OPTO6224 Perception Scott Stevenson...When the retina becomes distorted, for example by Central...
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OPTO6224 Perception Scott Stevenson Visual Direction The perception of an object’s direction, based on retinal image position, sensed eye position, and sensed head position. a = Oculocentric Visual Direction (OVD) a + b = Egocentric Visual Direction (EVD) a + b + c = Exocentric Visual Direction (XVD) Visual Axis: The line connecting the point of regard through the center of the pupil to the retinal fixation locus. Fixation locus: The point on the retina where images land, when we perceive that we are “looking right at them”. For most of us this is in the central fovea. Figure from Putnam et al. JoV 2005 Subjects were asked to hit a button when they were looking right at a dot. The red dots show where the target was on the retina when the button was hit. The black dot shows the very center of the foveola where cones are most dense. This subject, like others in the study, used a slightly eccentric point for fixation, about 10 arc minutes away from the center. Eccentric Fixation vs. Eccentric Viewing: Sometimes we see something better if we don’t “look right at it”. For example, in scotopic vision we have a foveal scotoma where there are no rods, so we must use Eccentric Viewing to see dim stars. We are looking at it, but not looking right at it. When a person uses a region away from the foveal center for all fixation, and believes they are “looking right at” the target, 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
Transcript of OPTO6224 Perception Scott Stevenson...When the retina becomes distorted, for example by Central...
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
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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..
