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Chapter 3

PSYCH 2H03 Chapter 3: Visual Perception


Department
Psychology
Course Code
PSYCH 2H03
Professor
Karin R Humphreys
Chapter
3

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PreviewofChapterThemes
● visionhuman’sdominantsensorymodality
○ discussmechanismsbywhichthevisualsystemdetectspatternsintheincoming
light
○ showcasetheactivityofthevissystem,ininterpretingandshapingincominginfo
● Waysinwhichperceptionofoneaspectoftheinputisshapedbyperceptionofother
aspectsdetectionofsimplefeaturesdependsonhowtheoverallformisorganized
○ perceptionofsizeisshapedbyperceiveddistance
● interpretationofthevisualinputisusuallyaccuratebutthesamemechanismscanlead
toillusionsandthestudyofthoseillusionscanoftenilluminatetheprocessesthrough
whichperceptionfunctions
TheVisualSystem
● visionisthedominantsenseforhumans,highlightedbyhowmuchofthebrainis
devotedtovisionincomparisontotheothersenses
● Thephotoreceptors
○ visionstartswithlightproducedbymanyobjectsinoursurroundings,that
directlycomestooureyeorbouncesofotherobjects
○ inmostcasesit’sreflectedlightthatlaunchestheprocessofvisualperception
■ someofthislighthitsthefrontsurfaceoftheeyeball,passesthroughthe
corneaandthelenstillithitsthelightsensitivetissueoftheretina
■ thecorneaandlensfocusthecominglightsothatasharp,backwards
andupsidedownimageiscastontheretina
■ thelensbudgetsfordistancebytighteningorlooseningabandofmuscle
■ whenflatitcanfocusfarimages,whenbulgingcloseimages
○ 2typesofphotoreceptorswhicharespecializedneuralcellsthatrespond
directlytoincominglight
■ rodsaresensitivetolowlevelsoflightandgoodwithperipheries,but
colourblindcanonlydistinguishdifferentlevelsoflight
■ conesareconcentratedinthemiddleoftheeye,needmoreincoming
light3differenttypesthatreacttodifferentwavelengths
● perceivecoloursthroughcomboofthedifferentconetypes
■ conesalsohelpyoutoseeinfinedetailvisualacuity
■ acuityismuchbetterforconesthanrods
■ thecenterofthefoveadoesn’thaverodsonlyconesthisistheareaof
theeyewithmostvisualacuity
3.1Foveation
● Cellsofthefoveaarethebestatseeingdetailwhyyouneedtolookstraightat
somethingtoreallyseeallthedetailfovealview
● whyyouneedtomoveyoureyesalottoreallytakeinascene
3.2Eyemovements
● mustconstantlymoveeyestoattainfovealviewofyoursurroundings
● toexploretheworldwerelyoneyemovementscalledsaccadestheseareabrupteye
movementsthathopfromplacetoplace

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● inperipheralportionsoftheretinatherodspredominateandtherearenoconesatall
thisexplainswhyit’seasiertoseedimlylitthingswhenusingyourperipheralvision
● LateralInhibition
○ Photoreceptorsstimulatebipolarcellswhichinturnexciteganglioncellswhich
arespreaduniformlyacrosstheentireretina
○ alloftheganglionaxonsconvergeontheopticnervewhichcarriesinfofromthe
eyeballtovarioussitesofthebrain
○ firstthisinfoissenttothelateralgeniculatenucleus(LGN)andfromthere
informationissenttotheoccipitallobe
3.3TheBlindSpotandtheActiveNatureofVIsion
● axonsfromtheretina’sganglioncellsgathertogethertoformtheopticnervewhich
carriesinfotothethalamusandthenthevisualcortex
● theareawherethisnerveexitscan’thaveanyrodsorconessoit’sablindspot
● thecellsthatlinktheretinatothebrainarealreadyengagedinprocessingvisualinput
likeinphenomenonoflateralinhibition
○ apatterninwhichcells,whenstimulated,inhibittheactivityofneighboringcells
3.4ABrightnessIllusion
● activenatureofvision:weapparentlyfillininformationoutsidetheretinasothatitseems
we’reseeingauniformlydetailedscene
● wefillininfothat’smissingfromvisualinputbecauseoftheblindspot
● canalsobeshownusingillusionareasoftheillusionmaybereceivingmorelateral
inhibitioncellsrespondtoreceivingdifferentinput
● withthegreyspotsinthemiddleoftheyellowbarsthoseareasarebeinginhibitedfrom
allsidescausingthemtoseemdarker
● thiscanalsobeseeninthephenomenonofedgeenhancementwhereattheedgesthe
cell'sresponseisincreasingduetoinputfromeitherside
SingleNeuronsandSingleCellRecording
● singlecellrecordingisaprocedurethroughwhichinvestigatorscanrecordmomentby
momentthepatternofelectricalchangeswithinacertainneuron
● whenaneuronsfireseachresponseisthesamesize,butneuronsdifferinhowoften
theyfire
● thisisusedalotinstudyingneuronstolookatwhat’sfiringinthebrainandopticnerve
etc.
● canfindwhichpatternsofvisualinputcausecertainneuronstofiredefiningthatcell’s
receptivefieldthesize,shapeandareainthevisualworldtowhichthatcellfires
● Multipletypesofreceptivefields
○ existenceofseveraltypesofneuronsinthebrainwithdifferentkindsofreceptive
fields/visualtriggers
○ thesecellsfireattheirmaximumratewhenlightispresentedinasmallroughly
circularareaandinaspecificpositionwithinthefieldofview
○ thesecellsareoftencalledcentersurroundcellswherelightpresentedinthe
centerofreceptivefieldhasoneinfluence,whileifit’spresentedinthe
surroundingareaithastheoppositeeffet
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