How does the visual system represent visual information? How does the visual system represent features of scenes? Vision is analytical - the system breaks down the scene into distinct kinds of features and represents them in functionally segregated pathways but… the spike timing matters too!

Visual Neuron Responses Unit recordings in LGN reveal a centre/surround receptive field many arrangements exist, but the typical RF has an excitatory centre and an inhibitory surround these receptive fields tend to be circular - they are not orientation specific How could the outputs of such cells be transformed into a cell with orientation specificity?

Visual Neuron Responses LGN cells converge on “simple” cells in V1 imparting orientation specificity

Visual Neuron Responses V1 maintains a map of orientations across the retina because each small area on the retina has a corresponding cortical module that contains cells with the entire range of orientation tunings

Visual Neuron Responses LGN cells converge on simple cells in V1 imparting orientation specificity Thus we begin to see how a simple representation - the orientation of a line in the visual scene - can be maintained in the visual system –increase in spike rate of specific neurons indicates presence of a line with a specific orientation at a specific location on the retina –Why should this matter? –Such cells also represent the presence of edges between two objects or an object and the background

Visual Neuron Responses This conceptualization of the visual system was “static” - it did not take into account the possibility that visual cells might change their response selectivity over time –Logic went like this: if the cell is firing, its preferred line/edge must be present and… –if the preferred line/edge is present, the cell must be firing We will encounter examples in which neither of these are true!

Visual Pathways V1 is, of course, not the only visual area (it turns out it’s not even always “primary”) What role does the rest of visual cortex play?

Visual Pathways Visual processing “culminates in formatting the representation of the stimulus so it matches (or not) information in memory” Alternatively, visual processing formats the representation in a way that can be stored for later comparison to other inputs…visual memory

The Role of “Extrastriate” Areas Consider two plausible models: 1.System is hierarchical: –each area performs some elaboration on the input it is given and then passes on that elaboration as input to the next “higher” area 2.System is analytic and parallel: –different areas elaborate on different features of the input

The Role of “Extrastriate” Areas Consider two plausible models: 1.System is hierarchical: –each area performs some elaboration on the input it is given and then passes on that elaboration as input to the next “higher” area 2.System is analytic and parallel: –different areas elaborate on different features of the input –Already by the retinal ganglion cells we see this “divide-and- conquer” strategy

The Role of “Extrastriate” Areas Different visual cortex regions contain cells with different tuning properties

The Role of “Extrastriate” Areas Other visual areas have cells that are specifically tuned for other features or properties in the scene E.g. V5 (area MT) has cells that respond to the presence of motion in their receptive fields –these cells are direction tuned but are non-specific for color –V5 cells even have velocity specificity! Unit Recordings

The Role of “Extrastriate” Areas Functional imaging (PET) investigations of motion and colour selective visual cortical areas Zeki et al. Subtractive Logic –stimulus alternates between two scenes that differ only in the feature of interest (i.e. colour, motion, etc.)

The Role of “Extrastriate” Areas Identifying colour sensitive regions Subtract Voxel intensities during these scans… …from voxel intensities during these scans …etc. Time ->

The Role of “Extrastriate” Areas result –voxels are identified that are preferentially selective for colour –these tend to cluster in anterior/inferior occipital lobe

The Role of “Extrastriate” Areas similar logic was used to find motion-selective areas Subtract Voxel intensities during these scans… …from voxel intensities during these scans …etc. Time -> MOVING STATIONARY MOVING STATIONARY

The Role of “Extrastriate” Areas result –voxels are identified that are preferentially selective for motion –these tend to cluster in superior/dorsal occipital lobe near TemporoParietal Junction –Akin to Human V5

The Role of “Extrastriate” Areas Visual areas elaborate on low-level input –in Enigma pattern, motion is perceived, but the low- level input is stationary –functional imaging reveals V5 but not V1 activation associated with viewing this stimulus