Visual Pathways W. W. Norton Primary cortex maintains distinct pathways – functional segregation M and P pathways synapse in different layers Ascending (i.e. feed-forward) projections synapse in middle layers Descending (i.e. feed-back) projections synapse in superfical and deep layers

Visual Pathways Visual scene is represented: –Retinotopically thus… –spatiotopically = Fovea Tootell R B H et al. PNAS 1998;95:

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

Visual Neuron Responses The notion of a receptive field is fundamental in vision science –A neuron’s receptive field is the region in space in which a stimulus will evoke a response from that neuron –Receptive field properties vary widely across visual neurons and are never just “ON” or “OFF” –Unit recordings in LGN reveal a centre/surround receptive field

Visual Neuron Responses Unit recordings in LGN reveal a centre/surround receptive field many arrangements exist, but the “classical” 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 (and location) specificity

Visual Neuron Responses LGN cells converge on “simple” cells in V1 imparting orientation (and location) specificity Again, information is physically seperated into a “map”

Visual Neuron Responses LGN cells converge on simple cells in V1 imparting orientation specificity Thus we begin to see how a simple representation – orientations of lines - 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 –Reality is that spike rate probably is only one part of the story: information is coded in many ways e.g. Relative timing Graded potentials

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

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 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 Thus PET studies doubly-dissociate colour and motion sensitive regions

The Role of “Extrastriate” Areas V4 and V5 are doubly-dissociated in lesion literature:

The Role of “Extrastriate” Areas V4 and V5 are doubly-dissociated in lesion literature: –achromatopsia (color blindness): there are many forms of color blindness cortical achromatopsia arises from lesions in the area of V4 singly dissociable from motion perception deficit - patients with V4 lesions have other visual problems, but motion perception is substantially spared

The Role of “Extrastriate” Areas V4 and V5 are doubly-dissociated in lesion literature: –akinetopsia (motion blindness): bilateral lesions to area V5 (extremely rare) severe impairment in judging direction and velocity of motion - especially with fast-moving stimuli visual world appeared to progress in still frames similar effects occur when M-cell layers in LGN are lesioned in monkeys

Visual Neuron Responses Edges are important because they are the boundaries between objects and the background or objects and other objects

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 these don’t apply! Representing boundaries must be more complicated than simple edge detection!

Visual Neuron Responses Boundaries between objects can be defined by color rather than brightness

Visual Neuron Responses Boundaries between objects can be defined by texture

Visual Neuron Responses Boundaries between objects can be defined by motion and depth cues