PHYSIOLOGY+OF+VISION+AND+THE+VISUAL+SYSTEM.docx

PHYSIOLOGY+OF+VISION+AND+THE+VISUAL+SYSTEM.docx

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PHYSIOLOGY+OF+VISION+AND+THE+VISUAL+SYSTEM

PHYSIOLOGYOFVISIONANDTHEVISUALSYSTEM

■Introduction

■LightDetectionandDarkAdaptation

■VisualAcuityandContrastSensitivity

■ElectrophysiologyoftheVisualSystem

■ColourVision

■VisualPerception

■DivisionofLabourintheVisualSystem

■ControlofOcularMovement

■Conclusion

INTRODUCTION

Itgoeswithoutsayingthattheabilitytodetect,recognize,anddiscriminateobjectsinspaceisfundamentaltosurvivalandisachievednotsimplythroughdetectionofluminanceorcontrastsensitivity（visualacuity）butalsobydiscriminationoftexture，colour，depth,andmotiondisparities．Mostofthesefunctionarelocatedinthehighercorticalcentreswheretheretinal‘sensation,（image）isconvertedintoaperceptionoftheoutsideworld.Whatisseenmaynotbethesameaswhatisperceivedandthelattermaybeextensivelyeditedthroughinputfromothernonvisualcentres.especiallymemoryandpreviousvisualexperience.

AsZeki（1992）hasputit：

'Toobtainknowledgeofwhatisvisible．thebrain（doesnot）merelyanalyzetheimagespresentedtotheretina；itmustactivelyconstructavisualworld'Aswillbeseenlater,thebrainbuildsupapictureofavisualscene，bysegregationoftheimageintoitscomponentparts：

thustherearecorticalcellsthataredirectionallyselective（motiondetectors），othersthatareorientationselective，andstillothersthatarespecificforcolour.Onfacevalueitmightbethoughtthatspatialresolutionwouldbemostimportanttosurvivalbutinfactithasbeenshownthatcolouriswhatweseebest.

Therearefurthersubtletiestothebusinessofseeing.Forinstance，howdowedetectform?

Isthissimplyacomputationofinputfromorientationselectiveneurons，ordoesitinvolveotherinput?

Whataboutthatmosthighlydevelopedfunction，recognitionoffaces?

Whatabouttexture?

Thischapteraddressessomeoftheseaspectsofthesensoryandpsycho-physicalresponsestovisualstimuli,butofnecessityinthemostsuperficialofdetail.

WHATDOWEMEANBYGOODVISION?

Itisimportantwehaveaconceptofthelimitsofourvisualcapabilities.Thevisualprocessisinitiatedbythedetectionofalightsignalbyphotoreceptorcellsintheouterretina.Thesecellsconvertlightenergytoanelectricstimulus，whichisthentransmittedtothebipolarcellsandonwardstotheganglioncellsintheretina（seeCh1）.Theinformationisfurthertransmittedintheaxonsofthesecells（theopticnerveswhich，after50％crossoverintheopticchiasma，becometheoptictracts）tothevisualthalamicorgan．thelateralgeniculatenucleus（LGN）.

SynapticcontactwithneuronesintheLGNthatprojecttothecerebralcortexpermitsonwardtransmissionofthesignalsviatheopticradiationtothevisualorstriatecortex（Vl），wheretheyinteractwithmanyotherneuronalconnectionsfromvisualcorticalcellsintheprestriatecortex（V3-5），andwhereparcellingoutandprocessingofthesignalstakesplacetobuildupthefinalperceivedvisualimageInputisalsoreceivedbythevisualcortexfrommanyotherareas，particularlythosecontrollinggeneralmotorfunctionandeyemovement，cerebellarandspatialsense,memory,andmanyotherfunctions.

Inbiophysicaltermsaphotoreceptoristheoreticallycapableofdetectingasinglephotonoflight（seebelow）.Butinpracticewhatarethelimitsofdetectionofavisualstimulus?

Thisdependsonthenatureofthestimulusandthenatureoftheambientconditionsinwhichitispresented.Sensinglightisafunctionofallregionsoftheretinabutthefovealregionisspecializedforhighspatialresolutionandcolourdetection．servedbythesmall’midget'，slowtransmittingganglioncells（theparvocellularorPsystem）.Incontrast，luminanceandmotiondetectionareservedbythelarge,fasttransmittingganglioncells（themagnocellularorMsystem）thatdominatetheremainingretinaandthusincorporatetheentirevisualfield.AccordingtoBarlow'ssingleneuronedoctrine，itshouldtakeonlyoneneuronetodetectavisualstimulus.HoweverpsychophysicalstudieshaveshownthatboththeluminanceandcolourthresholdsforvisionaredifferentbyordersofmagnitudeforPneuronesbetweenmonkeysandhumans，suggestingthatmorethanoneneuroneisinvolved.Infact，thecurrentViewisthatcontinuouspoolingofinformationoccursbothintheexcitatoryandinhibitoryneuronalactivitythatispresentatalltimesandthatafteravisualstimulusthechangesintheresponserateofmanyneuronesare'sampled'bythebrainuntiltheyreachacertainthresholdlevel.Atwhichpointtheyregisterandthestimulusis'recognized'（HurlbertandDerrington，1993）.Thisperhapsexplainshowwecansometimeslookatanobjectandyetnotseeit；furthermore.thesepsychophysicalconsiderationsarehighJyrelevanttomethodsfortestingvisionforinstancewithregardtosettingluminancethresholdsforstudiesofvisualfieldsusingsmalltransienttargets.

Flickercanbeusedtodeterminelimitsofvision

Detectionofastationarytargetorspotdependsonthesizeandbrightnessofthespotrelativetothebackground.Thelimitsofdetectabilityofthetargetarethereforedeterminedbythespatialresolutionandtheanatomicalrelationshipsbetweenstimulatedreceptors（seebelow）.Spatialresolutionishighestatthefoveaanddeclinessharplytowardstheperipheralretina；thisisclearlydemonstratedbythedetectionthresholdatdifferenteccentricitiesinthevisualfield.

Thethresholdforspatialresolutionishoweverconsiderablyhigherthanthatfordetectinglight;Thislatterparametercanbemeasuredbyflickerdetection,whichistheabilitytodetecttwostimuliseparatedintime.Thisfunctionisnormallysubservedbyrodphotoreceptors，whilespatialresolutionissubservedbyconeswithsomeinputfromrods.

Motiondetectionisalsoafeatureofrodvision

Itisclear,therefore,thattheabilitytodetectastandardsmallbrightspotinspecifiedregionsofthevisualfieldisinfactamuchmorecomplextaskthanwouldatfirstappear.Notonlydoesitdependontheabsolutebrightnessofthestimulus，butalsoonthebackgroundonwhichthestimulusispresentedandthusoncontrast.Italsodependsonwhetherthetargetismovingorstationaryand.ifstationary．Forhowlongthetargetispresented.Itsdetectiondependsonthedensityofphotoreceptorsandthustheregionofretinastimulated.Ifitisamovingtargetitwillstimulatedifferentcorticalneuronsdependingonwhichdirectionitismoving.Thisfunctionalsegregationofvisualinputisretainedatseverallevelswithinthecortexbeforeconstructionofthefinalvisualimage.

SENSINGCOLOUR

Colourisdetectedbyconephotoreceptors．ofwhichtherearethreetypes：

long（L,red）,medium（M,green），andshort（S，blue）wavelengthsearlyevidenceforthiswasbasedpartlyoncolourmatchingexperimentswhichshowthatthecolourofateststimuluscanbematchedbyaddingtogetherstimulicomposedofthethreeprimarycolours（seebelowcolourimetry）.ThisisknownasthetrichromatictheoryofcolourVision.Eachcolourhaspropertiessuchashueandchromaticity.

Therearemanyhuesbutonlythreeprimarycolours

Hueisanidealizedtermforthecolourproducedbylightofasinglewavelength.Inspiteofhavingonlythreeconephotoreceptors,wealeabletodistinguishmanyhuesofcolour,e.g.lilacandviolet.Itisthereforeclearthatanysinglecolourisrecognizedbyamixtureofthethreeprimariesandthattheremustbeoverlapinthespectralsensitivityforeachprimarycolour（seebelow）.Theoreticallyitshouldbepossibletoproducelightofasinglewavelengthusinganarrowslitonadevicesuchasamonochromater．butphotoreceptorsensitivityisalsosubjecttotheintensityofthelight，andnarrowwavebandsofthisdegreeofselectivityarenotsufficientlyintensetoproduceastimulus.

Thehue-discriminationcurve（Fig.5.1）describesthephysiologicallimitsatwhichashiftinwavelengthcanbediscriminatedasachangeincolourMonochromaticlightthereforeisnotapracticalreality，mostcoloursareinfacttints，i.etheyareunsaturatedhues，thedegreeofunsaturationbeingdeterminedbytheamountofadditionalwhitelighttheyrequiretomatchthemtoahue.

chromaticityissemiquantifiedcolouredness'

Chromaticityrefersto‘colouredness’anddependsonhue，saturation．andintensityof

light（luminosity）Indeedhueitselfisnotindependentoftheluminosityofthestimulusandchromaticshiftsoccurasthe_intensityincreasestillallhuesappearyellow-white（theBezold—Bruckephenomenon）orastheintensitydecreaseswhenallhuesappearachromatic（thePurkinjeshife;seebelow）.Anycolourcanthusbematchedbyamixtureofthethreeprimarycoloursplusorminusaproportionofwhitelighttoaccountforunsaturation;theseareformallydescribedinthechromaticitychart（Fig.5.2）.

VISIONISALSOABOUTSHAPE，FORM，ANDDEPTHPERCEPTION

Thediscriminationofshapeandformishighlydevelopedinprimatevisionandmuchresearchinthelastdecadehasbeendevotedtodeterminingthecorticallocalizationofthesefunctions.Theappreciationofthecomplexityandsophisticationofthisaspectofvisualperceptionhasinpartdevelopedfromtherealizationthatthebrainrecognizesandcancategorizeobjectsaccordingtoshapeirrespectiveoftheangleordistancefromwhichtheyareviewedoroftheambientlightingconditions，etc.

Shapeprocessingisachievedbyspecializedorientation-sensitivecellsinthevisualcortex．Buttheextradimensionofformrecognition，asinrecognitionoffacialfeatures,requiresadditionalprocessing,possiblyintheinferotemporalcortexStudiesofpatientswithspecificvisualdefectssuchasprosopagnosia（inabilit