The+Central+Visual+System.ppt

TheCentralVisualSystem,Transduction,Photoreceptorsreleasetheneurotransmitterglutamate(glu)whendepolarized.Depolarizedinthedark.Hyperpolarizedbylight.Onlyganglioncellshaveactionpotentials.Photoreceptorsproducegradedresponsethatprovidesinputaggregatedbybipolarcells.Magnoganglioncellsreceiveinputfromrods,parvoganglioncellsfromcones,铜球阀截止阀蝶阀,BipolarCellReceptiveFields,ThereceptivefieldistheareaoftheretinacapableofchangingthebipolarcellsmembranepotentialTwokindsofreceptivefields:OFFcellexcitatoryONcellinhibitoryOFFandONreferstolight,notthecellCenterandsurroundareopposites,EdgeDetection,Thecenter-surroundorganizationofthereceptivefieldsofganglioncellsexaggeratesthecontrastatborders.Visualprocesses“fillin”whatoccursbetweenborders(edges).Contrasteffectsoccurbecausewenoticevariations,notabsolutemagnitudesoflight.,ColorContrast,Conesrespondtospecificwavelengthsoflightthatdeterminehue.Colorcellshavecomplementarysurroundsthatheightencontrastandstrengthentheirsignal.Opponentsare:red/green,blue/yellow.,ColorOpponency,Certaincolorsareneverseenincombination:Reddishgreen,bluishyellow.Redandgreenmixtoformyellow;yellowandbluemixtoformwhite.Heringsopponentprocesstheoryperceptualcancellationoccursbecausecolorsareprocessedasopponentpairs.Colorcellshavecomplementarysurroundsthatheightencontrastandstrengthentheirsignal.,ColorProcessing,Thebraincomparesresponsesofthreetypesofconecells.Inputsfromthethreetypesofconesarecombinedindifferentways.Thebraincomputesresponsesofspecificconesbutalsoallconesintheretina(background)tocompensateforambientlight(constancy).AreaV4responsibleforcolorconstancydamageresultsinlossofcolorexperience.,VisualFields,Eacheyehasavisualfieldthatoverlapsthevisualfieldoftheothereye.Eacheyesvisualfieldisdividedinhalfcalledahemifield.Therighthemifieldofeacheyeisviewedbythelefthemisphereofthebrain.Thelefthemifieldofeacheyeisviewedbytherighthemisphereofthebrain.,SomeTerminology,Thesuffix“fugal”meanstofleeRetinofugalreferstowheretheaxonsoftheopticnervegoaftertheyleave(flee)theretina.Decussationcrossingofabundleoffibers(axons)fromonesideofthebraintotheother.Tractabundleoffibersgoingthesameway,RetinotopicMapping,Therelationshipbetweenanimageintheworld,itsimpactontheretina,andtheretinasprojectiontothecortexismaintained.Thisiscalledtopographicmapping.StimulationofneighboringretinallocationsresultsinstimulationofcorrespondingareasoftheLGN,superiorcolliculus,andoccipitalcortex(primaryvisualcortex).Relationshipsbetweenareasaremaintained.,TypesofGanglionCells,Magnocellular(Mcells)largecellsthatreceiveinputfromrods.Parvocellular(Pcells)smallcellsthatreceiveinputfromcones.Blobpathwayconcernedwithcolorperception.Interblobpathwayconcernedwithshape/form.Koniocellular(nonM-nonP)smallcellsinvolvedincolorvision(notwellunderstood).,MappingWithintheLGN,OpticnervecarriesinformationfromgangliatoLGN.Crossesatopticchiasm.Separatelayersaremaintainedforeacheyeandforeachtypeofcell(MandP).InterneuronsprojectfromareasoftheLGNtostriatecortex(alsocalledprimaryvisualcortexorV1).,MappingintheStriateCortex,SeparatelayersfromLGNtostriatecortexaremaintainedinoculardominancecolumns.M,P,&non-M/Pcellsenterthecortexatdifferentlevelsoflayer4ofthevisualcortex.Informationiscombinedbypyramidalcellsthatsynapseathigherlevelsinthestriatecortex.Inputfrombotheyesiscombinedatlayer3.,StagesinEdgeDetection,Retinalbipolarcellshavecenter-surroundreceptivefields.LGNganglioncellsrespondtocontrastandchangeinvisualinput.Center-surround(on-off)receptivefield.Neuronsinthevisualcortexhaverectilinearreceptivefieldswithexcitatoryandinhibitoryzones.,EdgeDetectors,Hubel&Weiselfoundsimplecellsrespondingtoedgesatdifferentorientations.Complexcellsinthevisualcortexcollecton-offdatafrommultiplecellstoformedges.Complexcellsprovidepositionalinvariance.M-channelcellsareorientationanddirectionselective,formotiondetection.P-IBchannelcellsanalyzeobjectshape.,ExtrastriatePathways,Parallelprocessingofvisualinformationfromthestriatecortex.Threepathways:ColorprocessingPblobcells,goesfromV1toV2,thenV4,theninferiortemporalcortex.Shapeprocessing,depthperceptionPinterblobcells,gofromV1tointeriortemporalcortex.Motion&spatialrelationsMcells,V1toV2,thenMT(V5),toparietalcortex.,Equiluminance,Holdingbrightnessconstantpermitsthestudyofthecontributionofcolortoperception.Results:Brightness,notcolor,isimportanttomotiondetection,perspective,relativesizes,depthperception,figure-groundrelations,visualillusions.Motionisacuefordistinguishingamongobjects.Thingsthatmovetogetherbelongtogether.,ComplexForms,Motion,Processingofformoccursoutsidethevisualcortexinferiortemporalcortex.Notorganizedretinotopically.10%selectiveforspecificimages(hands,faces).Processingofmotionoccursinmiddletemporalarea(MTorV5),thenparietallobe.Usedforseeingmovingobjects,pursuiteyemovements,guidanceofbodilymovement,BindingMechanisms,Howisinformationfromtheseparate,parallelpathwaysbroughttogetherandassociated?Cellsmayidentifypatternsofsynchronousactivity.Treisman&Juleszcombinationrequiresattention.Apre-attentiveprocessdetectsthemajoroutlineofanobject.Anattentiveprocessnotices,selects&highlightscombinationsoffeatures.,VisualAgnosias,Existenceofdistinctagnosiasforaspectsofperceptionsuggeststhattheseabilitiesarelocalizedtoareasselectivelydamaged.Achromatopsiagoodperceptionofformdespiteinabilitytodistinguishhues.Prosopagnosiainabilitytorecognizefacesasparticularpeople(identity