PHYSIOLOGY+OF+VISION+AND+THE+VISUAL+SYSTEM.docx
- 文档编号:10109854
- 上传时间:2023-05-23
- 格式:DOCX
- 页数:74
- 大小:3.05MB
PHYSIOLOGY+OF+VISION+AND+THE+VISUAL+SYSTEM.docx
《PHYSIOLOGY+OF+VISION+AND+THE+VISUAL+SYSTEM.docx》由会员分享,可在线阅读,更多相关《PHYSIOLOGY+OF+VISION+AND+THE+VISUAL+SYSTEM.docx(74页珍藏版)》请在冰点文库上搜索。
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
- 配套讲稿:
如PPT文件的首页显示word图标,表示该PPT已包含配套word讲稿。双击word图标可打开word文档。
- 特殊限制:
部分文档作品中含有的国旗、国徽等图片,仅作为作品整体效果示例展示,禁止商用。设计者仅对作品中独创性部分享有著作权。
- 关 键 词:
- PHYSIOLOGY OF VISION AND THE VISUAL SYSTEM