Vision Lunch 2008

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Contents

[edit] January 2008

2 WINTER BREAK

9 Discussion: Parietal effects on visual processing

Saalmann et. al., 2007

Neural mechanisms of visual attention: how top-down feedback highlights relevant locations.

Attention helps us process potentially important objects by selectively increasing the activity of sensory neurons that represent the relevant locations and features of our environment. This selection process requires top-down feedback about what is important in our environment. We investigated how parietal cortical output influences neural activity in early sensory areas. Neural recordings were made simultaneously from the posterior parietal cortex and an earlier area in the visual pathway, the medial temporal area, of macaques performing a visual matching task. When the monkey selectively attended to a location, the timing of activities in the two regions became synchronized, with the parietal cortex leading the medial temporal area. Parietal neurons may thus selectively increase activity in earlier sensory areas to enable focused spatial attention.

16 Discussion: Color Globs

Conway et. al. 2007

Specialized color modules in macaque extrastriate cortex.

Imaging studies are consistent with the existence of brain regions specialized for color, but electrophysiological studies have produced conflicting results. Here we address the neural basis for color, using targeted single-unit recording in alert macaque monkeys, guided by functional magnetic resonance imaging (fMRI) of the same subjects. Distributed within posterior inferior temporal cortex, a large region encompassing V4, PITd, and posterior TEO that some have proposed functions as a single visual complex, we found color-biased fMRI hotspots that we call "globs," each several millimeters wide. Almost all cells located in globs showed strong luminance-invariant color tuning and some shape selectivity. Cells in different globs represented distinct visual field locations, consistent with the coarse retinotopy of this brain region. Cells in "interglob" regions were not color tuned, but were more strongly shape selective. Neither population was direction selective. These results suggest that color perception is mediated by specialized neurons that are clustered within the extrastriate brain.

23 Ken Nakayama, Harvard: Normal and abnormal face processing in human observers

We humans spend countless hours staring at faces and its perhaps no surprise that scientists now realize that the perception of human faces is one of the more important functions of our visual system. Our laboratory has devoted itself to studying human face perception in both normal observers and those who have severe difficulties in recognizing faces. Behavioral studies on normal face processing show that fine features of the face are important for recognition, MEG studies also show the importance of face parts insofar as they by themselves can cause adaptation of the M170 face response. Studies on developmental prosopagnosia indicate that it is much more widespread than previously assumed, that as many as 2% of the population has serious face recognition problems. Such individuals, while suffering major difficulties in recognition perform normally in other subtle face specific judgments such as gender, attractiveness and emotions. In addition we show that developmental prosopagnosia comes in two forms, pure and iconic and that these segregate in different families. This suggests that prosopagnosia and its variants has a likely genetic basis.

30 Discussion: Objects in the dorsal stream

Konen & Kastner, 2007

Two hierarchically organized neural systems for object information in human visual cortex.

The primate visual system is broadly organized into two segregated processing pathways, a ventral stream for object vision and a dorsal stream for space vision. Here, evidence from functional brain imaging in humans demonstrates that object representations are not confined to the ventral pathway, but can also be found in several areas along the dorsal pathway. In both streams, areas at intermediate processing stages in extrastriate cortex (V4, V3A, MT and V7) showed object-selective but viewpoint- and size-specific responses. In contrast, higher-order areas in lateral occipital and posterior parietal cortex (LOC, IPS1 and IPS2) responded selectively to objects independent of image transformations. Contrary to the two-pathways hypothesis, our findings indicate that basic object information related to shape, size and viewpoint may be represented similarly in two parallel and hierarchically organized neural systems in the ventral and dorsal visual pathways.

[edit] February 2008

6 Nikola Stikov, EE MRSRL: t1 relaxometry and cross-relaxation (myelin) mapping

13 Elena Rykhlevskaia: DTI fiber clustering

Will discuss issues related to whole brain fiber clustering: methods and possible applications.

20 Samy Bengio, Google: A Discriminative Kernel-based Model to Rank Images from Text Queries

I will introduce a discriminative model for the retrieval of images from text queries. Our approach formalizes the retrieval task as a ranking problem, and introduces a learning procedure optimizing a criterion related to the ranking performance. The proposed model hence addresses the retrieval problem directly and does not rely on an intermediate image annotation task, which contrasts with previous research. Moreover, our learning procedure builds upon recent work on the online learning of kernel-based classifiers. This yields an efficient, scalable algorithm, which can benefit from recent kernels developed for image comparison. The experiments performed over stock photography data show the advantage of our discriminative ranking approach over state-of-the-art alternatives. Further analysis of the results shows that our model is especially advantageous over difficult queries such as queries with few relevant pictures or multiple-word queries.

27 Field Trip to Jean Suh's thesis defense || Special Time: 10:30 AM. || Special Place: Astrophysics room 102

http://hepl.stanford.edu/visitors.html

The new physics building which is across the lawn area besides the math building. Go up the little steps from the lawn area, and enter the building thru the front door (facing physics main office). The conference room (room 102) is on the left of the main office.

The open session is supposed to take about an hour. Refreshments at 10:15.

The role of local feature processing in object perception

One of the outstanding questions in the study of human visual perception of objects is how local feature processing affects object perception. In this thesis, we addressed this fundamental question by examining human perceptual performance on images containing whole objects or partial images of objects (containing one, two or three features) for two perceptual tasks: detection ("something" vs. random dot noise) and classification (perception of the object category, e.g., car vs. other objects). Images were embedded in visual noise and we measured human subjects' performance in a range of noise levels close to subjects' perceptual threshold. First, we found that human detection performance increases when more object area (A) is revealed and when the noise variance (N) decreases. The contribution of these factors can be characterized as a function of Ln(A/N), which we will refer to as the Area-to-Noise ratio (ANR). When the ANR was equated in the subsequent experiments, comparison of detection performance on partial vs. whole images of objects revealed the dynamic role of local feature processing in object detection: i) detection based on a single useful feature was better than detection based on a whole object suggesting the effectiveness of local feature processing, ii) however, detection of a whole object did not require detection of its features, and iii) detection performance varied across features; useful features (such as eyes for faces and wheels for cars) yielded better performance than suboptimal features (such as nose and mouth for faces). This pattern of results did not significantly vary across two methods we used to select features (features based on semantic judgments and a computer algorithm), and two object categories (faces and cars). For classification, results were largely similar, except that in low ANR levels (below the level sufficient for successful detection of a whole object) classification performance was better on a single useful feature than on a whole object, and in higher ANR levels the reverse was true, indicating that global processing becomes advantageous for object classification at higher ANR levels. Overall, these data suggest the dynamic role of local feature processing in object perception, which can be explained by the contribution of two mechanisms: probability summation of information in local features and spatial summation of information across whole objects.

[edit] March 2008

5 No Vision Lunch

12 Papers: David Melcher on Spatiotopy, eye movements, and adaptation

1: Melcher D: Predictive remapping of visual features precedes saccadic eye movements. Nat Neurosci. 2007 Jul;10(7):903-7.

2: Melcher D. Spatiotopic transfer of visual-form adaptation across saccadic eye movements. Curr Biol. 2005 Oct 11;15(19):1745-8.


19 Paper discussion: Adelson & Bergen, 1985

We will also have a short visit (~15 min) at 11:30 from the Green Library curator to find out about our research needs and habits.

William J. Wheeler, Ph.D.|| Curator for the Social & Behavioral Sciences || 123A, Social Science Resource Center, Green Library


[edit] April 2008

2 Netta Levin: Hemianopic patient’s functional imaging & vision restoration therapy

9 Kendrick Kay from UCB: Using computational models of voxels to identify images seen by an observer

16 Marie Smith, University of Glasgow: Inverse mapping the neuronal substrates of categorization

In the search for a deeper understanding of the workings of the human brain, researchers are increasingly looking to brain imaging methods to provide the answers. Thanks to these methodologies the framework of distributed processing networks has emerged, with neuronal oscillations potentially playing an important role for interactions among network nodes. Of critical importance in understanding the functional role of these networks is to establish the information that they process and how this information is distributed and transferred throughout the different brain areas involved. Through the careful control of cognitive parameters during experimentation and by making use of advanced multi-voxel pattern analysis methods, considerable insight into what information is represented by the brain has been possible. However, while subtle differences in experimental conditions (e.g. a happy vs. a fearful face) may tell us which brain areas respond to a particular condition, they cannot inform us about the underlying neural representations and the information processing that subsumes these critical categorizations. Here we sought to address these questions by establishing an analysis methodology that allows us to ascribe specific information processing content to temporally and spatially resolved MEG signals. In essence, with centimeter and millisecond precision, we decode the brain activity into the underlying specific stimulus information and track how this processing evolves over time across the cortex. To illustrate our approach, we apply it to the study of facial expressiveness (happy vs. neutral) and gender categorizations.

23 Auditory lunch! Vivienne L. Ming, Mind, Brain & Computation, Stanford University & Redwood Center for Theoretical Neuroscience, UC Berkeley

Title to be announced

30 OPEN


[edit] May 2008

7 Glenn C. Cockerham, MD (Ophthalmology, Stanford) - TBA. Hosted by Bob Dougherty.

16 (Note special Friday date after VSS) Ichiro Kuriki and Ryusuke Hayashi. Hosted by Kaoru Amano.

This talk will be in room 102 starting at 1:30.

Speaker1: Ichiro Kuriki (Vision and Cognition Systems Laboratory, Research Institute of Electrical Communication, Tohoku University, Japan)

An fMRI study on the variability of hue selectivity in human visual cortex

The representation of color signal begins with relative differences among the activities of three cone classes. It has been pointed out in the electrophysiological studies in Macaque monkeys that the color signal is coded in the form of a luminance channel and two chromatic channels up to the level of lateral geniculate nucleus (LGN), and the variation of selectivity to color increase remarkably at the level of primary visual cortex. Several psychophysical studies in humans pointed out the possible existence of multiple hue-selective systems in human visual system, but there is no direct evidence showing the variability of hue selectivity in the human visual cortex. We made an attempt on clarifying the variability of hue selectivity in the human visual cortex by using BOLD functional MRI (fMRI) technique. As a result, we found that the voxels around calcarine sulcus show the variability of hue selectivity similar to that suggested in the electrophysiological studies in monkeys.


Speaker2: Ryusuke Hayashi (Laboratory for Integrative Neural Systems, RIKEN Brain Science Institute, Japan)

Paradoxical motion perception observed through contrast-alternating multiple -slit-viewing

It is known that the human visual system can extract motion signals although a moving pattern is presented only through an array of slits (Multiple-Slit-Viewing, MSV). Here we introduce a new motion display, in which the contrast of a pattern moving behind the array of slits flips from the original to negative polarity alternatively (contrast-alternating MSV), and demonstrate that such motion stimuli produce intriguing motion perception phenomena: they appear to move in the opposite direction first and then move to the forward direction as the interslit interval increases. We will show that our visual system uses the limited frequency band for making a decision on the perceived motion direction and this bias can account for the motion perception of contrast-alternating MSV.


21 Kate Crookes, Australian National University: Childhood improvements in behavioural face task performance result from general cognitive development not changes in face perception

Despite the remarkable face processing abilities observed in infants, performance on face perception and memory tasks improves across childhood and into adolescence and fMRI reveals late changes in face-selective areas. The important question for understanding the developmental origins of face processing is: Why does performance on behavioural face tasks improve with age? There are two possible explanations: (1) the 'special' aspect of face perception (holistic processing) is developing due to extended experience with faces (face specific perceptual development theory) or (2) the improvement seen on face tasks is due entirely to the development of general cognitive factors (general cognitive development theory). Previous studies attempting to differentiate these two theories have suffered methodological issues (e.g., baseline differences across age groups, poorly matched comparison object class, differences in task difficulty across conditions). The present study presents converging evidence from three independent approaches which strongly support the general cognitive development theory, and argue that, behaviourally, there is no development in face perception between early childhood and adulthood.


[edit] June 2008

4 Valentin Dragoi

11 Spatiotemporal object continuity in human ventral visual cortex
Do-Joon Yi*‡, Nicholas B. Turk-Browne†, Jonathan I. Flombaum†, Min-Shik Kim*, Brian J. Scholl†, Marvin M. Chun PDF
Coherent visual experience requires that objects be represented as the same persisting individuals over time and motion. Cognitive science research has identified a powerful principle that guides such processing: objects must trace continuous paths through space and time. Little is known, however, about how neural representations of objects, typically defined by visual features, are influenced by spatiotemporal continuity. Here we report the consequences of spatiotemporally continuous vs. discontinuous motion on perceptual representations in human ventral visual cortex. In experiments employing both dynamic occlusion and apparent motion, face-selective cortical regions exhibited significantly less activation when faces were repeated in continuous vs. discontinuous trajectories, suggesting that discontinuity caused featurally-identical objects to be represented as different individuals. These results indicate for the first time that spatiotemporal continuity modulates neural representations of object identity, influencing judgments of object persistence even in the most staunchly ‘featural’ areas of ventral visual cortex.

18 Alison Kevan Predicting early reading skills from preliterate dorsal stream functioning

Despite the large body of literature indicating that many dyslexic readers possess a magnocellular fed dorsal stream deficit, the relationship between dorsal stream sensitivity, reading ability and indeed reading failure remains largely unknown. As such, my research examines the visual functioning of preliterate children to determine whether visual processing deficits are present before children learn to read or whether they develop as the result of poor phonological processing and subsequent reading failure. The dorsal processing skills (as assessed by coherent motion and frequency doubling illusion) of two groups of pre-reading children (unselected and at- risk) were compared at two time points: prior to formal reading instruction (start of kindergarten) and 18 months later (Grade 1). Results indicate that at-risk children demonstrate specific dorsal stream deficits before they display any reading impairment, and that pre-reading measures of dorsal stream functioning can predict early reading skills.

25 A voice region in the monkey brain. Petkov CI, Kayser C, Steudel T, Whittingstall K, Augath M, Logothetis NK. Max Planck Institute for Biological Cybernetics, Spemannstrasse 38, 72076 Tübingen, Germany. chris.petkov@tuebingen.mpg.de

For vocal animals, recognizing species-specific vocalizations is important for survival and social interactions. In humans, a voice region has been identified that is sensitive to human voices and vocalizations. As this region also strongly responds to speech, it is unclear whether it is tightly associated with linguistic processing and is thus unique to humans. Using functional magnetic resonance imaging of macaque monkeys (Old World primates, Macaca mulatta) we discovered a high-level auditory region that prefers species-specific vocalizations over other vocalizations and sounds. This region not only showed sensitivity to the 'voice' of the species, but also to the vocal identify of conspecific individuals. The monkey voice region is located on the superior-temporal plane and belongs to an anterior auditory 'what' pathway. These results establish functional relationships with the human voice region and support the notion that, for different primate species, the anterior temporal regions of the brain are adapted for recognizing communication signals from conspecifics.

[edit] July 2008

9 Discussion: Mapping the Structural Core of Human Cerebral Cortex

Hagmann et al., 2008

16 Discussion: microstimulation of face patches

PDF: Moeller et al., 2008

Patches with links: a unified system for processing faces in the macaque temporal lobe. Moeller S, Freiwald WA, Tsao DY.

Institute for Brain Research and Center for Advanced Imaging, University of Bremen, Post Office Box 330440, D-28334 Bremen, Germany.

The brain processes objects through a series of regions along the ventral visual pathway, but the circuitry subserving the analysis of specific complex forms remains unknown. One complex form category, faces, selectively activates six patches of cortex in the macaque ventral pathway. To identify the connectivity of these face patches, we used electrical microstimulation combined with simultaneous functional magnetic resonance imaging. Stimulation of each of four targeted face patches produced strong activation, specifically within a subset of the other face patches. Stimulation outside the face patches produced an activation pattern that spared the face patches. These results suggest that the face patches form a strongly and specifically interconnected hierarchical network.


23 OPEN


30 Discussion: The effect of a serotonin-induced dissociation between spiking and perisynaptic activity on BOLD functional MRI.

Alexander Rauch, Gregor Rainer, Nikos K Logothetis

The relationship of the blood oxygen-level-dependent (BOLD) signal to its underlying neuronal activity is still poorly understood. Combined physiology and functional MRI experiments suggested that local field potential (LFP) is a better predictor of the BOLD signal than multiunit activity (MUA). To further explore this relationship, we simultaneously recorded BOLD and electrophysiological activity while inducing a dissociation of MUA from LFP activity with injections of the neuromodulator BP554 into the primary visual cortex of anesthetized monkeys. BP554 is a 5-HT1A agonist acting primarily on the membrane of efferent neurons by potassium-induced hyperpolarization. Its infusion in visual cortex reliably reduced MUA without affecting either LFP or BOLD activity. This finding suggests that the efferents of a neuronal network pose relatively little metabolic burden compared with the overall presynaptic and postsynaptic processing of incoming afferents. We discuss implications of this finding for the interpretation of BOLD activity.


[edit] August 2008

6 High-field fMRI unveils orientation columns in humans.

Yacoub E, Harel N, Ugurbil K. Proc Natl Acad Sci U S A. 2008 Jul 29;105(30):10607-12.

http://www.ncbi.nlm.nih.gov/pubmed/18641121?dopt=Abstract


13 no vision lunch


20 Multivariate Patterns in Object-Selective Cortex Dissociate Perceptual and Physical Shape Similarity

Johannes Haushofer, Margaret S. Livingstone, Nancy Kanwisher

http://biology.plosjournals.org/perlserv/?request=get-document&doi=10.1371/journal.pbio.0060187

27 Bradley Buchsbaum & Andrew Kayser UC Berkeley

Decision-making and Evidence Accumulation in a Random Dot Motion Coherence Task

Our ability to make rapid decisions based on sensory information belies the complexity of the underlying computations. Unraveling them involves understanding both the responses of the individual components in the neural circuitry and the relation of these responses to each other. Recently, "accumulator" models of decision-making, such as Ratcliff's diffusion model, have been shown to explain the activity of neurons in the parietal cortex as macaques make judgments in a visual motion task. In our study of the decision process in humans, we demonstrate that the diffusion model predicts responses in the middle intraparietal sulcus (mIPS) as measured by functional MRI, and that these responses are distinct from those of the middle temporal area (MT). Significantly, the metrics used to distinguish these two areas reveal a processing continuum that includes the early sensory detectors in visual cortex, the visuo-motor integration system of the IPS, and the centers of cognitive control in the prefrontal cortex.

[edit] September 2008

3 Discussion: Tuned responses of astrocytes and their influence on hemodynamic signals in the visual cortex. Schummers J, Yu H, Sur M.

http://www.sciencemag.org/cgi/content/full/320/5883/1638

10 Discussion: Neural repetition suppression reflects fulfilled perceptual expectations Summerfield C, Trittschuh EH, Monti JM, Mesulam MM, Egner T.

http://www.nature.com/neuro/journal/v11/n9/abs/nn.2163.html


24 Discussion: The cortical column: a structure without a function Horton JC, Adams DL.

http://journals.royalsociety.org/content/p01xkq6eeh9ad84t

[edit] October 2008

1 No Vision Lunch

8 Discussion continued: The cortical column: a structure without a function Horton JC, Adams DL.

15 Brian Avants: surface-based analysis of MRI data, and other topics

Brian is a grad student from U. Penn who is working with James Gee at the Penn Image and Computing Science lab, as well as with David Brainard. His work is mainly on software development. This software covers a number of topics which are of interest to people in our labs, including:

  • Surface-based analysis and cross-subjects normalization

(http://www.picsl.upenn.edu/project_insane.aspx)

(http://www.picsl.upenn.edu/project_retinotopicmapping.aspx)

  • Segmentation and analysis of functional data in the hippocampus

(http://www.picsl.upenn.edu/project_hippocampusSPM.aspx)

He is also one of the contributors to ITK-Snap, MR viewing software which we have adapted into our current segmentation tool, ITK-Gray.


22 Geoffrey Karl Aguirre, M.D., Ph.D.: The geometry of neural similarity spaces studied with fMRI

http://cfn.upenn.edu/aguirre

Assistant Professor of Neurology Hospital of the University of Pennsylvania Philadelphia, PA

What is the relationship between the perceptual similarity of sensory experiences and the similarity of the neural responses that encode them? In a series of experiments we have studied the neural representation of objects and shapes in human visual cortex using this question as a guiding principle. With continuous carry-over, functional MRI (Aguirre, Neuroimage, 2007), we can measure the similarity of evoked neural responses to objects on either a focal (within voxel adaptation) or distributed (across voxel pattern) spatial scale. For simple two-dimension shapes and for faces we have found that perceptual similarity predicts neural response similarity, but that the visual information represented at focal and distributed scales differ, both within and across visual areas (Drucker & Aguirre, Submitted). Specifically, the dorsolateral potion of the “object responsive” visual area LOC represents a subset of object features with a spatially coarse code, while ventral LOC contains focal populations of neurons that represent the entire object appearance.

These studies show that neural population coding of object appearance within ventral LOC reflects perceptual similarity. The precise metric properties of perceptual similarity may further predict the stimulus axes along which these representations are organized. We hypothesize that integral perceptual axes (perceived as a composite with a Euclidean distance metric) are represented by populations of neurons that are conjointly tuned to the axes, while separable axes (defined by a rectilinear metric) are represented by independently tuned neural populations. Using fMRI we may measure the geometric properties of neural adaptation to distinguish between conjoint or independent tuning for a population of neurons (Drucker, Kerr, Aguirre, Submitted). For both two-dimensional shapes and for faces we find that neural tuning within ventral visual areas reflects the metric properties of perception. For shapes, curvature and thickness are independently represented while two arbitrary dimensions of shape variation have conjoint representation. For faces, left ventral areas are tuned to represent face features, while right ventral cortex modulates tuning between face features and wholes depending upon stimulus context and reflecting behavioral measures (Harris & Aguirre, Submitted).


29 Bart Anderson, U Sydney. Photometric and geometric constraints in the perception of surfaces

http://www.psych.usyd.edu.au/staff/barta/


[edit] November 2008

5 Break

12 SFN practice talks: 10 AM - 1:30 PM

19 SFN

26 Thanksgiving break

[edit] December 2008

3 Davie: 'Anatomical Evidence for Classical and Extra-classical Receptive Field Completion Across the Discontinuous Horizontal Meridian Representation of Primate Area V2'

Jeffs J, Ichida JM, Federer F, Angelucci A

Cereb Cortex. 2008 Sep 16

In primates, a split of the horizontal meridian (HM) representation at the V2 rostral border divides this area into dorsal (V2d) and ventral (V2v) halves (representing lower and upper visual quadrants, respectively), causing retinotopically neighboring loci across the HM to be distant within V2. How is perceptual continuity maintained across this discontinuous HM representation? Injections of neuroanatomical tracers in marmoset V2d demonstrated that cells near the V2d rostral border can maintain retinotopic continuity within their classical and extra-classical receptive field (RF), by making both local and long-range intra- and interareal connections with ventral cortex representing the upper visual quadrant. V2d neurons located <0.9-1.3 mm from the V2d rostral border, whose RFs presumably do not cross the HM, make nonretinotopic horizontal connections with V2v neurons in the supra- and infragranular layers. V2d neurons located <0.6-0.9 mm from the border, whose RFs presumably cross the HM, in addition make retinotopic local connections with V2v neurons in layer 4. V2d neurons also make interareal connections with upper visual field regions of extrastriate cortex, but not of MT or MTc outside the foveal representation. Labeled connections in ventral cortex appear to represent the "missing" portion of the connectional fields in V2d across the HM. We conclude that connections between dorsal and ventral cortex can create visual field continuity within a second-order discontinuous visual topography.

10 Ariel Rokem

Cholinergic modulation of attention and learning in the human visual system:

The human visual system is anisotropic. For example, direction discrimination is better around cardinal directions (up/down, right/left) relative to oblique directions (off-cardinal diagonals). In the first part of my talk, I will present a model of neural population coding, which accounts for this so-called "oblique effect" in motion direction discrimination. I will present preliminary data from an fMRI experiment which tests the physiological predictions of the model.

In the second part of my talk, I will discuss changes in the oblique effect due to perceptual learning. Multiple animal studies have shown that learning and attention are modulated by release of Acetylcholine (Ach) in primary sensory cortex. We have studied modulation of attention and perceptual learning in the visual system of humans, by administering donepezil (trade name: Aricept), a drug which raises the levels of acetylcholine in the nervous system, by inhibiting the cholinesterase which breaks down Ach.

We have found that administration of donepezil augments perceptual learning and enhances the effect of voluntary attention in a spatial cueing task.


17 OPEN

24 Christmas: no Vision Lunch

31 New Years: no Vision Lunch

Imaging studies are consistent with the existence of brain regions specialized for color, but electrophysiological studies have produced conflicting results. Here we address the neural basis for color, using targeted single-unit recording in alert macaque monkeys, guided by functional magnetic resonance imaging (fMRI) of the same subjects. Distributed within posterior inferior temporal cortex, a large region encompassing V4, PITd, and posterior TEO that some have proposed functions as a single visual complex, we found color-biased fMRI hotspots that we call "globs," each several millimeters wide. Almost all cells located in globs showed strong luminance-invariant color tuning and some shape selectivity. Cells in different globs represented distinct visual field locations, consistent with the coarse retinotopy of this brain region. Cells in "interglob" regions were not color tuned, but were more strongly shape selective. Neither population was direction selective. These results suggest that color perception is mediated by specialized neurons that are clustered within the extrastriate brain.

23 Ken Nakayama, Harvard: Normal and abnormal face processing in human observers

We humans spend countless hours staring at faces and its perhaps no surprise that scientists now realize that the perception of human faces is one of the more important functions of our visual system. Our laboratory has devoted itself to studying human face perception in both normal observers and those who have severe difficulties in recognizing faces. Behavioral studies on normal face processing show that fine features of the face are important for recognition, MEG studies also show the importance of face parts insofar as they by themselves can cause adaptation of the M170 face response. Studies on developmental prosopagnosia indicate that it is much more widespread than previously assumed, that as many as 2% of the population has serious face recognition problems. Such individuals, while suffering major difficulties in recognition perform normally in other subtle face specific judgments such as gender, attractiveness and emotions. In addition we show that developmental prosopagnosia comes in two forms, pure and iconic and that these segregate in different families. This suggests that prosopagnosia and its variants has a likely genetic basis.

30 Discussion: Objects in the dorsal stream

Konen & Kastner, 2007

Two hierarchically organized neural systems for object information in human visual cortex.

The primate visual system is broadly organized into two segregated processing pathways, a ventral stream for object vision and a dorsal stream for space vision. Here, evidence from functional brain imaging in humans demonstrates that object representations are not confined to the ventral pathway, but can also be found in several areas along the dorsal pathway. In both streams, areas at intermediate processing stages in extrastriate cortex (V4, V3A, MT and V7) showed object-selective but viewpoint- and size-specific responses. In contrast, higher-order areas in lateral occipital and posterior parietal cortex (LOC, IPS1 and IPS2) responded selectively to objects independent of image transformations. Contrary to the two-pathways hypothesis, our findings indicate that basic object information related to shape, size and viewpoint may be represented similarly in two parallel and hierarchically organized neural systems in the ventral and dorsal visual pathways.

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