Prof. Greg Francis1PSY 310: Sensory and Perceptual ProcessesPurdue UniversityFunctional anatomyPSY 310Greg FrancisLecture 06That’s my daughter!Purdue UniversityDifference of Gaussians Model of ganglion cell receptive fieldsPurdue UniversityDifference of Gaussians What do we do with such a model? Describe a spatial pattern of light mathematically I(x,y)Purdue UniversityDifference of Gaussians What do we do with such a model? Describe a spatial pattern of light mathematically I(x,y)Purdue UniversityCell response The receptive field of the cell “weights” the light input Multiply point by point Add up across all pointsPurdue UniversityCell response Strength of response depends on position of receptivefield relative to stimulusProf. Greg Francis2PSY 310: Sensory and Perceptual ProcessesPurdue UniversityCell response Strength of response depends on position of receptivefield relative to stimulusPurdue UniversityCell response Strength of response depends on the spacing of the barsPurdue UniversityCell response Strength of response depends on the spacing of the barsPurdue UniversityModel behavior Try bar gratingsof manydifferentfrequencies Set parametersto match realganglion cells Dots are for areal cell Blue line ismodelPurdue UniversityCell response Of course, there is more than one ganglion cell Multiple cells with different centersPurdue UniversityCell response Of course, there is more than one ganglion cell Multiple cells with different centers And they overlapProf. Greg Francis3PSY 310: Sensory and Perceptual ProcessesPurdue UniversityCell response Of course, there is more than one ganglion cell Multiple cells with different centers And they overlap A lot!Purdue UniversityLayers It helps to think of each stage of the visual system as akind of “plane of activity”ImagePurdue UniversityLayers It helps to think of each stage of the visual system as akind of “plane of activity”ImageRetinaGanglion cellsPurdue UniversityLayers Each ganglion cell has a center to its receptive field On the retina We’ll let that indicate the “pixel position” of the ganglion cell inthe plane We’ll let the response (number of action potentials) of theganglion cell indicate the “intensity” of a picture we’ll draw onthe plane This gives us a way of understanding what the neurons aredoingGanglion cellsPurdue UniversityReceptive field So, if you have a bar grating as an imagePurdue UniversityReceptive field So, if you have a bar grating as an image Suppose the receptive fields are small relative tothe thickness of the barsProf. Greg Francis4PSY 310: Sensory and Perceptual ProcessesPurdue UniversityReceptive field So, if you have a bar grating as an image If the receptive fields are a bit biggerPurdue UniversityReceptive field So, if you have a bar grating as an image If the receptive fields are a lot biggerPurdue UniversityBrightness perception The properties of ganglion cells seem to be relatedto several interesting aspects of brightnessperception How bright is a region that you are looking at? Brightness contrastPurdue UniversityBrightness perception The properties of ganglion cells seem to be related toseveral interesting aspects of brightness perception How bright is a region that you are looking at? Brightness contrastPurdue UniversityBrightness perception Ganglion cell responses show differences that aresimilar to our perceptsPurdue UniversityBrightness perception Hermann gridProf. Greg Francis5PSY 310: Sensory and Perceptual ProcessesPurdue UniversityBrightness perception Hermann grid With receptive fields about this size… More surround inhibition at intersectionsPurdue UniversityBrightness perception Hermann grid You get weaker responses at the intersectionsPurdue UniversityImplications For ganglion cells to provide an explanation of thesebrightness illusions you can to assume The brightness percept can be related to the responses of thesecells and not cells with different sized receptive fields The rest of the visual system (e.g., the brain) doesn’t muck upthings Notice that many cells do not respond well tohomogeneous light that covers the whole receptive field Inhibition balances out excitationPurdue UniversityAn image like this With lots of detailsPurdue UniversityAn image like this Is treated like thisby ganglion cellsthat are quite smallPurdue UniversityAn image like this Is treated like thisby ganglion cellsthat are fairly largeProf. Greg Francis6PSY 310: Sensory and Perceptual ProcessesPurdue UniversityAn image like this Is treated like thisby ganglion cellsthat are very largePurdue UniversityWhich is the percept? None of these response patterns seem to match ourpercept Clearly the brain must somehow combine theinformation in a significant wayPurdue UniversityConclusions Ganglion cells Center-surround receptive fields They respond well to edges Parse the retinal pattern in a peculiar way Accounts for some brightness illusions Other parts of the brain are necessary forperceptionPurdue UniversityNext time Visual cortex Orientation sensitive cells Decomposing the visual
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