Transcript Vision 2
Human Neurobiology 207 & part of 210 Vision 2 Stuart Bunt (standing in for Alan Harvey) 207 only 1999 Dr Stuart Bunt [email protected] Human Neurobiology 207 & part of 210 Retinal Histology • 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) Dr Stuart Bunt [email protected] Human Neurobiology 207 & part of 210 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. Dr Stuart Bunt [email protected] Human Neurobiology 207 & part of 210 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 Dr Stuart Bunt [email protected] Human Neurobiology 207 & part of 210 RGCs • Respond to contrast • respond to movement/changes • processed in retina where comparisons are accurate. Dr Stuart Bunt [email protected] Human Neurobiology 207 & part of 210 Visual pathways dLGN to V1 • V1 is the area 17, primary visual cortex • Note the large area devoted to the macula Dr Stuart Bunt [email protected] Human Neurobiology 207 & part of 210 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 Dr Stuart Bunt [email protected] Human Neurobiology 207 & part of 210 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 Dr Stuart Bunt [email protected] Human Neurobiology 207 & part of 210 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? Dr Stuart Bunt [email protected] Human Neurobiology 207 & part of 210 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 Dr Stuart Bunt [email protected] Human Neurobiology 207 & part of 210 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 Dr Stuart Bunt [email protected] Human Neurobiology 207 & part of 210 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? Dr Stuart Bunt [email protected] Human Neurobiology 207 & part of 210 Parallel and Hierarchical pathways • A number of serial pathways may function in parallel • Separate pathways process different sensory modalities • Colour and motion are separate Dr Stuart Bunt [email protected] Human Neurobiology 207 & part of 210 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 Dr Stuart Bunt [email protected] Human Neurobiology 207 & part of 210 Motion Detection • The detection of motion occurs in the secondary visual areas in parietal cortex (V5) as shown here by blood flow measurements (fMRI?) Dr Stuart Bunt [email protected]