Opto6224Perception
ScottStevenson
Perceptionoftiltandorientation.
Whichwayisup?Ourorientationwithrespecttogravityiscriticalforsafelymovingaround
intheworld.Fallsareamajorcauseofinjuryanddeath,especiallyintheelderly.
http://www.cdc.gov/homeandrecreation
alsafety/falls/adultfalls.html
Withthebaby-boomergeneratingaging,
andpeoplelivinglonger,accidentalinjury
fromfallsisbecominganincreasing
publichealthissue.
Onefactoristheperceptionofone’sown
orientationrelativetogravity.Visualand
vestibularcuesareusedtomaintainan
uprightposture,andasthesesenses
degradewithage,sodoestheriskof
falling.
JustaswithoursenseofPerceivedVisualDirection,oursenseofPerceivedVisualOrientation
dependsonthreesourcesofinformation:First,weneedinformationfromtheretinalimage
(OculocentricVisualOrientation-OVO).Thisiscombinedwithinformationaboutthetorsional
postureoftheeyes,togiveusasenseoforientationwithrespecttothehead(Egocentric
VisualOrientation–EVO).Thisisthencombinedwithinformationabouttheorientationof
theheadtogiveusaperceptionoforientationwithrespecttogravity(eXocentricVisual
Orientation–XVO).
OculocentricVisualOrientation-OVO
Ourperceptionoflinesandedges,andtheirorientations,isbasedonresponsesofcellsin
cortex.
GanglionCellsintheretinaandLGN
cellsarenotstronglytunedtothe
orientationoflinesoredgesfalling
ontheretina.Theyhaveacircular
receptivefield,withanONorOFF
centerandanOFForONsurround.
Inprimaryvisualcortex(V1)arow
ofLGNcellsinastraightlineadd
togetherintoaSimpleCell,which
prefersaparticularorientationof
lineoredge.
Likemotionsensitivecells,ourorientationsensitivecellswilladapt,andshowaftereffects.
TiltAftereffect
Fixatetheblackbarontheright,lookingbackand
forthalongthebar.Thetiltedlinesaboveandbelow
fixationwillbegintoadaptcellssensitivetothattilt.
Afteraseveralsecondsofadaptation,lookoverat
theblackbarontheleft,andobservetheapparent
oppositetiltoftheverticallines.
Likewithmotion,adaptationtotiltinonedirection
causesabiasforperceivingtheopposite.
Whendifferent,overlappingfeatureshavesimilarorientation,weperceiveabiggerdifference
inorientationthatisactuallythere.ThisiscalledOrientationContrast,anditgivesriseto
someillusionsinwhichparallellinesdon’tlookparallel.
TheWundt(above)andZollner(left)illusions
illustrateOrientationContrast.
Theperceptionofthelonglinesisbiasedby
thelinesthatcrossthem.InWundt,the
horizontallinesappearcurved.InZollner,the
diagonallinesappearslightlyoffthetrue
diagonal.
Likemotionsignals,orientationsignalsaredeterminedlocally,ateachpointinthevisualfield.
Theselocalsignalsarecombinedtoproduceaglobalperceptofoverallorientation.
Adaptationandcontrastalterthelocalsignals,resultinginglobalillusions.Anotherexample
oflocaltoglobaltilteffectsisinthetwistedcordillusion,orFraser’sspiral.
Inbothfiguresabove,localtiltsaremakingcircularshapesappeartospiral.Adynamic
versionofthisisathttp://intranet.opt.uh.edu/coursematerials/newcoursefiles/opto6224/index_files/image002.gif
EgocentricVisualOrientation–EVO
Withtheheadfixed,andalineimageontheretina,theperceivedorientationofthatlinewill
dependonboththevisualinformationandoursenseofoureye’sorientation.Inthiscase,itis
eyetorsionthatwemustsense.Torsionisnotgenerallyundervoluntarycontrol,andsowe
don’thaveasgoodasenseofthecommandsthatcontrolit.Nevertheless,whentheeyemakes
torsionthereisacompensationinourperceptionsothattheworldappearsupright.
eXocentricVisualOrientation–XVO
Ourperceptionofwhichwayisupisfinallyacombinationoftheretinalorientationand
eye/headorientation.HeadorientationissensedbytheVestibularSystem,comprisingthe
semicircularcanalsandotoliths.Thecanalsprovideinformationaboutthedynamicrotation
ofthehead,whiletheotolithsprovideinformationaboutthestatictiltoftheheadandalso
aboutlinearacceleration.
Semicircularcanalsrespondtorotationoftheheadinanydirection.Haircellsintheotolith
organssensethedirectionofgravitywhentheheadistilted.
Inmicrogravity,ontheISSforexample,theotolithsnolongerprovideareferencefor
perceivedvertical.Astronautsonthespacestationdonothaveasensethatanyparticular
directionis“up”,butratherthatwhicheverwaytheyareoriented,“up”isabovethem.
OurperceptionofverticalandXVOgenerallyisheavilybiasedbyvisualinformation.Whenwe
perceivethatweareinanatural,familiarenvironment,perceptionisanchoredtovisual
featuresthatareapparentlyhorizontalorverticalwithrespecttogravity.Balancingwiththe
eyesclosedismuchmoredifficultthanbalancingwiththeeyesopen.
VisualVestibularMismatchandSeaSickness
Theotoliths,whichtellusthedirectionofgravity,aremoreeasilyignoredthanthe
semicircularcanals,whichtelluswhenwearerotating.Insideashipthatisrockingonthe
oceanwaves,thecanalstellusthatwearerotatingbutvisiontellsuswearenotbecausethe
roomisrotatingwithus.Thismismatchcancauseseasickness,withsymptomsofnausea,
sweating,headache,etc.
Themechanismsbehindseasickness,andtherelatedphenomenonofspacesickness,arestill
notfullyunderstood.Onetheoryholdsthatthemismatchbetweentheintendedmovement
andthesensedmovementistheprimarycauseofdiscomfort.Forexample,whenyouturn
yourheadslightlyonewaybutsensethatithasturnedmoredramaticallyadifferentway.In
automobiles,passengersaremorelikelytobecarsickthandrivers,perhapsbecausedrivers
haveabetterexpectationoftheturns.
Insituationsthatleadtoseaorcarsickness,itisusuallybettertolookoutthewindowandsee
thehorizon,sothatvisionandvestibularsignalsagree.Peoplewhohavelostvestibular
function,forexamplefromuseofmycinantibiotics,whichkillhaircells,willnotexperience
seasickness.However,theydoexperienceperceivedrotationoftheworldaroundthemwhen
theyturntheirheadsquickly.
TiltedandSwingingRooms
Artificialenvironmentsthataltertherelationshipbetweengravityandthevisualcuesto
verticalcanproduceprofoundillusions,andsometimesdiscomfort.Thisiswhytiltedrooms
giveastrongillusionofleaning.Thesignalsfromotolithsareoverriddenbythefamiliarvisual
cues.
http://www.puzzlingworld.co.nz/gallery/
Tiltedroomillusionmovie
https://www.youtube.com/watch?v=1BM
SYXK4-AI
Ourabilitytobalanceisverymuchcontrolledbyvision,andinsituationsofunsteadybalance
wemayfalloverifthevisualworldmovessuddenly.
IntheSwingingRoomexperiments,
subjectsstoodinaroomwithastable
floor,butmovingwallsandceiling.
Whenthevisualenvironmentwas
moved,forexampleforward,the
subjectsperceivedthattheyhadmoved
backward.Theyleanedforwardto
compensate.
Toddlers,whoaregenerallyunsteady,
willfalloverinthissituation.Adults
generallywillnot,unlesstheyaretrying
tobalanceonanarrowsupport.
Thisdemonstratestheimportanceof
visionincontrollingbalance,andinour
perceptionofverticalposture.
https://www.youtube.com/watch?v=F4xenIulg_8
Objectperceptionandorientation
Theperceptionofobjectsisstronglyinfluencedbyorientation.Welearntorecognizeobjects
incertainorientations,andsomeobjectschangetheiridentigywhentheyrotate.Inparticular,
considerthelettersuandn,qandb,xand+,whichdifferonlybyrotation.Withoutan
accurateperceptionoforientationtheidentitymaybemisperceived.Othercommonobjects
aremosteasilyprocessedinthemostcommon,or“canonical”orientation.
Ourperceptionoffacesisstronglyinfluencedbyorientation.
ThatcherIllusion:
Thesefourphotosareallbasedonthe
sameoriginalpictureofformerUK
primeministerMargaretThatcher.
Theoriginalisbottomleft.
Themouthandeyesarerotated180
degreesforagruesomefaceinthe
bottomright.
Thetoprowhasthesetwoinverted180
degrees,sointhetoprightpicturethe
eyesandmouthareactuallyinthe
uprightorientation.
Themainpointisthattheupsidedown
faceishardtoprocess,andsothe
inversionoftheeyesandmouthhardly
matters.
Thesetwopicturesareinfactthesame,justrotatedby90degrees.
Afrog?
Ahorse?