately localized to Brodmann area 3b in all members of the control group and in seven of the patients; however, the sources were shifted in three patients relative to locations outside the primary somatosensory cortex (SI). Compared with the affected (tumor) hemispheres in the patient group, N20m amplitudes and the strengths of the current sources were significantly lower in the unaffected hemispheres and in both hemispheres of the control group. These results were consistent for both dSPM and sLORETA approaches. Conclusion
Tumors in the sensorimotor cortex lead to cortical functional reorganization and an increase in N20m amplitude and current-source strengths. Noise-normalized approaches for MEG analysis that are integrated with MRI show accurate and reliable localization of sensorimotor function.
... In those cases, the distances have been in the expected range (>3 mm for humans). 3,8 The agreement of the fMRI somatotopic ordering with the model somatotopy is shown in Table 1 for the various evaluation strategies. Strategies are ordered from upper to lower row with an increasing average value of the ordering in each row. ...
... 32 Furthermore, the results agreed with previous investigations of the right hand. 2,8,18,33 However, comparisons with literature are tedious because the results depend profoundly on the applied methodology. ...
To introduce a standardized and automatized method for functional MRI (fMRI) examinations of the cortical sensory somatotopy in large samples for investigations of the fingertip somatotopy in the primary somatosensory cortex.
At 3 Tesla, T2* (spin-spin relaxation time) weighted images (gradient-echo echo planar imaging, voxel size 1.5 × 1.5 × 2 mm(3) ) were acquired during stimulation of the finger tips for thumb, index and middle finger on both hands, in a group of 18 healthy participants. In addition, structural T1 weighted (magnetization prepared rapid gradient echo, isotropic voxel size 1 mm) and MR-angiography (time of flight, voxel size 0.26 × 0.26 × 0.5 mm(3) ) images were recorded. Boundary based register served to combine movement correction and registration in FreeSurfer Functional analysis stream (FS-Fast), resulting in fine scale corrections, as revealed with FSL Possum (FSL FMRIB Software Library Physics-Oriented Simulated Scanner for Understanding MRI) simulations. Automated data analysis was achieved by inclusion of cytoarchitectonic probability maps for calculation of functional activation in Brodmann area 3b. Draining vessel artifacts were identified using the peak value approach and the MR-angiography. Distances were computed as the shortest connection within the gray matter.
The fMRI somatotopic maps agreed with the expected fingertip somatotopy in 63% of the investigated subjects, an improvement of 34% compared with FS-Fast. Artifacts have been removed completely. Adjacent fingertips showed average distances of 8 ± 4.3 mm, and between thumb and middle finger 13.4 ± 4.8 mm was found. Distances for both hands were similar as expected from the characteristics of the fingertip spatial tactile resolution.
The introduced evaluation procedure allowed automated analysis of the fingertip representation in excellent agreement with preceding results. J. Magn. Reson. Imaging 2015.
© 2015 Wiley Periodicals, Inc.
... The latero-medial organization from the thumb to the little finger in somatosensory cortex was originally observed by Penfield using intraoperative electrical stimulation of humans 24,25 and has been confirmed in multiple species [26][27][28][29][30] including non-human primates [18][19][20]31 . This somatotopic finger organization can also be observed in humans using non-invasive imaging techniques -functional magnetic resonance imaging (fMRI) and magnetoencephalography (MEG) [32][33][34][35][36][37][38][39][40] . Importantly, several studies have demonstrated highly preserved finger representations in people who were deafferented through amputation [41][42][43][44] or spinal-cord injury (SCI) 45 ; the two groups of people most likely to benefit from BCIs to restore upper limb function. ...
Intracortical microstimulation (ICMS) is a method for restoring sensation to people with paralysis as part of a bidirectional brain-computer interface to restore upper limb function. Evoking tactile sensations of the hand through ICMS requires precise targeting of implanted electrodes. Here we describe the presurgical imaging procedures used to generate functional maps of the hand area of the somatosensory cortex and subsequent planning that guided the implantation of intracortical microelectrode arrays. In five participants with cervical spinal cord injury, across two study locations, this procedure successfully enabled ICMS-evoked sensations localized to at least the first four digits of the hand. The imaging and planning procedures developed through this clinical trial provide a roadmap for other brain-computer interface studies to ensure successful placement of stimulation electrodes.
... According to Broadmann's classification, the S1 of primates can be divided into three regions, called 1, 2, and 3 (Brodmann, 1909), subsequently divided into area 3a and 3b by Vogt and Vogt's classification (Vogt & Vogt, 1919). All regions appear to possess somatotopic maps of the contralateral half of the body, whether we consider non-human primates (Kaas et al., 1979;Kaas, 1983;Merzenich et al., 1978;Nelson et al., 1980;Pons et al., 1985;Sur et al., 1980) or humans (Burton et al., 1997;Francis et al., 2000;Gelnar et al., 1998;Kurth et al., 1998Kurth et al., , 2000Lin et al., 1996;Moore et al., 2000). The same was observed for the SII area, located in the upper bank of the lateral sulcus, and more precisely in the region of the parietal operculum: also in this case, electrophysiological studies on the monkey (Krubitzer et al., 1995; C. J. Robinson & Burton, 1980b;Whitsel et al., 1969) and neuroimaging in humans (Disbrow et al., 2000;Francis et al., 2000;Polonara et al., 1999;Ruben et al., 2001) reported neurons with somatotopically organized somatosensory receptive fields. ...
Forty years have passed since the coining of the term "peripersonal space" (PPS), that region of space in which our daily life takes place, in which we can interact with the objects and people around us. The first studies of the electrophysiological literature of this spatial representation have observed in specific regions of the macaque’s brain the existence of multisensory neurons capable of encoding tactile, visual and / or auditory stimuli according to their distance from specific parts of the body. These bi- or trimodal neurons, indeed, show tactile receptive fields centered on a specific part of the body, such as the face or hand, and visual and / or auditory receptive fields overlapping spatially with the formers. In this way, the same neurons are able to respond to tactile, visual and auditory stimulations delivered on or close to a specific body-part. Furthermore, these multisensory receptive fields are "anchored" to each other: the movement of the monkey's hand involves a coherent displacement not only of the tactile receptive fields, but also of the visual ones. This body-part centered reference frame of the coding of multisensory stimuli within PPS allows to keep the information relating to the position of the different parts of the body and surrounding objects always updated, with the aim of planning and implementing effective actions. Neurophysiological and behavioral studies on patients suffering from extinction and neglect following brain lesions of the right hemisphere have allowed to highlight, even in humans, the existence and modularity of the PPS. Subsequent neuroimaging studies have brought support to this evidence, highlighting a network of fronto-parietal and subcortical regions capable of coding multi-modal stimulations according to their distance from the body. The functions of this spatial representation are manifold: mediate the relationship between the perception of external stimuli and the execution of goal-directed actions, monitoring the space around the body in order to identify potential threats and implement defensive reactions, organize and manage the space between us and others in case of different types of social interaction and allow us to identify ourselves with our body, giving it a localization in space. However, despite the great scientific interest that this region of space has elicited over the past forty years, a direct comparison of its neural underpinnings in non-human primates and humans is still missing. For this reason, in the first chapter of this doctoral dissertation we will report the results of an fMRI study, conducted on human and macaque participants, which investigated the neural response patterns to stimulations close to or far from different body-parts, minimizing the differences among the experimental protocols used in the two species. For the first time PPS is tested in two different species but with the same experimental protocol, highlighting similarities and differences between the human and simian PPS circuit but also between the response patterns associated with the stimulation of different bodily districts. Starting from the second chapter we will instead focus our interest only on human participants, to try to shed light on a defining problem that has overlapped the concept of PPS representation to that of a second spatial representation: the arm reaching space (ARS). The latter, considered as the space around the body that we can reach by extending our arm, over time has often been used as a synonym for the PPS representation, leading to define PPS as ARS or to test the two spatial representations with the same experimental protocols. However, the different neural bases and the different characteristics of the encoding of stimuli within these two regions of space suggest their distinction. In chapter II, to this purpose, we will present a series of five behavioral experiments that investigated the differences and similarities between PPS and ARS .. [etc]
... An intriguing picture emerges when we contrast these results with different stimulation modalities, such as mechanical vibration. The widespread activation pattern, observed in sufficiently powered focal pressure stimulation studies, is consistent with studies using rather broad-area vibrotactile stimulation 89,90 or muscle stimulation 91,92 and far exceeds the cortical maps of relatively circumscribed finger vibrotactile stimulation in other studies [93][94][95] . Although qualitatively different stimuli are not directly comparable, this illustrates that pressure stimulation can be associated with robust motor activations that provide the neuroanatomical substrate for sensorimotor interactions and motor aftereffects of stimulation. ...
Peripheral afferent input is critical for human motor control and motor learning. Both skin and deep muscle mechanoreceptors can affect motor behaviour when stimulated. Whereas some modalities such as vibration have been employed for decades to alter cutaneous and proprioceptive input, both experimentally and therapeutically, the central effects of mechanical pressure stimulation have been studied less frequently. This discrepancy is especially striking when considering the limited knowledge of the neurobiological principles of frequently used physiotherapeutic techniques that utilise peripheral stimulation, such as reflex locomotion therapy. Our review of the available literature pertaining to pressure stimulation focused on transcranial magnetic stimulation (TMS) and neuroimaging studies, including both experimental studies in healthy subjects and clinical trials. Our search revealed a limited number of neuroimaging papers related to peripheral pressure stimulation and no evidence of effects on cortical excitability. In general, the majority of imaging studies agreed on the significant involvement of cortical motor areas during the processing of pressure stimulation. Recent data also point to the specific role of subcortical structures, such as putamen or brainstem reticular formation. A thorough comparison of the published results often demonstrated, however, major inconsistencies which are thought to be due to variable stimulation protocols and statistical power. In conclusion, localised peripheral sustained pressure is a potent stimulus inducing changes in cortical activation within sensory and motor areas. Despite historical evidence for modulation of motor behaviour, no direct link can be established based on available fMRI and electrophysiological data. We highlight the limited amount of research devoted to this stimulus modality, emphasise current knowledge gaps, present recent developments in the field and accentuate evidence awaiting replication or confirmation in future neuroimaging and electrophysiological studies.
... With fMRI, Moore et al. (2000) have demonstrated the differential projections of somaesthetic modalities to the several postcentral fields in the human brain. 6. Functional MRI studies with vibratory and other stimuli applied to different fingertips have provided evidence for separate representations in areas 3a, 3b, 1 and 2 (Lin et al. 1996;Gelnar et al. 1998;Francis et al. 2000;Burton 2002). 7. Recordings of scalp-recorded evoked potentials also support the conclusion that in humans the maps of receptor surfaces in the anterior parietal cortex are organized much as they are in monkeys . ...
The somatosensory system has by far the largest number of receptor types of any of the primate sensory systems, including mechanoreceptors, chemoreceptors, nociceptors and thermoreceptors. The sensation of touch is mainly mediated by mechanoreceptors, but there are a number of other processing channels within the somatosensory system for proprioception, pain and temperature. The classic view of two independent channels for somatosensory information from the trunk and the extremities, i.e. the dorsal column-medial lemniscus system for tactile sensitivity and position sense and the anterolateral or spinothalamic system for pain and temperature sensitivity, has been modified through the discovery of additional spinal pathways for the transmission of sensory impulses to the brain and by new views on pain mechanisms. Somatosensory information from the face is transmitted via the trigeminal nerve.
... It has revealed the retinotopic organization of the visual cortex (DeYoe et al., 1996;Engel, 1997;Sereno et al., 1995;Wandell et al., 2007), tonotopic organization in the auditory cortex (Da Costa et al., 2011;Formisano et al., 2003;Moerel et al., 2018Moerel et al., , 2012Saenz and Langers, 2014) and the cortical representation of body parts (Akselrod et al., 2017;Sanchez Panchuelo et al., 2018), particularly the digits of the hand (Besle et al., 2013;Sanchez Panchuelo et al., 2010;Schweisfurth et al., 2015Schweisfurth et al., , 2014Stringer et al., 2011;van der Zwaag et al., 2015) in the primary somatosensory cortex (S1). Due to the fine architecture of the cortical representation of the digits of the hand in the post-central gyrus (Geyer et al., 2000), somatotopic mapping is more challenging than 50 retinotopic and tonotopic mapping, in terms of both the spatial resolution of cortical maps and the statistical power (Francis et al., 2000;Gelnar et al., 1998;Huang and Sereno, 2007;Kurth et al., 2000;Nelson and Chen, 2008;Overduin and Servos, 2004;Weibull et al., 2008). With the advent of ultra-high-field (UHF) MR scanners, operating at 7 Tesla (7 T) and above, high spatial resolution fMRI has provided robust maps of the representation of all the digits of the hand in primary somatosensory cortex in individual subjects (Besle et al., , 2013Martuzzi et al., 2014;Sanchez Panchuelo et al., 2010). ...
With the advent of ultra-high field (7T), high spatial resolution functional MRI (fMRI) has allowed the differentiation of the cortical representations of each of the digits at an individual-subject level in human primary somatosensory cortex (S1). Here we generate a probabilistic atlas of the contralateral SI representations of the digits of both the left and right hand in a group of 22 right-handed individuals. The atlas is generated in both volume and surface standardised spaces from somatotopic maps obtained by delivering vibrotactile stimulation to each distal phalangeal digit using a travelling wave paradigm.
Metrics quantify the likelihood of a given position being assigned to a digit (full probability map) and the most probable digit for a given spatial location (maximum probability map). The atlas is validated using a leave-one-out cross validation procedure. Anatomical variance across the somatotopic map is also assessed to investigate whether the functional variability across subjects is coupled to structural differences. This probabilistic atlas quantifies the variability in digit representations in healthy subjects, finding some quantifiable separability between digits 2, 3 and 4, a complex overlapping relationship between digits 1 and 2, and little agreement of digit 5 across subjects. The atlas and constituent subject maps are available online for use as a reference in future neuroimaging studies.
... Basic demonstrations of tactile stimulation eliciting cortical activation within human S1 were shown using Positron Emission Tomography (PET) (Fox et al., 1987;Greenberg et al., 1981). Using functional magnetic resonance imaging (fMRI), it later became possible to resolve this activity with such detail that the responses could be attributed to the stimulation of individual fingers (Francis et al., 2000;Gelnar et al., 1998). More recently, high-resolution fMRI has borne evidence that human S1 actually contains multiple orderly somatotopic maps of the fingers, both across (Martuzzi et al., 2014;Sanchez-Panchuelo et al., 2010) and within (Sanchez-Panchuelo et al., 2012) digits. ...
Somatosensation is fundamental to our ability to sense our body and interact with the world. Our body is continuously sampling the environment using a variety of receptors tuned to different features, and this information is routed up to primary somatosensory cortex. Strikingly, the spatial organization of the peripheral receptors in the body are well maintained, with the resulting representation of the body in the brain being referred to as the somatosensory homunculus. Recent years have seen considerable advancements in the field of high-resolution fMRI, which have enabled an increasingly detailed examination of the organization and properties of this homunculus. Here we combined advanced imaging techniques at ultra-high field (7T) with a recently developed Bayesian population receptive field (pRF) modeling framework to examine pRF properties in primary somatosensory cortex. In each subject, vibrotactile stimulation of the fingertips (i.e., the peripheral mechanoreceptors) modulated the fMRI response along the post-central gyrus and these signals were used to estimate pRFs. We found the pRF center location estimates to be in accord with previous work as well as evidence of other properties in line with the underlying neurobiology. Specifically, as expected from the known properties of cortical magnification, we find a larger representation of the index finger compared to the other stimulated digits (middle, index, little). We also show evidence that the little finger is marked by the largest pRF sizes, and that pRF size increases from anterior to posterior regions of S1. The ability to estimate somatosensory pRFs in humans provides an unprecedented opportunity to examine the neural mechanisms underlying somatosensation and is critical for studying how the brain, body, and environment interact to inform perception and action.
... Functional imaging studies on proprioception during either vibration induced illusory motion or passive extremity movements revealed involvement of a number of brain regions partially overlapping with those discussed in the context of temporal discrimination (Weiller et al., 1996;Gelnar et al., 1998;Mima et al., 1999;Francis et al., 2000;Romaiguere et al., 2003;Naito et al., 2007;Kavounoudias et al., 2008). It is beyond the scope of our study to speculate about the neural underpinnings of both phenomena in the CNS. ...
Background
Increasing attention is payed to the contribution of somatosensory processing in motor control. In particular, temporal somatosensory discrimination has been found to be altered differentially in common movement disorders. To date, there have only been speculations as to how impaired temporal discrimination and clinical motor signs may relate to each other. Prior to disentangling this relationship, potential confounders of temporal discrimination, in particular age and peripheral nerve conduction, should be assessed, and a quantifiable measure of proprioceptive performance should be established.
Objective
To assess the influence of age and polyneuropathy (PNP) on somatosensory temporal discrimination threshold (STDT), temporal discrimination movement threshold (TDMT), and behavioral measures of proprioception of upper and lower limbs.
Methods
STDT and TDMT were assessed in 79 subjects (54 healthy, 25 with PNP; age 30–79 years). STDT was tested with surface electrodes over the thenar or dorsal foot region. TDMT was probed with needle electrodes in flexor carpi radialis (FCR) and tibialis anterior (TA) muscle. Goniometer-based devices were used to assess limb proprioception during (i) active pointing to LED markers, (ii) active movements in response to variable visual cues, and (iii) estimation of limb position following passive movements. Pointing (or estimation) error was taken as a measure of proprioceptive performance.
Results
In healthy subjects, higher age was associated with higher STDT and TDMT at upper and lower extremities, while age did not correlate with proprioceptive performance. Patients with PNP showed higher STDT and TDMT values and decreased proprioceptive performance in active pointing tasks compared to matched healthy subjects. As an additional finding, there was a significant correlation between performance in active pointing tasks and temporal discrimination thresholds.
Conclusion
Given their notable impact on measures of temporal discrimination, age and peripheral nerve conduction need to be accounted for if STDT and TDMT are applied in patients with movement disorders. As a side observation, the correlation between measures of proprioception and temporal discrimination may prompt further studies on the presumptive link between these two domains.
... Based on this, the advanced technology of fMRI was developed to provide information about biological function, in addition to the concomitant anatomical information, which uses the naturally occurring paramagnetic deoxyhemoglobin in venous blood as a contrast agent. 22 The technology of fMRI has been used by a large number of researchers to study brain activities related to various kinds of tactile perception on electrical stimulation, heat stimulation, sponge brush stimulation, 23 vibratory stimulation (a plastic cylinder), 24 and so on. For example, Hammeke et al. 25 studied the brain signal changes of subjects when their palms received tactile stimulation from the experimenter stroking in a tickling fashion. ...
Fabric hand is most frequently used by consumers and researchers to evaluate the touch feeling of textiles. Academically, many methods have been developed to characterize it psychologically and physically, and the relationship between the hand attributes of fabrics and their physical properties are well understood. However, in physiological terms, the cognitive mechanism of the brain on different attributes of fabric hand is not clear. Previous studies have shown that the sensory or discrimination information from fabric touch can be detected by the technology of functional magnetic resonance imaging (fMRI). In this study, further fMRI experiments were carried out, attempting to find the relationship between the cerebral cortices of various brain areas and different hand attributes of fabrics. The subtle atlas of Automated Anatomical Labeling (AAL) was used to display and analyze the blood oxygenation level dependent signals completely and conveniently. The results showed that when the subjects touched two samples with distinct fabric hand in a specified way, activation information and the index of the mean signal in every related brain areas can distinguish them, and several brain regions in the AAL atlas are linked to different fabric hand attributes. The technology of fMRI was proved again to be a promising tool for studying the cognitive mechanism of the brain on fabric touch.
... The topographic organisation of the sensorimotor cortices is well established (Penfield and Boldrey, 1937), and in recent years functional magnetic resonance imaging (fMRI) has allowed a non-invasive means to map the brain's response to stimulation of different body parts (Maillard et al., 2000;Sakai et al., 1995;Stippich et al., 2002), individual digits of the hand (Francis et al., 2000;Gelnar et al., 1998;Sanchez-Panchuelo et al., 2010), and the within digit functional parcellation of Brodmann areas (Sanchez-Panchuelo et al., 2012). This mapping, which separates functionally specialised brain regions lying only millimetres apart, has been made possible by the high spatial resolution of high-and ultra-high field fMRI. ...
The spatial topology of the human motor cortex has been well studied, particularly using functional Magnetic Resonance Imaging (fMRI) which allows spatial separation of haemodynamic responses arising from stimulation of different body parts, individual digits and even spatially separate areas of the same digit. However, the spatial organisation of electrophysiological responses, particularly neural oscillations (rhythmic changes in electrical potential across cellular assemblies) has been less well studied. Mapping the spatial signature of neural oscillations is possible using magnetoencephalography (MEG), however spatial differentiation of responses induced by movement of separate digits is a challenge, because the brain regions involved are separated by only a few millimetres. In this paper we first show, in simulation, how to optimise experimental design and beamformer spatial filtering techniques to increase the spatial specificity of MEG derived functional images. Combining this result with experimental data, we then capture the organisation of the post-movement beta band (13-30 Hz) oscillatory response to movement of digits 2 and 5 of the dominant hand, in individual subjects. By comparing these MEG results to ultra-high field (7T) fMRI, we also show significant spatial agreement between beta modulation and the blood oxygenation level dependent (BOLD) response. Our results show that, when using an optimised inverse solution and controlling subject movement (using custom fitted foam padding) the spatial resolution of MEG can be of order 2-5 mm. The method described offers exciting potential to understand better the cortical organisation of oscillations, and to probe such organisation in patient populations where those oscillations are known to be abnormal.
... Early demonstrations of tactile stimulation eliciting focal neuronal activation within human S1 were performed using Positron Emission Tomography (PET) (Fox et al., 1987;Greenberg et al., 1981). Using fMRI, it became possible to further resolve this activation to individuate the responses to stimulation of individual fingers (Francis et al., 2000;Gelnar et al., 1998;Schweisfurth et al. 2014;Schweizer et al. 2008). More recently, research employing high-resolution fMRI has revealed that human S1 contains multiple orderly somatotopic maps of the fingers, both across digits (Maldjian et al., 1999;Martuzzi et al., 2014;Sanchez-Panchuelo et al., 2010) and within digits (Sanchez-Panchuelo et al., 2012). ...
Attention to sensory information has been shown to modulate the neuronal processing of that information. For example, visuospatial attention acts by modulating responses at retinotopically appropriate regions of visual cortex (Puckett and DeYoe, 2015; Tootell et al. 1998). Much less, however, is known about the neuronal processing associated with attending to other modalities of sensory information. One reason for this is that visual cortex is relatively large, and therefore easier to access non-invasively in humans using tools such as functional magnetic resonance imaging (fMRI). With high-resolution fMRI, however, it is now possible to access smaller cortical areas such as primary somatosensory cortex (Martuzzi et al., 2014; Sanchez-Panchuelo et al., 2010; Schweisfurth et al. 2014; Schweizer et al. 2008). Here, we combined a novel experimental design and high-resolution fMRI at ultra-high field (7T) to measure the effects of attention to tactile stimulation in primary somatosensory cortex, S1. We find that attention modulates somatotopically appropriate regions of S1, and importantly, that this modulation can be measured at the level of the cortical representation of individual fingertips.
... Les capteurs cutanés étant très sensibles aux vibrations, l'exploration du réseau de la perception tactile, en particulier de la sensibilité à la vibration, est souvent réalisée en IRMf en appliquant un stimulus vibratoire amagnétique sur la peau, consistant le plus souvent en un système de plaquettes piézo-électriques (Gelnar et al., 1998;Maldjian et al., 1999;Francis et al., 2000;McGlone et al., 2002;Nelson et al., 2004;Burton et al., 2004;Albanese et al., 2009;Li Hegner et al., 2010;Chung et al., 2013). Globalement, l'ensemble de ces études s'accordent sur un réseau d'activations reliées à la stimulation tactile cutanée incluant SI, SII, l'insula notamment antérieure et le cortex pariétal postérieur. ...
... Bao et al. 2012;Nakagoshi et al. 2005). We argue that this issue is less relevant for the hand, where the respective representations are larger and thus more likely to overlap across subjects, compatible with previously described somatotopic S1 organization of the upper limb at the group level(Gelnar et al. 1998;Kurth et al. 1998, 2000; Maldjian et al. 1999; van Westen et al. 2004). Studies targeting plasticity-dependent changes in S1 (Muret et al. 2016; Pleger et al. 2003), clinical research in amputees or spinal cord injury related to potential S1 changes in chronic pain (Freund et al. 2011; Henderson et al. 2014), and studies on phantom limb pain (Flor et al. 1995; Makin et al. 2013) should most likely pursue an approach that is based on high resolution single subject analysis. ...
Primary somatosensory cortex (S1) processes somatosensory information and is composed of multiple subregions. In particular, tactile information from the skin is encoded in three subregions, namely Brodmann areas (BAs) 3b, 1 and 2, with each area representing a complete map of the contralateral body. Although, much is known about the somatotopic organization of the hand in human S1, less research has been carried out regarding the somatotopic maps of the foot and leg in S1. Moreover, a latero-medial S1 organization along the superior part of the postcentral gyrus has been reported when moving from hip to toes, yet to date there is no study investigating leg/foot maps within the different subregions of S1. Using ultra-high field MRI (7 T), we mapped six cortical representations of the lower limb (hip to toes) at the single subject level and performed this analysis separately for BAs 3b, 1 and 2. Analyzing the BOLD responses associated with tactile stimulations of the mapped foot and leg regions on each side, we quantified the extent and the strength of activation to determine somatotopic organization. In addition, we investigated whether each mapped representation also responded to the stimulation of other body parts (i.e. response selectivity) and conducted dissimilarity analysis relating these anatomical and functional properties of S1 to the physical structure of the lower limbs. Our data reveal somatotopy for the leg, but not for the foot in all investigated BAs, with large inter-subject variability. We found only minor differences between the properties of the three investigated BAs, suggesting that S1 maps for the lower limbs differ from those described for the hand. We also describe greater extent/strength of S1 activation for the big toe representation (compared to the other mapped representations) within all BAs, suggesting a possible homology between the first digit of upper and lower extremity in humans, and report different patterns of selectivity in the foot representations (i.e. lower selectivity) compared to the other leg representations (i.e. greater selectivity). These data provide a detailed description of human S1 subregions for the foot and leg, highlight the importance of high-resolution mapping studies and of single subject analysis, and indicate potential differences between the lower and the upper limb.
... First, S2 (homologous with the parietal operculum in the human brain) and pIns are central regions in pain perception. For example, both regions (also termed the operculoinsular region along the Sylvian fissure in humans) have been proposed as early pain-specific processing regions, and have been linked to subjective recognition of pain (Maihofner et al., 2006;Peyron et al., 2002;Strigo et al., 2005;Timmermann et al., 2001;Treede et al., 2000), encoding of pain intensity (Petrovic et al., 2000;Peyron et al., 1999;Timmermann et al., 2001), learning and memory of pain-related events (Dong et al., 1994;Ploner et al., 1999;Treede et al., 2000), and possibly the generation and maintenance of chronic pain states in humans ( (Coghill et al., 1999;Coghill et al., 1994;Derbyshire et al., 1997;Gelnar et al., 1998;Greenspan et al., 1999;Kim et al., 2007;Mazzola et al., 2006;Meyer et al., 1991;Morrow and Casey, 1992;Talbot et al., 1991;Torquati et al., 2002); for reviews, see Apkarian et al. (2005), Treede et al. (2000)). PIns, as a part of the broadly defined operculoinsular region, has also been implicated as a pain-specific cortical region, even though its role remains debatable due to the lack of specificity of fMRI signals (Davis et al., 2015;Segerdahl et al., 2015). ...
Numerous functional imaging and electrophysiological studies in humans and animals indicate that the two contiguous areas of secondary somatosensory cortex (S2) and posterior insula (pIns) are core regions in nociceptive processing and pain perception. In this study, we tested the hypothesis that the S2-pIns connection serves as a hub for connecting distinct sensory and affective nociceptive processing networks in the squirrel monkey brain. At 9.4 T, we first mapped the brain regions that respond to nociceptive heat stimuli with high-resolution fMRI, and then used seed-based resting-state fMRI (rsfMRI) analysis to delineate and refine the global intrinsic functional connectivity circuits of the proximal S2 and pIns regions. In each subject, nociceptive (47.5 °C) heat-evoked fMRI activations were detected in many brain regions, including primary somatosensory (S1), S2, pIns, area 7b, anterior cingulate cortex (ACC), primary motor cortex, prefrontal cortex, supplementary motor area, thalamus, and caudate. Using the heat-evoked fMRI activation foci in S2 and pIns as the seeds, voxel-wise whole-brain resting-state functional connectivity (rsFC) analysis revealed strong functional connections between contralateral S2 and pIns, as well as their corresponding regions in the ipsilateral hemisphere. Spatial similarity and overlap analysis identified each region as part of two distinct intrinsic functional networks with 7% overlap: sensory S2-S1-area 7b and affective pIns-ACC-PCC networks. Moreover, a high degree of overlap was observed between the combined rsFC maps of nociceptive S2 and pIns regions and the nociceptive heat-evoked activation map. In summary, our study provides evidence for the existence of two distinct intrinsic functional networks for S2 and pIns nociceptive regions, and these two networks are joined via the S2-pIns connection. Brain regions that are involved in processing nociceptive inputs are also highly interconnected at rest. The presence of robust and distinct S1-S2-area 7b and pIns-ACC-PCC rsFC networks under anesthesia underscores their fundamental roles in processing nociceptive information.
... Significant interaction was observed in the inhibitory responses of the V-left with-touch condition against the V-left without-touch condition along with the magnitude of TIVS. The detected region was the right S2 as well as the insula ( Table 2, Fig. 2B), which was also considered to be involved in tactile processing 42,43 . These results indicate that neural activations in the cortical areas related to somatosensory and visual processing were primarily involved in TIVS. ...
Crossmodal studies have demonstrated inhibitory as well as facilitatory neural effects in higher sensory association and primary sensory cortices. A recent human behavioral study reported touch-induced visual perceptual suppression (TIVS). Here, we introduced an experimental setting in which TIVS could occur and investigated brain activities underlying visuo-tactile interactions using a functional magnetic resonance imaging technique. While the suppressive effect of touch on vision was only found for half of the participants who could maintain their baseline performance above chance level (i.e. TIVS was not well replicated here), we focused on individual differences in the effect of touch on vision. This effect could be suppressive or enhancement, and the neuronal basis of these differences was analyzed. We found larger inhibitory responses in the anterior part of the right visual cortex (V1, V2) with higher TIVS magnitude when visuo-tactile stimuli were presented as spatially congruent. Activations in the right anterior superior temporal region, including the secondary somatosensory cortical area, were more strongly related to those in the visual cortex (V1, V2) with higher TIVS magnitude. These results indicate that inhibitory neural modulations from somatosensory to visual cortices and the resulting inhibitory neural responses in the visual cortex could be involved in TIVS.
... In the studies of Gelnar, Harrington, and Stippich et al., brain activation within the postcentral gyrus and superior and inferior parietal lobule have been found (Gelnar et al., 1998a;Harrington et al., 2000a;Stippich et al., 1999a). Furthermore, brain activation within Brodmann area 3a was detected due to somatosensory stimulation (Geyer et al., 1999;Geyer et al., 2000;Kurth et al., 2000), which can be explained by the fact that Brodmann area 3a receives input from the deep and from the proprioceptive receptors (Ibanez et al., 1989;Iwamura et al., 1993;Kaas et al., 1979;Maldjian et al., 1999a;Recanzone et al., 1992;Tharin and Golby, 2007). ...
... This network encompasses several areas the involvement of which remained controversial so far. Indeed, beside primary somatosensory cortices and SII/insula region (39), we revealed the activation of motor and premotor cortex (13), SMA (10, 40), superior and inferior parietal lobules (12,41,42). Thus, despite the very stringent response criteria we used, computed maps indicate a widespread network (over 10% of the surface in both hemispheres) devoted to processing median nerve input. ...
Significance
Here, we show how anatomical and functional data recorded from patients undergoing stereo-EEG can be combined to generate highly resolved four-dimensional maps of human cortical processing. We used this technique, which provides spatial maps of the active cortical nodes at a millisecond scale, to depict the somatosensory processing following electrical stimulation of the median nerve in nearly 100 patients. The results showed that human somatosensory system encompasses a widespread cortical network including a phasic component, centered on primary somatosensory cortex and neighboring motor, premotor, and inferior parietal regions, as well as a tonic component, centered on the opercular and insular areas, lasting more than 200 ms.
... The group analysis result is consistent with other imaging study results that electrotactile stimulation activates the contralateral SI area and bilateral SII areas (Boakye et al., 2000;Francis et al., 2000;Kurth et al., 1988 Lobule, which are almost consistent with the recent preceding study result using the neuroimaging technique (Burton et al., 1993;Disbrow et al., 2000;Gelnar et al., 1998;Ledburg et al., 1995;Polonara et al., 1999;Reed et al., 1998;Ruben et al., 2001). ...
Objective: The aim of this study is to investigate 1) somatotopic arrangement of the second and third fingers in SI area 2) difference of neural activation in the SI area produced by stimulation with different frequencies 3) correlation between the intensity of tactile perception by different stimulus intensity and the level of brain activation measurable by means of fMRI. Background: Somatosensory cortex can obtain the information of environmental stimuli about "where" (e.g., on the left palm), "what" (e.g., a book or a dog), and "how" (e.g., scrub gently or scrub roughly) to organism. However, compared to visual sense, the neural mechanism underlying the processing of specific electrotactile stimulus is still unknown. Method: 10 right-handed subjects participated in this study. Non-painful electrotactile stimuli were delivered to two different finger tips of right hand. Functional brain images were collected from 3.0T MRI using the single-shot EPI method. The scanning parameters were as follows: TR and TE were 3000, 35ms, respectively, flip angle 60, FOV 24{\times}24cm, matrix size 64{\times}64, slice thickness 4mm (no gap). SPM5 was used to analyze the fMRI data. Results: Significant activations produced by the stimulation were found in the SI, SII, the subcentral gyrus, the precentral gyrus, and the insula. In all participants, statistically significant activation was observed in the contralateral SI area and the bilateral SII areas by the stimulation on the fingers but ipsilaterally dominant. The SI area representing the second finger generally located in the more lateral and inferior side than that of the third finger across all the subjects. But no difference in brain area was found for the stimulation of the fingers by different frequencies. And two typical patterns were observed on the relationship between the perceived psychological intensity and the amount of voxels in the primary sensory cortex during the stimulation. Conclusion: It was possible to discriminate the representation sites in the SI by electrotactile stimulation of digit2 and digit3. But we could not find the differences of the brain areas according to different stimulation frequencies from 3 to 300Hz. Application: The results of the study can provide a deeper understanding of somatosensory cortex and offer the information for tactile display for blinds.
... The main reason for this has been the limited spatial resolution available in these fMRI neuroimaging studies, where MRI field strengths of 1.5 T and 3 T have been used. Initially, to achieve sufficiently large blood oxygen level-dependent (BOLD) responses, large voxel sizes, spatial filtering, or averaging of results across subjects was used, limiting the spatial resolution of the maps (Francis et al., 2000;Gelnar, Krauss, Szeverenyi, & Apkarian, 1998;Kurth et al., 2000;Maldjian, Gottschalk, Patel, Detre, & Alsop, 1999;Nelson & Chen, 2008;Overduin & Servos, 2008;Weibull et al., 2008). ...
The human somatosensory system processes signals relating to fine touch, pressure, and vibration. It also underlies proprioception (sense of joint position), as well as the sense of temperature and pain. This article presents a brief overview of somatosensory processing, how recent advances in high-field fMRI have allowed the mapping of primary somatosensory cortex in vivo, and current research areas of particular interest.
... Moreover, meta-analyses indicated the participation of PFt in action observation and imitation networks (59)(60)(61). In humans somatosensory activation of the posterior insula has been observed during simple stimulation paradigms, e.g., estimation of the roughness of gratings and TOR, suggesting a role in somatosensory processing (62)(63)(64)(65). Multisensory processing in the posterior insula has also been observed in primate experiments with responses also to auditory, baroreceptive and painful stimuli (66,67). ...
Aim: To describe structural covariance networks of gray matter volume (GMV) change in 28 patients with first-ever stroke to the primary sensorimotor cortices, and to investigate their relationship to hand function recovery and local GMV change.
Methods: Tensor-based morphometry maps derived from high-resolution structural images were subject to principal component analyses to identify the networks. We calculated correlations between network expression and local GMV change, sensorimotor hand function and lesion volume. To verify which of the structural covariance networks of GMV change have a significant relationship to hand function, we performed an additional multivariate regression approach.
Results: Expression of the second network, explaining 9.1% of variance, correlated with GMV increase in the medio-dorsal (md) thalamus and hand motor skill. Patients with positive expression coefficients were distinguished by significantly higher GMV increase of this structure during stroke recovery. Significant nodes of this network were located in md thalamus, dorsolateral prefrontal cortex, and higher order sensorimotor cortices. Parameter of hand function had a unique relationship to the network and depended on an interaction between network expression and lesion volume. Inversely, network expression is limited in patients with large lesion volumes.
Conclusion: Chronic phase of sensorimotor cortical stroke has been characterized by a large scale co-varying structural network in the ipsilesional hemisphere associated specifically with sensorimotor hand skill. Its expression is related to GMV increase of md thalamus, one constituent of the network, and correlated with the cortico-striato-thalamic loop involved in control of motor execution and higher order sensorimotor cortices. A close relation between expression of this network with degree of recovery might indicate reduced compensatory resources in the impaired subgroup.
... Moreover, meta-analyses indicated the participation of PFt in action observation and imitation networks (59)(60)(61). In humans somatosensory activation of the posterior insula has been observed during simple stimulation paradigms, e.g., estimation of the roughness of gratings and TOR, suggesting a role in somatosensory processing (62)(63)(64)(65). Multisensory processing in the posterior insula has also been observed in primate experiments with responses also to auditory, baroreceptive and painful stimuli (66,67). ...
Aim
To describe structural covariance networks of gray matter volume (GMV) change in 28 patients with first-ever stroke to the primary sensorimotor cortices, and to investigate their relationship to hand function recovery and local GMV change.
Methods
Tensor-based morphometry maps derived from high-resolution structural images were subject to principal component analyses to identify the networks. We calculated correlations between network expression and local GMV change, sensorimotor hand function and lesion volume. To verify which of the structural covariance networks of GMV change have a significant relationship to hand function, we performed an additional multivariate regression approach.
Results
Expression of the second network, explaining 9.1% of variance, correlated with GMV increase in the medio-dorsal (md) thalamus and hand motor skill. Patients with positive expression coefficients were distinguished by significantly higher GMV increase of this structure during stroke recovery. Significant nodes of this network were located in md thalamus, dorsolateral prefrontal cortex, and higher order sensorimotor cortices. Parameter of hand function had a unique relationship to the network and depended on an interaction between network expression and lesion volume. Inversely, network expression is limited in patients with large lesion volumes.
Conclusion
Chronic phase of sensorimotor cortical stroke has been characterized by a large scale co-varying structural network in the ipsilesional hemisphere associated specifically with sensorimotor hand skill. Its expression is related to GMV increase of md thalamus, one constituent of the network, and correlated with the cortico-striato-thalamic loop involved in control of motor execution and higher order sensorimotor cortices. A close relation between expression of this network with degree of recovery might indicate reduced compensatory resources in the impaired subgroup.
... The practice of relating neural data recorded from nonhuman primates to psychophysical data recorded from humans dates to the seminal work of Mountcastle and colleagues (Mountcastle et al. 1969;Talbot et al. 1968). Many similarities in somatosensory anatomy and physiology have been demonstrated across primate species, including similarities in peripheral receptors, and cortical organization and topography (Gelnar et al. 1998;Johnson et al. 2000). Notably, by testing trained monkeys on a tactile letter discrimination task, Hsiao et al. (1993) showed that tactile spatial acuity is also similar in monkeys and humans. ...
The ability to resolve the orientation of edges is crucial to daily tactile and sensorimotor function, yet the means by which edge perception occurs is not well understood. Primate cortical area 3b neurons have diverse receptive field (RF) spatial structures that may participate in edge orientation perception. We evaluated five candidate RF models for macaque area 3b neurons previously recorded while an oriented bar contacted the monkey's fingertip. We used a Bayesian classifier to assign each neuron a best-fit RF structure. We generated predictions for human performance by implementing an ideal observer that optimally decoded stimulus-evoked spike counts in the model neurons. The ideal observer predicted a saturating reduction in bar orientation discrimination threshold with increasing bar length. We tested 24 humans on an automated, precision-controlled bar orientation discrimination task and observed performance consistent with that predicted. We next queried the ideal observer to discover the RF structure and number of cortical neurons that best matched each participant's performance. Human perception was matched with a median of 24 model neurons firing throughout a 1 s period. The ten lowest-performing participants were fit with RFs lacking inhibitory sidebands, whereas 12 of the 14 higher-performing participants were fit with RFs containing inhibitory sidebands. Participants whose discrimination improved as bar length increased to 10 mm were fit with longer RFs; those who performed well on the 2 mm bar, with narrower RFs. These results suggest plausible RF features and computational strategies underlying tactile spatial perception and may have implications for perceptual learning.
... However, we do not see this as a full explanation since the described motor activation from these muscle groups have been observed more anteriorly (at the same axial level) during pain (Casey et al., 1996;Svensson et al., 1997;Xu et al., 1997;Peyron et al., 1998). In addition, results from a fMRI study, have indicated several activations in the somatosensory association areas (S2), ranging from 28 to 228 in the y-axis and from the inner surface of the parietal operculum to the lateral surface of postcentral gurus (BA40/43) during non-painful somatosensory stimulation of the ®ngers (Gelnar et al., 1998). These borders are in good agreement with the coordinates of the peak activations due to pain and the peak interactions in this region observed in this study. ...
It has previously been suggested that the activity in sensory regions of the brain can be modulated by attentional mechanisms during parallel cognitive processing. To investigate whether such attention-related modulations are present in the processing of pain, the regional cerebral blood ¯ow was measured using [ 15 O]butanol and positron emission tomography in conditions involving both pain and parallel cognitive demands. The painful stimulus consisted of the standard cold pressor test and the cognitive task was a computerised perceptual maze test. The activations during the maze test reproduced ®ndings in previous studies of the same cognitive task. The cold pressor test evoked signi®cant activity in the contralateral S1, and bilaterally in the somatosensory association areas (including S2), the ACC and the mid-insula. The activity in the somatosensory association areas and periaqueductal gray/midbrain were signi®cantly modi®ed, i.e. relatively decreased, when the subjects also were performing the maze task. The altered activity was accompanied with signi®cantly lower ratings of pain during the cognitive task. In contrast, lateral orbitofrontal regions showed a relative increase of activity during pain combined with the maze task as compared to only pain, which suggests the possibility of the involvement of frontal cortex in modulation of regions processing pain.
... L'étude de Sanchez-Panchuelo et co-auteurs (2010) démontre l'existence d'une représentation somatotopique des doigts à la fois dans l'aire 3b et dans l'aire 1, même si dans cette dernière, les données de l'imagerie présentent un recouvrement entre les représentations des doigts adjacents Nelson & Chen 2008 ;Kurth et al. 2000 ;Moore et al. 2000). Certains auteurs rapportent également des activations significatives au niveau de l'aire 2 (Nelson & Chen, 2008) et de l'aire 4 (Gelnar et al. 1998), sans pour autant retrouver une organisation somatotopique dans ces deux aires. ...
Thanks to the high-density electro-encephalography (HD-EEG) technique, this work aimed to evaluate the human hand somatosensory representation plasticity induced by a non-invasive neurostimulation . Increasing interest in cortical plasticity has prompted the growing use of somatosensory evoked potentials (SEPs) to estimate changes in the cortical representation of body regions. Here we first tested the effect of different sites of hand stimulation and of the density of spatial sampling in the quality of estimation of somatosensory sources. Sources of two SEP components from the primary somatosensory cortex (N20/P20 and P45) were estimated using two levels of spatial sampling (64- vs. 128-channel) and stimulation of four distal sites in the upper limbs, including single digits (1st vs. 5th) and distal nerves with comparable cortical projection (superficial branch of the radial nerve and distal ulnar nerve). The most robust separation of somatosensory sources was achieved by comparing the cortical representations of the 1st digit and the distal ulnar nerve territories on the N20/P20 component of SEPs. While both the 64- and the 128-electrode montages correctly discriminated these two areas, only the 128-electrode montage was able to significantly separate sources in the other cases, notably when using 1st vs. 5th digit stimulation. Trustworthy somatotopic distinction of cortical representations was not obtainable for the P45 component, probably because of greater activation volume, radial orientation of sources in areas 1-2 and increased variability with attention and vigilance, which may need a denser sampling and better covering of the lower part of the head. Assessment of tangential SEP components to stimulation of 1st digit vs. ulnar nerve appears the best option to assess plastic somatosensory changes, especially when using relatively low electrode sampling. Abnormal reorganization of the primary somatosensory cortex (SI) after nerve damage, in particular shrinking of deafferented regions, is thought to participate in the emergence of pain, and pain-relieving procedures have been reported to induce normalization of altered somatotopic maps. Repetitive transcranial magnetic stimulation (rTMS) of the motor cortex is able to lessen neuropathic pain, but there is no direct evidence that it may also induce plastic somatotopic changes related to pain sensation. In the second study we assessed the ability of two modes of rTMS to induce such plastic phenomena in the primary somatosensory cortex (S1), and the possibility for this effect to be correlated with variations in pain perception in healthy subjects. Source reconstruction of S1 responses elicited by peripheral electrical stimulation and recorded with high-density (128-channel) EEG revealed significant expansion of the cortical representation of the hand following a single 20-min session of high-frequency (20 Hz) rTMS. This was associated with a specific increase of pain thresholds in this same hand. A similar trend was observed using a "theta-burst" rTMS paradigm, but changes in hand representation were significantly lesser, and not associated to pain threshold increase. No plastic somatotopic changes were observed after a sham rTMS session. These results demonstrate in humans the capability of motor cortex stimulation to induce rapid plastic somatosensory changes that may counterbalance those induced by a nerve lesion, and substantiate the use of this technique as a powerful measure to treat human pain.
Cortical processing pathways for sensory information in the mammalian brain tend to be organized into topographical representations that encode various fundamental sensory dimensions. Numerous laboratories have now shown how these representations are organized into numerous cortical field maps (CMFs) across visual and auditory cortex, with each CFM supporting a specialized computation or set of computations that underlie the associated perceptual behaviors. An individual CFM is defined by two orthogonal topographical gradients that reflect two essential aspects of feature space for that sense. Multiple adjacent CFMs are then organized across visual and auditory cortex into macrostructural patterns termed cloverleaf clusters. CFMs within cloverleaf clusters are thought to share properties such as receptive field distribution, cortical magnification, and processing specialization. Recent measurements point to the likely existence of CFMs in the other senses, as well, with topographical representations of at least one sensory dimension demonstrated in somatosensory, gustatory, and possibly olfactory cortical pathways. Here we discuss the evidence for CFM and cloverleaf cluster organization across human sensory cortex as well as approaches used to identify such organizational patterns. Knowledge of how these topographical representations are organized across cortex provides us with insight into how our conscious perceptions are created from our basic sensory inputs. In addition, studying how these representations change during development, trauma, and disease serves as an important tool for developing improvements in clinical therapies and rehabilitation for sensory deficits.
Functional magnetic resonance imaging (fMRI) is a powerful tool for imaging somatosensory cortex, providing a means to non-invasively measure cortical activity in awake and behaving humans. Notably, this technique has permitted the homunculus—a hallmark of primary somatosensory cortex (S1) organization—to be examined with unprecedented detail. With the development of high-resolution fMRI (mostly at ultra-high field, 7 Tesla), it is now possible to investigate the finer topographic details of the sensory homunculus in almost any individual. Moreover, fMRI can be used to investigate other various bottom-up response properties as well as more top-down perceptual and cognitive processes (e.g., attention and prediction) across a wide range of experimental conditions. This chapter mainly focuses on tactile experiments, outlining a number of experimental paradigms and analysis techniques; practical and participant-specific difficulties are noted. Although we focus on fMRI for imaging primary somatosensory cortex, this technique can also be used to image cortical activity in other areas involved in somatosensory processing, such as secondary somatosensory cortex (S2), insular cortex, or the cerebellum.Key words
Somatotopic mapping
High-resolution Neuroimaging Hemodynamic response Digits
Receptive fields
Functional magnetic resonance imaging (fMRI) is a powerful tool for imaging somatosensory cortex, providing a means to non-invasively measure cortical activity in awake and behaving humans. Notably, this technique has permitted the homunculus – a hallmark of primary somatosensory cortex (S1) organization – to be examined with unprecedented detail. With the development of high-resolution fMRI (mostly at ultra-high field, 7 Tesla), it is now possible to investigate the finer topographic details of the sensory homunculus in almost any individual. Moreover, fMRI can be used to investigate other various bottom-up response properties as well as more top-down perceptual and cognitive processes (e.g., attention and prediction) across a wide range of experimental conditions. This chapter mainly focuses on tactile experiments, outlining a number of experimental paradigms and analysis techniques; practical and participant-specific difficulties are noted. Although we focus on fMRI for imaging primary somatosensory cortex, this technique can also be used to image cortical activity in other areas involved in somatosensory processing such as secondary somatosensory cortex (S2), insular cortex, or the cerebellum.
Vibratory (e.g., piezoelectric) devices can stimulate cortical responses from the somatosensory area during functional magnetic resonance imaging. Twelve healthy, right-handed subjects (7 males and 5 females) were scanned with a 3.0 T magnetic resonance imaging scanner and stimulated at 30-240 Hz using a piezoelectric vibrator attached to the subjects’ index fingers. The functional images were analysed to determine the brain activation region by performing random effects analyses at the group level. One-way analysis of variance was used to measure changes in frequency on brain activity. The activated regions were identified with WFU PickAtlas software, and the images were thresholded at Puncorrected < 0.001 for multiple comparisons. The average effect of frequency revealed significant activations in the right insula and right middle frontal gyrus; the corresponding region in the somatosensory area may act as a top-down control signal to improve sensory targets. Results revealed significant differences between frequencies; 90 Hz > 120 Hz activated right inferior parietal gyrus, 120 Hz > 150 Hz activated right cerebellum, and 60 Hz > 90 Hz activated right supramarginal gyrus and bilateral inferior frontal gyrus pars triangularis. Findings indicated the role of secondary somatosensory areas and the cerebellum in performing higher-order functions and discriminating various frequencies during vibratory stimulation. Increasing the patient sample size and testing higher frequencies in future experiments will contribute to furthering brain mapping of somatosensory areas.
The retinotopic map depicts the cortical neurons’ response to visual stimuli on the retina and has contributed significantly to our understanding of human visual system. Although recent advances in high field functional magnetic resonance imaging (fMRI) have made it possible to generate the in vivo retinotopic map with great detail, quantifying the map remains challenging. Existing quantification methods do not preserve surface topology and often introduce large geometric distortions to the map. In this study, we developed a new framework based on computational conformal geometry and quasiconformal Teichmüller theory to quantify the retinotopic map. Specifically, we introduced a general pipeline, consisting of cortical surface conformal parameterization, surface-spline-based cortical activation signal smoothing, and vertex-wise Beltrami coefficient-based map description. After correcting most of the violations of the topological conditions, the result was a “Beltrami coefficient map” (BCM) that rigorously and completely characterizes the retinotopy map by quantifying the local quasiconformal mapping distortion at each visual field location. The BCM provided topological and fully reconstructable retinotopic maps. We successfully applied the new framework to analyze the V1 retinotopic maps from the Human Connectome Project (n=181), the largest state of the art retinotopy dataset currently available. With unprecedented precision, we found that the V1 retinotopic map was quasiconformal and the local mapping distortions were similar across observers. The new framework can be applied to other visual areas and retinotopic maps of individuals with and without eye diseases, and improve our understanding of visual cortical organization in normal and clinical populations.
The common knowledge of the functional organization of the human primary somatosensory cortex (S1) had been primarily established by Penfield who electrically stimulated the exposed surface [referred as Brodmann area (BA)1] of S1 under neurosurgical conditions. Nevertheless, the functional information regarding the deep surface (BA 2 and 3) of S1 is poorly understood. We retrospectively analyzed all the clinical manifestations induced by extra-operative cortical electrical stimulation (ES) in 33 patients with medically intractable epilepsy who underwent stereo-electroencephalography (SEEG) monitoring for presurgical assessment. Demographic and clinical data were gathered and evaluated to delineate the determinants of the occurrence of positive responses, types of responses, and size of body regions involved. The stimulation of 244 sites in S1 yielded 198 positive sites (81.1%), most of which were located in the sulcal cortex. In multivariable analyses, no clinical or demographic factors predicted the occurrence of responses or their threshold levels. The size of body region involved in the responses had ordinal association with the stimulated BA sites (p < 0.001). Various types of responses elicited from the S1 were documented and classified, and the predictors of those responses were also assessed. Our analysis revealed the functional characteristics of the entire S1 and proved the multiplicity of functions of S1.
Objective
The classical homunculus of the human primary somatosensory cortex (S1) established by Penfield has mainly portrayed the functional organization of convexial cortex, namely Brodmann area (BA) 1. However, little is known about the functions in fissural cortex including BA2 and BA3. We aim at drawing a refined and detailed somatosensory homunculus of the entire S1.
Methods
We recruited 20 patients with drug-resistant focal epilepsy who underwent stereo-electroencephalography for preoperative assessments. Direct electrical stimulation was performed for functional mapping. Montreal Neurological Institute coordinates of the stimulation sites lying in S1 were acquired.
Results
Stimulation of 177 sites in S1 yielded 149 positive sites (84%), most of which were located in the sulcal cortex. The spatial distribution of different body-part representations across the S1 surface revealed that the gross medial-to-lateral sequence of body representations within the entire S1 was consistent with the classical “homunculus”. And we identified several unreported body-part representations from the sulcal cortex, such as forehead, deep elbow and wrist joints, and some dorsal body regions.
Conclusions
Our results reveal general somatotopical characteristics of the entire S1 cortex and differences with the previous works of Penfield.
Significance
The classical S1 homunculus was extended by providing further refinement and additional detail.
Identifying the plastic and stable components of the visual cortex after retinal loss is an important topic in visual neuroscience and neuro-ophthalmology. Humans with juvenile macular degeneration (JMD) show significant blood-oxygen-level-dependent (BOLD) responses in the primary visual area (V1) lesion projection zone (LPZ), despite the absence of the feedforward signals from the degenerated retina. Our previous study reported that V1 LPZ responds to full-field visual stimuli during the one-back task (OBT), not during passive viewing, suggesting the involvement of task-related feedback signals. Aiming to clarify whether visual inputs to the intact retina are necessary for the LPZ responses, here, we measured BOLD responses to tactile and auditory stimuli for both JMD patients and control participants with and without OBT. Participants were in- structed to close their eyes during the experiment for the purpose of eliminating retinal inputs. Without OBT, no V1 responses were detected in both groups of participants. With OBT, to the contrary, both stimuli caused substantial V1 responses in JMD patients, but not controls. Furthermore, we also found that the task- dependent activity in V1 LPZ became less pronounced when JMD patients opened their eyes, suggesting that task-related feedback signals can be partially suppressed by residual feedforward signals. Modality-in- dependent V1 LPZ responses only in the task condition suggest that V1 LPZ responses reflect task-related feedback signals rather than reorganized feedforward visual inputs.
This thesis provides a systematic analysis of registration algorithms for application to medical images. We divide our survey into four parts, each of which concentrates on a particular aspect of the algorithms. Linear methods are reviewed in respect to their selection of corresponding features, and their application to the inter-modality case. Non-linear methods are extensively analysed to understand the transition from a conceptual physical or statistical model to the mathematical model and its implementation. A chapter is devoted to hierarchical methods and their application to solving the local minima problem. Constructions of hierarchies are grouped as temporal variations in data complexity, in warp complexity and in model complexity. These divisions are paralleled in the classification of inhomogeneous methods, where the application of an algorithm varies spatially within the image. Thus we identify variances in relevance of data, in deformability and in chosen model type. In respect of these divisions, we have introduced a nomenclature to describe the restriction or otherwise of the deformation of selected regions in the image. We distinguish between passive and actively-deforming regions, between strongly and weakly deformable regions, and describe two specialisations of rigid regions, namely those which are motionless and those which are independently moving. The second main contribution of this work is in presenting three inhomogeneous variants to the viscous fluid registration algorithm, one for each of the three classes of inhomogeneity an algorithm may exhibit. In particular one of the variants exhibits a varying viscosity over the image. They are all tested for their ability to restrict the deformation of a specified region independently of the information contained within it. Finally we evaluate a selection of non-linear registration algorithms using both global and local registration metrics in a variety of tests. The dissertation concludes with three interesting suggestions for future projects.
The utility of functional magnetic resonance (fMR) imaging in patients with implanted thalamic electrodes has not yet been determined. The aim of this study was to establish the safety of performing fMR imaging in patients with thalamic deep brain stimulators and to determine the value of fMR imaging in detecting cortical and subcortical activity during stimulation.
Functional MR imaging was performed in three patients suffering from chronic pain and two patients with essential tremor. Two of the three patients with pain had undergone electrode implantation in the thalamic sensory ventralis caudalis (Vc) nucleus and the other had undergone electrode implantation in both the Vc and the periventricular gray (PVG) matter. Patients with tremor underwent electrode implantation in the ventralis intermedius (Vim) nucleus. Functional MR imaging was performed during stimulation by using a pulse generator connected to a transcutaneous extension lead. Clinically, Vc stimulation evoked paresthesias in the contralateral body, PVG stimulation evoked a sensation of diffuse internal body warmth, and Vim stimulation caused tremor arrest.
Functional images were acquired using a 1.5-tesla MR imaging system. The Vc stimulation at intensities provoking paresthesias resulted in activation of the primary somatosensory cortex (SI). Stimulation at subthreshold intensities failed to activate the SI. Additional stimulation-coupled activation was observed in the thalamus, the secondary somatosensory cortex (SII), and the insula. In contrast, stimulation of the PVG electrode did not evoke paresthesias or activate the SI, but resulted in medial thalamic and cingulate cortex activation. Stimulation in the Vim resulted in thalamic, basal ganglia, and SI activation.
An evaluation of the safety of the procedure indicated that significant current could be induced within the electrode if a faulty connecting cable (defective insulation) came in contact with the patient. Simple precautions, such as inspection of wires for fraying and prevention of their contact with the patient, enabled the procedure to be conducted safely. Clinical safety was further corroborated by performing 86 MR studies in patients in whom electrodes had been implanted with no adverse clinical effects.
This is the first report of the use of fMR imaging during stimulation with implanted thalamic electrodes. The authors' findings demonstrate that fMR imaging can safely detect the activation of cortical and subcortical neuronal pathways during stimulation and that stimulation does not interfere with imaging. This approach offers great potential for understanding the mechanisms of action of deep brain stimulation and those underlying pain and tremor generation.
Pain was first considered to be a hard-wired system in which noxious input was passively transmitted along sensory channels to the brain. However, today it is generally accepted that the experience of pain is not simply driven by noxious stimulus characteristics, but that the brain is the structure where the subjective perception of pain emerges and is critically linked with other cognitive processes.
The field of pain research has progressed immensely due to the advancement of brain imaging techniques. The initial goal of this research was to expand our understanding of the cerebral mechanisms underlying the perception of pain; more recently the research objectives have shifted toward chronic pain—understanding its origins, developing methods for its diagnosis, and exploring potential avenues for its treatment. While several different neuroimaging approaches have certain advantages for the study of pain, fMRI has ultimately become the most widely utilized imaging technique over the past decade because of its noninvasive nature, high-temporal and spatial resolution, and general availability; thus, the following chapter will focus on fMRI and the special aspects of this technique that are particular to pain research.
The relationship between consciousness and brain imaging is discussed from the vantage point of pain perception. It is argued that taking into consideration the temporal variations in perceived pain can pinpoint the brain circuitry specifically related to conscious, subjective perception of pain. Moreover, that this procedure is general enough to be used in examining clinical pain conditions. Brain regions activated with painful stimuli have now been described by a variety of laboratories using either positron emission tomography (PET) or functional magnetic resonance imaging (fMRI). These studies have identified a large cortical network with many components. Pinpointing the functional roles of the different components of this network is a major focus of our current fMRI studies. To this end we examine the cortical circuitry of pain along the stimulus-perception dimension, and also demonstrate that small changes in painful stimulus parameters result in dramatic reorganization of the cortical network underlying pain perception. We also examine this cortical network underlying pain in different pain patient groups: one patient with large fiber polyneuropathy over the whole body below the neck, chronic reflex sympathetic dystrophy (RSD or CRPS I) patients, chronic back pain patients, and patients with syringomyelia. Cortical activations with painful stimuli or following the exacerbation of the pain that the patients suffer from results in giving us different insights to the spatio-temporal dynamics of the cortical network underlying pain and its re-organization in time.
BACKGROUND AND PURPOSE: Our objective was to map by means of a conventional mid-field (1.0 T) MR imaging system the somatosensory areas activated by unilateral tactile stimulation of the hand, with particular attention to the areas of the ipsilateral hemisphere. METHODS: Single-shot echo-planar T2*-weighted imaging sequences were performed in 12 healthy volunteers to acquire 10 contiguous 7-mm-thick sections parallel to the coronal and axial planes during tactile stimulation of the hand. The stimulation paradigm consisted of brushing the subjects' palm and fingers with a rough sponge at a frequency of about 1 Hz. RESULTS: Stimulation provoked a signal increase (about 2% to 5%) that temporally corresponded to the stimulus in several cortical regions of both hemispheres. Contralaterally, activation foci were in the anterior parietal cortex in an area presumably corresponding to the hand representation zone of the first somatosensory cortex, in the posterior parietal cortex, and in the parietal opercular cortex forming the upper bank of the sylvian sulcus and probably corresponding to the second somatosensory cortex. Activation foci were also observed in the frontal cortex. Ipsilaterally, activated areas were in regions of the posterior parietal and opercular cortices roughly symmetrical to those activated in the contralateral hemisphere. The same activation pattern was observed in all subjects. CONCLUSION: The activated areas of the somatosensory cortex described in the present study corresponded to those reported in other studies with magnetoelectroencephalography, positron emission tomography, and higher-field functional MR imaging. An additional area of activation in the ipsilateral parietal operculum, unnoticed in other functional MR imaging studies, was also observed.
This chapter focuses on a key characteristic of the somatosensory as well as the motor system, that is, its somatotopic organization. The good spatial resolution of functional MRI (fMRI) as compared to previous functional neuroimaging approaches such as positron emission tomography has allowed noninvasive studies at hitherto unknown spatio-anatomical precision. While early fMRI studies mostly were confirmatory of previously known features of sensorimotor organization, recent studies are shaping a new concept of cortical representations. Besides the somatotopic arrangement, they also take into account the pronounced overlap of body representations, that is, its network characters, particularly for functional units such as the hand. In addition to elucidating general features of sensorimotor neurophysiology, fMRI is ideally suited for identifying individual neuroplasticity. It allows to monitor the cerebral effects of learning processes, such as learning to play an instrument, but also the reaction to pathological events such as peripheral and central lesions to the nervous system.
Unlabelled:
Studies of human primary somatosensory cortex (S1) have placed a strong emphasis on the cortical representation of the hand and the propensity for plasticity therein. Despite many reports of group differences and experience-dependent changes in cortical digit somatotopy, relatively little work has considered the variability of these maps across individuals and to what extent this detailed functional architecture is dynamic over time. With the advent of 7 T fMRI, it is increasingly feasible to map such detailed organization noninvasively in individual human participants. Here, we extend the ability of ultra-high-field imaging beyond a technological proof of principle to investigate the intersubject variability of digit somatotopy across participants and the stability of this organization across a range of intervals. Using a well validated phase-encoding paradigm and an active task, we demonstrate the presence of highly reproducible maps of individual digits in S1, sharply contrasted by a striking degree of intersubject variability in the shape, extent, and relative position of individual digit representations. Our results demonstrate the presence of very stable fine-grain somatotopy of the digits in human S1 and raise the issue of population variability in such detailed functional architecture of the human brain. These findings have implications for the study of detailed sensorimotor plasticity in the context of both learning and pathological dysfunction. The simple task and 10 min scan required to derive these maps also raises the potential for this paradigm as a tool in the clinical setting.
Significance statement:
We applied ultra-high-resolution fMRI at 7 T to map sensory digit representations in the human primary somatosensory cortex (S1) at the level of individual participants across multiple time points. The resulting fine-grain maps of individual digits in S1 reveal the stability in this fine-grain functional organization over time, contrasted with the variability in these maps across individuals.
Competing principles for the functional-anatomical organization of human primary motor cortex include first, whether muscles or movements are represented in the cortex; and second, whether the large-scale somatotopic principle also applies within the representation of limbs or whether it is replaced by mosaical network arrangement. We review evidence supporting a composite arrangement, wherein both pairs of the seemingly opposite mapping principles can coexist. Presence of somatotopic gradients within motor cortex allows for occasional clinical observations of relatively focal motor pareses. In contrast, there is much less controversy regarding somatotopic maps in the primary somatosensory cortex. Motor and somatosensory cortices show certain similarities, but also significant differences in their functional topography. While the distributed character dominates in M1 and S1, a somatotopic arrangement exists for both M1 and S1 hand representations, with the S1 somatotopy being more discrete and segregated in contrast to the more integrated and overlapping somatotopy in M1. Overall, the different topographic organization may reflect an organization optimized for three-dimensional control of a diverse repertoire of movements (M1) or for predominantly two-dimensional localization of somatosensory input (S1).
The field of pain research has progressed immensely due to the advancement of brain imaging techniques. The initial goal of this research was to expand our understanding of the cerebral mechanisms underlying the perception of pain; more recently the research objectives have shifted toward chronic pain - understanding its origins, developing methods for its diagnosis, and exploring potential avenues for its treatment. While several different neuroimaging approaches have certain advantages for the study of pain, fMRI has ultimately become the most widely utilized imaging technique over the past decade because of its noninvasive nature, high-temporal and spatial resolution, and general availability; thus, the following chapter will focus on fMRI and the special aspects of this technique that are particular to pain research. Subheading 1 begins with a brief review on the spinal pathways and neuroanatomical regions involved in pain processing, and highlights the novel information that has been gained about these structures and their function through the use of fMRI and other neuroimaging techniques. Subheading 2 reviews a few of the aspects associated with the blood-oxygen-level-dependent signal commonly used in fMRI, as they apply to the particular challenges of pain research. Likewise, Subheading 3 summarizes some of the special considerations of experimental design and statistical analysis that are encountered in pain research and their applications to fMRI studies. Subheading 4 reviews special applications of fMRI for the study of higher cognitive processes implicated in pain processing, including pain empathy and cognitive reappraisal of one's own pain perception. The chapter concludes with Subheading 5, exploring some of the future prospects of fMRI techniques and new applications related to pain research.
Neurophysiological pathology in CRPS type I is characterized by somatosensory and motor deficits. Transcranial magnetic stimulation (TMS) and functional MRI reveal decreased short intracortical inhibition (SICI) and smaller representation distance in the hand region of primary somatosensory cortex (S1) between the first and fifth digit in CRPS. Interestingly, an application of anesthetic cream to the forearm increases spatial tactile resolution (STR) and also SICI of hand muscles in healthy participants. As such, this intervention might be suited to reverse pathologic findings in CRPS patients and tested this in a double blinded placebo controlled study. In addition we used high spatial resolution fMRI of the thumb (D1) and pinky (D5) S1 BA 3b representation before and after intervention. Patients showed decreased STR over D1 (tip of thumb) on their affected side if compared to the unaffected side ( t =2.31; p t =3.0; p t =2.22; p
This chapter provides an overview of the organization of the somatosensory system of humans. It emphasizes on the components of somatosensory system that are important in identifying objects and features of surfaces by touch. Even though small shapes can be perceived with information solely from tactile receptors, most discrimination involves an active process of tactile exploration with multiple contacts on the skin and an integration of cutaneous and proprioceptive information as well as efferent control. Thus, this chapter concentrates on the pathways and neural centers for processing information from the low-threshold mechanoreceptors of the skin that provide information about touch, and the deeper receptors in joints and especially muscles that provide information about position. It is observed that conclusions are based on both studies in humans and studies in other primates, especially the frequently studied macaque monkeys. Moreover, the early stages of processing are likely to be similar in humans and monkeys, but humans appear to have a more expanded cortical network for processing somatosensory information.
1. Single unit impulses were recorded with percutaneously inserted tungsten needle electrodes from the median nerve in conscious human subjects. 2. A sample of 334 low threshold mechanoreceptive units innervating the glabrous skin area of the hand were studied. In accordance with earlier investigations, the units were separated into four groups on the basis of their adaptation and receptive field properties: RA, PC, SA I and SA II units. 3. The locations of the receptive fields of individual units were determined and the relative unit densities within various skin regions were calculated. The over-all density was found to increase in the proximo-distal direction. There was a slight increase from the palm to the main part of the finger and an abrupt increase from the main part of the finger to the finger tip. The relative densities in these three regions were 1, 1.6, 4.2. 4. The differences in over-all density were essentially accounted for by the two types of units characterized by small and well defined receptive fields, the RA and SA I units, whereas the PC and SA II units were almost evenly distributed over the whole glabrous skin area. 5. The spatial distribution of densities supports the idea that the RA and SA I units account for spatial acuity in psychophysical tests. This capacity is known to increase in distal direction along the hand. 6. On the basis of histological data regarding the number of myelinated fibres in the median nerve, a model of the absolute unit density was proposed. It was estimated that the density of low threshold mechanoreceptive units at the finger tip is as high as 241 u./cm2, whereas in the palm it is only 58 u./cm2.
We report that visual stimulation produces an easily detectable (5-20%) transient increase in the intensity of water proton magnetic resonance signals in human primary visual cortex in gradient echo images at 4-T magnetic-field strength. The observed changes predominantly occur in areas containing gray matter and can be used to produce high-spatial-resolution functional brain maps in humans. Reducing the image-acquisition echo time from 40 msec to 8 msec reduces the amplitude of the fractional signal change, suggesting that it is produced by a change in apparent transverse relaxation time T*2. The amplitude, sign, and echo-time dependence of these intrinsic signal changes are consistent with the idea that neural activation increases regional cerebral blood flow and concomitantly increases venous-blood oxygenation.
Progress has recently been made in implementing magnetic resonance imaging (MRI) techniques that can be used to obtain images
in a fraction of a second rather than in minutes. Echo-planar imaging (EPI) uses only one nuclear spin excitation per image
and lends itself to a variety of critical medical and scientific applications. Among these are evaluation of cardiac function
in real time, mapping of water diffusion and temperature in tissue, mapping of organ blood pool and perfusion, functional
imaging of the central nervous system, depiction of blood and cerebrospinal fluid flow dynamics, and movie imaging of the
mobile fetus in utero. Through shortened patient examination times, higher patient throughput, and lower cost per MRI examination,
EPI may become a powerful tool for early diagnosis of some common and potentially treatable diseases such as ischemic heart
disease, stroke, and cancer.
Knowledge of regional cerebral hemodynamics has widespread application for both physiological research and clinical assessment because of the well-established interrelation between physiological function, energy metabolism, and localized blood supply. A magnetic resonance technique was developed for quantitative imaging of cerebral hemodynamics, allowing for measurement of regional cerebral blood volume during resting and activated cognitive states. This technique was used to generate the first functional magnetic resonance maps of human task activation, by using a visual stimulus paradigm. During photic stimulation, localized increases in blood volume (32 +/- 10 percent, n = 7 subjects) were detected in the primary visual cortex. Center-of-mass coordinates and linear extents of brain activation within the plane of the calcarine fissure are reported.
Microelectrode mapping methods were used to define and describe 3 representations of the body surface in somatosensory cortex of marmosets: S-I proper or area 3b of anterior parietal cortex, S-II, and the parietal ventral area (PV) of the upper bank of the lateral sulcus. In the same animals, injections of anatomical tracers were placed into electrophysiologically determined sites in area 3b or S-II. Mapping results and patterns of connections were later related to architectonic fields that were delimited in sections cut parallel to the surface of manually flattened cortex and stained for myelin. There were several major results. (1) Recordings from area 3b revealed a characteristic somatotopic organization of foot to face in a mediolateral sequence as previously reported in other members of the marmoset family (Carlson et al., 1986). (2) Multiple injections of WGA-HRP in area 3b demonstrated dense, patchy interconnections with ipsilateral S-II, PV, area 3a, and area 1, less dense interconnections with primary motor cortex (M-I), the supplementary motor area (SMA), limbic cortex of the medial wall (L), and rostrolateral parietal cortex of the lateral sulcus (PR), and callosal connections with areas 3b, S-II, and PV. Injections of 3 different tracers into the representation of 3 body regions in area 3b indicated that the connections with areas 3a, 3b, 1, S-II, and PV are topographically organized. (3) Recordings from cortex on the upper bank of the lateral sulcus demonstrated a somatotopic representation of the body surface that matches that of S-II of other mammals. S-II immediately adjoined areas 3b along the dorsal lip of the lateral sulcus. The face representation in S-II was adjacent to the face representation in 3b while the trunk, hindlimb, and forelimb were represented in a caudorostral sequence deeper in the sulcus. (4) Injections in S-II revealed ipsilateral connections with areas 3a, 3b, 1, a presumptive area 2, PV, PR, M-I, SMA, limbic cortex, the frontal eye fields, and the frontal ventral visual area. Dense callosal connections were with S-II and PV. (5) The recordings also revealed a systematic representation just rostral to S-II that has not been previously described in primates.(ABSTRACT TRUNCATED AT 400 WORDS)
1. The anatomic generators of human median nerve somatosensory evoked potentials (SEPs) in the 40 to 250-ms latency range were investigated in 54 patients by means of cortical-surface and transcortical recordings obtained during neurosurgery. 2. Contralateral stimulation evoked three groups of SEPs recorded from the hand representation area of sensorimotor cortex: P45-N80-P180, recorded anterior to the central sulcus (CS) and maximal on the precentral gyrus; N45-P80-N180, recorded posterior to the CS and maximal on the postcentral gyrus; and P50-N90-P190, recorded near and on either side of the CS. 3. P45-N80-P180 inverted in polarity to N45-P80-N180 across the CS but was similar in polarity from the cortical surface and white matter in transcortical recordings. These spatial distributions were similar to those of the short-latency P20-N30 and N20-P30 potentials described in the preceding paper, suggesting that these long-latency potentials are generated in area 3b of somatosensory cortex. 4. P50-N90-P190 was largest over the anterior one-half of somatosensory cortex and did not show polarity inversion across the CS. This spatial distribution was similar to that of the short-latency P25-N35 potentials described in the preceding paper and, together with our and Goldring et al. 1970; Stohr and Goldring 1969 transcortical recordings, suggest that these long-latency potentials are generated in area 1 of somatosensory cortex. 5. SEPs of apparently local origin were recorded from several regions of sensorimotor cortex to stimulation of the ipsilateral median nerve. Surface and transcortical recordings suggest that the ipsilateral potentials are generated not in area 3b, but rather in other regions of sensorimotor cortex perhaps including areas 4, 1, 2, and 7. This spatial distribution suggests that the ipsilateral potentials are generated by transcallosal input from the contralateral hemisphere. 6. Recordings from the periSylvian region were characterized by P100 and N100, recorded above and below the Sylvian sulcus (SS) respectively. This distribution suggests a tangential generator located in the upper wall of the SS in the second somatosensory area (SII). In addition, N125 and P200, recorded near and on either side of the SS, suggest a radial generator in a portion of SII located in surface cortex above the SS. 7. In comparison with the short-latency SEPs described in the preceding paper, the long-latency potentials were more variable and were more affected by intraoperative conditions.
FUNCTIONAL MAGNETIC RESONANCE imaging (FMRI) has detected changes in regional cerebral blood flow and volume in response to motor movements, visual stimuli, and auditory stimuli in each of their respective primary cortices. This experiment was conducted to determine whether signal changes in the somatosensory cortex secondary to tactile stimulation could be demonstrated. The palm of the right hand was periodically stimulated while the subject was undergoing echo-planar imaging with a 1.5-T magnetic resonance scanner equipped with local gradient and radio frequency coils. Sagittal and coronal images of 10- to 15-mm slice thickness were selected to include the postcentral gyrus and surrounding regions. Temporally correlated signal changes of 1% to 5% occurred in the peri-rolandic region in each of six subjects. The time course of signal changes was comparable to that found in other primary sensory and motor cortices. The results provide preliminary evidence of the sensitivity of FMRI to activation of the somatosensory cortex with tactile stimulation and support FMRI as a promising noninvasive technique for study of the functional organization and integrity of the cerebrum.
Recently published psychophysical studies have shown that the capacity of human subjects to identify direction of tactile stimuli that move in a linear path across the thenar eminence and the upper arm is a function of velocity, of the distance traversed by the moving stimulus (“traverse length”) and of the cutaneous innervation density (Dreyer, Hollins, and Whitsel, 1976; Dreyer, Duncan, and Wong, 1978a; Dreyer, Hollins, and Whitsel, 1978b). Moreover, a subject’s capacity to identify direction of linear movement on either the hairy or glabrous skin is independent of the orientation of the stimulus path (Whitsel and Dreyer, unpublished observations).
ABSTRACT Microelectrode multiunit,mapping,studies of parietal cortex in owl monkeys,indicate that the classical “primary” somatosensory,region (or “SI”) including the separate architectonic fields 3a, 3b, 1, and 2 contains as many as four separate representations of the body rather than one, An analysis of receptive field locations for extensive arrays of closely placed recording sites in parietal cortex which,were later related , contains a sys-
We report that visual stimulation produces an easily detectable (5-20%) transient increase in the intensity of water proton magnetic resonance signals in human primary visual cortex in gradient echo images at 4-T magnetic-field strength. The observed changes predominantly occur in areas containing gray matter and can be used to produce high-spatial-resolution functional brain maps in humans. Reducing the image-acquisition echo time from 40 msec to 8 msec reduces the amplitude of the fractional signal change, suggesting that it is produced by a change in apparent transverse relaxation time T*2. The amplitude, sign, and echo-time dependence of these intrinsic signal changes are consistent with the idea that neural activation increases regional cerebral blood flow and concomitantly increases venous-blood oxygenation.
Positron emission tomography (PET) measurements of brain blood flow were used to monitor changes in the human primary and secondary somatosensory cortices during the period when somatosensory stimuli were expected. In anticipation of either focal or innocuous touching, or localized, painful shocks, blood flow decreased in parts of the primary somatosensory cortex map located outside the representation of the skin area that was the target of the expected stimulus. Specifically, attending to an impending stimulus to the fingers produced a significant decrease in blood flow in the somatosensory zones for the face, whereas attending to stimulation of the toe produced decreases in the zones for the fingers and face. Decreases were more prominent in the side ipsilateral to the location of the expected stimulus. No significant changes in blood flow occurred in the region of the cortex representing the skin locus of the awaited stimulation. These results are concurrent with a model of spatial attention in which potential signal enhancement may rely on generalized suppression of background activity.
Changes in the regional cerebral blood flow (rCBF) are often used as an indicator of changes in local neuronal activity of the brain. We present here quantitative measurements with positron emission tomography (PET) of rCBF with a freely diffusible flow tracer 15O-butanol in control and activation states, and a pixel-by-pixel statistical parametric analysis of the rCBF changes combined with a cluster analysis. Anatomically standardized rCBF activation data from 39 normal subjects were analyzed for the occurrence of clustered voxels. Noise data was obtained from repeat measurements of rCBF with the brain in the activated state and from simulations. The variance in test-control images was largest outside the skull, and large in soft tissue regions around the brain. It was moderate but inhomogenous in gray matter, and low and homogenous in white matter. A special picture was generated of the conformation of sampled data with a normal distribution. In the gray and white matter, the pixel values were fount to conform to a normal distribution, permitting calculation of Student's t-images. In the cluster analysis, activations are detected as clusters of voxels with high t-values. The clusters in activated regions, however, were considerably larger than the full width half-maximum of spatial autocorrelation function and were easily detected. Tables of the empirical Poisson-like distributions of the number of clusters of different sizes are provided, from which P values of the significance of the occurrence of clusters of different sizes can be estimated. © 1993 Wiley-Liss, Inc.
This study of somatosensory discrimination of rectangular parallelepipeds with the right hand had three purposes: (i) to describe the exploratory finger movements; (ii) to reveal the anatomical brain structures specifically engaged in the production of exploratory finger movements; and (iii) to reveal the anatomical structures specifically engaged in the discrimination of tactually sensed shape. The thumb was the most active finger, moving with a mean exploration frequency of 2.4 Hz, as evident from videotape records of the exploratory finger movements. The cerebral structures activated during somatosensory discrimination were mapped by measurements of regional cerebral blood flow (rCBF) in six healthy male volunteers with positron emission tomography (PET) and the use of the computerized brain atlas of Greitz et al. (1991, J. Comp. Ass. Tomogr., 15, 26–38). The rCBF changes caused by somatosensory discrimination were compared point-to-point to a PET-study on right-hand finger movements and a PET-study on vibration stimulation of the right hand. From these results the following conclusions were drawn. The rCBF increase in the left superior parietal lobule indicated the site engaged in the analysis of shape. The rCBF increases in the left supplementary sensory area, bilaterally in premotor areas, in the left putamen, the right dentate nucleus and bilaterally in the posterior cerebellum were related to the control of the tactile exploratory finger movements. The rCBF increases in the right homologue of Broca's area, bilaterally in the superior prefrontal cortex and in the right midfrontal cortex probably resulted from working memory, the direction of attention, and the discrimination process.
The sections in this article are:
Detailed microelectrode maps of the hand representation were derived in cortical areas 3b and 1 from a series of normal adult owl and squirrel monkeys. While overlap relationships were maintained, and all maps were internally topographic, many map features varied significantly when examined in detail. Variable features of the hand representations among different monkeys included (a) the overall shapes and sizes of hand surface representations; (b) the actual and proportional areas of representations of different skin surfaces and the cortical magnifications of representations of specific skin surfaces, which commonly varied severalfold in area 3b and manyfold in area 1; (c) the topographic relationships among skin surface representations, with skin surfaces that were represented adjacently in some monkeys represented in locations many hundreds of microns apart in others; (d) the internal orderliness of representations; (e) the completeness of representations of the dorsal hand surfaces; and (f) the skin surfaces represented along the borders of the hand representation.
Owl monkey maps were, in general, internally more strictly topographic than squirrel monkey maps. In both species, area 3b was more strictly topographic and less variable than was area 1.
The degree of individual variability revealed in these experiments is difficult to reconcile with the hypothesis that details of cortical maps are ontogenetically specified during a period in early life. Instead, we propose that differences in the details of cortical map structure are the consequence of individual differences in lifelong use of the hands. This conclusion is consistent with earlier studies of the consequences of peripheral nerve transection and digital amputation, which revealed that cortical maps are dynamically maintained and are alterable as a function of use or nerve injury in these monkeys (Merzenich et al., '83a, b, '84a; Merzenich, '86; Jenkins et al., '84; Jenkins and Merzenich, '87).
Microelectrode multiunit mapping studies of parietal cortex in owl monkeys indicate that the classical “primary” somatosensory region (or “SI”) including the separate architectonic fields 3a, 3b, 1, and 2 contains as many as four separate representations of the body rather than one. An analysis of receptive field locations for extensive arrays of closely placed recording sites in parietal cortex which were later related to cortical architecture led to the following conclusions: (1) There are two large systematic representations of the body surface within “SI”. Each is activated by low threshold cutaneous stimuli; one representation is coextensive with Area 3b and the other with Area 1. (2) While each of these representations contain regions of cortex with topological or “somatotopic” transformations of skin surface, the representations have many discontinuities where adjoining skin surfaces are adjoining in the representations. Thus, the representations can be considered as composites of somatotopically organized regions, but cannot be accurately depicted by simple continuous homunculi. Lines of discontinuity often cut across dermatomes and seldom follow dermatomal boundaries, i.e., neither cutaneous representation constitutes a systematic representation of dermatomal skin fields. (3) While the two cutaneous fields are basically similar in organization and are approximate mirror images of each other, they differ in important details, i.e., lines of discontinuity in the representations and the sites of representations of different specific skin surfaces differ significantly in the two representations. (4) The two cutaneous representations also differ in size and in the relative proportions in each representation differ, they cannot both be simple reflections of overall peripheral innervation density. (5) All or part of Area 2 contains a systematic representation of deep body structures.
The representation of the hand and fingers in area 1 of the first somatosensory cortex was studied in conscious monkeys by recording single neuronal activity. The results are as follows.(1)
We found multi-finger type receptive fields which cover more than one finger discontinuously or wide-field type ones which cover both finger and palmar skin or two halves of the palmar skin together. Multi-finger type receptive fields were also found in some joint manipulation neurons. Multifinger or wide-field type receptive fields were found in nearly 40% of area 1 neurons. The rate was even higher, up to 70%, in the medial part of the cortical finger region. Consequently, the finger representation in area 1 was less discretely somatotopic than that in area 3b.
(2)
The submodality content of area 1 was almost identical to that of area 3b: 74.5% and 20.9% of identified neurons were, respectively, cutaneous and deep. The distribution of neurons with different submodalities overlapped in area 1.
(3)
Among area 1 neurons with multi-finger type receptive fields, response characteristics of those with inhibitory receptive fields, those with directional selectivity to moving stimuli, and those with converging afferent inputs, were studied in detail. Evidence is presented to suggest that information from different parts of the body, or from the same body parts but different afferent sources, is integrated in area 1.
(4)
It is proposed that, within the SI, area 1 is the initial stage of integration of sensory information coming from the thalamus and from area 3a or 3b via cortico-cortical connections.
We have compared spatial patterns of somatosensory evoked magnetic fields (SEFs) to stimulation of the ulnar and median nerves at the wrist. An oddball paradigm was used additionally to examine whether an infrequent change in the stimulation site would alter the field pattern. The response consisted of 3 parts: an early small deflection at 22-28 msec, a large deflection peaking between 34 and 86 msec, and a late deflection at 110-180 msec. The wave forms and amplitudes of the responses to ulnar and median nerve stimulation were similar, without any additional deflections for the infrequent stimuli. The field patterns, which were interpreted in terms of the dipole model, could be explained by activation of the primary sensorimotor cortex during all peaks of the response. For the early parts of the response at 22-46 msec, the locations of the equivalent sources for median and ulnar nerve stimulation differed from each other, in agreement with the known somatotopy of SI. No somatotopical order was found for the sources of the later deflections.
This book is based on the Lane Medical Lectures which Penfield gave in 1947. Four hundred craniotomies performed between 1928 and 1947 presented the opportunity to stimulate various parts of the cerebral cortex with electrical currents and to record the objective (movements) and subjective effects. The results of these studies are presented clearly and with the necessary details. Objective results and interpretation are sharply separated. The investigations give a very complete description of the organization of the sensorimotor cortex. The primitive character of the movements is emphasized. They are "not more complicated than those the newborn infant is able to perform." Evidence of the existence of a secondary motor cortex is also presented. A certain muscle may show widely separated cortical foci when it is used in different functional groupings. Central overlap exists clearly in precentral and postcentral gyrus. The authors assume that the diencephalon plays an important role in
The temporal (time-analysis) component of the statistical parametric mapping of the method of Friston et al. ([1994]: Human Brain Mapping 1:153-171) is evaluated using stimulated time-course data sets and images acquired during a motor-task activation study. The results indicate that their method for estimating hemodynamic delay parameters is unreliable when the hemodynamic delay time is greater than 1-2 image frames. Problems also arise when system noise is present in addition to physiologic noise. These conditions apply for most task-activation studies with image acquisition rates (TRs) shorter than about 3 sec, and for images acquired at field strengths below 3-4 tesla. Modifications are suggested that may make their method usable under these conditions.
We have used functional magnetic resonance imaging (fMRI) to study the changes in cerebellar activation that occur during the acquisition of motor skill in human subjects presented with a new task. The standard paradigm consisted of a center-out movement in which subjects used a joystick to superimposed a cursor onto viusual targets. Two variations of this paradigm were introduced: (1) a learning paradigm, where the relationship between movement of the joystick and cursor was reversed, requiring the learning of a visuomotor transformation to optimize performance and (2) a random paradigm, where the joystick/cursor relationship was changed randomly for each trial. Activation in the cerebellum was highest during the random paradigm and during the early stages of the learning paradigm. In the early stages of learning and during the random paradigm performance was poor with a decrease in the number of completed movements, and an increase in the time and length of movements. With repeated practice at the learning paradigm performance improbed and reached the same level of proficiency as in the standard task. Commensurate with the improbement in performance was a decrease in cerebellar activation, that is, activation in the cerebellum changed in a parallel, but inverse relationship with performance. Linear regression analysis demonstarated that the inverse correlation between cerebellar activation and motor performance was significant. Repeated practice at the random paradigm did not produce improvements in performance and cerebellar activity remained high. The data support the hypothesis that the cerebellum is strongly activated when motor performance is inaccurate, consistent with a role for the cerebellum in the detection of, and correction for visuomotor errors.
This study is an investigation into the reproducibility of brain activation in the sensorimotor cortex obtained with 3D "PRESTO" fMRI on eleven normal subjects. During one session, two series of functional scans were acquired while the subjects performed a finger opposition task (2 Hz). Nine subjects were tested once more on a different day. Each individual motor trial was analyzed separately, with a conservative zt-based method. Using these results, the agreement between repeated series was examined in a number of ways, comparing the two series within one session, and the two series across sessions. In 28 of the 31 series (90%) significant signal change was found in the contralateral primary sensorimotor cortex (PSM). Overall, 0.20% of all voxels (total about 11,000) in the scanned volume reached significance, and approximately 60% of the significant positive signal changes were located in the PSM (P<5x10(-7) for a chance occurrence). Comparisons within and across sessions yielded similar results: there was a 20-30% overlap of the clusters of activated voxels in the PSM (chance overlap within the PSM: P<0.01). The mean distance between zt-weighted centers of mass was 4.0-4.4 mm (chance distance within the PSM: P=0.033 and 0.058, respectively). No significant difference was found between series in the magnitude of significant signal change. Whereas the number of activated voxels in the PSM was not consistently correlated between series, the ratio of this number over the total number of activated voxels in the scanned volume was significantly correlated (rho=0.75-0.79, P<0.05). These results indicate that activation in sensorimotor cortex associated with oppositional finger movement is reliably mapped with 3D PRESTO fMRI.
In: Journal f. Psychol. u. Neurol, Bd. 35 Berlin, Med. Diss., 1928.
The findings obtained in neurophysiological and psychophysical investigations using tactile stimuli that move at constant velocity across the skin are reviewed. For certain neurons in the postcentral gyrus of the cerebral cortex (S-I) of macaque monkeys, direction of stimulus motion is a "trigger feature" i.e., moving tactile stimuli evoke vigorous discharge activity in these neurons only if the stimuli are moved in a particular direction across the receptive field. This directional selectivity is maximal when stimulus velocity is between 5 and 50 cm/sec, and falls off rapidly at lower or higher velocities. The capacity for human subjects to correctly identify the direction of stimulus motion on the skin exhibits a similar dependence on stimulus velocity. The similar effects of velocity on neural and psychophysical measures of directional sensitivity support the idea that direction of stimulus motion on the skin can only be recognized if the moving stimulus optimally activates the group of S-I neurons for which that directions of simulus motion is the trigger feature.
1. In the hand area of the post-central gyrus of three alert Macaca speciosa monkeys neurones related to cutaneous receptors but not activated by simple touch on the receptive field were recorded using the transdural micro-electrode recording technique. Thirty-six cells were found to have complex cutaneous receptive field properties. These neurones were subdivided into the following three groups. 2. Nine neurones were not activated by punctate stimuli on the receptive fields but responded well to movement along the skin. The activity of these neurones was not affected by the direction of movement; nor was it sensitive to different textures of the moving surface. 3. Eighteen neurones responded to cutaneous movement along the skin surface in a particular direction giving no response to stimulation in the opposite direction and intermediate responses to intermediate directions. Similar responses were evoked from different subparts of the receptive field. 4. Nine neurones responded well to an edge placed on the skin in an optimal orientation or moved along the skin in a direction perpendicular to the edge. A maximal response was produced by stimuli of the same optimal orientation in different parts of the receptive field. The significance of the stimuli to the monkey had only a minor influence on the magnitude of the responses of these neurones and no influence on the receptive field properties. 5. The occurrence of the complex cutaneous cells increased from anterior to posterior within the post-central gyrus and most of them were found in Brodmann's area 2. Thus we postulate that the complex receptive field properties arise as a consequence of cortical processing in a network in which postsynaptic one-way lateral inhibition generates the directional properties of the neurones. 6. The complex cutaneous neurones constituted only 6% of the neurones studied in the hand area of the post-central gyrus. Thus the prevalence of neurones with elongated and direction-selective receptive fields is low in the primary somatosensory cortex in comparison with the visual cortex. These neurones may, however, serve the sterognostic capcity of the hand by contributing information about stimulus motion, orientation and direction of movement on the skin.
Microelectrode mapping experiments indicate that the classical primary somatosensory cortex of monkeys consists of as many as four separate body representations rather than just one. Two complete body surface representations occupy cortical fields 3b and 1. In addition, area 2 contains an orderly representation of predominantly "deep" body tissues. Area 3a may constitute a fourth representation.
This paper reports and illustrates in figurine style results obtained by electrical stimulation of the cortex in 20 patients and by recording of cortical evoked potentials (EPs) in 13 of these patients, whose surgery required wide exposure of the Rolandic or paracentral regions of the cortex. This study is unique in that cutaneous receptive fields related to specific cortical sites were defined by mechanical stimulation, as is done in animals, in contrast to electrical stimulation of peripheral nerves at fixed sites, as in scalp EP recordings. Observations were made on pre- and postcentral gyri, on the second somatic sensory-motor area, on the supplementary motor area, and on the supplementary sensory area. In two patients with phantom limb pain, the pain was elicited in one on stimulation of the postcentral arm area, and in the other on stimulation of the supplementary sensory leg area. Surgical removal of these areas had the immediate effect of abolishing the phantoms and the pain. Long-term follow-up review was not possible. In one patient with severe Parkinson's disease, stimulating currents subthreshold for the elicitation of movement resulted in disappearance of tremor and rigidity for short periods after stimulation of the precentral gyrus. The possible patterns of organization of the human pre- and postcentral areas are considered and compared with those of the chimpanzee and other primates. In patients in whom data from pre- and postcentral gyri were adequate, it appeared that the precentral face-arm boundary is situated 1 to 2 cm higher than the corresponding postcentral boundary.
The posterior nuclear complex of the thalamus in rhesus, pigtailed and squirrel monkeys consists of the combined suprageniculate‐limitans nucleus and an ill defined region of heterogeneous cell types extending anteriorly from the dorsal lobe of the medial geniculate body towards the posterior pole of the ventral nuclear complex. This region is referred to as the posterior nucleus.
The cortical projections of each of these nuclei, together with those of the adjacent ventral, pulvinar and medial geniculate complexes, have been studied by means of the autoradiographic tracing technique. The suprageniculate‐limitans nucleus, the main input to which is the superior colliculus, projects upon the granular insular area of the cortex. The medial portion of the posterior nucleus projects to the retroinsular field lying posterior to the second somatic sensory area. There is clinical and electrophysiological evidence to suggest that the retroinsular area may form part of a central pain pathway. The lateral portion of the posterior nucleus which is closely related to certain elements of the medial geniculate complex, projects to the postaditory cortical field. The ventroposterioinferior nucleus, which may be involved in vestibular function, projects to the dysgranular insular field.
The principal medial geniculate nucleus can be subdivided into a ventral division that projects to field AI of the auditory cortex and a dorsal division that merges with the posterior nucleus; it is further subdivided into an anterodorsal component that projects to two fields on the superior temporal gyrus, together with a posterodorsal component in which separate cell populations project to areas lying anterior and medial to AI. The magnocellular medial geniculate nucleus, sometimes considered a part of the posterior complex, appears to project diffusely to layer I of all the auditory fields.
The Auditory fields are bounded on three sides by the projection field of the medial nucleus of the pulvinar which also extends into the upper end of the lateral sulcus to bound the fields receiving fibers from the posterior nucleus. The topography of the areas receiving fibers from the posterior, medial geniculate and pulvinar complexes, taken in conjunction with the rotation of the primate temporal lobe, permits all of these fields to be compared with similar, better known areas in the cat brain.
Magnetic resonance imaging (MRI) studies of human brain activity are described. Task-induced changes in brain cognitive state were measured using high-speed MRI techniques sensitive to changes in cerebral blood volume (CBV), blood flow (CBF), and blood oxygenation. These techniques were used to generate the first functional MRI maps of human task activation, by using a visual stimulus paradigm. The methodology of MRI brain mapping and results from the investigation of the functional organization and frequency response of human primary visual cortex (V1) are presented.
To assess cortical activity during pain perception, regional cerebral blood flow (rCBF) studies were done in humans using single photon emission computed tomography (SPECT) with the radiotracer Tc99m-HMPAO and magnetic resonance imaging localization. Normalized SPECT data were analyzed by region of interest and change distribution. Contralateral somatosensory rCBF was decreased when the digits of the hand were immersed in a hot water bath for 3 min which was rated as moderately painful (persistent pain). No decrease was observed when the hand was immersed in tepid water (control). In contrast, cortical rCBF was increased during vibratory and sensorimotor tasks, in the contralateral somatosensory and sensorimotor areas, respectively. These results indicate that pain perception in man is associated with somatosensory cortical inhibition.
The aim of this study was to examine the hypothesis, if the activation of some cerebral structures due to physiological stimulation is accompanied by deactivations of other structures elsewhere in the brain. A vibratory stimulus was applied to the right hand palm of healthy volunteers and the regional cerebral blood flow (rCBF) and regional cerebral oxygen metabolism (rCMRO2) were measured with positron emission tomography (PET). Regional analysis and voxel-by-voxel plots indicated that the stimulation induced increases and decreases of the rCBF were coupled to increases and decreases of the rCMRO2. The increases were localized in the left primary somatosensory area (SI), the left secondary somatosensory area (SII), the left retroinsular field (RI), the left anterior parietal cortex, the left primary motor area (MI), and the left supplementary motor area (SMA). The decreases occurred bilaterally in the superior parietal cortex, in paralimbic association areas, and the left globus pallidus. The increases and decreases of the rCBF and rCMRO2 were balanced in such a way that the mean global CBF and CMRO2 did not change compared with rest. We conclude that the decreases of the cerebral oxidative metabolism indicated regional depressions of synaptic activity.
Five somatosensory fields were defined in the grey-headed flying fox by using microelectrode mapping procedures. These fields are: the primary somatosensory area, SI or area 3b; a field caudal to area 3b, area 1/2; the second somatosensory area, SII; the parietal ventral area, PV; and the ventral somatosensory area, VS. A large number of closely spaced electrode penetrations recording multiunit activity revealed that each of these fields had a complete somatotopic representation. Microelectrode maps of somatosensory fields were related to architecture in cortex that had been flattened, cut parallel to the cortical surface, and stained for myelin. Receptive field size and some neural properties of individual fields were directly compared. Area 3b was the largest field identified and its topography was similar to that described in many other mammals. Neurons in 3b were highly responsive to cutaneous stimulation of peripheral body parts and had relatively small receptive fields. The myeloarchitecture revealed patches of dense myelination surrounded by thin zones of lightly myelinated cortex. Microelectrode recordings showed that myelin-dense and sparse zones in 3b were related to neurons that responded consistently or habituated to repetitive stimulation respectively. In cortex caudal to 3b, and protruding into 3b, a complete representation of the body surface adjacent to much of the caudal boundary of 3b was defined. Neurons in this area habituated rapidly to repetitive stimulation. We termed this caudal field area 1/2 because it had properties of both area 1 and area 2 of primates.
The experiments of this study demonstrate that relatively modest rates of repetitive tactile stimulation are accompanied by rapid and reversible modifications (either increases or decreases) in the response of SI neurons. Complete recovery occurs in a few minutes following cessation of stimulation. The modifications are reproducible (1) if stimulus parameters remain the same and (2) if time for recovery is provided between successive exposures. In contrast, repetitive tactile stimuli identical to those that modify SI neuron response rarely lead to changes in the response of cutaneous mecha-noreceptive afferents.
SI neuron functional properties conventionally regarded as immutable [e.g., directional selectivity, and distribution of sensitivity within the receptive field (RF)] also modify with repetitive stimulation. While the changes in RF organization differ in detail from one neuron to the next, they are similar in form: the response generated by stimulus contact with one (or more rarely, several) RF region(s) becomes enhanced relative to the response the same stimulus evokes from neighboring regions. Neurons in the same column (sampled in the same radial penetration) exhibit very similar changes in the distribution of sensitivity within the RF, whereas neurons sampled in tangential penetrations exhibit diverse, apparently unrelated changes in RF organization in response to the same repetitive stimulus. Simultaneous multichannel recordings reveal that a repetitive tactile stimulus exerts similar effects on the response and RFs of the neurons within local (no more than 100 μm) neuron groupings.
A model that incorporates a manner of SI topographical organization (segregate organization) and welt-known aspects of neocortical cellular, neurotransmitter/recep-tor, and connactional architecture accounts for the changes in SI neuron behavior observed during repetitive stimulation.
A 14-channel cryogenic magnetometer system (BTi) was used to record the magnetic fields over the left hemisphere of 3 human subjects in order to locate the sources of responses to tactile stimulation of the index, the thumb and the little finger of the right hand. The locations of the active dipole sources determined using the spherical model were then projected onto the magnetic resonance image (MRI) of the individual subjects providing an anatomical localization. The MRI slices were also used to construct a 3-dimensional image to enhance visualization of the area of the calculated sources. The locations of the dipole sources from the 3 fingers were distinct from one another in all subjects. An analysis of variance ('ANOVA') showed the most significant (P less than 0.05) difference in source location between the little finger and the thumb with the former being superior to the sources of the other 2 fingers in all of the subjects. In all cases, the sources were found to be located on the postcentral gyrus. The strength of the equivalent dipole sources and the amplitudes of the responses to stimulation for all 3 fingers showed a consistent trend among all of the 3 subjects, with the thumb having the largest response. In general, no signs of habituation were found.
In order to investigate functional topography of human hand somatosensory cortex we recorded somatosensory evoked fields (SEFs) on MEG during the first 40 ms after stimulation of median nerve, ulnar nerve, and the 5 digits. We applied dipole modeling to determine the three-dimensional cortical representations of different peripheral receptive fields. Median nerve and ulnar nerve SEFs exhibited the previously described N20 and P30 components with a magnetic field pattern emerging from the head superior and re-entering the head inferior for the N20 component; the magnetic field pattern of the P30 component was of reversed orientation. Reversals of field direction were oriented along the anterior-posterior axis. SEFs during digit stimulation showed analogous N22 and P32 components and similar magnetic field patterns. Reversals of field direction showed a shift from lateral inferior to medial superior for thumb to little finger. Dipole modeling yielded good fits at these peak latencies accounting for an average of 83% of the data variance. The cortical digit representations were arranged in an orderly somatotopic way from lateral inferior to medial superior in the sequence thumb, index finger, middle finger, ring finger, and little finger. Median nerve cortical representation was lateral inferior to that of ulnar nerve. Isofield maps and dipole locations for these components are consistent with neuronal activity in the posterior bank of central fissure corresponding to area 3b. We conclude that SEFs recorded on MEG in conjunction with source localization techniques are useful to investigate functional topography of human hand somatosensory cortex non-invasively.
1. To clarify the generators of human short-latency somatosensory evoked potentials (SEPs) thought to arise in sensorimotor cortex, we studied the effects on SEPs of surgical excision of somatosensory or motor cortex in humans and monkeys. 2. Normal median nerve SEPs (P20-N30, N20-P30, and P25-N35) were recorded from the cortical surface of a patient (G13) undergoing a cortical excision for relief of focal seizures. All SEPs were abolished both acutely and chronically after excision of the hand area of somatosensory cortex. Similarly, excision of the hand area of somatosensory cortex abolished corresponding SEPs (P10-N20, N10-P20, and P12-N25) in monkeys. Excision of the crown of monkey somatosensory cortex abolished P12-N25 while leaving P10-N20 and N10-P20 relatively unaffected. 3. After excision of the hand area of motor cortex, all SEPs were present when recorded from the cortical surface of a patient (W1) undergoing a cortical excision for relief of focal seizures. Similarly, all SEPs were present in monkeys after excision of the hand area of motor cortex. 4. Although all SEPs were present after excision of motor cortex in monkeys, variable changes were observed in SEPs after the excisions. However, these changes were not larger than the changes observed after excision of parietal cortex posterior to somatosensory cortex. We concluded that the changes were not specific to motor cortex excision. 5. These results support two major conclusions. 1) Median nerve SEPs recorded from sensorimotor cortex are produced by generators in two adjacent regions of somatosensory cortex: a tangentially oriented generator in area 3b, which produces P20-N30 (human) and P10-N20 (monkey) [recorded anterior to the central sulcus (CS)] and N20-P30 (human) and N10-P20 (monkey) posterior to the CS; and a radially oriented generator in area 1, which produces P25-N35 (human) and P12-N25 (monkey) recorded from the postcentral gyrus near the CS. 2) Motor cortex makes little or no contribution to these potentials.
In human primary somatosensory cortex, the cerebral blood flow response to vibrotactile stimulation of the fingers (110 Hz), as measured by positron emission tomography and H2(15)O, was 13% higher (p less than 0.025) when the subjects attended to the stimulus, compared to when they were simultaneously engaged in a distraction task. This suggests that the physiological response of a primary cortical area can be modulated by the attentive behavior of the subject.
Positron emission tomographic (PET) studies of human attention have begun to dissect isolable components of this complex higher brain function, including a midline attentional system in a region of the anterior cingulate cortex. The right hemisphere may play a special part in human attention; neglect, an important phenomenon associated with damage to attentional systems, is more severe, extensive and long-lasting after lesions to the right hemisphere. Here we use PET measurements of brain blood flow in healthy subjects to identify changes in regional brain activity during simple visual and somatosensory tasks of sustained attention or vigilance. We find localized increases in blood flow in the prefrontal and superior parietal cortex primarily in the right hemisphere, regardless of the modality or laterality of sensory input. The anterior cingulate was not activated during either task. These data localize the vigilance aspects of normal human attention to sensory stimuli, thereby clarifying the biology underlying asymmetries of attention to such stimuli that have been reported in clinical lesions.
Paramagnetic deoxyhemoglobin in venous blood is a naturally occurring contrast agent for magnetic resonance imaging (MRI). By accentuating the effects of this agent through the use of gradient-echo techniques in high fields, we demonstrate in vivo images of brain microvasculature with image contrast reflecting the blood oxygen level. This blood oxygenation level-dependent (BOLD) contrast follows blood oxygen changes induced by anesthetics, by insulin-induced hypoglycemia, and by inhaled gas mixtures that alter metabolic demand or blood flow. The results suggest that BOLD contrast can be used to provide in vivo real-time maps of blood oxygenation in the brain under normal physiological conditions. BOLD contrast adds an additional feature to magnetic resonance imaging and complements other techniques that are attempting to provide positron emission tomography-like measurements related to regional neural activity.
1. Multiple microelectrode maps of the hand representation within and across the borders of cortical area 3b were obtained before, immediately after, or several weeks after a period of behaviorally controlled hand use. Owl monkeys were conditioned in a task that produced cutaneous stimulation of a limited sector of skin on the distal phalanges of one or more fingers. 2. Analysis of microelectrode mapping experiment data revealed that 1) stimulated skin surfaces were represented over expanded cortical areas. 2) Most of the cutaneous receptive fields recorded within these expanded cortical representational zones were unusually small. 3) The internal topography of representation of the stimulated and immediately surrounding skin surfaces differed greatly from that recorded in control experiments. Representational discontinuities emerged in this map region, and "hypercolumn" distances in this map sector were grossly abnormal. 4) Borders between the representations of individual digits and digit segments commonly shifted. 5) The functionally defined rostral border of area 3b shifted farther rostralward, manifesting either an expansion of the cutaneous area 3b fingertip representation into cortical field 3a or an emergence of a cutaneous input zone in the caudal aspect of this normally predominantly deep-receptor representational field. 6) Significant lateralward translocations of the borders between the representations of the hand and face were recorded in all cases. 7) The absolute locations--and in some cases the areas or magnifications--of representations of many skin surfaces not directly involved in the trained behavior also changed significantly. However, the most striking areal, positional, and topographic changes were related to the representations of the behaviorally stimulated skin in every studied monkey. 3. These experiments demonstrate that functional cortical remodeling of the S1 koniocortical field, area 3b, results from behavioral manipulations in normal adult owl monkeys. We hypothesize that these studies manifest operation of the basic adaptive cortical process(es) underlying cortical contributions to perception and learning.
1. The anatomic generators of human median nerve somatosensory evoked potentials (SEPs) in the 40 to 250-ms latency range were investigated in 54 patients by means of cortical-surface and transcortical recordings obtained during neurosurgery. 2. Contralateral stimulation evoked three groups of SEPs recorded from the hand representation area of sensorimotor cortex: P45-N80-P180, recorded anterior to the central sulcus (CS) and maximal on the precentral gyrus; N45-P80-N180, recorded posterior to the CS and maximal on the postcentral gyrus; and P50-N90-P190, recorded near and on either side of the CS. 3. P45-N80-P180 inverted in polarity to N45-P80-N180 across the CS but was similar in polarity from the cortical surface and white matter in transcortical recordings. These spatial distributions were similar to those of the short-latency P20-N30 and N20-P30 potentials described in the preceding paper, suggesting that these long-latency potentials are generated in area 3b of somatosensory cortex. 4. P50-N90-P190 was largest over the anterior one-half of somatosensory cortex and did not show polarity inversion across the CS. This spatial distribution was similar to that of the short-latency P25-N35 potentials described in the preceding paper and, together with our and Goldring et al. 1970; Stohr and Goldring 1969 transcortical recordings, suggest that these long-latency potentials are generated in area 1 of somatosensory cortex. 5. SEPs of apparently local origin were recorded from several regions of sensorimotor cortex to stimulation of the ipsilateral median nerve. Surface and transcortical recordings suggest that the ipsilateral potentials are generated not in area 3b, but rather in other regions of sensorimotor cortex perhaps including areas 4, 1, 2, and 7. This spatial distribution suggests that the ipsilateral potentials are generated by transcallosal input from the contralateral hemisphere. 6. Recordings from the periSylvian region were characterized by P100 and N100, recorded above and below the Sylvian sulcus (SS) respectively. This distribution suggests a tangential generator located in the upper wall of the SS in the second somatosensory area (SII). In addition, N125 and P200, recorded near and on either side of the SS, suggest a radial generator in a portion of SII located in surface cortex above the SS. 7. In comparison with the short-latency SEPs described in the preceding paper, the long-latency potentials were more variable and were more affected by intraoperative conditions.(ABSTRACT TRUNCATED AT 400 WORDS)
Positron emission tomography measurements of regional cerebral blood flow were used to detect focal neuronal activation in the first somatosensory cortex (SI) of humans induced by cutaneous vibratory stimulation. Intravenously administered water labeled with oxygen-15 (H2(15)O) was used as a blood flow tracer to obtain five stimulated-state and two resting-state blood flow images in each of eight normal volunteers. Three cutaneous surfaces were tested: lips, fingers, and toes. Intense, highly focal SI responses were seen during all 39 stimulated-state trials. The SI responses from the three stimulation sites were anatomically distinct and formed a medial-to-lateral homonculus in every subject. Response magnitudes (increase in local blood flow) and response locales (expressed as proportionately measured bicommissural stereotaxic coordinates) were highly consistent among subjects and on repeated trials for each subject. These findings suggest that eliciting cerebral blood flow responses by cutaneous vibration provides a safe, rapid, and reproducible tool for locating and assessing the functional status of somatosensory cortex, and offers potential clinical and research utility. This study has established normative values for future applications of this experimental paradigm.