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Integrating electrophysiology and neuroimaging in the study of brain function
... In response (Cohen et al., 2002), it was suggested that these observations might reflect top-down activation, such as imaging the word "duck" in association with a picture of a duck. While blood flow measures lack the time resolution to distinguish between these two possibilities, it might be possible to do so with the millisecond time resolution of ERPs (Mangun, Hopfinger, & Jha, 2000;Mangun & Heinze, 1995). ...
Event-related potential (ERP) studies of semantic processing have generally focused on the N400, a component that peaks at about 400 ms in response to words and which is larger when words are incongruent with the preceding sentence context. An earlier left-lateralized posterior N2(p3) has also been found to be correlated with an "unexpectedness" rating for incongruent sentence endings [Dien, Frishkoff, Cerbone, and Tucker, 2003, Parametric analysis of event-related potentials in semantic comprehension: evidence for parallel brain mechanisms, Cognitive Brain Research, 15: 137-153]. Because the incongruent endings were too odd to be explicitly predicted, we here hypothesize that this rating, and hence the N2(p3), reflects the degree of automatic spreading activation (ASA) in the visual lexical network rather than semantic expectancy, an interpretation also consistent with the early latency of this ERP (208 ms). In order to identify the brain systems involved in these linguistic processes, functional magnetic resonance imaging (fMRI) was utilized in a replication of the ERP study [Dien, Frishkoff, Cerbone, and Tucker, 2003, Parametric analysis of event-related potentials in semantic comprehension: evidence for parallel brain mechanisms, Cognitive Brain Research, 15: 137-153]. We found that activation in the fusiform semantic area (FSA), an area that converges with the source solution for the N2(p3), responded to the "unexpectedness" parameter in the same manner as the N2(p3) component. These findings suggest that the FSA helps mediate ASA processes and that the N2(p3) can serve as an index of ASA. Furthermore, close effects were found in the superior frontal gyrus and the inferior frontal gyrus that could reflect subvocalization and semantic selection processes respectively.
... The scalp distribution of this effect is shown at top right, by plotting the difference map for the symbol task minus the passive viewing condition. Adapted from[43]with permission from Lippincott,Williams, & Wilkins, 2000.Multiple brain regions that had not shown selective attention effects in the previous direction-specific analyses were revealed by the current analysis as being involved in aspects of attentional engagement. For example, this analysis revealed activation in the right pulvinar (stereotactic coordinates 10, − 24, 8; Z =3.57; P B 0.001 uncorrected for multiple comparisons;Fig. ...
Research into the neural mechanisms of attention has revealed a complex network of brain regions that are involved in the execution of attention-demanding tasks. Recent advances in human neuroimaging now permit investigation of the elementary processes of attention being subserved by specific components of the brain's attention system. Here we describe recent studies of spatial selective attention that made use of positron emission tomography (PET), functional magnetic resonance imaging (fMRI), and event-related brain potentials (ERPs) to investigate the spatio-temporal dynamics of the attention-related neural activity. We first review the results from an event-related fMRI study that examined the neural mechanisms underlying top-down attentional control versus selective sensory perception. These results defined a fronto-temporal-parietal network involved in the control of spatial attention. Activity in these areas biased the neural activity in sensory brain structures coding the spatial locations of upcoming target stimuli, preceding a modulation of subsequent target processing in visual cortex. We then present preliminary evidence from a fast-rate event-related fMRI study of spatial attention that demonstrates how to disentangle the potentially overlapping hemodynamic responses elicited by temporally adjacent stimuli in studies of attentional control. Finally, we present new analyses from combined neuroimaging (PET) and event-related brain potential (ERP) studies that together reveal the timecourse of activation of brain regions implicated in attentional control and selective perception.
... Their results suggest that multiple sessions for single subjects may be necessary in order to avoid erroneous conclusions about the particular locations of activations based on a single session from multiple subjects. Other researchers have examined the reproducibility of fMRI and PET data, including similar activation paradigms across laboratories (Casey et al., 1998), imaging modalities (Mangun, Hopfinger, & Jha, 2000;Ojemann et al., 1998), and sessions (Cohen & DuBois, 1999;Tegeler, Strother, Anderson, & Kim, 1999;Rombouts, Barkhof, Hoogenraad, Sprenger, & Scheltens, 1998;Le & Hu, 1997;Noll et al., 1997). Our study differs from these previous neuroimaging studies on variability in two ways. ...
The localization of brain functions using neuroimaging techniques is commonly dependent on statistical analyses of groups of subjects in order to identify sites of activation, particularly in studies of episodic memory. Exclusive reliance on group analysis may be to the detriment of understanding the true underlying cognitive nature of brain activations. In the present study, we found that the patterns of brain activity associated with episodic retrieval are very distinct for individual subjects from the patterns of brain activity at the group level. These differences go beyond the relatively small variations due to cyctoarchitectonic differences or spatial normalization. We quantify this individual variability by cross-correlating volumes of brain images. We demonstrate that individual patterns of brain activity are reliable over time despite their extensive variability. We suggest that varied but reliable individual patterns of significant brain activity may be indicative of different cognitive strategies used to produce a recognition response. We believe that individual analysis in conjunction with group analysis may be critical to fully understanding the relationship between retrieval processes and underlying brain regions.
Interactions between Vision and Emotion, two major adaptive functions, have been largely investigated. It is also well known that visual performance decreases with retinal eccentricity. Therefore, studies aiming to explore emotional processing always presented stimuli in central vision (CV) but never at eccentric positions, in peripheral vision (PV). The main purpose of this work was to examine behavioral and cerebral responses following the presentation of emotional stimulations in PV. Evoked related potentials (ERP) were recorded and analyzed by the use of spatio-temporal principal component analyses (PCA). Considering the saliency character of emotional information and its facility to capture attentional resources, present study was expected to optimize PV responses in order to better explore PV mechanisms. Furthermore, peripheral emotional stimulations could help to draw new strategies devoted to the reeducation of CV pathologies mainly due to central scotomas. Indeed, in these pathologies, PV represents the only available visual resources.
First, we confirmed that natural scenes were processed faster and better in CV than in PV. We showed for the first time an emotional modulation of reaction times and ERP components specifically generated by peripheral presentations. These results allowed us to extend to PV classical data about differential processing of neutral and emotional information described for the CV.
Second, keeping in mind possibilities of neurovisual reeducation, we looked for specific stimulations which could increase PV reactivity when associated with emotional information. Accordingly, because of their relevant role in social communication, emotional faces (fearful, neutral, happy) were used to stimulate the PV. Results showed that faces are processed in PV and their emotional expression modulates behavioral and cerebral responses. Indeed, participants responded faster for emotional faces than for neutral ones and evoked components were larger when emotional faces appeared.
Finally, some studies suggest that PV performance could be improved following the peripheral presentation of dynamic stimuli. Therefore, saliency of emotional faces could be strengthened by the animation of their expression. In this third experiment, we presented static and dynamic emotional faces. Results of the second experiment were confirmed but no advantage could be found for presentations of dynamic faces.
In a prospective view, emotional and neutral faces were presented to patients with central scotomas. The behavioral results evidenced that emotional faces were better processed than neutral ones and suggest that such stimulations could be used to increase peripheral visual resources.
In conclusion, this work provides new behavioral and electrophysiological data on emotional processing in PV and brings new insights concerning the use of emotional information in neurovisual reeducation of CV pathologies.
We used fMRI to investigate how moment-to-moment neural activity contributes to success or failure on individual trials of a visual working memory (WM) task. We found that different nodes of a distributed cortical network were activated to a greater extent for correct compared to incorrect trials during stimulus encoding, memory maintenance during delays, and at test. A logistic regression analysis revealed that the fMRI signal amplitude during the delay interval in a network of frontoparietal regions predicted successful performance on a trial-by-trial basis. Differential delay activity occurred even for only those trials in which BOLD activity during encoding was strong, demonstrating that it was not a simple consequence of effective versus ineffective encoding. Our results indicate that accurate memory depends on strong sustained signals that span the delay interval of WM tasks.
Event-related potentials (ERPs) and functional magnetic resonance imaging are both measures of functional brain activation that have been applied to developmental questions. However, the two measures are not identical and reflect different aspects of neuronal function at a physiological level. There is increasing interest in using these methods in combination because of the complementary information they provide about brain activation. ERPs provide more detailed information about the timing of neural activity, while fMRI provides more detailed information about its spatial location. Thus, their combined use may provide more detailed spatio-temporal information than either method alone. In this paper, we review the basic principles of ERP and fMRI and present selected studies from our own work to illustrate their strengths for studies of development. In addition, we discuss the potential benefits and special challenges of the combined use of electrophysiological and functional MRI techniques.
We used fMRI to investigate how moment-to-moment neural activity contributes to success or failure on individual trials of a visual working memory (WM) task. We found that different nodes of a distributed cortical network were activated to a greater extent for correct compared to incorrect trials during stimulus encoding, memory maintenance during delays, and at test. A logistic regression analysis revealed that the fMRI signal amplitude during the delay interval in a network of frontoparietal regions predicted successful performance on a trial-by-trial basis. Differential delay activity occurred even for only those trials in which BOLD activity during encoding was strong, demonstrating that it was not a simple consequence of effective versus ineffective encoding. Our results indicate that accurate memory depends on strong sustained signals that span the delay interval of WM tasks.
The temporal dynamics of the effects of lateralized visual selective attention within the lower visual field were studied with the combined application of event-related potentials (ERPs) and positron emission tomography (15O PET). Bilateral stimuli were rapidly presented to the lower visual field while subjects either passively viewed them or covertly attended to a designated side to detect occasional targets. Lateralized attention resulted in strongly enhanced PET activity in contralateral dorsal occipital cortex, while ERPs showed an enhanced positivity (P1 effect, 80-160 ms) for all stimuli (both non-targets and targets) over contralateral occipital scalp. Dipole modeling seeded by the dorsal occipital PET foci yielded an excellent fit for the peak P1 attention effect. However, more detailed ERP modeling throughout the P1 latency window (90-160 ms) suggested a spatial-temporal movement of the attention-related enhancement that roughly paralleled the shape of the dorsal occipital PET attention-related activations-likely reflecting the sequential attention-related enhancement of early visual cortical areas. Lateralized spatial attention also resulted in a longer-latency contralateral enhanced negativity (N2 effect, 230-280 ms) with a highly similar distribution to the earlier P1 effect. Dipole modeling seeded by the same dorsal occipital PET foci also yielded an excellent fit. This pattern of results provides evidence for re-entrance of attention-enhanced activation to the same retinotopically organized region of dorsal extrastriate cortex. Finally, target stimuli in the attended location elicited an additional prolonged enhancement of the longer-latency negativity over contralateral occipital cortex. The combination of PET activation and dipole modeling suggested contribution from a ventral-occipital generator to this target-related activity.
The effects of spatial selective attention on sensory processing in visual cortical areas were investigated by means of visual evoked potential (VEP) recordings and source localization techniques. Patterned stimuli were rapidly presented in random order to the left and right visual fields while subjects maintained central fixation and attended to one visual field at a time. Attended stimuli evoked enhanced P1 (100130 msec) and N1 (120200 msec) components of the VEP, whereas no effects of attention were observed on the C1 (50100 msec) or P2 (200240 msec) components. Spatiotemporal dipole modeling of the early VEP sources was carried out in relation to MRI-defined cortical anatomy. The dipolar generator of the C1 component was found to lie in calcarine cortex, the human homologue of area V1, whereas the attention-sensitive P1 generator was localized to ventral-lateral occipital cortex, within extrastriate area 19. These results support the hypothesis that spatial attention does not affect the initial activity evoked in area V1 but rather produces an enhancement within extrastriate visual areas of sensory signals arising from stimuli at attended locations.
Event-related brain potentials (ERPs) were recorded in response to unilateral arrays of letters flashed in rapid, randomized
sequences to left and right visual field locations. Subjects were required to focus attention exclusively on either left or
right field stimuli, or to divide attention in different proportions between the two fields, with the aim of detecting infrequent
target letters. Both d’ and percent hits for target detections increased significantly as attentional allocation to a stimulus
location increased. Attention operating characteristic (AOC) curves for the target detection scoreswere highly similar in
form to those for the amplitudes of the long-latency, endogenous ERP components—N350-650 and P400-800 (P300). All of
these measures showed gradual, nearly rectangular tradeoff functions. In contrast, the AOC curves for the early sensoryevoked
components displayed steep, nearly linear amplitude tradeoffs as attention was increasingly allocated to one visual field
at the expense of the other. The early and late ERP components were considered as indices of separate but interacting levels
of attentional selection having different operating principles.
Reaction time (RT) differences to visual stimuli as a function of expectancy have been attributed to changes in perceptual processing or entirely to shifts in decision and response criteria. To help distinguish between these competing interpretations, event-related brain potentials (ERPs) were recorded to lateralized flashes delivered to visual field locations precued by a central arrow (valid stimuli) or not precued (invalid stimuli). Validly cued stimuli in both simple and choice RT tasks elicited consistent amplitude enhancements of the early, sensory-evoked PI component of the ERP recorded at scalp sites overlying lateral prestriate visual cortex (90-130 ms poststimulus). In contrast, the subsequent N1 component (150-200 ms) was enhanced by validly cued stimuli in the choice RT task condition only. These electrophysiological findings support models proposing that the behavioral effects of precuing expected target locations are due, at least in part, to changes in sensory-perceptual processing. Furthermore, these data provide specific information regarding the neural mechanisms underlying such effects.
The mechanism by which visual-spatial attention affects the detection of faint signals has been the subject of considerable debate. It is well known that spatial cuing speeds signal detection. This may imply that attentional cuing modulates the processing of sensory information during detection or, alternatively, that cuing acts to create decision bias favoring input at the cued location. These possibilities were evaluated in 3 spatial cuing experiments. Peripheral cues were used in Experiment 1 and central cues were used in Experiments 2 and 3. Cuing similarly enhanced measured sensitivity, P(A) and d', for simple luminance detection in all 3 experiments. Under some conditions it also induced shifts in decision criteria (beta). These findings indicate that visual-spatial attention facilitates the processing of sensory input during detection either by increasing sensory gain for inputs at cued locations or by prioritizing the processing of cued inputs.
Event-related potentials were elicited by bilateral and unilateral stimulus arrays flashed in rapid sequence in order to investigate both focused attention and attentional orienting. Subjects attended selectively to the stimuli on one side of the bilateral arrays and were required to discriminate infrequent target stimuli on either the attended side (no switch of attentional focus) or unattended side of the array (switch of attentional focus). The ERPs to the bilateral stimuli elicited an occipital P1 component that was larger in amplitude over scalp regions contralateral to the attended visual half-field. The ERPs to the unilateral stimuli on the attended side also showed an amplitude enhancement of early P1 components, followed by a positive shift that lasted until 200 msec latency over the contralateral occipital scalp. No enhancement of the N1 component was observed to attended-side stimuli. These patterns were not different for conditions requiring or not requiring a spatial switch of the attentional focus. In conjunction with ERP signs of focused spatial attention, significant differences in discrimination performance (d') were obtained for the attended vs unattended-side targets; no changes in measures of criterion (beta) were obtained. These data support the idea that the early occipital P1 attention effect represents a facilitation of visual inputs that occur at attended locations in the visual field.
The borders of human visual areas V1, V2, VP, V3, and V4 were precisely and noninvasively determined. Functional magnetic
resonance images were recorded during phase-encoded retinal stimulation. This volume data set was then sampled with a cortical
surface reconstruction, making it possible to calculate the local visual field sign (mirror image versus non-mirror image
representation). This method automatically and objectively outlines area borders because adjacent areas often have the opposite
field sign. Cortical magnification factor curves for striate and extrastriate cortical areas were determined, which showed
that human visual areas have a greater emphasis on the center-of-gaze than their counterparts in monkeys. Retinotopically
organized visual areas in humans extend anteriorly to overlap several areas previously shown to be activated by written words.
Visual-spatial attention is an essential brain function that enables us to select and preferentially process high priority information in the visual fields. Several brain areas have been shown to participate in the control of spatial attention in humans, but little is known about the underlying selection mechanisms. Non-invasive scalp recordings of event-related potentials (e.r.ps) in humans have shown that attended visual stimuli are preferentially selected as early as 80-90 ms after stimulus onset, but current e.r.p. methods do not permit a precise localization of the participating cortical areas. In this study we combined neuroimaging (positron emission tomography) with e.r.p. recording in order to describe both the cortical anatomy and time course of attentional selection processes. Together these methods showed that visual inputs from attended locations receive enhanced processing in the extrastriate cortex (fusiform gyrus) at 80-130 ms after stimulus onset. These findings reinforce early selection models of attention.
Three experiments were conducted to determine whether attention-related changes in luminance detectability reflect a modulation of early sensory processing. Experiments 1 and 2 used peripheral cues to direct attention and found substantial effects of cue validity on target detectability; these effects were consistent with a sensory-level locus of selection but not with certain memory- or decision-level mechanisms. In Experiment 3, event-related brain potentials were recorded in a similar paradigm using central cues, and attention was found to produce changes in sensory-evoked brain activity beginning within the 1st 100 ms of stimulus processing. These changes included both an enhancement of sensory responses to attended stimuli and a suppression of sensory responses to unattended stimuli; the enhancement and suppression effects were isolated to different neural responses, indicating that they may arise from independent attentional mechanisms.
The goal of our research is to develop an experimental and analytical framework for spatiotemporal imaging of human brain function. Preliminary studies suggest that noninvasive spatiotemporal maps of cerebral activity can be produced by combining the high spatial resolution (millimeters) of functional MRI (fMRI) with the high temporal resolution (milliseconds) of electroencephalography (EEG) and magnetoencephalography (MEG). Although MEG and EEG are sensitive to millisecond changes in mental activity, the ability to resolve source localization and timing is limited by the ill-posed "inverse" problem. We conducted Monte Carlo simulations to evaluate the use of MRI constraints in a linear estimation inverse procedure, where fMRI weighting, cortical location and orientation, and sensor noise statistics were realistically incorporated. An error metric was computed to quantify the effects of fMRI invisible ("missing") sources, "extra" fMRI sources, and cortical orientation errors. Our simulation results demonstrate that prior anatomical and functional information from MRI can be used to regularize the EEG/MEG inverse problem, giving an improved solution with high spatial and temporal resolution. An fMRI weighting of approximately 90% was determined to provide the best compromise between separation of activity from correctly localized sources and minimization of error caused by missing sources. The accuracy of the estimate was relatively independent of the number and extent of the sources, allowing for incorporation of physiologically realistic multiple distributed sources. This linear estimation method provides an operator-independent approach for combining information from fMRI, MEG, and EEG and represents a significant advance over traditional dipole modeling.
The present study investigated the effect of visual selective attention upon neural processing within functionally specialized regions of the human extrastriate visual cortex. Field potentials were recorded directly from the inferior surface of the temporal lobes in subjects with epilepsy. The experimental task required subjects to focus attention on words from one of two competing texts. Words were presented individually and foveally. Texts were interleaved randomly and were distinguishable on the basis of word colour. Focal field potentials were evoked by words in the posterior part of the fusiform gyrus. Selective attention strongly modulated long-latency potentials evoked by words. The attention effect co-localized with word-related potentials in the posterior fusiform gyrus, and was independent of stimulus colour. The results demonstrated that stimuli receive differential processing within specialized regions of the extrastriate cortex as a function of attention. The late onset of the attention effect and its co-localization with letter string-related potentials but not with colour-related potentials recorded from nearby regions of the fusiform gyrus suggest that the attention effect is due to top-down influences from downstream regions involved in word processing.
When we expect important stimuli at a particular spatial location, how does our perceptual sensitivity change over space? Subjects were cued to expect a target stimulus at one location and then required to perform one of the following tasks at that and three other locations: luminance detection, brightness discrimination, orientation discrimination, or form discrimination. The analysis of subjects' performance according to signal detection theory revealed changes in both sensitivity and bias for each of these tasks. Sensitivity was maximally enhanced at the location where a target stimulus was expected and generally decreased with distance from that location. Factors that influenced the gradient of sensitivity were (a) the type of task performed and (b) the spatial distribution of the stimuli. Sensitivity fell off more steeply over distance for orientation and form discrimination than for luminance detection and brightness discrimination. In addition, it fell off more steeply when stimuli were near each other than when they were farther apart. (PsycINFO Database Record (c) 2012 APA, all rights reserved)
Event-related brain potentials (ERPs) were recorded from subjects as they attended to colored bars that were flashed in random
order to the left or right of fixation. The task was to detect slightly smaller target bars having a specified color (red
or blue) and location (left or right). The ERP elicited by stimuli at an attended location contained a sequence of phasic
components (P122/N168/N264) that was highly distinct from the sequence associated with selection on the basis of color (N150-350/P199/P400-500).
These findings suggested that spatially focused attention involves a gating or modulation of evoked neural activity in the
visual pathways, whereas color selection is manifested by an endogenous ERP complex. When the stimulus locations were widely
separated, the ERP signs of color selection were hierarchically dependent upon the prior selection for spatial location. In
contrast, when the stimulus locations were adjacent to one another, the ERP signs of color selection predominated over those
of location selection. These results are viewed as supporting “early selection” theories of attention that specify the rejection
of irrelevant inputs prior to the completion of perceptual processing. The implications of ERP data for theories of multidimensional
stimulus processing are considered.
Event-related potentials (ERPs) were recorded from the scalp while subjects attended to sequences of bilaterally symmetrical arrays of 4 letters (2 in each visual half-field) that were flashed briefly at intervals of 280-520 msec. These sequences also included randomized presentations of unilateral 'probe' stimuli consisting of irrelevant bars (experiment 1) or potentially relevant letter pairs (experiment 2). The task was to pay attention to the letter pairs in either the left or the right half-field on a given run and to press a button when the two letters matched one another (targets). The ERPs to the bilateral arrays included an early positive wave (P1, peaking at 135 msec) that was enhanced over posterior scalp sites contralateral to the attended visual field. Both types of probe stimulus also elicited a larger early positivity in the P1 latency range (100-200 msec) when delivered to the attended half-field, followed in some cases by a more prolonged positive deflection. Notable for its absence was any sign of an enlarged posterior N1 component (160-200 msec), which was prominent in the ERP to attended-field stimuli in previous studies using randomized sequences of unilateral stimuli. Attended-field targets elicited large N2 and P3 (P300) components, which were greatly reduced or absent when targets occurred in the unattended field. The observed ERP effects were interpreted in terms of early sensory selection during visual spatial attention.
The spatial distribution of visual attention was investigated by measuring target detectability (d') and event-related brain potentials (ERPs) to stimuli at varying distances from an attended locus. Vertical bars were flashed rapidly in random order to 1 of 3 locations: one in each of the lateral visual fields and one on the vertical meridian above the fixation point. Subjects maintained eye fixation while directing their attention to 1 of the 3 locations for the duration of each 1.75 min run. Their primary task was to detect infrequent, shorter target bars at the attended location. A secondary task was to respond to shorter target bars at either of the 2 unattended locations if they 'happened to notice them' (without trying to detect them). ERPs and d' scores were obtained to the lateral field stimuli both when they were specifically attended (primary task), as well as when attention was focused upon midline or opposite-field flashes (secondary task). Both d' scores and the amplitudes of the P135 and N190 waves decreased progressively as attention was directed to locations increasingly distant from a given lateral stimulus. These results support 'gradient' models of the spatial distribution of visual attention.
The effects of focussed attention to peripherally and centrally located visual stimuli were compared via an analysis of event-related brain potentials (ERPs) while subjects detected the direction of motion of a white square in a specified location. While attention to both peripheral and foveal stimuli produced enhancements of the early ERP components, the distribution over the scalp of the attention-related changes varied according to stimulus location. The attention-related increase in the amplitude of the N1 wave (157 ms) to the peripheral stimuli was greater over the parietal region of the hemisphere contralateral to the attended visual field. By contrast, the largest effects of foveally directed attention occurred over the occipital regions where the increase was bilaterally symmetrical. Additionally, the effects of attention on the ERPs were significantly larger for moving than for stationary stimuli, and this effect was greater for peripheral than for central attention. A long-latency positive displacement component (300-600 ms) was larger over the right than the left hemisphere during attention to the lateral visual fields, but was symmetrical in amplitude when central stimuli were attended. These results suggest that different pathways are modulated when attention is deployed to different regions of the visual fields. Further, they suggest that the special role of the right hemisphere in spatial attention may be limited to analysis of information in the visual periphery.
Neurons in a subdivision of the pulvinar resemble those in parietal cortex: many respond to visual stimuli, some of these have a spatial selection mechanism, and some have signals about the occurrence of eye movements. These properties suggest a role in visual spatial attention. Injection of GABA-related drugs into this part of the pulvinar alters animals' performance on an attentional task. These data support our hypothesis that the pulvinar contributes to visual spatial attention.
Event-related potentials (ERPs) were recorded in two experiments involving selective visual processing. In Experiment 1, subjects attended to light flashes emanating from one visual field, in order to detect occasional slightly deviant "targets", while ignoring equiprobable stimuli from the opposite field. ERPs elicited by stimuli in an attended field were characterised by larger posteriorly distributed P120 and N170 components, and a larger anteriorly distributed N145 component. In addition, these ERPs were, in comparison to those elicited by unattended stimuli, more negative-going in the latency region of approx. 200-400 msec. This late effect had a marked fronto-central distribution. In Experiment 2 subjects attended to either horizontal or vertical bars, displayed equiprobably in the same spatial location. No enhancement of early components was observed as a function of attention but, as in Experiment 1, a late, sustained, fronto-centrally distributed negative shift was observed in ERPs elicited by "attended" compared to "unattended" stimuli. It was concluded that the enhancement of P120 (P1) observed in Experiment 1 reflects the engagement of attentional mechanisms specific to the selection of stimuli on the basis of spatial cues. The later sustained negative shift seen in both experiments was considered to reflect a feature of within-channel processing common to both spatial and non-spatial selective tasks.
Hemispheric differences in a negative brain potential associated with selectively attending the location and type of stimulation were investigated. The earlier portion of this negativity (between 125 and 222 msec after stimulation) was associated with attending the location of the stimulus. It was symmetrical in the central scalp regions but was greater in the hemisphere contralateral to the attended visual field in the posterior scalp region. The latter portion of this negativity (from 222 to 272 msec after stimulation) primarily was associated wih attending one of the different types of stimuli presented at a given location and was greater over the left posterior regions of the scalp. These results were interpreted in relationship to the time-course of different types of information processing in the left and right hemisphere.
Visual selective attention improves our perception and performance by modifying sensory inputs at an early stage of processing. Spatial attention produces the most consistent early modulations of visual processing, which can be observed when attention is voluntarily allocated to locations. These effects of spatial attention are similar when attention is cued in a trial-by-trial, or sustained, fashion and are manifest as changes in the amplitudes, but not the latencies, of evoked neural activity recorded from the intact human scalp. This modulation of sensory processing first occurs within the extrastriate visual cortex and not within the striate or earlier subcortical processing stages. These relatively early spatial filters alter the inputs to higher stages of visual analysis that are responsible for feature extraction and ultimately object perception and recognition, and thus provide physiological evidence for early precategorical selection during visual attention. Moreover, the physiological evidence extends early selection theories by providing neurophysiologically precise information about the stages of visual processing affected by attention.
Perception, action, cognition, and emotion can now be mapped in the brain by a growing family of techniques. Positron emission tomography, functional magnetic resonance imaging, event-related electrical potentials, event-related magnetic fields, and other non-invasive imaging techniques are rapidly evolving and providing an increasingly rich literature on the functional organization of the human brain. Although no two techniques map identical physiological processes or physical parameters, replications of functionally specific maps by different techniques indicate sufficient common ground for multimodality integration. The process of integration is multi-tiered. Recent advances in integration range from simple image fusion, to model-based synthetic analyses, to collective databases for neural-system modeling. Spatially, temporally, physiologically, and cognitively accurate computational models of the neural systems of human behavior are the ultimate objective of functional brain mapping. This objective will be reached only through integrating the diversity of modern brain-mapping methods.
Stimuli attended-to locations in visual space are usually detected with higher speed and accuracy than stimuli at unattended positions. It has been argued that this effect is due to "sensory gating" mechanisms that modulate the flow of perceptual information from attended and unattended positions. In the present experiments, event-related potentials (ERPs) were recorded to stimuli that were preceded either by a valid or by an invalid positional cue (trial-by-trial cuing). When overt responses were required only to infrequent target stimuli on valid trials (Experiment 1) or to all validly cued stimuli (Experiment 2B), but not to invalid trials, systematic enhancements of early sensory-evoked potentials were found. These effects were smaller when both validly and invalidly cued stimuli required a response (Experiment 2A). These findings are interpreted as evidence that sensory gating processes are activated during the trial-by-trial cuing of spatial attention. Furthermore, valid stimuli elicited a greater negativity than invalid stimuli at midline electrodes following the early enhancements of sensory-evoked potentials. This possibly reflects an additional enhanced processing of attended-to locations.
In ERP literature on visual selective attention evidence has been provided that selectively directing attention to a spatial frequency affects the visual processing of the attended frequency, and of unattended frequencies within the same channel bandwidth, starting at a relatively late level of post-stimulus processing, i.e., after about 150 msec. Nevertheless, little knowledge is available about the topographic distribution of these attention effects. This study investigated attentional selection of stimulus relative size at occipital and latero-occipital sites, as well as at fronto-lateral sites. ERPs from posterior scalp electrode sites showed that attention to check sizes enhanced the early sensory components, thus indicating that feature-based attention may result in a modulation of sensory processing. Comparisons of the ERPs to relevant and irrelevant patterns showed an enhanced latero-occipital P90 positivity as well as an occipital N115 negativity to relevant patterns, thus also suggesting possible differential mechanisms of early attentional selectivity at these locations. Later effects of attention consisted of a selection negativity to relevant patterns at posterior electrodes, and a selection positivity at latero-frontal sites. A larger late positivity to irrelevant patterns at anterior sites also suggested an active suppression of attentional response to irrelevant information. Moreover, right-and left-sided asymmetries were found to be respectively consistent for the P90 and N115 with left hemispheric specialization for high, and right hemispheric specialization for low spatial frequencies. A stronger left-sided attentional selectivity has also been found.
A recent development in the cognitive modelling of visual selective attention is the incorporation of design principles derived from the neuroanatomy and neurophysiology of the primate visual system. In this paper, we describe these recent 'neurocognitive' models in more detail, point out the underlying neurobiological principles, and show that in all cases attention is implemented as an energetical resource which can be directed to representations and pathways in the system. In the second part of the paper, we specify the predictions derived from this 'energy hypothesis', and evaluate available data pertaining to this issue. We present new analyses of electrophysiological data in order to directly test the hypothesis that attention modulates feature-specific representations. It will be shown that in the case of sustained spatial attention, the data are in agreement with this hypothesis, whereas in the case of nonspatial attention, there is no evidence of a modulation of feature-specific pathways by attention.
Human functional-anatomic correlates of object repetition were explored in a cohort of 20 subjects using fMRI. Subjects performed an object classification task where the target objects were either novel or repeated. Objects appeared rapidly, one every 2 s, in a randomly intermixed task design similar to traditional behavioral, event-related potential (ERP), and single-unit physiological studies. Recently developed event-related fMRI methods were used to analyze the data. Clear effects of repetition were observed. Brain areas in midlevels of the processing hierarchy, including extrastriate visual cortex extending into inferotemporal cortex and left dorsal prefrontal cortex, showed reductions in the amount of activation after repetition. By contrast, early visual areas and output motor areas were activated equally by both novel and repeated objects and did not show effects of repetition, suggesting that the observed correlates of repetition were anatomically selective. We discuss these findings in relation to previous positron emission tomography (PET) and fMRI studies of item repetition and single-unit physiological studies; we also address the broad impact that rapid event-related fMRI is likely to have on functional neuroimaging.
This study characterized patterns of brain electrical activity associated with selective attention to the color of a stimulus. Multichannel recordings of event-related potentials (ERPs) were obtained while subjects viewed randomized sequences of checkerboards consisting of isoluminant red or blue checks superimposed on a grey background. Stimuli were presented foveally at a rapid rate, and subjects were required to attend to the red or blue checks in separate blocks of trials and to press a button each time they detected a dimmer target stimulus of the attended color. An early negative ERP component with an onset latency of 50 ms was sensitive to stimulus color but was unaffected by the attentional manipulation. ERPs elicited by attended and unattended stimuli began to diverge after approximately 100 ms following stimulus onset. Inverse dipole modelling of the attended-minus-unattended difference waveform indicated that an initial positive deflection with an onset latency of 100 ms had a source in lateral occipital cortex, while a subsequent negative deflection with an onset at 160 ms had a source in inferior occipito-temporal cortex. Longer-latency attention-sensitive components were localized to premotor frontal areas (onset at 190 ms) and to more anterior regions of the fusiform gyrus (onset at 240 ms). These source localizations correspond closely with cortical areas that were identified in previous neuroimaging studies as being involved in color-selective processing. The present ERP data thus provide information about the time course of stimulus selection processes in cortical areas that subserve attention to color.
Typical natural visual scenes contain many objects, which need to be segregated from each other and from the background. Present theories subdivide the processes responsible for this segregation into a pre-attentive and attentive system. The pre-attentive system segregates image regions that 'pop out' rapidly and in parallel across the visual field. In the primary visual cortex, responses to pre-attentively selected image regions are enhanced. When objects do not segregate automatically from the rest of the image, the time-consuming attentive system is recruited. Here we investigate whether attentive selection is also associated with a modulation of firing rates in area V1 of the brain in monkeys trained to perform a curve-tracing task. Neuronal responses to the various segments of a target curve were simultaneously enhanced relative to responses evoked by a distractor curve, even if the two curves crossed each other. This indicates that object-based attention is associated with a response enhancement at the earliest level of the visual cortical processing hierarchy.