OPTO6224PerceptionScottStevenson

VisualDirectionTheperceptionofanobject’sdirection,basedonretinalimageposition,sensedeyeposition,andsensedheadposition.

a=OculocentricVisualDirection(OVD)a+b=EgocentricVisualDirection(EVD)

a+b+c=ExocentricVisualDirection(XVD)

VisualAxis:Thelineconnectingthepointofregardthroughthecenterofthepupiltotheretinalfixationlocus.Fixationlocus:Thepointontheretinawhereimagesland,whenweperceivethatweare“lookingrightatthem”.Formostofusthisisinthecentralfovea.

FigurefromPutnametal.JoV2005Subjectswereaskedtohitabuttonwhentheywerelookingrightatadot.Thereddotsshowwherethetargetwasontheretinawhenthebuttonwashit.Theblackdotshowstheverycenterofthefoveolawhereconesaremostdense.Thissubject,likeothersinthestudy,usedaslightlyeccentricpointforfixation,about10arcminutesawayfromthecenter.

EccentricFixationvs.EccentricViewing:Sometimesweseesomethingbetterifwedon’t“lookrightatit”.Forexample,inscotopicvisionwehaveafovealscotomawheretherearenorods,sowemustuseEccentricViewingtoseedimstars.Wearelookingatit,butnotlookingrightatit.Whenapersonusesaregionawayfromthefovealcenterforallfixation,andbelievestheyare“lookingrightat”thetarget,this

Head direction in space

Eye direction in head

Target direction in eye

Direction of travel

In order to know the relative direction of an object, you need to account for ...a) retinal eccentricity (target direction in eye)b) gaze deviation (eye direction in head)c) head deviation (head direction in space)

a

b

c

iscalledEccentricFixation.TheseindividualswillseeMaxwell’sspotdisplacedfromthepointoffixation.PatientswhodevelopscotomashavetouseEccentricViewing,andadoptanewpointontheretinacalledthePreferredRetinalLocusofFixation(PRL).UsingthePRLmayormaynotbecomeperceivedas“lookingrightat”attarget.OculocentricVisualDirection(OVD):Theangleformedbetweenthevisualaxisandavisuallinetoaparticularobject.Alternatively,itistheretinaleccentricityofanimagepoint.ThinkofOVDasthedirectionyouwouldhavetoturnyoureyeinordertolookrightatsomething.ThereferencepointforOVDistheretinalfixationlocus,sothatpointisdefinedasadirectionof0degrees.Pointstotherightanduparepositiveangles,pointsdownandleftarenegativeangles.Whenwemakeajudgmentaboutshape,wearecomparingOVDs.Forexample,whichoftheseisactuallysquare?

Toanswerthat,youmightcomparetheOVDsforthefoursidesandjudgerelativedistances.Likewise,ifyoumakeaVernieracuityjudgment,youarecomparingtheretinalpositionoftwolines,sothisisanOVDcomparison.Whentheretinabecomesdistorted,forexamplebyCentralSerousChorio-Retinopathy(CSR)thejudgmentofOVDscanbedistorted.

ThiscartoonofanAmslerGridshowsaapparentdistortionthatmightoccurwithaCSR(below)orotherretinalpucker.TheconesbecomedisplacedbuttheperceptionofOVDisreferencedtotheconearraysotheimageappearsdistorted.

Inregionswheretheconesgetstretchedapart,animageappearssmaller(“micropsia”)andwheretheconesgetcompressedtogethertheimageappearslarger(“macropsia”).

EgocentricVisualDirection(EVD):Theangleformedbetweenthedirectiontheheadpointsandavisuallinetoaparticularobject.TheperceivedEVDisfoundbycomparingOVDandsensedeyeposition.ThinkofEVDasthedirectionyouwouldhavetoturnyourheadinordertofacesomething.InflowandOutflow:Inordertoknowthedirectionofaseenobject(EVDorXVD)wecombineinformationaboutwhereitlandsontheretina,relativetoourfovea,alongwithinformationaboutwhereoureyesarepointing.Howdoweknowwhereoureyesarepointing?

FoundationsofVisionFigure10.17

InflowandOutflowtheoriesofhowmotionanddirectionareperceived.Inordertoknowthedirectionofaseenobject,wemustcombinethelocationontheretinawiththedirectiontheeyesarepointing.Thesourceofourknowledgeaboutwheretheeyespointmightcomefromsensorsinthemuscles(INFLOW)orfromthemotorcommandsthatcontrolthemuscles(OUTFLOW).InflowsignalsarecalledProprioception.OutflowsignalsarecalledEfferenceCopy.

FromDrBedell,sourceunknown

Theeyepokedemonstration.Gentlyplaceyourfingeragainsttheoutercanthusofyoureye.Closeyourothereye.Pressquicklyontheeyeandrelease.Observethattheentireworldseemstojumpbackandforthastheeyeismovedbythefinger.Nowlookbackandforththesamedistancebymovingyoureyeinthenaturalway.Thesameretinalimagemotionproducesadifferentpercept,dependingonwhetheryourbrainhascommandedtheeyetorotate.

Theeyepokedemonstrationshowsthatpassivemotionoftheeyedoesnotproduceastrongsensethattheeyehasrotated,becausetheworldappearstoshiftastheretinalimagemoves.Thestretchreceptorsinthemusclesshouldbestimulatedbytheeyepoke,soInflowshouldstillbepresent.Thecommandtomovetheeyeisnot

present,soOutflowismissing.ThisdemoisusedtoarguethatOutflowismoreimportantthatInflowinourperceptionofmotion.PastPointingisaphenomenonthatoccurswhenthebraincommandstheeyetomovebuttheeyedoesnotmoveormoveslessthannormal.Theextraeffortcausesamisperceptionofdirection.

Thepatientinthisfigurehasaleftlateralrectusparesis.Hiseyeisabletoturnleftbutittakesextraefforttodosobecauseofnerveormuscledamage.Hishandishiddenfromhimundertheboard,andheisaskedtopointinthedirectionofthepencilheisfixating.Ontherightside,hepointsaccurately.Ontheleftside,hepointstoofarbecausehisefforttellshisbrainthathiseyeislookingfarthertotheleftthanitactuallyis.PastpointingismoreevidencefortheimportanceofOutflow.Ifthehandisnothiddenbytheboard,thepatientwillnotmakethiserror.Artificialparalysiswillalsoproducemisperceptionofvisualdirection.ErnstMachcrammedputtyinhiseyesockettomechanicallypreventeyemotion.–Uponattempted(butunsuccessful)eyemovement,thevisualworldappeareddisplaced.Paralyzingtheeyemuscles,forexamplewithcurare,yieldsgenerallysimilarresults,consistentwithsignalsofsensedeyepositionfromoutflow(e.g.,Stevensetal.,1976;Matinetal.,1982).OtherevidencepointstoaminorroleforInflow,however.ItismostlikelythatbothInflowandOutflowcontributetoourperceptionofdirectionandmotion,withOutflowbeingmoreimportant.ExocentricVisualDirection(XVD):Thedirectionofanobjectintheworld,relativetowhereweare,regardlessofheaddirectionoreyedirection.TheperceivedXVDisfoundbycomparingOVD,sensedeyeposition,andsensedheadposition.ThinkofXVDasthedirectionyouwouldhavetoreachoutinordertograbsomething,orthedirectionyouwouldtravelinordertoreachsomething.OursenseofheadpositionchangecomesprimarilyfromtheVestibularSystem.Whentheheadturns,theVestibuloOcularReflex(VOR)causestheeyestoturnintheoppositedirection.Ourcombinedsensethattheheadhasturnedandtheeyeshaveturnedintheoppositedirectiontellsusthatthepointwearefixatinghasnotmoved.Whenyouspininonedirectionforalongtime,thesituationchanges.TheVestibularsystemnolongerreportsthemotioncorrectlybecausethefluidcatchesup.Whenyoustop,thefluidkeepsspinningandyougetanerroneoussignalthatyourheadis

turning.Theeyesmove,theretinalimagemoves,buttheheadisnotactuallymoving.Theperceptionisthattheworldisspinningaroundyou!Oscillopsia:Generally,anytimetheeyesorheadmovesandthebraindoesnothavegoodinformationaboutit,theperceptionwillbethattheworldismoving.Whenapersonacquiresanystagmus,theyusuallyexperienceoscillopsia:anillusorymotionoftheworld.Whenapersonisbornwithnystagmusanddevelopswithitfromanearlyage,theyusuallydonotexperienceoscillopsia.Patientswithinfantilenystagmushavesomehowlearnedtosensetheirdramaticbackandfortheyemotionsothatperceptionoftheworldremainsstable.SensedEyepositionisnotperfect:Therearemanydemonstrationstoshowthatoursenseofeyepositionisflawed,resultinginillusionsofdirectionormotion.TheAutokineticEffect:Youwillneedacompletelydarkroomandasinglelightsource.Fixatethelightsource,andafteralittlebitoftime,thelightwillappeartomoveoffinonedirection.Eachpersonseesadifferentmotion.Themotionperceptioncomesfromsmalldriftimbalanceintheeyethatthebraindoesnotsense.Whenyoureyedriftsoff,youdon’tsenseit.Whenyoucorrecttheerrorwitharefixation,youdosenseit.Soallthosesensedcorrectionsmakeyouperceivethatyouarechasingamovingspot.Saccadicmislocalization:Whenwemoveoureyesquicklywithasaccade,oursenseofeyepositionchangesmoreslowlythantheeyeactuallymoves.Ifalightisflashedbrieflyrightbeforeyoumoveyoureye,itappearsinthewrongdirection.Forexample,aflashjustabovethestartingpointofthesaccademayappeartobeattheendingpoint.YoucansometimesnoticethiswithanLEDclockdisplayonamicrowaveorstereo.Makesaccadesbackandforthneartheclock,andsometimesitwillappearinthewrongplace.Afterimagemotion:Putastrongafterimageintoyourfovea.Inadarkroom,lookbackandforthbetweentwodirectionsandyouwillnoticetheafterimagemoveswithyoureye.Thatis,itchangesperceiveddirectionbecauseyoureyehaschangeddirectionandyousensethat.Butnowlookbackandforthquickly,asfastasyoucan,severaltimes.Theafterimageappearstomovelessthanitdid.Yoursenseofeyepositiondoesnotkeepupwiththerapidbackandforthmotion.BinocularCombinationofDirection:TheOVDisbasedonretinaleccentricity,butsometimesapointinspaceprojectstoadifferentlocationinthetwoeyes.Theimagefallsonnon-correspondingpoints.Whenthedifference,orretinaldisparity,issmall,theobjectisseeninasingledirection,itisfused.ThemergingoftwodifferentOVDsintoasinglebinoculardirectioniscalledallelotropia.Whenthedisparityislarge,theobjectisseenintwodirectionsatonce.Itisdiplopic,ordouble.Whenbotheyesareopenandobjectsarediplopic,youseeoneobjectintwodifferentdirections,butyoualsoseetwodifferentobjectsinthesamedirection.Thisiscalledconfusion.

Thephenomenadescribedaboveareeasilydemonstratedwithtwothumbs.Raiseyourtwothumbsatarmslength,sidebyside,andfixateoneofthem.Nowbringtheotherthumbcloserandcloser,andnoticethatitdoesnotbecomedoubleimmediately.Onceitisdouble,youseethesamethumbintwodifferentdirections.Eachofthesethumbsisconfusedwithsomepartofthebackground,andyoumayseerivalrythere.

Diplopicthumb.WhenIholdmythumbupinfrontofamoredistantbackgroundthatIamfixating(here,aneyechart),thedistantobjectissingleandthethumbisdouble.EacheyehasadifferentOculocentricVisualDirectionforthethumb.Whencombined,Iseetwothumbs,eachwithitsownCyclopeanVisualDirection.AreaswherethethumbcombineswithpartofthebackgroundisVisualConfusion.Twodifferentthingsinthesameperceiveddirection.

EgocenterandCyclopeanEye:Inthelate1800s,EwaldHeringdescribedaframeworkforunderstandinghowvisualdirectiongetscombinedfromthetwoeyes,whichwecallHering’sLawsofVisualDirection.Thecentralconceptbehindhisframeworkisthe“CyclopeanEye”.Heproposedanimaginaryeyethatisinbetweenthetwoeyes,andthatourperceptionofegocentricvisualdirectionisbasedthere,whenbotheyesareopen.

Thereality:twoeyesseeonethumbfromtwodifferentdirections

Theperception:Asinglecyclopeaneyeperceivesthesamethumbintwodifferentdirections.

Trythissimpledemonstration.Takeapenorpencilandholditoutinfrontofyou,aimed“rightatyou”.Thisideaof“aimedrightatyou”issubjective,becausethepenisnotaimedateitheroneofyoureyesbutsomewhereinbetween.ThepointbetweenyoureyeswherethepenisaimedrightatyouiscalledtheEgocenter.Theegocenteristheplacewheresomethingpointswhenitis“aimedrightatyou”.Withthepenaimedatyouregocenter,closeoneeyeandthentheother,andyoumayfind

thatthepenisoffsetmoretowardoneeyethantheother.Thisisonemeasureofeyedominance.BinocularSensedEyePosition:Thevisualimagesfromtwoeyesarefusedintoasingleperception,buttheEgocentricVisualDirectionthatthingsappearinalsorequiressensedeyeposition.Thepositionofeacheyecontributestothesensedeyeposition.Trythisdemonstration:Coveroneeyewithyourhand,preferablythemoredominanteye(theoneclosertoyouregocenter).Holdupyourthumbclosetoyourface,about40cm.Nowfocusbackandforthquicklybetweenyourthumbandadistantobjectrightaboveyourthumb.Youwillperceivethatthewholeworldisshiftingleftandright.Nowuncoverthedominanteyeandlookbackandforthbetweenthethumbandthesamedistantpoint.Youwillperceivethatyouhaveshiftedgazeleftandright,eventhoughonlyoneeyeisturning.Explanation:Whenyoushiftyourfocusfromfartonear,thecoveredeyemovesinandoutbecauseofaccommodativeconvergence.Thecombineddirectionoftheopenandcoveredeyeisshiftedpartiallybythismovement,andsotheEgocentricVisualDirectionofobjectsseenwiththeopeneyeappearstoshift.EVDisperceivedrelativetotheCyclopeanEye’sdirection.

Whenthelefteyeisoccluded,theperceivedlineofsight(red)isstillacombinationofleftandrighteyedirections.TheshiftofgazefromAtoBcausesthelefteyetoturnin,andsodoesthecyclopeanlineofsight.Thisproducesanillusionthatobjectsseenbytherighteyehavemovedtowardtheleft..