視覺與大腦課程

視覺與大腦課程第1頁/共41頁Outline(Ch.3andCh.4)Neurons,ReceptiveFields&HumanBrainFromRetinatoVisualCortexPrimaryVisualCortex(V1)SecondaryVisualCortex(V2)-optional第2頁/共41頁Outline(Ch.3andCh.4)Neurons,ReceptiveFields&HumanBrainFromRetinatoVisualCortexPrimaryVisualCortex(V1)SecondaryVisualCortex(V2)第3頁/共41頁MajorDivisionofHumanBrainNeuronsandSynapses-optionalReceptiveFieldsMajorDivisionofHumanBrainVisualCortex第4頁/共41頁NeuronsWhilethefundamentalstructuralunitofthebrainistheneuron,thecurrencyofthebrainistheactionpotentialorspike,whichtravelsatconductionvelocitiesfrom1to120meters(3to380feet)persecond.Ifaninsulatingmyelinsheath(signalbooster)iswrappedaroundtheaxonthentheactionpotentialpropagatesby“jumping”betweengapsinthemyelinsheath,otherwisetheactionpotentialdecaysexponentially.第5頁/共41頁SynapsesSynapse.Eachneuronreceivesinputsviaitsdendrites.Iftheseinputsaresufficiently

largethenanoutputintheformofanactionpotentialappearsintheneuron’saxon.Thiscauseschemicalneurotransmittertobereleasedatthesynapsestootherneurons.CourtesyCreativeCommons.第6頁/共41頁BackwardretinaThelightraysthatcontributetotheretinalimagehavetopassthroughlayerof

nervefibers

andthebloodvesselsbeforereachingthelight-sensitivephotoreceptors.第7頁/共41頁ReceptiveFields-TiltingtheRetinaPhotoreceptors(pixelarray)ThisreceptorissharedbytwoganglioncellsCablesshowconnectionsforonlythreereceptivefieldsGanglioncellseachphotoreceptorcanbelongtoabouttenreceptivefields第8頁/共41頁ReceptiveFields–MexicanHat?Ganglioncellreceptivefields.(a)On-center/off-surround.(b)Off-center/on-surround.(c)Three-dimensionalgraphshowingthesensitivitytolightspotsplacedindifferentregionsofthereceptivefieldofacellwithanon-centerreceptivefield.Thespikeinthemiddleshowswherelightexcitesthecell,andthetrougharoundthisspikeshowswherelightinhibitsthecell.(d)ThesecellsaresometimessaidtohaveMexicanhatreceptivefields.abcd第9頁/共41頁ReceptiveFields–SpatialFrequencyNarrowstripesonthezebra’sfrontlegcorrespondtoahighspatialfrequency,whereaswidestripesonitsbacklegcorrespondtoalowspatialfrequencyFourieranalysis.Thecomplicatedcurvein(d)isthesumofcorrespondingpointsinthesinusoidalcurvesin(a–c).第10頁/共41頁ReceptiveFields–Logan’stheorem第11頁/共41頁MajorDivisionofHumanBrain第12頁/共41頁Outline(Ch.3andCh.4)Neurons,ReceptiveFields&HumanBrainFromRetinatoVisualCortexPrimaryVisualCortex(V1)SecondaryVisualCortex(V2)第13頁/共41頁FromRetinatoVisualCortexAFewInterestingDiscoveriesFromRetinatoLGNLayersintheLGNsFromLGNtoStriateCortex(V1)

basicsonly第14頁/共41頁AFewInterestingDiscovery“Thedumbertheanimal,thesmartertheretina”(DenisBaylorofStanfordMedicalSchool,inMontgomery,1995).Neurons,knownas“bugdetectors,”thatrespondalmostexclusivelytotheerraticmotionofaflyaresynthesizedinretinalganglioncells(Lettvinetal.1959)Mostofthecomputationalmachineryresponsibleforvisioninthefrogresidesintheretina.Thehumanbrain

delaysdetailedanalysisoftheretinalimageuntilaftertheinformationhaslefttheretinaviatheopticnerve.Whilethisistrue,itisnotthewholetruth.Thedecompositionoftheretinalimageintokeycomponents,suchascolorandmotion,beginsattheretina.第15頁/共41頁FromRetinatoLGNHowthevisualworldgetstransformedbythepathwaysofthevisualsystem.ModifiedfromFrisby&Stone2010.Cross-overSimilarvisualhemifieldLGN:lateralgeniculatenucleiLeftandright6layers第16頁/共41頁LayersintheLGNsLeftLGN:R,L,R,L,L,RretinasRightLGN:L,R,L,R,R,LretinasRatherthanusingthelabelsLandR,amoregeneralterminologyisipsilateral(I)andcontralateral(C),sobothhaveC,I,C,I,I,CMagnolayers1and2atthebase(fromrodsand10%ganglioncellswithlargereceptivefields,solow-luminance,B-Wandmagno(M)processingstreamformotion)Parvolayers

3–6atthetop(fromconesand80%ganglioncellswithsmallreceptivefields,soR/Gcolorandparvo(P)processingstreamforspatialandstaticforms).Atthebaseofeachmagnoandparvolayer,athinnerkoniolayerfrom10%ofganglioncellsrespondingtoblue(B)andyellow.第17頁/共41頁FromLGNtoStriateCortex(V1)Macaquemonkeystriatecortex(right)showingactivityevokedbyvisualstimulus(left).Correspondingregionsinthevisualfieldandthevisualcortexareindicatedbylettersandnumbers:S=superior(upper)visualfield;F=fovea;I=inferior(lower)visualfield;H=horizontalmeridian.ReproducedwithmodificationsfromTootell

etal.1988.第18頁/共41頁FromLGNtoStriateCortex(V1)Theneocortexissheetofneurons,a1.7mmthickand1,300cm2

inarea,thatconstitutestheoutermostpartofthetwocerebralhemispheresofthebrain.Everymm3

ofneocortexcontains100,000neurons,andthewholeneocortexcontainsatotalofapproximately25billionneurons,eachofwhichreceivesinputsvia4,000synapses.Theglobalstructureofthevisualcortexisorganizedretinotopically;thatis,adjacentpointsintheretinalimageusuallyprojecttoadjacentpointsinstriatecortexNotethatthecentralfovealregionoftheretinaprojectstoarelativelylargeareaintheLGNsandstriatecortex.第19頁/共41頁FromLGNtoStriateCortex(V1)Unpackingthestriatecortex.TheoutputsofdifferentganglioncellsineacheyeprojecttospecificlayersoftheLGNs(C=contralateral,I=ipsilateral).Thesethenprojecttostriatecortex(V1),whichprojecttodiscretestripesofV2.TheseprojecttoareasV3–V5,whicharespecializedforprocessingdifferenttypesofinformation.第20頁/共41頁FromLGNtoStriateCortex(V1)ThedivisionoflaborthatbeganwiththedifferenttypesofphotoreceptorsandcontinuedwiththeretinalganglioncellsandtheLGNsextendsintotheprimaryvisualcortex.Specifically,theprojectionsofthemagnocellular

LGNlayersterminateinasublayeroflayer4(4Cα),whereasprojectionsoftheparvocellularLGNlayersterminateinsublayer(4Cβ)Collaterals(subbranches)oftheaxonsfromcellsintheLGNalsoinnervatethepyramidalcellsincorticallayer6,whichsendprojectionsbacktotheLGNcellsthatinnervatethem.第21頁/共41頁FromLGNtoStriateCortex(V1)Monkeystriatecortex.Microscopicenlargementsofsectionsstainedtoemphasizecellbodiesin(a)andnervefibersin(b).Thenumbersattherightrefertovariouslayersthatcanbedistinguishedmicroscopically.CourtesyLeVay,Hubel,andWiesel(1975).第22頁/共41頁FromLGNtoStriateCortex(V1)Thereisacomplete,andquitespecific,setoffeedbackconnectionsfromstriatecortextoLGNs.Infact,mostoftheinputconnectionstotheLGNcome,notfromtheretina,butfromtheseV1feedbackconnections.Thefunctionoftheseconnectionsisnotknown,butinterferingwiththemseemstodecreasethespecificityofLGNcells.第23頁/共41頁Outline(Ch.3andCh.4)Neurons,ReceptiveFields&HumanBrainFromRetinatoVisualCortexPrimaryVisualCortex(V1)SecondaryVisualCortex(V2)第24頁/共41頁StriateCortex(V1)SimpleCellsTemporalReceptiveCellsMapsinPrimaryVisualCortex(V1)HypercolumnsPicturesinHead?ThePackingProblem第25頁/共41頁SimpleCellsDavidHubeland

TorstenWiesel(1981NobelPrize)werethefirsttodiscovercellsinstriatecortex(V1)thatrespondedtoelongatedstripsofcontrastintheformofeitheralineorluminanceedge,whichtheynamed“simplecells”.第26頁/共41頁TemporalReceptiveCellsWealreadyknowthatthereceptivefieldofasimplecellhasaspatialstructure

definedoverasmallregionontheretina.Butthatfieldalsohasatemporalstructuredefinedoverasmallintervaloftimefollowingtheonsetofastimulus.Aseachcellonlyrespondstomotionofaparticularspatialpattern,thereexistsaspatiotemporalstimulusthatevokesthebestresponsefromeachcell,andthatstimulusisusuallytheonethatmatchesthespatiotemporalstructureofthecell’sreceptivefield.第27頁/共41頁MapsinPrimaryVisualCortex(V1)Howinformationfromthetwoeyesiscombinedinrightoccipitallobeofmonkeyvisualcortex.Lookingdownonthesurfaceofthecortex,inputfromthelefteyeisrepresentedbyblackstripes,andinputfromtherighteyebywhitestripes.Theright-handsiderepresentsthefovea.Obtainedbyinjectingradioactiveaminoacidintooneeye,whichisthentransportedalongthevisualpathwaytoeye-specificcellsinstriatecortex.RedrawncourtesyoftheNobelFoundationfromHubel1981.第28頁/共41頁Hypercolumns?Todetectalineofanyorientationwithinanyregionoftheretinalimage,thebrainmusthavea“completeset”ofsimplecellsthathavepreferredorientationsthatspantherangefrom0to180degreesforeverysmallpatchoftheretinalimage.Anexampleofasimplifiedhypotheticalhypercolumnthatcandetectonlyorientation,size,andocularity

isshowninthediagramatthetopoffigure4.14.第29頁/共41頁PicturesinHead?thebrain’srepresentationoftheretinalimageneedn’tlookanythinglikethatimage.Moregenerally,there’snoreasontosupposethatthebrain’srepresentationofanythingshouldresemblethatthing,anymorethanthewordsheepshouldresembleawoollymammal.第30頁/共41頁ThePackingProblem126Mphotoreceptors->1Mganglioncells->10BV1cellsIfalltheinformationimplicitinaretinalimageof126Mcellsisrepresentedintheoutputsofamillionganglioncellsandifthecellsofthevisualcortexcannotaddnewinformationabouttheretinalimage(andinformationtheoryassuresustheycannot),thenwhyarethere10billioncellsintheprimaryvisual(striate)cortex(among25billionneurons)?第31頁/共41頁ThePackingProblem126Mphotoreceptors->1Mganglioncells->10BV1cellsThetaskofthevisualcortexisnottoaddinformation(whichitcannotdo),buttomakeexplicitthatwhichismerelyimplicitOrientationTextureMotionContrastDepthColorBrightness第32頁/共41頁Outline(Ch.3andCh.4)Neurons,ReceptiveFields&HumanBrainFromRetinatoVisualCortexPrimaryVisualCortex(V1)SecondaryVisualCortex(V2)-optional第33頁/共41頁SecondaryVisualCortex(V2)AnOverviewColorCortexMotionCortexLosingRetinotopyInferotemporalCortex(grandmothercell)第34頁/共41頁SecondaryVisualCortex(V2)InformationinV2isorganizedintoparallelstripes.Thethinstripesreceivetheirinputsfromcolorblobsinstriatecortex.Thethickstripesreceivetheirinputsfromlayer4Bofstriatecortex,whichispartofthemagno(M)pathway,thusspecificorientations,andbinocular.ThepaleinterstripesofV2receivetheirinputsfromthecomplexandhypercomplex

cellsfromparvolayersoftheLGN,theyrespondbesttoorientedlines.Despitethisdivision,adjacentstripesrespondtothesameregionofretina,ensuringretinotopicmapinstriatecortex(V1)isapproximatelypreservedinV2.第35頁/共41頁Color(andShape)CortexThethinstripesandinterstripesofV2,bothofwhichbelongtotheparvocellular

(P)stream,projecttothe“colorarea”V4.V4seemsnottohaveanaccurateretinotopic

map,probablybecauseitsprimaryconcerniscolorandshape,ratherthanretinallocation.Thedominantorganizingprincipleforagivencorticalmapiscalleditsprimaryindex.Forexample,withinstriatecortex,theprimaryindexisspatial

(becausethestructureofthismapisdominatedbyretinalposition),whereasitssecondaryindicesincludecolorandorientation.第36頁/共41頁Motion(andStereo)CortexThethickstripesofV2,whichbelongtothemagnocellular(M)stream,projecttothe“motionarea”V5.CellsinV5notonlyrespondtomotion;theyalsorespondtootherparameters,especiallystereodisparityJustasV4hasreducedretinotopy

(point-for-pointmappingofcorticaltoretinalneurons),sotherelativelackofretinotopyinV5mayexemplify