OPTO6224Perception
ScottStevenson
Scotomasandperceptualfilling-in.
Holdyourrighthandoutatarmslength,spreadyourfingers,andfixateyourthumb.
Closeyourlefteyeandnoticethatyourlittlefingeroftherighthandhasvanished.
Whathastakenitsplace?
Withbotheyes,fixatethethumbandyou Closethelefteye,thepinkydisappears
stillseeyourpinky.
butthebackgroundfillsinthegap!
Ascotomaisaregionofthevisualfieldthatislocallyblind.Thewordisfromthe
Greek“skótos”meaningdarkness,andhasthesamerootasscotopic.Thepluralis
eitherscotomasorscotomata.Thetermscotomaisusuallyappliedwhenthereis
visionlosssurroundedbyintactvision,asopposedtoafieldlossthatoccursatthe
edgeofthevisualfield.
Ascotomamaybemonocularorbinocular,physiologicalorpathological,absoluteor
relative,positiveornegative.
Aphysiologicalscotomaisduetonaturalstructuresintheeye.
Apathologicalscotomaisduetodiseaseprocessesthatcauseblindnessinalocal
area.
Examplesofphysiologicalscotomas:
Theopticdiscineacheyeproducesamonocularscotoma,the“physiologicalblind
spot”oftheeye.
Bloodvesselsblockingthephotoreceptorsthroughoutthevisualfieldproduce
“angioscotomas”.
(http://www.medicine.uiowa.edu/uploadedIm
ages/Departments/Ophthalmology/Content/P
atient_Care/Imaging_Services/FA_Fig2(1).jpg
Fluoresceinangiographyimageofblood
vessels.Largervesselsproduceabsolute
scotomas,smalleronesproducerelative
scotomas.
Theopticdiskisanabsolutescotoma,because
therearenophotoreceptorsthere.
Thelackofrodsinthecentralfoveaproducesafovealscotopicscotomaindimlight,
ThisfigurefromCrosslandetal.2011
showsresultsfrommicroperimetry
testingofdarkadaptedsubjects.The
testedregionwascenteredonthefovea,
andtheredareainthefundusimage
showsthescotomaintherodfreearea
ofthecentralfovea.
TheMP-1isaninstrumentbyNidekthat
trackstheeye,allowingforprecise
microperimetry.
Anabsolutescotomareferstoacompletelossofvisioninaparticularregion.A
relativescotomareferstoalossofvisionwithsomesensitivityremaining.Arelative
scotomaisusuallydescribedassomanylogunits(ordb)ofsensitivityloss.
Apositivescotomaisonethatisperceivedasa(usually)darkregionthatobscures
vision.Anegativescotomaisnotperceiveddirectly,becauseof“filling-in”.Patients
areonlyawareofnegativescotomaswhentheynoticeobjectsdisappearingintothem.
Anexampleofanegativescotomaisthephysiologicalblindspot.Peoplearenotaware
ofblindnessintheregionoftheopticdiscuntilitisspecificallypointedouttothem.
Pathologicalscotomasareusuallynegativescotomas.
Anexampleofa(temporary)positivescotomaisaverystrongafterimage.Peopleare
awareofthelossofvisionbecauseitappearsasafloatingblobthatcoversobjects.
Positivescotomasmayoccurrightafterlaserphoto-coagulationsurgery,butthe
perceptionofthescotomawillusuallyfade.Anotherexampleofapositivescotomais
thescintillationscotomathatcomeswithmigraineauras.Thepatientisawareofthe
blindnessbecausetheareaisfilledwithadynamicpatternoftwinklinglines.
Thetermscotomaisusedmoregenerallysometimes.Forexample,“Ringscotoma”
hastwodifferentmeanings:
1)itdescribesapairofarcuatescotomasthatareoneithersideofthefovea,also
calledanannularscotomaor
2)itdescribestheblockedareaproducedbyastrongconvexlens,forexampleasina
hyperopicspectaclecorrection.Thereisaregionofthevisualworldthatisnotseenat
therimofthelensduetotheprismaticeffectsofthelens.
Cartoonofanarcuatescotoma.Theblindarea
followstheganglioncellaxonfibersastheycollectto
leavetheeyeattheopticdisc.Damagetothefibersat
thediskmarginorintheopticnervewillproduce
visionlossinalltheretinallocationswherethefibers
originate.
Amentalscotomaisafigurativeblindspotinyourpsyche,usedtodescribecases
whensomeonelacksinsight.
Filling-inisatermthatdescribestheperceptionofpatternandcolorinaregionthat
isactuallyblind.Filling-inpreventsusfromnoticingthatwehaveascotoma,whichis
abenefitbecausewearenotdistractedorconfusedbytheappearanceofblindspots.
Ithasacostclinically,becausepatientsdonotnoticethattheyarelosingvisionuntilit
isquiteadvanced.
Thephenomenonoffilling-inismoregeneralthanjustscotomas.Perceptionfillsin
informationinmanyways,inordertogiveusthebestestimateofthetruestructureof
theworld.
TheKanizsatriangleisanexampleofthe
brainfillingindetailsthatarenotinthe
picture.Thesolidwhitetriangleappears
tohaveedgesallthewayaround,
becausethebrainextendstheedges
wherethedisciscovered.
Areasofsolidcolorarefilledinfromthe
visibleedges.Otherwise,adapatation
wouldmakethemlookgray.
Fillinginofcolorisillustratedinthis
figure,similartoonemadebyJohn
KrauskopfofBellLabs.Fixatetheblack
dotsteadily.Thegreenringwillvanish
andbereplacedbyred.Thefadingofthe
greenisduetoadaptation.The
replacementwithredisduetofillingin.
Thereddoesnotfadebecausetinyeye
movementscausethesharpedgesto
shiftontheretina.Fillinginoccursfrom
theedgesin.
TheAmslerGridisusedforpatientsto
self-screenforAMD.Patientslooksfor
areasofdistorsionormissinglines.
Filling-inofpatternscanmaketheAmsler
Gridlookcomplete,evenifthereisa
scotomablockingpartofit.
TheAmslergridfillsinfor~75%of
patientswithretinalscotomas<6°
diameter(Schuchard,1993).