1 Early processing in human vision retina optic nerve LGN primary visual cortex light neuron neural signal 1 Rods vs. cones cones: • sensitive to color • operate in daylight • adapt quickly to changing light rods: • not sensitive to color • operate at low light levels • adapt slowly to changing light In the night sky: why do stars appear to disappear when you look directly at them? why do all stars look white? (new figure) 2 + Retinal ganglion cells “on-center” ganglion cell “off-center” ganglion cell • receptive fields have center-surround structure • cross-section ~ difference of two Gaussians light on neural response over time flash spot in center flash spot in surround light on neural response over time center surround + 3 Emergence of center-surround receptive field retinal ganglion cells receptive field center receptive field surround photoreceptors bipolar cell + - - horizontal cell direct excitation of bipolar cell from photoreceptors inhibition of bipolar cell via horizontal cell resulting bipolar cell response 4
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1
Earlyprocessinginhumanvision
retina opticnerve
LGN primaryvisualcortex
light
neuron neuralsignal
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Rodsvs.cones
cones:•sensitivetocolor•operateindaylight•adaptquicklytochanginglight
rods:•notsensitivetocolor•operateatlowlightlevels•adaptslowlytochanginglight
Inthenightsky:whydostarsappeartodisappearwhenyoulookdirectlyatthem?whydoallstarslookwhite?
(newfigure)
2
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Retinalganglioncells
“on-center”ganglioncell “off-center”ganglioncell
•receptivefields havecenter-surround structure•cross-section~differenceoftwoGaussians
lighton
neuralresponseovertime
flashspotincenter
flashspotinsurround
lighton
neuralresponseovertime
center surround
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Emergenceofcenter-surroundreceptivefield
retinalganglioncells
receptivefieldcenter
receptivefieldsurround
photoreceptors
bipolarcell
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horizontalcell
directexcitationofbipolarcellfromphotoreceptors
inhibitionofbipolarcellviahorizontalcell
resultingbipolarcellresponse
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Analyzingintensitychangesina2Dimageimageaftersmoothing&secondderivativeconvolutionwithÑ2Goperator
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•on-centercellscarrypositivepart•off-centercellscarrynegativepart
~convolutionoftheretinalimageispasseduptheopticnerve
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secondderivative
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firingrate=(numberofspikes)/time
excitation:higherfiringrate inhibition:lowerfiringrate
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Detectingintensitychangesatmultiplescaleshumanvision:•multiplereceptivefieldsizesinthesameregionofthevisualfield
•receptivefieldsizesincreasewitheccentricity(distancefromthecenteroftheeye)
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Spatialfrequencydecomposition
frequencye.g.cycles/deg
Anyrealsignal,suchasI(x),canbedescribedasthesumofsinusoidalwavesofdifferentfrequency,amplitude,andphase
frequency
amplitude
low high
FourierTransformI(x)
Twowaystodescribe1Dor2Dsignals
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Spatialfrequencychannels
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Funwithspatialfrequencies:Hybridimages
Oliva,Torralba&Schyns,2006
HybridImages@MITGallery:http://olivalab.mit.edu/hybrid_gallery/gallery.html
Hybridimagescombinethelowspatialfrequenciesofonepicturewiththehighspatialfrequenciesofanother
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”Spatialfrequencychannels”inhumanvision
Campbell&Robson,1968
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Orientationtuningcurves
Manyneurons:
orientation angle in degrees-180 1800
60ra
te (H
z)
0
orientation (deg)
aver
age
firin
g ra
te (H
z)
Singleneuron:
(preferredorientation)
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