Vision 2

1. Vision 2 Stuart Bunt (standing in for Alan Harvey) 207 only 1999

2. Rods high sensitivity single photons saturate in daylight low temporal resolution slow response long integration time low acuity convergent achromatic Cones less photopigment less amplification only saturate in intense light fast response high acuity concentrated in the fovea chromatic, three types (in primates) Retinal Histology

3. Circuitry • Retina is an extension of the brain • Photoreceptors graded polarization • RGCs output cell, action potentials • bipolar, horizontal and amacrine cells interneurons of the retina.

4. Classes of RGC • CAT • On/off cells • sustained X/transient Y • not much colour • MONKEY • On/off • Parvo (P) colour (red/green) and form • Magno (M) motion

5. RGCs • Respond to contrast • respond to movement/changes • processed in retina where comparisons are accurate.

6. Visual pathways dLGN to V1 • V1 is the area 17, primary visual cortex • Note the large area devoted to the macula

7. Monkey dLGN • 6 layers • M (layers 1 and 2) • P (layers 3-6) • ipsi (layers 2,3,5) • contra (layers 1,4,6) • On and Off in P pathways • Receptive fields in LGN similar to the retina

8. Parallel and Serial Pathways • There are multiple visual areas, most with retinotopic maps • Processing proceeds serially along a pathway • At each level more complex features are identified

9. Some cells are very specific In the cortex the retinal ganglion cell receptive fields are combined to form edges and lines Progressive convergence may allow some feature extraction The Grandmother cell?

10. Face recognition • Prosopagnosia • inability to recognize faces. • Bilateral damage to parahippocampal gyri can lead to loss of memory of faces • damage to secondary visual areas can lead to Prosopagnosia

11. Ocular Dominance in V1 • A number of maps are overlaid on the visual cortex. • Alternative columns receive major input from each eye • Orientation columns are overlaid on this • “Blobs” for colour

12. Multiple Overlaid Maps • Ocular dominance columns • Orientation columns • These can be revealed by direct visualisation using activity activated fluorescent dyes • horizontal connections between these areas. • Cytochrome oxidase: significance unknown?

13. Parallel and Hierarchical pathways • A number of serial pathways may function in parallel • Separate pathways process different sensory modalities • Colour and motion are separate

14. Human Regional Cerebral Blood Flow • Measurements of blood flow show cerebral activity. • Colour is clearly processed separately from movement in temporal areas (V4) • In both cases V1 & V2 receive input and are active

15. Motion Detection • The detection of motion occurs in the secondary visual areas in parietal cortex (V5) as shown here by blood flow measurements (fMRI?)