Dorsal Visual Cortex Research Articles

7 tesla FMRI reveals systematic functional organization for binocular disparity in dorsal visual cortex.

The binocular disparity between the views of the world registered by the left and right eyes provides a powerful signal about the depth structure of the environment. Despite increasing knowledge of the cortical areas that process disparity from animal models, comparatively little is known about the local architecture of stereoscopic processing in the human brain. Here, we take advantage of the high spatial specificity and image contrast offered by 7 tesla fMRI to test for systematic organization of disparity representations in the human brain. Participants viewed random dot stereogram stimuli depicting different depth positions while we recorded fMRI responses from dorsomedial visual cortex. We repeated measurements across three separate imaging sessions. Using a series of computational modeling approaches, we report three main advances in understanding disparity organization in the human brain. First, we show that disparity preferences are clustered and that this organization persists across imaging sessions, particularly in area V3A. Second, we observe differences between the local distribution of voxel responses in early and dorsomedial visual areas, suggesting different cortical organization. Third, using modeling of voxel responses, we show that higher dorsal areas (V3A, V3B/KO) have properties that are characteristic of human depth judgments: a simple model that uses tuning parameters estimated from fMRI data captures known variations in human psychophysical performance. Together, these findings indicate that human dorsal visual cortex contains selective cortical structures for disparity that may support the neural computations that underlie depth perception.

Retinotopic organization of extrastriate cortex in the owl monkey--dorsal and lateral areas.

Dense retinotopy data sets were obtained by microelectrode visual receptive field mapping in dorsal and lateral visual cortex of anesthetized owl monkeys. The cortex was then physically flatmounted and stained for myelin or cytochrome oxidase. Retinotopic mapping data were digitized, interpolated to a uniform grid, analyzed using the visual field sign technique-which locally distinguishes mirror image from nonmirror image visual field representations-and correlated with the myelin or cytochrome oxidase patterns. The region between V2 (nonmirror) and MT (nonmirror) contains three areas-DLp (mirror), DLi (nonmirror), and DLa/MTc (mirror). DM (mirror) was thin anteroposteriorly, and its reduced upper field bent somewhat anteriorly away from V2. DI (nonmirror) directly adjoined V2 (nonmirror) and contained only an upper field representation that also adjoined upper field DM (mirror). Retinotopy was used to define area VPP (nonmirror), which adjoins DM anteriorly, area FSTd (mirror), which adjoins MT ventrolaterally, and TP (mirror), which adjoins MT and DLa/MTc dorsoanteriorly. There was additional retinotopic and architectonic evidence for five more subdivisions of dorsal and lateral extrastriate cortex-TA (nonmirror), MSTd (mirror), MSTv (nonmirror), FSTv (nonmirror), and PP (mirror). Our data appear quite similar to data from marmosets, though our field sign-based areal subdivisions are slightly different. The region immediately anterior to the superiorly located central lower visual field V2 varied substantially between individuals, but always contained upper fields immediately touching lower visual field V2. This region appears to vary even more between species. Though we provide a summary diagram, given within- and between-species variation, it should be regarded as a guide to parsing complex retinotopy rather than a literal representation of any individual, or as the only way to agglomerate the complex mosaic of partial upper and lower field, mirror- and nonmirror-image patches into areas.

The vertical occipital fasciculus: a century of controversy resolved by in vivo measurements.

The vertical occipital fasciculus (VOF) is the only major fiber bundle connecting dorsolateral and ventrolateral visual cortex. Only a handful of studies have examined the anatomy of the VOF or its role in cognition in the living human brain. Here, we trace the contentious history of the VOF, beginning with its original discovery in monkey by Wernicke (1881) and in human by Obersteiner (1888), to its disappearance from the literature, and recent reemergence a century later. We introduce an algorithm to identify the VOF in vivo using diffusion-weighted imaging and tractography, and show that the VOF can be found in every hemisphere (n = 74). Quantitative T1 measurements demonstrate that tissue properties, such as myelination, in the VOF differ from neighboring white-matter tracts. The terminations of the VOF are in consistent positions relative to cortical folding patterns in the dorsal and ventral visual streams. Recent findings demonstrate that these same anatomical locations also mark cytoarchitectonic and functional transitions in dorsal and ventral visual cortex. We conclude that the VOF is likely to serve a unique role in the communication of signals between regions on the ventral surface that are important for the perception of visual categories (e.g., words, faces, bodies, etc.) and regions on the dorsal surface involved in the control of eye movements, attention, and motion perception.

Investigating category- and shape-selective neural processing in ventral and dorsal visual stream under interocular suppression.

Recent behavioral and neuroimaging studies using continuous flash suppression (CFS) have suggested that action-related processing in the dorsal visual stream might be independent of perceptual awareness, in line with the "vision-for-perception" versus "vision-for-action" distinction of the influential dual-stream theory. It remains controversial if evidence suggesting exclusive dorsal stream processing of tool stimuli under CFS can be explained by their elongated shape alone or by action-relevant category representations in dorsal visual cortex. To approach this question, we investigated category- and shape-selective functional magnetic resonance imaging-blood-oxygen level-dependent responses in both visual streams using images of faces and tools. Multivariate pattern analysis showed enhanced decoding of elongated relative to non-elongated tools, both in the ventral and dorsal visual stream. The second aim of our study was to investigate whether the depth of interocular suppression might differentially affect processing in dorsal and ventral areas. However, parametric modulation of suppression depth by varying the CFS mask contrast did not yield any evidence for differential modulation of category-selective activity. Together, our data provide evidence for shape-selective processing under CFS in both dorsal and ventral stream areas and, therefore, do not support the notion that dorsal "vision-for-action" processing is exclusively preserved under interocular suppression.

Global processing in amblyopia: a review.

Amblyopia is a neurodevelopmental disorder of the visual system that is associated with disrupted binocular vision during early childhood. There is evidence that the effects of amblyopia extend beyond the primary visual cortex to regions of the dorsal and ventral extra-striate visual cortex involved in visual integration. Here, we review the current literature on global processing deficits in observers with either strabismic, anisometropic, or deprivation amblyopia. A range of global processing tasks have been used to investigate the extent of the cortical deficit in amblyopia including: global motion perception, global form perception, face perception, and biological motion. These tasks appear to be differentially affected by amblyopia. In general, observers with unilateral amblyopia appear to show deficits for local spatial processing and global tasks that require the segregation of signal from noise. In bilateral cases, the global processing deficits are exaggerated, and appear to extend to specialized perceptual systems such as those involved in face processing.

Perceptual Integration for Qualitatively Different 3-D Cues in the Human Brain

The visual system's flexibility in estimating depth is remarkable: We readily perceive 3-D structure under diverse conditions from the seemingly random dots of a "magic eye" stereogram to the aesthetically beautiful, but obviously flat, canvasses of the Old Masters. Yet, 3-D perception is often enhanced when different cues specify the same depth. This perceptual process is understood as Bayesian inference that improves sensory estimates. Despite considerable behavioral support for this theory, insights into the cortical circuits involved are limited. Moreover, extant work tested quantitatively similar cues, reducing some of the challenges associated with integrating computationally and qualitatively different signals. Here we address this challenge by measuring fMRI responses to depth structures defined by shading, binocular disparity, and their combination. We quantified information about depth configurations (convex "bumps" vs. concave "dimples") in different visual cortical areas using pattern classification analysis. We found that fMRI responses in dorsal visual area V3B/KO were more discriminable when disparity and shading concurrently signaled depth, in line with the predictions of cue integration. Importantly, by relating fMRI and psychophysical tests of integration, we observed a close association between depth judgments and activity in this area. Finally, using a cross-cue transfer test, we found that fMRI responses evoked by one cue afford classification of responses evoked by the other. This reveals a generalized depth representation in dorsal visual cortex that combines qualitatively different information in line with 3-D perception.

Integration of motion and disparity cues to depth in monkey brain

Event Abstract Back to Event Integration of motion and disparity cues to depth in monkey brain Marcelo Armendariz1*, Andrew Welchman2, Hiroshi Ban2 and Wim Vanduffel1 1 KU Leuven, Neuroscience, Belgium 2 University of Birmingham, School of Psychology, United Kingdom Recognising and interacting with objects depends on knowing the three-dimensional (3D) structure of the world around us. Estimating the depth structure of the environment is a principal function of the visual system in primates, enabling many key computations, such as segmentation, object recognition, material perception and the guidance of movements. The brain exploits a range of depth cues to estimate depth, combining information from shading and shadows to linear perspective, motion and binocular disparity. To solve the depth perception there are two computationally distinct ways: independence or fusion. Traditionally, this process has been broadly conceived in modular terms, with the independent processing of individual cues followed by a combination stage in which the influence of each cue reflects the reliability with which it is encoded. However, a recent study with humans (Ban et al., 2012), using fMRI for decoding depth from binocular disparity and motion parallax cues, provided evidence that dorsal visual areas are involved in the fusion mechanism that combines the two cues into a single depth perception. They found that fMRI responses are more discriminable when two cues (binocular disparity and relative motion) concurrently signal depth, since the variance is significantly reduced. Our present study aims to investigate cue integration in monkey (Macaca mulatta) with fMRI. We presented the monkey a set of stimuli representing near or far depths defined by motion parallax, binocular disparity and the combination of both in congruent and incongruent ways. We performed statistical data analysis to obtain all the activation areas in the brain involved in depth processing: V1, V2, V3, V3A, V4, MST, MT, FST, F5 and MIP. Once the regions of interest are defined, we will use multivoxel pattern analysis (MVPA) to reveal depth selectivity in macaque cortex (Preston et al., 2008) and assess the fusion mechanism for cue integration. References Ban, H., Preston, T.J., Meeson, A.,& Welchman, A.E. The integration of motion and disparity cues to depth in dorsal visual cortex. Nature Neuroscience 15, 636–643 (2012). Preston, T.J., Li, S., Kourtzi, Z. & Welchman, A.E. Multivoxel pattern selectivity for perceptually relevant binocular disparities in the human brain. J. Neurosci. 28, 11315–11327 (2008). Keywords: fMRI, monkey, Depth Perception, binocular diparity, motion parallax Conference: Imaging the brain at different scales: How to integrate multi-scale structural information?, Antwerp, Belgium, 2 Sep - 6 Sep, 2013. Presentation Type: Poster presentation Topic: Poster session Citation: Armendariz M, Welchman A, Ban H and Vanduffel W (2013). Integration of motion and disparity cues to depth in monkey brain. Front. Neuroinform. Conference Abstract: Imaging the brain at different scales: How to integrate multi-scale structural information?. doi: 10.3389/conf.fninf.2013.10.00042 Copyright: The abstracts in this collection have not been subject to any Frontiers peer review or checks, and are not endorsed by Frontiers. They are made available through the Frontiers publishing platform as a service to conference organizers and presenters. The copyright in the individual abstracts is owned by the author of each abstract or his/her employer unless otherwise stated. Each abstract, as well as the collection of abstracts, are published under a Creative Commons CC-BY 4.0 (attribution) licence (https://creativecommons.org/licenses/by/4.0/) and may thus be reproduced, translated, adapted and be the subject of derivative works provided the authors and Frontiers are attributed. For Frontiers’ terms and conditions please see https://www.frontiersin.org/legal/terms-and-conditions. Received: 23 Aug 2013; Published Online: 31 Aug 2013. * Correspondence: Mr. Marcelo Armendariz, KU Leuven, Neuroscience, Leuven, Belgium, m.zimarron@gmail.com Login Required This action requires you to be registered with Frontiers and logged in. To register or login click here. Abstract Info Abstract The Authors in Frontiers Marcelo Armendariz Andrew Welchman Hiroshi Ban Wim Vanduffel Google Marcelo Armendariz Andrew Welchman Hiroshi Ban Wim Vanduffel Google Scholar Marcelo Armendariz Andrew Welchman Hiroshi Ban Wim Vanduffel PubMed Marcelo Armendariz Andrew Welchman Hiroshi Ban Wim Vanduffel Related Article in Frontiers Google Scholar PubMed Abstract Close Back to top Javascript is disabled. Please enable Javascript in your browser settings in order to see all the content on this page.

FMRI responses in dorsal visual cortex relate to binocular depth perception for both signal-in-noise and feature difference tasks

Event Abstract Back to Event fMRI responses in dorsal visual cortex relate to binocular depth perception for both signal-in-noise and feature difference tasks Matthew L. Patten1, 2 and Andrew E. Welchman2, 3* 1 University of Sydney, School of Psychology, Australia 2 University of Birmingham, School of Psychology, United Kingdom 3 University of Cambridge, Department of Psychology, United Kingdom Background. Binocular disparities are formed by the horizontal separation of the eyes and are used by the visual system to provide a powerful cue to depth and 3D structure. Disparity representations are widespread in the visual cortex, and it is an open challenge to understand the functional relevance of these signals. One framework posits a functional distinction based on disparity magnitude, where coarse and fine disparities are processed in the dorsal and ventral visual pathways respectively. However, evidence that supports this conclusion has typically mixed differences in disparity magnitude with different limiting factors on behavioural performance (internal vs. external noise). To dissociate these factors, we performed two types of disparity tasks that have been used previously but presented stimuli of comparable disparity magnitudes. Method. Participants viewed random-dot stereograms that contained a disparity-defined target plane and judged its depth position (near vs. far). We simultaneously recorded event-related fMRI responses in regions of the visual, temporal and parietal cortices. For the signal-in-noise task, we varied the percentage of dots that defined the position of the target plane in relation to dots with randomly assigned disparities (i.e., an external noise manipulation). For the feature difference task, we titrated the disparity difference between a target plane and its surround (i.e., a task limited by internal noise). Results. We used multi-voxel pattern analysis to decode depth positions from the fMRI activity and constructed ‘fMR-metric’ functions that described decoding performance as a function of signal magnitude. We then generated fMR-metric functions using the parameters from the psychometric results so that we could directly compare behavioural performance and fMRI activity. We found evidence of an association between judged depth and fMRI activity in later dorsal regions: V7, VIPS, POIPS for the signal-in-noise task and VIPS and POIPS for the feature difference task. In contrast, activity in higher ventral cortex was only weakly associated with performance on either task. Discussion. These results indicate that later dorsal regions support perceptually-relevant decisions and reflect a higher stage of disparity processing, quite possibly reading out signals from lower portions of the processing hierarchy to support visual judgments and motor actions. Acknowledgements The research leading to these results received funding from the European Community’s Seventh Framework Programme FP7/2007-2013 (grant agreements PITN-GA-2008-214728 and PIIF-GA-2011-299610) and the Wellcome Trust (095183/Z/10/Z). Keywords: binocular disparity, Depth Perception, external noise, multi-voxel pattern analysis, fMRI, psychophysics. Conference: ACNS-2013 Australasian Cognitive Neuroscience Society Conference, Clayton, Melbourne, Australia, 28 Nov - 1 Dec, 2013. Presentation Type: Poster Topic: Sensation and Perception Citation: Patten ML and Welchman AE (2013). fMRI responses in dorsal visual cortex relate to binocular depth perception for both signal-in-noise and feature difference tasks. Conference Abstract: ACNS-2013 Australasian Cognitive Neuroscience Society Conference. doi: 10.3389/conf.fnhum.2013.212.00061 Copyright: The abstracts in this collection have not been subject to any Frontiers peer review or checks, and are not endorsed by Frontiers. They are made available through the Frontiers publishing platform as a service to conference organizers and presenters. The copyright in the individual abstracts is owned by the author of each abstract or his/her employer unless otherwise stated. Each abstract, as well as the collection of abstracts, are published under a Creative Commons CC-BY 4.0 (attribution) licence (https://creativecommons.org/licenses/by/4.0/) and may thus be reproduced, translated, adapted and be the subject of derivative works provided the authors and Frontiers are attributed. For Frontiers’ terms and conditions please see https://www.frontiersin.org/legal/terms-and-conditions. Received: 15 Oct 2013; Published Online: 25 Nov 2013. * Correspondence: Dr. Andrew E Welchman, University of Birmingham, School of Psychology, Birmingham, United Kingdom, aew69@cam.ac.uk Login Required This action requires you to be registered with Frontiers and logged in. To register or login click here. Abstract Info Abstract The Authors in Frontiers Matthew L Patten Andrew E Welchman Google Matthew L Patten Andrew E Welchman Google Scholar Matthew L Patten Andrew E Welchman PubMed Matthew L Patten Andrew E Welchman Related Article in Frontiers Google Scholar PubMed Abstract Close Back to top Javascript is disabled. Please enable Javascript in your browser settings in order to see all the content on this page.

Differentiated parietal connectivity of frontal regions for “what” and “where” memory

In a previous meta-analysis across almost 200 neuroimaging experiments, working memory for object location showed significantly stronger convergence on the posterior superior frontal gyrus, whereas working memory for identity showed stronger convergence on the posterior inferior frontal gyrus (dorsal to, but overlapping with Brodmann’s area BA 44). As similar locations have been discussed as part of a dorsal frontal—superior parietal reach system and an inferior frontal grasp system, the aim of the present study was to test whether the regions of working-memory related “what” and “where” processing show a similar distinction in parietal connectivity. The regions that were found in the previous meta-analysis were used as seeds for functional connectivity analyses using task-based meta-analytic connectivity modelling and task-independent resting state correlations. While the ventral seed showed significantly stronger connectivity with the bilateral intraparietal sulcus (IPS), the dorsal seed showed stronger connectivity with the bilateral posterior inferior parietal and the medial superior parietal lobule. The observed connections of regions involved in memory for object location and identity thus clearly demonstrate a distinction into separate pathways that resemble the parietal connectivity patterns of the dorsal and ventral premotor cortex in non-human primates and humans. It may hence be speculated that memory for a particular location and reaching towards it as well as object memory and finger positioning for manipulation may rely on shared neural systems. Moreover, the ensuing regions, in turn, featured differential connectivity with the bilateral ventral and dorsal extrastriate cortex, suggesting largely segregated bilateral connectivity pathways from the dorsal visual cortex via the superior and inferior parietal lobules to the dorsal posterior frontal cortex and from the ventral visual cortex via the IPS to the ventral posterior frontal cortex that may underlie action and cognition.

The integration of disparity and shading cues to 3D shape in dorsal visual cortex

The integration of disparity and shading cues to 3D shape in dorsal visual cortex

Interactive processes in viual perception: Evidence from neuropsychology

Event Abstract Back to Event Interactive processes in viual perception: Evidence from neuropsychology Glyn Humphreys1* 1 Oxford University, Experimental Psychology, United Kingdom Lesions of extra-striate visual cortex are associated with a range of perceptual problems, including deficits in visual object recognition in which patients can be selectively impaired at responding either to the local or global properties of objects. Standard ‘subtraction’ accounts of neuropsychological cases suggest that such impairments reflect the simple loss of a visual process. Here I will present evidence from two contrasting patients, with complementary lesions of extra-striate cortex, which indicate that, rather than there being a simple loss of visual processing, there is a dynamic change in processing produced by the lesion. Neuropsychological fMRI demonstrates that, following impairments to local processing regions in ventral extra-striate cortex, there is an enhanced response to global shape in dorsal extra-striate cortex. Similarly, following damage to dorsal extra-striate cortex, there is an enhanced response to local shape. The data indicate the important role of interactions between visual areas in determining visual perception. Acknowledgements Stroke Association (UK) Keywords: local-global perception, fMRI, Agnosia, ventral visual cortex, dorsal visual cortex Conference: Belgian Brain Council, Liège, Belgium, 27 Oct - 27 Oct, 2012. Presentation Type: Oral Presentation (only for invited speakers) Topic: Higher Brain Functions in health and disease: cognition and memory Citation: Humphreys G (2012). Interactive processes in viual perception: Evidence from neuropsychology. Conference Abstract: Belgian Brain Council. doi: 10.3389/conf.fnhum.2012.210.00097 Copyright: The abstracts in this collection have not been subject to any Frontiers peer review or checks, and are not endorsed by Frontiers. They are made available through the Frontiers publishing platform as a service to conference organizers and presenters. The copyright in the individual abstracts is owned by the author of each abstract or his/her employer unless otherwise stated. Each abstract, as well as the collection of abstracts, are published under a Creative Commons CC-BY 4.0 (attribution) licence (https://creativecommons.org/licenses/by/4.0/) and may thus be reproduced, translated, adapted and be the subject of derivative works provided the authors and Frontiers are attributed. For Frontiers’ terms and conditions please see https://www.frontiersin.org/legal/terms-and-conditions. Received: 03 Sep 2012; Published Online: 12 Sep 2012. * Correspondence: Prof. Glyn Humphreys, Oxford University, Experimental Psychology, Oxford, OX1 3UD, United Kingdom, glyn.humphreys@frontiersin.org Login Required This action requires you to be registered with Frontiers and logged in. To register or login click here. Abstract Info Abstract The Authors in Frontiers Glyn Humphreys Google Glyn Humphreys Google Scholar Glyn Humphreys PubMed Glyn Humphreys Related Article in Frontiers Google Scholar PubMed Abstract Close Back to top Javascript is disabled. Please enable Javascript in your browser settings in order to see all the content on this page.

Decoding of coherent but not incoherent motion signals in early dorsal visual cortex

When several scattered grating elements are arranged in such a way that their directions of motion are consistent with a common path, observers perceive them as belonging to a globally coherent moving object. Here we investigated how this coherence changes the representation of motion signals in human visual cortex using functional magnetic resonance imaging (fMRI) and multivariate voxel pattern decoding, which have the potential to reveal how well a stimulus is encoded in different contexts. Only during globally coherent motion was it possible to reliably distinguish fMRI signals evoked by different directions of motion in early visual cortex. This effect was specific to the retinotopic representation of the visual field quadrant in V1 traversed by the coherent element path and could not simply be attributed to a general increase in signal strength. Decoding was more reliable for cortical areas corresponding to the lower visual field. Because some previous studies observed poorer speed discrimination when motion was grouped, we also conducted behavioural experiments to investigate this with our stimuli, but did not reveal a consistent relationship between coherence and perceived speed. Taken together, these data show that neuronal populations in early visual cortex represent information that could be used for interpreting motion signals as unified objects.

Retinotopic Coding of Extraretinal Pursuit Signals in Early Visual Cortex

During smooth pursuit, the image of the target is stabilized on the fovea, implying that speed judgments made during pursuit must rely on an extraretinal signal providing precise eye speed information. To characterize the introduction of such extraretinal signal into the human visual system, we performed a factorial, functional magnetic resonance imaging experiment, in which we manipulated the factor eye movement, with "fixation" and "pursuit" as levels, and the factor task, with "speed" and "form" judgments as levels. We hypothesized that the extraretinal speed signal is reflected as an interaction between speed judgments and pursuit. Random effects analysis yielded an interaction only in dorsal early visual cortex. Retinotopic mapping localized this interaction on the horizontal meridian (HM) between dorsal areas visual 2 and 3 (V2/V3) at 1-2 degrees azimuth. This corresponded to the position the pursuit target would have reached, if moving retinotopically, at the time of the subject's speed judgment. Because the 2 V2/V3 HMs are redundant, both may be involved in speed judgments, the ventral one involving judgments based on retinal motion and the dorsal one judgments requiring an internal signal. These results indicate that an extraretinal speed signal is injected into early visual cortex during pursuit.

Opposite Neural Signatures of Motion-Induced Blindness in Human Dorsal and Ventral Visual Cortex

Motion-induced blindness (MIB) is a visual phenomenon in which a salient static target spontaneously fluctuates in and out of visual awareness when surrounded by a moving mask pattern. It has been hypothesized that MIB reflects an antagonistic interplay between cortical representations of the static target and moving mask. Here, we report evidence for such antagonism between human ventral and dorsal visual cortex during MIB. Functional magnetic resonance imaging (fMRI) responses in ventral visual area V4 decreased with the subjective disappearance of the target. These response decreases were specific for the cortical subregion corresponding retinotopically to the target, occurred early in time with respect to the perceptual report, and could not be explained by shifts of attention in reaction to target disappearance. At the same time, responses increased in mask-specific subregions in dorsal visual areas in and around the intraparietal sulcus. These opposite responses in ventral and dorsal visual areas occurred only during subjective target disappearance, not when the target was physically removed. Perceptual reports of target disappearance were furthermore associated with a "global" modulation of activity, which was delayed in time, and evident throughout early visual cortex, for both subjective target disappearance and physical target removal. We conclude that awareness of the target is tightly linked to the strength of its representation in ventral visual cortex, and that the mask representation in dorsal visual cortex plays a crucial role in the spontaneous suppression of the target representation during MIB.

Functional Cerebral Reorganization for Auditory Spatial Processing and Auditory Substitution of Vision in Early Blind Subjects

Early blind (EB) individuals can recognize bidimensional shapes using a prosthesis substituting vision with audition (PSVA) and activate right dorsal extrastriate visual cortex during the execution of this task. The present study used repetitive transcranial magnetic stimulation (rTMS) to further examine the functional role of this structure in the successful use of the PSVA. Moreover, we investigated which auditory parameter used in the prosthesis (pitch, intensity, or spatial location) might contribute to this occipital activation. Results revealed that rTMS applied to right dorsal extrastriate cortex in EB subjects interferes with both the PSVA use and the auditory spatial location task but not with pitch and intensity discriminations. By contrast, rTMS targeting the same cortical areas in sighted subjects did not affect performance on any auditory tasks. Early visual deprivation thus leads to functional cerebral cross-modal reorganization in the processing of auditory information and auditory-to-visual sensory substitution. The findings also point to the specific involvement of the dorsal visual stream for auditory spatial processing in blind subjects. Moreover, this suggests that sensory substitution prostheses can be developed using these additional neural resources to perform tasks that partially compensate for the loss of vision.

Preserved implicit form perception and orientation adaptation in visual form agnosia

Visual form agnosia is mainly characterized by profound deficits in visual form and shape discrimination. Previous studies have shown that patients retain the capacity for coordinated motor behaviors, color naming and implicit letter perception. However, it is unknown to what extent other visual functions, such as implicit form and orientation perception, are preserved. To address these questions, we investigated a single visual form agnosic patient, X.F., in two distinct experiments. X.F.'s visual lesions were mainly localized in the bilateral occipitotemporal cortex, with the dorsal visual stream and early visual cortex largely spared. In Experiment 1, X.F. named the color of different forms across 12 blocks of trials. After the first six blocks, the combinations of a form with its color were changed and the new combination was presented for the remaining six blocks. X.F.'s reaction time increased during the switch block and was significantly greater than the overall RT changes between adjacent, non-switch blocks. This indicates that X.F. retained the ability to perceive changes in form despite her inability to discriminate the forms. In Experiment 2, X.F. showed selective orientation adaptation effects to different spatial frequencies; that is, her contrast threshold was significantly higher when the adapting and test orientations were the same than when they were orthogonal, although her orientation discrimination performance was severely impaired. These data provide evidence of a functional dissociation between explicit and implicit visual abilities, and suggest that the residual early visual cortex mediates form and orientation processing in the absence of awareness.

Neural correlates of implied motion.

Current views of the visual system assume that the primate brain analyses form and motion along largely independent pathways; they provide no insight into why form is sometimes interpreted as motion. In a series of psychophysical and electrophysiological experiments in humans and macaques, here we show that some form information is processed in the prototypical motion areas of the superior temporal sulcus (STS). First, we show that STS cells respond to dynamic Glass patterns, which contain no coherent motion but suggest a path of motion. Second, we show that when motion signals conflict with form signals suggesting a different path of motion, both humans and monkeys perceive motion in a compromised direction. This compromise also has a correlate in the responses of STS cells, which alter their direction preferences in the presence of conflicting implied motion information. We conclude that cells in the prototypical motion areas in the dorsal visual cortex process form that implies motion. Estimating motion by combining motion cues with form cues may be a strategy to deal with the complexities of motion perception in our natural environment.

Dorsal visual cortex activity elicited by posture change in a visuo-tactile matching task.

To investigate the process of crossmodal spatial recognition, we examined the effect of posture change on the recognition of a tactile stimulus position. The task was to judge whether a visual and a tactile stimulus, presented to the left or right, were on the same or different sides while subjects crossed or uncrossed their hands. Under a condition which removed the effect of response bias to the left and right, the dorsal visual cortex (area 18/19) and the precuneus were more activated in the crossed hands condition. The dorsal visual cortex activation suggests that the activity of brain areas classically considered to be visual cortex is affected by posture change, and reflects the reciprocal process across different modalities in spatial recognition.

Development of response timing and direction selectivity in cat visual thalamus and cortex.

Single-unit recordings were made in the dorsal lateral geniculate nucleus (LGN) and visual cortex of kittens that were 4-13 weeks of age. Responses to visual stimuli were analyzed to determine the relationship between two related facets of the behaviors of the cells: direction selectivity (DS) and timing. DS depends on timing differences within the receptive field. Cortical DS was present at all ages, but its temporal frequency tuning changed. In kittens, DS was more common at high (approximately 4 Hz) than low ( approximately 1 Hz) temporal frequencies. This is in contrast to adults, in which DS is tuned to low frequencies, more common at 1 Hz than at 4 Hz (Saul and Humphrey, 1992a). In adult cats, the LGN provides the cortex with a wide range of timings that are also observable in cortical receptive fields (Saul and Humphrey, 1990, 1992b; Alonso et al., 2001). In kittens, LGN and cortical timing were immature. Most cells showed long-latency sustained responses. At low temporal frequencies, the variance in timing was small, but at higher frequencies, all timings were well represented. The timing data thus matched the temporal frequency tuning of DS. Kittens show DS at high temporal frequencies because of the abundance of inputs with different timing at high frequencies. As cells in the LGN mature, more low-frequency timing differences become available to the cortex, allowing DS at low frequencies to become possible for more cortical cells.

Development of Response Timing and Direction Selectivity in Cat Visual Thalamus and Cortex

Single-unit recordings were made in the dorsal lateral geniculate nucleus (LGN) and visual cortex of kittens that were 4–13 weeks of age. Responses to visual stimuli were analyzed to determine the relationship between two related facets of the behaviors of the cells: direction selectivity (DS) and timing. DS depends on timing differences within the receptive field. Cortical DS was present at all ages, but its temporal frequency tuning changed. In kittens, DS was more common at high (∼4 Hz) than low (∼1 Hz) temporal frequencies. This is in contrast to adults, in which DS is tuned to low frequencies, more common at 1 Hz than at 4 Hz (Saul and Humphrey, 1992a). In adult cats, the LGN provides the cortex with a wide range of timings that are also observable in cortical receptive fields (Saul and Humphrey, 1990,1992b; Alonso et al., 2001). In kittens, LGN and cortical timing were immature. Most cells showed long-latency sustained responses. At low temporal frequencies, the variance in timing was small, but at higher frequencies, all timings were well represented. The timing data thus matched the temporal frequency tuning of DS. Kittens show DS at high temporal frequencies because of the abundance of inputs with different timing at high frequencies. As cells in the LGN mature, more low-frequency timing differences become available to the cortex, allowing DS at low frequencies to become possible for more cortical cells.