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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)

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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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