Visual Pathways

1. Visual Pathways • 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 W. W. Norton

2. Visual Pathways • Visual scene is represented: • Retinotopically thus… • spatiotopically = Fovea Tootell R B H et al. PNAS 1998;95:811-817

3. 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

4. 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

5. 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?

6. Visual Neuron Responses • LGN cells converge on “simple” cells in V1 imparting orientation (and location) specificity

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

8. 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

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

10. The Role of “Extrastriate” Areas • Consider two plausible models: • 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 • System is analytic and parallel: • different areas elaborate on different features of the input

11. 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.)

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

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

14. 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. STATIONARY STATIONARY MOVING MOVING Time ->

15. 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

16. The Role of “Extrastriate” Areas • Thus PET studies doubly-dissociate colour and motion sensitive regions

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

18. 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

19. 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

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

21. 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!

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

23. Visual Neuron Responses • Boundaries between objects can be defined by texture

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