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The motor-sensory homunculus redrawn. (A) The proportions of this first homunculus correspond to those of the original reproduced in Fig. 1. All other homunculi in B-D are derived from an average of the motor and somatosensory maps produced in Fig. 2 and Supplementary Fig. 1. (B) Homunculus generated from the surface maps. (C) Homunculus derived from the vertical length measurements. (D) Homunculus derived from the number of stimulation points. All measurements are from Penfield and Boldrey (1937).
Source publication
Eighty years ago, Penfield and Boldrey introduced the homunculus in a paper published in Brain. In a reappraisal of the iconic aide-mémoire, Marco Catani reanalyses the original data, and argues that through its extended network the homunculus holds the key to the precise coding that results in coordinated activation of peripheral muscles.
Context in source publication
Context 1
... tions of the homunculus started to appear in popular text- books, where its altered proportions were thought to reflect the extension of the area of each body part (Fig. 2B). To rectify this historical error, different versions of a modern motor-sensory homunculus have been generated using the data available in the original paper and presented in Fig. 4. A comparison of the different homunculi shows that the original version was not proportionally scaled accord- ing to the measures reported in the 1937 paper. For exam- ple, the size of the tongue was clearly exaggerated in the first homunculus, a misrepresentation that Penfield reme- died in a following publication (Penfield and ...
Citations
... The results are detailed by brain region, cluster size, and MNI coordinates of the voxel with the highest t-value within each cluster. Notably, for both tasks, fMRI activity was significantly greater in OA compared to YA, with no regions showing greater activity in YA C contralateral to side producing force, I ipsilateral to side producing force, MNI Montreal Neurological Institute, PMd dorsal premotor cortex control (Catani 2017;Christou et al. 2003;Duchateau and Enoka 2022). When interpreting the results, one needs to keep in mind that the MVC was maintained at 15% to facilitate comparison with other studies, minimize head motion during the MRI scans, and enable a direct comparison of force control between limbs. ...
Despite the widespread use of older adults (OA) as controls in movement disorder studies, the specific effects of aging on the neural control of upper and lower limb movements remain unclear. While functional MRI paradigms focusing on hand movements are widely used to investigate age-related brain changes, research on lower limb movements is limited due to technical challenges in an MRI environment. This study addressed this gap by examining both upper and lower limb movements in healthy young adults (YA) vs. OA. Sixteen YA and 20 OA, matched for sex, dominant side, and cognitive status, performed pinch grip and ankle dorsiflexion tasks, each requiring 15% of their maximum voluntary contraction. While both groups achieved the target force and exhibited similar force variability and accuracy, OA displayed distinct differences in force control dynamics, with a slower rate of force increase in the hand task and a greater rate of force decrease in the foot task. Imaging results revealed that OA exhibited more widespread activation, extending beyond brain regions typically involved in movement execution. In the hand task, OA showed increased activity in premotor and visuo-motor integration regions, as well as in the cerebellar hemispheres. During the foot task, OA engaged the cerebellar hemispheres more than YA. Collectively, results suggest that OA may recruit additional brain regions to manage motor tasks, possibly to achieve similar performance. Future longitudinal studies that track changes over time could help clarify if declines in motor performance lead to corresponding changes in brain activation.
... First, the stimulating electrode may have been placed closer to the primary motor cortex of the upper extremities. The area for the movement of the lower extremities is located close to the midline, while that of the upper extremities is located on the lateral side [15], which could cause contradictory responses in the upper and lower extremities. Second, infants' immature development of nervous system structures could be related to attenuating action potential conduction on nerves. ...
Purpose
The influence of anesthetic interactions on motor-evoked potentials in infants has rarely been reported. In infants, adding a small dose of sevoflurane to propofol-based total intravenous anesthesia is reasonable for reducing propofol administration. We collected preliminary data regarding the effect of low-dose sevoflurane in propofol-based total intravenous anesthesia on motor-evoked potentials in infants.
Methods
This pilot interventional study included 10 consecutive infants requiring motor-evoked potentials between January 2023 and March 2024. The motor-evoked potential amplitudes in the upper and lower extremities were recorded twice when general anesthesia was maintained using (1) propofol-based total intravenous anesthesia and (2) 0.1–0.15 age-adjusted minimum alveolar concentration sevoflurane + propofol-based total intravenous anesthesia.
Results
The motor-evoked potential amplitude in the right upper extremity was not significantly different after the addition of a small dose of sevoflurane [192 (75.3–398) μV, 121 (57.7–304) μV, P = 0.19]. All the motor-evoked potential amplitudes in the right lower extremity (quadriceps femoris, anterior tibialis, and gastrocnemius muscles) were significantly attenuated by adding a small dose of sevoflurane (median [interquartile range]: 47.9 [35.4–200] μV, 25.2 [12.4–55.3] μV, P = 0.014; 74.2 [51.9–232] μV, 31.2 [2.7–64] μV, P = 0.0039; 29.8 [20–194] μV, 9.9 [3.8–92.4] μV, P = 0.0039, respectively). Similar results were observed in the left lower extremities.
Conclusion
Adding even 0.1–0.15 age-adjusted minimum alveolar concentration sevoflurane to propofol-based total intravenous anesthesia attenuated the motor-evoked potential amplitudes in the lower extremities. A further prospective interventional study with an appropriate sample size is required to investigate the study hypothesis.
... However, this somatotopic organization is not as segregated as indicated by the classical motor homunculus (Penfield and Boldrey 1937). Evidence has revealed that the motor representations of the digits, wrist, forearm, and proximal arm are partially overlapping (Schieber and Hibbard 1993;Catani 2017) and can be adaptively altered via extensive hand motor training (Nudo et al. 1996a(Nudo et al. , 1996b. Furthermore, the topographical pattern of motor representations can change within minutes following motor nerve injury (Sanes et al. 1988) owing to the recruitment of existing neural pathways that are normally inhibited (Jacobs and Donoghue 1991). ...
... A key limitation of the first study was its focus on a single stimulation point, while the second study, although it included an additional brain stem auricular acupoint, did not include points for 80 anatomically distant body parts. In the present study, we examined the theory that there is a topographic map of acupuncture points on the ear to the same cortical region on the primary somatosensory cortex (S1) as the specific body parts [36][37][38] , specifically the shoulder and thumb points 35 . Two previous fMRI studies 9, 10 have suggested that this mapping exists, as evidenced by evoked brain changes observed in regions of S1 corresponding to the actual thumb, contralaterally 85 in 6 subjects and bilaterally in 3 subjects 9 , and in the secondary somatosensory cortex bilaterally when stimulating the thumb point and in the limbic and cortical areas of the pain matrix, with stronger activity ipsilaterally, when stimulating the brainstem point 10 in six subjects. ...
Significance
Auriculotherapy is a technique based on stimulation applied to specific ear points. Its mechanism of active and clinical efficacy remain to be established. This study aims to assess the role that primary somatosensory cortex may play to validate auriculotherapy mechanisms.
Aim
This study examined whether tactile stimulation at specific auricular points is correlated with distinct cortical activation in the primary somatosensory cortex.
Approach
Seventeen healthy adults participated in the study. Tactile stimuli were delivered to the thumb, shoulder, and skin master points on the ear using von Frey filaments. Functional near-infrared spectroscopy was used to measure and spatially map cortical responses.
Results
This study revealed distinct hemodynamic activity patterns in response to ear point stimulation, consistent with the classic homunculus model of somatotopic organization. Ipsilateral stimulation showed specific cortical activations for the thumb and shoulder points, while contralateral stimulation showed less significant activity. Functional near-infrared spectroscopy effectively captured localized cortical responses to ear tactile stimuli, supporting the somatotopic mapping hypothesis.
Conclusion
These findings enhance the understanding of sensory processing with auricular stimulation and supports the concepts of auricular cartography that underpins some schools of auriculotherapy practice. Future research should explore bilateral cortical mapping and the integration of other neuroimaging techniques.
... Conversely, the deactivation of upper motor centers can elicit the activation of mid-level motor centers, potentially resulting in episodes of hallucination [159,160]. As per Wilder Penfield's experimental findings, electrical stimulation in certain cortical or subcortical structures could induce different forms of hallucinations [161,162]. Notably, the occurrence of hallucination is intricately linked to changes in neuroplasticity, particularly within key brain regions such as primary and secondary sensory cortices, basal ganglia, and limbic system including the hippocampus [163]. Among different neurotransmitter-based hypotheses, varying levels of dopamine in the limbic system have been strongly implicated in the development of hallucinations [164]. ...
Hallucination is a sensory perception that occurs in the absence of external stimuli during abnormal neurological disturbances and various mental diseases. Hallucination is recognized as a core psychotic symptom and is particularly more prevalent in individuals with schizophrenia. Strikingly, a significant number of subjects with Alzheimer’s disease (AD), Parkinson’s disease (PD), Huntington’s disease (HD), and other neurological diseases like cerebral stroke and epileptic seizure also experience hallucination. While aberrant neurotransmission has been linked to the neuropathogenic events of schizophrenia, the precise cellular mechanism accounting for hallucinations remains obscure. Neurogenesis is a cellular process of producing new neurons from the neural stem cells (NSC)-derived neuroblasts in the brain that contribute to the regulation of pattern separation, mood, olfaction, learning, and memory in adulthood. Impaired neurogenesis in the hippocampus of the adult brain has been linked to stress, anxiety, depression, and dementia. Notably, many neurodegenerative disorders are characterized by the mitotic and functional activation of neuroblasts and cell cycle re-entry of mature neurons leading to a drastic alteration in neurogenic process, known as reactive neuroblastosis. Considering their neurophysiological properties, the abnormal integration of neuroblasts into the existing neural network or withdrawal of their connections can lead to abnormal synaptogenesis, and neurotransmission. Eventually, this would be expected to result in altered perception accounting for hallucination. Thus, this article emphasizes a hypothesis that aberrant neurogenic processes at the level of reactive neuroblastosis could be an underlying mechanism of hallucination in schizophrenia and other neurological diseases.
... Based on an improved understanding of the functional architecture of the right hemisphere, intraoperative Direct Electric Stimulation (DES) mapping in awake patients [20, [4,26]. Also, DES on the medial and posterior segments of STG, the posterior segment of MGT, the Uncinate Fasciculus (UF), and SMG produce interference for facial emotion recognition (i.e., mentalizing) [30,37,48]. ...
The right hemisphere has been underestimated by being considered as the non-dominant hemisphere. However, it is involved in many functions, including movement, language, cognition, and emotion. Therefore, because lesions on this side are usually not resected under awake mapping, there is a risk of unfavorable neurological outcomes. The goal of this study is to compare the functional and oncological outcomes of awake surgery (AwS) versus surgery under general anesthesia (GA) in supratentorial right-sided gliomas. A systematic review of the literature according to PRISMA guidelines was performed up to March 2023. Four databases were screened. Primary outcome to assess was return to work (RTW). Secondary outcomes included the rate of postoperative neurological deficit, postoperative Karnofsky Performance Status (KPS) score and the extent of resection (EOR). A total of 32 articles were included with 543 patients who underwent right hemisphere tumor resection under awake surgery and 294 under general anesthesia. There were no significant differences between groups regarding age, gender, handedness, perioperative KPS, tumor location or preoperative seizures. Preoperative and long-term postoperative neurological deficits were statistically lower after AwS (p = 0.03 and p < 0.01, respectively), even though no difference was found regarding early postoperative course (p = 0.32). A subsequent analysis regarding type of postoperative impairment was performed. Severe postoperative language deficits were not different (p = 0.74), but there were fewer long-term mild motor and high-order cognitive deficits (p < 0.05) in AwS group. A higher rate of RTW (p < 0.05) was documented after AwS. The EOR was similar in both groups. Glioma resection of the right hemisphere under awake mapping is a safer procedure with a better preservation of high-order cognitive functions and a higher rate of RTW than resection under general anesthesia, despite similar EOR.
... However, few studies have investigated the distinct cortical loci considering hierarchical representations in the cortical homunculus for the movement of the ankle, toe, or knee, which are feasible movements of the lower limb extremities for fMRI acquisition because head motion is potentially more controllable compared with hip joint movement [11]. The identification of cortical loci specific to these lower limb movements in the median wall of the sensorimotor area mainly in the paracentral lobule (PCL) region is more challenging than for upper limb movements because the motor cortex associated with the lower limbs is smaller in volume than that for the upper limbs based on the cortical homunculus [16]. In addition, isolation of individual lower limb movements is more demanding due to the potentially greater head motion [10,[17][18][19]. ...
Background
Identification of cortical loci for lower limb movements for stroke rehabilitation is crucial for better rehabilitation outcomes via noninvasive brain stimulation by targeting the fine-grained cortical loci of the movements. However, identification of the cortical loci for lower limb movements using functional MRI (fMRI) is challenging due to head motion and difficulty in isolating different types of movement. Therefore, we developed a custom-made MR-compatible footplate and leg cushion to identify the cortical loci for lower limb movements and conducted multivariate analysis on the fMRI data. We evaluated the validity of the identified loci using both fMRI and behavioral data, obtained from healthy participants as well as individuals after stroke.
Methods
We recruited 33 healthy participants who performed four different lower limb movements (ankle dorsiflexion, ankle rotation, knee extension, and toe flexion) using our custom-built equipment while fMRI data were acquired. A subgroup of these participants (Dataset 1; n = 21) was used to identify the cortical loci associated with each lower limb movement in the paracentral lobule (PCL) using multivoxel pattern analysis and representational similarity analysis. The identified cortical loci were then evaluated using the remaining healthy participants (Dataset 2; n = 11), for whom the laterality index (LI) was calculated for each lower limb movement using the cortical loci identified for the left and right lower limbs. In addition, we acquired a dataset from 15 individuals with chronic stroke for regression analysis using the LI and the Fugl–Meyer Assessment (FMA) scale.
Results
The cortical loci associated with the lower limb movements were hierarchically organized in the medial wall of the PCL following the cortical homunculus. The LI was clearer using the identified cortical loci than using the PCL. The healthy participants (mean ± standard deviation: 0.12 ± 0.30; range: – 0.63 to 0.91) exhibited a higher contralateral LI than the individuals after stroke (0.07 ± 0.47; – 0.83 to 0.97). The corresponding LI scores for individuals after stroke showed a significant positive correlation with the FMA scale for paretic side movement in ankle dorsiflexion (R² = 0.33, p = 0.025) and toe flexion (R² = 0.37, p = 0.016).
Conclusions
The cortical loci associated with lower limb movements in the PCL identified in healthy participants were validated using independent groups of healthy participants and individuals after stroke. Our findings suggest that these cortical loci may be beneficial for the neurorehabilitation of lower limb movement in individuals after stroke, such as in developing effective rehabilitation interventions guided by the LI scores obtained for neuronal activations calculated from the identified cortical loci across the paretic and non-paretic sides of the brain.
... Normally, the mind´s body schema (Sattin et al., 2023;Vignemont et al., 2021) reflects the standard anatomical setup, i.e., a bilateral body structure with two arms, hands, legs, and so forth. The cortical areas processing the corresponding sensory input are known as the Homunculus (Catani, 2017;Corniani & Saal, 2020;. In some but not all amputees, incongruence between body and its representation by the mind occurs. ...
The visible human body is composed of flesh and bones for the most part, yet an invisible orchestra of sensations and perceptions creates a virtual or phantom body that behaves like a shadow following every movement and gesture of its anatomical complement. This shadow becomes only “visible” to the individual when bodily integrity is affected, anatomically or cognitively. Phantom limbs have been known for a long time. They refer to the felt presence of a missing hand, leg, or other body part as if it was still in place. Reciprocally and of a supposedly cognitive origin, phantom extremities are reported by some patients that feel the virtual presence of a supernumerary limb – signifying anatomical “overcompleteness.” However, other patients feel as one of their limbs does not belong to their body – signifying “foreignness”. Various shades of the so-called body integrity identity disorder exemplify the assumed complex signification processes within the human body. The Peircean theory of signs and the Uexküllian concept of endosemiosis are combined to approach the still poorly understood phantom phenomena in light of representation and embodiment.
... All physicians know the distorted Penfield and Boldrey's homunculus, whose shape reflects the cortical representation of somatic muscles and cutaneous sensory areas, 45 the core hardware of person-world relations. this homunculus, provided with huge hands, feet, mouth and tongue, is consistent with the prM perspective (figure 1a). ...
Modern medicine tends to privilege disciplines promising “objective“ laws governing body parts (from molecules to organs). studies on a per-son’s illness and disability are (apparently) confined to “subjectivity.” The Specialty of Physical and Rehabilitation Medicine is often regarded as a humanitarian approach, belonging at best to the family of “soft,” “qualitative,” or “quasi-experimental” sciences. This specialty often claims specificity by labelling itself as “functional” and “holistic.” However, it is shown here that the former term is acceptable, yet redundant, and the second misleading. When human behaviors and perceptions are at stake, “function” indicates a person’s relationship with the outer world (already tackled by the definitional term “physical” from the Greek “physis”). The word “holistic” emphasizes mind-body unity and person-environment interdependence but, in current usage, overshadows the complementary need for an analytic, experimental approach to any function. Medicine aims at fighting disease and disability in single persons. This endeavor requires knowing body parts and mechanisms and understanding how interventions on “parts” affect the “whole.” This understanding rests on the experimental method. For instance, returning to a given societal role (participation) may require restoration of walking (activity), which may require reinforcement of weakened muscular groups (impairment). Working only on holistic bio-psycho-social “wholes” may miss the therapeutic mission of medicine.
... They reported that the activity of the latter region may be ascribed to tongue movements involved in articulation, whereas the strong activity of the former reflects the significant contribution of phonation to speech production (Brown et al., 2009). Since then, there has been a continuing debate regarding the origin, function and evolutionary significance of these areas (Belyk et al., 2021;Catani, 2017;Simonyan, 2014). ...
... I would like to suggest that the division and relocation of the LMCd in man is rather due to the nonproportional growth of head, face, mouth, lips and tongue motor areas in the ventral part of the human motor homunculus. The altered proportions of the neural representations of the body parts is attributed to the frequency of their use, reflecting the amount of the cortex and the degree of innervation dedicated to their specialized functions (Catani, 2017) due to their significance in modern humans. The enlargement of above-mentioned areas appears to have been resulted in upward migration of the LMCd and backward dislocation of the LMCv into the Rolandic operculum. ...
... To support this idea, it is important to note that according to comparative anatomical studies, hot spots of "regional expansion" are found most frequently in temporoparietal, ventrolateral, and medial frontal cortices (Mars et al., 2018). The first comprehensive localization map of the human brain was provided by Penfield and Boldrey (Penfield and Boldrey, 1937) as a distorted human-like little man, the famous homunculus (Catani, 2017) (Fig. 2). This figure demonstrates obviously that the growth of cortical areas representing the tongue, mouth, jaws Fig. 1. ...
