ChapterSix

VisionVisualCodingandRetinalReceptorsReception-absorptionofphysicalenergybyreceptorsTransduction-theconversionofphysicalenergytoanelectrochemicalpatternintheneuronsCoding-one-to-onecorrespondencebetweensomeaspectofthephysicalstimulusandsomeaspectofthenervoussystemactivityVisualCodingandRetinalReceptorsFromNeuronalActivitytoPerceptioncodingofvisualinformationinthebraindoesnotduplicatethestimulusbeingviewedGeneralPrinciplesofSensoryCodingMullerandthelawofspecificenergies-anyactivitybyaparticularnervealwaysconveysthesamekindofinformationtothebrainQualificationsoftheLawofSpecificEnergiestherateoffiringorpatternoffiringmaysignalindependentstimulitimingofactionpotentialsmaysignalimportantinformationindicatingsuchthingsasmovementthemeaningofoneneurondependsonwhatotherneuronsareactiveatthesametimeVisualCodingandRetinalReceptorsTheEyeandItsConnectionstotheBrainPupil-openinginthecenteroftheeyethatallowslighttopassthroughLens-focusesthelightontheretinaRetina-backsurfaceoftheeyethatcontainsthephotoreceptorsTheFovea-pointofcentralfocusontheretinaTheRouteWithintheRetinaphotoreceptors-rodsandconesbipolarcells-receiveinputfromrodsandconesganglioncells-receiveinputfrombipolarcellsopticnerve-madeupofaxonsofganglioncellsblindspot-thepointwheretheopticnerveleavestheeyeFigure6.2

CrosssectionofthevertebrateeyeNotehowanobjectinthevisualfieldproducesaninvertedimageontheretina.Figure6.4

VisualpathwithintheeyeballThereceptorssendtheirmessagestobipolarandhorizontalcells,whichinturnsendmessagestotheamacrineandganglioncells.Theaxonsoftheganglioncellslooptogethertoexittheeyeattheblindspot.Theyformtheopticnerve,whichcontinuestothebrain.AnimationFigure6.6

TwodemonstrationsoftheblindspotoftheretinaCloseyourlefteyeandfocusyourrighteyeontheointhetoppart.Movethepagetowardyouandaway,noticingwhathappenstothex.Atadistanceofabout25cm(10inches),thexdisappears.Nowrepeatthisprocedurewiththebottompart.Atthatsamedistancewhatdoyousee?VisualReceptors:RodsandConesRodsabundantintheperipheryoftheretinabestforlowlightconditionsseeblack/whiteandshadesofgrayConesabundantaroundfoveabestforbrightlightconditionsseecolorTransductionBothRodsandConescontainphotopigments(chemicalsthatreleaseenergywhenstruckbylight)11-cis-retinalistransformedintoall-trans-retinalinlightconditionsthisresultsinhyperpolarizationofthephotoreceptorthenormalmessagefromthephotoreceptorisinhibitoryLightinhibitstheinhibitoryphotoreceptorsandresultsindepolarizationofbipolarandganglioncellsColorVisionTheTrichromatic(Young-Helmholtz)Theoryweperceivecolorthroughtherelativeratesofresponsebythreekindsofcones,eachkindmaximallysensitivetoadifferentsetofwavelengthsTheOpponent-ProcessTheoryweperceivecolorintermsofpairedoppositesTheRetinexTheoryWheninformationfromvariouspartsoftheretinareachesthecortex,thecortexcompareseachoftheinputstodeterminethebrightnessandcolorperceptionforeachareaFigure6.12

PossiblewiringforonebipolarcellShort-wavelengthlight(whichweseeasblue)excitesthebipolarcelland(bywayoftheintermediatehorizontalcell)alsoinhibitsit.However,theexcitationpredominates,sobluelightproducesnetexcitation.Red,green,oryellowlightinhibitthisbipolarcellbecausetheyproduceinhibition(throughthehorizontalcell)withoutanyexcitation.Thestrongestinhibitionisfromyellowlight,whichstimulatesboththelong-andmedium-wavelengthcones.Thereforewecandescribethisbipolarcellasexcitedbyblueandinhibitedbyyellow.Whitelightproducesasmuchinhibitionasexcitationandthereforenoneteffect.(Actually,receptorsexcitebydecreasingtheirusualinhibitorymessages.Herewetranslatethatdoublenegativeintoexcitationforsimplicity.)ColorVisionDeficiencyColorVisionDeficiency-inabilitytoperceivecolordifferencesGenerallyresultsfrompeoplelackingdifferentsubsetsofconesNeuralBasisofVisualPerceptionAnOverviewoftheMammalianVisualSystemRodsandConessynapsetoamacrinecellsandbipolarcellsBipolarcellssynapsetohorizontalcellsandganglioncellsAxonsoftheganglioncellsleavethebackoftheeyeTheinsidehalfoftheaxonsofeacheyecrossoverintheopticchiasmPassthroughthelateralgeniculatenucleusTransferredtovisualareasofcerebralcortexProcessingVisualStimuliMechanismsofProcessingintheVisualSystemReceptiveField-thepartofthevisualfieldtowhichanyoneneuronrespondsTheyhavebothexcitatoryandinhibitoryregionsLateralInhibition-thereductionofactivityinoneneuronbyactivityinneighboringneuronsHeightenscontrasts-thosereceptorsjustinsidetheborderaremostexcitedandthoseoutsidetheborderaretheleastresponsiveFigure6.16

ReceptivefieldsThereceptivefieldofareceptorissimplytheareaofthevisualfieldfromwhichlightstrikesthatreceptor.Foranyothercellinthevisualsystem,thereceptivefieldisdeterminedbywhichreceptorsconnecttothecellinquestion.Figure6.17

Blocksonasurfaceofgelatin,analogoustolateralinhibitionEachblockpushesgelatindownandthereforepushesneighboringblocksup.Blocksattheedgearepusheduplessthanthoseinthecenter.Figure6.18

AnillustrationoflateralinhibitionDoyouseedarkdiamondsatthe“crossroads〞?NeuralBasisofVisualPerceptionConcurrentPathwaysintheVisualSystemIntheRetinaandLateralGeniculateTwocategoriesofGanglioncellsParvocellular-smallercellbodiesandsmallreceptivefields,locatednearfovea;detectvisualdetails,colorMagnocellular-largercellbodiesandreceptivefields,distributedfairlyevenlythroughoutretina;respondtomovingstimuliandpatternsIntheCerebralCortexV1-PrimaryVisualCortex-responsibleforfirststagevisualprocessingV2-SecondaryVisualCortex-conductsasecondstageofvisualprocessingandtransmitstheinformationto additionalareasVentralstream-visualpathsinthetemporalcortexDorsalstream-visualpathintheparietalcortexFigure6.20

Threevisualpathwaysinthecerebralcortex(a)Apathwayoriginatingmainlyfrommagnocellularneurons.(b)Amixedmagnocellular/parvocellularpathway.(c)Amainlyparvocellularpathway.Neuronsareheavilyconnectedwithotherneuronsintheirownpathwaybutonlysparselyconnectedwithneuronsofotherpathways.AreaV1getsitsprimaryinputfromthelateralgeniculatenucleusofthethalamus;theotherareasgetsomeinputfromthethalamusbutmostfromcorticalareas.(Sources:BasedonDeYoe,Felleman,VanEssen,&McClendon,1994;Ts’o&Roe,1995;VanEssen&DeYoe,1995)NeuralBasisofVisualPerceptionTheCerebralCortex:TheShapePathwayHubelandWiesel’sCellTypesinthePrimaryVisualCortexSimpleCellshasfixedexcitatoryandinhibitoryzonesinitsreceptivefieldComplexCellsreceptivefieldscannotbemappedintofixedexcitatoryandinhibitoryzonesRespondtoapatternoflightinaparticularorientationHypercomplexcells(End-stoppedcells)Resemblecomplexcellsbuthaveastronginhibitoryareaatoneendofitsbar-shapedreceptivefieldFigure6.23

ThereceptivefieldofacomplexcellinthevisualcortexItislikeasimplecellinthatitsresponsedependsonabaroflight’sangleoforientation.Itisunlikeasimplecellinthatitsresponseisthesameforabarinanypositionwithinthereceptivefield.NeuralBasisofVisualPerceptionTheColumnarOrganizationoftheVisualCortexColumnaregroupedtogetherbyfunctionEx:cellwithinagivencolumnrespondbesttolinesofasingleorientationAreVisualCortexCellsFeatureDetectors?FeatureDetectors-neuronswhoseresponsesindicatethepresenceofaparticularfeatureShapeAnalysisBeyondAreasV1andV2InferiorTemporalCortex(V3)-detailedinformationaboutstimulusshape(V4)-ColorConstancy;VisualAttention(V5)-SpeedandDirectionofMovementNeuralBasisofPerceptionDisordersofObjectRecognitionVisualAgnosia-InabilitytoRecognizeObjectsProsopagnosia-InabilitytorecognizefacesNeuralBasisofVisualPerceptionTheCerebralCortex:TheColorPathwayParvocellulartoV1(blobs)toV2,V4,andPosteriorInferiorTemporalCortexTheCerebralCortex:TheMotionandDepthPathwaysStructuresImportantforMotionPerceptionMiddle-temporalcortex-V5-speedanddirectionofmovementMotionBlindness-InabilitytodetectobjectsaremovingNeuralBasisofVisualPerceptionVisualAttentionAttentionalProcessescandeterminewhatisseenTheBindingProblemRevisited:VisualConsciousnessHowareallaspectsofanobjectbroughttogether?AnimationDevelopmentoftheVisualSyst