OPTO6124PerceptionScottStevensonEntopticPhenomenaVisionissupposedtotellusabouttheworldbeyondourselves,butsometimeswhatweseeiscausedbythevisualapparatusitself.Becausetheycomefrom“inside”theyarecalled“Entoptic”phenomena.Perceptionoftenfilterstheseimagesout,butwhentheyhavesuddenonsetorbecomeannoying,patientswillcomplainaboutthem.Manyofthesephenomenaareproducedbyshadowsfallingontheretinafromopaqueobjectsintheeye.Shadowsincollimatedlightaresharp,howevernearorfartheyarefromthescreen.

Placeapinholeattheanteriorfocalpointoftheeye,andopacitieswithintheeyewillcastsharpshadows,whethertheobjectsareintheanteriororposteriorregionoftheretina.

Featuresfromtheanteriorsegmentappearasshadowswhenviewedwithapinholeattheanteriorfocalpointoftheeye.Asmallpinhole,alargepupil,andaverybrightbackgroundwillenhancetheeffect.

Shadowsfromextendedsourceshavetwocomponents,anUmbraandaPenumbra.

Here,lightfromtheSunhitstheMoon,whichcastsashadowontheEarth.PartoftheearthisinsidetheUmbra,andseesatotaleclipse.TherestoftheEarthisinsidethePenumbraandseesapartialeclipse.

Insidetheeye,withalargebrightbackgroundlikethebluesky,thepupilactslikeanextendedsource.Objectsclosetotheretinawillgeneratedistinctshadowsbecausetheyarecloseenoughfortheumbratohittheretina.Objectsfartherdonotgeneratedistinctshadows,andonlyreducetheoverallaveragelighthittingtheretina.Withalargepupil(left)theumbraisshort,butwithasmallpupilorpinhole,theumbraextendsandsoobjectsmaybecomevisiblethatareinsidethevitreous.Note,objectsineyearenottoscale.Floaters,aka“muscaevolitantes”(Latinfor“flyingbugs”)“Floatersarelittle“cobwebs”orspecksthatfloataboutinyourfieldofvision.Theyaresmall,dark,shadowyshapesthatcanlooklikespots,thread-likestrands,orsquigglylines.Theymoveasyoureyesmoveandseemtodartawaywhenyoutrytolookatthemdirectly.Theydonotfollowyoureyemovementsprecisely,andusuallydriftwhenyoureyesstopmoving.“[“FactsaboutFloaters”https://nei.nih.gov/health/floaters/floaters]

movie“Floater_WhiteBloodCells.mp4”

https://upload.wikimedia.org/wikipedia/commons/b/b2/Floaters.pngSimulatedimageoffloatersfromtheWikimediaCommons,astheyappearagainstabrightbluesky.Notethattheysometimesdonotappearinsharpfocus.

CausesofFloatersPrimarycauseistheliquefactionofthevitreousgelwithaging.Inparticular,asthevitreousshrinkswithage,itbecomesdetachedfromtheretina(PVD:PosteriorVitrealDetachment),allowingafreeflowofmaterialbetweenthevitreousgelandtheretina.Collagenfibersthatwerepartofthevitreousbecomelooseclumpsthatmovearoundastheeyemoves.Othercausesincludematerialleftoverfromthehyaloidarterythatbreaksdownduringthethirdtrimester,andsometimesmaterialfromtheretinaitselfthatbreaksoffduringavitrealdetachment.AppearanceofFloatersWhentheyareveryclosetotheretina,floaterscausesharpshadowsordiffractionpatterns.Whentheyarefartherfromtheretinatheyproduceblurryshadowsandarelessdistinct.Shadowscastontheretinaarenotvisibleunlesstheymove.Whenshadowsarestationary,theretinaandbrainadapttothelowerlightlevelthereandtheshadowisnotnoticed.Shadowsmovewhena)theobjectmoves,orb)thedirectionoflightchanges.Inthecaseofa),rapidmovementoftheeyecausesthevitreousbodytoshift,displacingthefluidbetweenvitreousandretina.Clumpsofdifferentsizemoveatdifferentspeeds,resultinginanappearanceofobjectsmovingacrossthevisualfield.Astheeyesettlesdown,thevitreoustendstosink,displacingthefluidanddebrisupward.Becausetheretinalimageisinvertedrelativetogravity,thefloatersappeartosink.Infact,theyaremovingupward.Inthecaseofb),ifapinholeismovedacrossalargepupil,thenmanyshadowsontheretinabecomenoticeable,particularlytheonescausedbybloodvessels.Collagenfiberclumps,“floaters”,alsobecomemorenoticeableinthiscase.Awarenessoffloaters:Althoughthedebrisareconstantlypresent,ourawarenessoffloatersvariesdependingonseveralthings:1)Iftheydon’tmovemuch,weadapttothedimmerretinalilluminationundertheshadow2)Iftheyareintheperiphery,ouracuityistoolowtoseethediffractionpatterncausedbytheverysmallfibers.3)Highlevelattentionmechanismsfilteroutthefloaterimagesbecausetheyarenotpartoftheworld“outthere”thatwearetryingtosee.Assoonassomeonementionsfloaters,however,attentionturnstothemandtheymaybecomevisibleandannoying.Thisissimilartothewayeyeblinksbecomenoticeablewhensomeonementionsthem.Clinicalsignificanceoffloaters:Asuddenincreaseinfloatersmaybeasignofinfection,inflammation(uveitis),hemorrhaging,retinaltears,orotherinjurytotheeye.Sufficientlydensefloaterscanobscurefovealvisionenoughtoaffectacuity,ormaybecomesoannoyingthatpatientsseektreatment.Whenapatientcomplainsoffloaters,itisimportanttoruleoutrecentonsetsignificantissueslikeretinaldetachment,beforegivingassurancethattheyarecommonandbenign.

Treatmentforfloaters:Mostcasesarenottreated.Patientsaretolditisanaturalpartofagingandthereisnosimpleproceduretoremovethem.Moreseverecasescanbetreatedwithvitrectomy.Removalofthevitreousbodyresolvesthefloaters,butbringswithitothercomplicationslikecataractandpossibleretinaldetachment.“Floaterectomy”isatermusedtodescribepartialvitrectomytoresolvefloaters.“FloaterectomyVersusConventionalParsPlanaVitrectomyForVitreousFloaters”http://www.djo.harvard.edu/site.php?url=/physicians/oa/1004SomedoctorsmaytryYAGlasertreatmenttovaporizethefibers,orenzymestobreakthemup.WhiteBloodCells:Whitebloodcellsarelargeenoughtocompletelyfillthesmallcapillariesintheretina.Sincetheydisplacethesmallerredbloodcells,whichabsorblightandcastashadow,andsincetheyaremostlywater,theymakethebloodvesseltemporarilyclear.Inretinalimaging,whenawhitebloodcellmovesthroughweareabletoseethephotoreceptorsbehindthecapillary.Visually,theymayappearassmallwhitespotsmovinginthevisualfield.ThisiscalledScheerer’sphenomenon.Wikipediahasanicesimulationofhowtheyappearagainstabrightsky.https://en.wikipedia.org/wiki/Blue_field_entoptic_phenomenonhttps://upload.wikimedia.org/wikipedia/commons/7/70/Blue_field_entoptic_phenomenon_animation.gifTounderstandwhytheyappearassmallwhitespots,watchthedemoWBCsim.mp4.Fixatethecenterspotuntilthedarkbandfades,andthemovinggapinthebandwillappearasawhitespot.Foramoreelaborateversionofthis,watchthedemoTroxlerDemo3Hz.mp4.Spotsmovearoundacircle,appearingfirstasgapsintheringthenasobjectsofoppositecolor.PurkinjeTree:ThePurkinjetreeisashadowofthesuperficialretinalvasculature.Itisnormallynotvisiblebecauseofadaptationandcentralinattention.Thereisalsoevidenceoflocalamblyopiaforcellsshadowedbythebloodvessels.AdamsDL,HortonJC."Shadowscastbyretinalbloodvesselsmappedinprimaryvisualcortex."Science2002;298:572-6.

ThisfigurefromAdamsandHortonshowsthattherearefewercellsincortexdevotedtotherighteyeinlocationsthatareshadowedbybloodvessels.Dataarefromasquirrelmonkey.ThePurkinjetreebecomesvisiblewhenthelightsourcemoves.Thiscanbeachievedbyviewingwithapinholepupil,whenthenaturalpupilislarge.Movingthepinholeacrossthenaturalpinholemakestheshadowsofthevesselsmoveenoughthattheybecomevisible.AbettermethodtovisualizethePurkinjetreeistoilluminatetheeyethroughthesclera.Placeapenlightagainsttheclosedlidnearthelimbusandmoveitinsmallcircles.Thiswillmaketheshadowsfallonreceptorsthatarenotusuallyshadowed,andthetreebecomesmorevisible.Themorepinpointthelightsource,thesharpertheshadowswillbe,andthussmallerbloodvesselscanbeseen.

Phosphenes:Phosphenesareperceivedflashesoflightthatarisebecauseofinternalactivityintheretinaorhighervisualsystem.Phosphenescanbecausedbymechanicalactionontheretina,asoccursduringvitreousdetachment.Moore’slightningstreaksareflashesoflightthatresemblelightningbolts.Theyusuallyappearinthetemporalvisualfield,andarefairlycommoninmiddleageandolderpatientsastheyexperiencePosteriorVitrealDetachment(PVD).Thevitreousputstractionontheretinaandthemechanicalactioncausesthecellstodischarge.FlickPhosphenesareproducedbyrapid,large,saccadicmovementsoftheeye.Theyaremorecommonlynoticedbyolderpatients,suggestingthatthepartiallyorfullydetachedvitreousmaybetuggingand/orbumpingontheretina.Theyaremostnoticeablewhendarkadapted.PressurePhosphenesareproducedbydirectmechanicalstimulationoftheeye.Closeyoureye,turnyoureyetowardthenose,andpressonthetemporalsidethroughyourlid.Youwillseeaspotappearinyournasalvisualfield.

FromFresco,Ophthalmology,1998

TheplotontheleftshowsacorrelationbetweenthepressurerequiredtoproduceaphospheneandtheintraocularpressuremeasuredwithGoldmanntonometry.Inthephoto,asimplepressuresensorispushedagainsttheeyeuntilaphospheneisseen.

ElectricalPhosphenesareproducedbyelectricalstimulationoftheretinaorcortex.ThesearesometimesnoticedbysubjectsbeingfittedwithEOG(EloctroOculoGram)electrodeswhentheimpedenceisbeingtested.Examinersrunasmallamountofcurrentthroughtheelectrodestomakesurethecontactisgood,andthiscurrentstimulatestheretina.CorticalPhosphenesarelikewiseproducedbyelectricalstimulationofthecortex.AnewtechniquecalledTranscranialMagneticStimulationisgaininginterestasatoodforstudyingcorticalfunction,andthissometimesproducesflashesoflight.Someneurologistshaveexperimentedwithimplantedelectrodearraysonthevisualcortexinordertoproducephosphenesinblindpatients.Stimulationelectrode

arraysimplantedintheeyearecurrentlyunderdevelopmentaswell.ThemostadvancedoftheseistheARGUSIIfromSecondSight

ARGUSIIelectrodearray.FromLuo & Cruz (2016) Prog. Ret. Eye. Res.

Left: A patient wearing the apparatus that

records video and transmits it by radio into the implanted ARGUS II device.

Above: An image of the photoreceptor array, with a white circle to indicate the relative size of one electrode in the ARGUS II device. The spatial resolution of current implant devices is

extremely poor.

InthecontextofPerception,theimportantthingaboutPhosphenesisthattheyareperceivedaslight,andcanbeusedtorecognizeforms.ThisisanexampleofMüller’slawofspecificnerveenergies.Activationoftheopticnervefibersisperceivedaslightbecausethosenervesareassociatedwithvision.

Migraineauras:Migrainesufferersoftenexperienceasensoryphenomenonpriorto,orinconjunctionwithasevereheadache.Thesephenomenacanaffectanyofthesenses,butvisualmigraineaurasarefairlycommon.Theaurashavebeencalled“fortificationillusions”becausetheyappearascriss-crossedlinesforminga“fence”thatmovesacrossthevisualfield,leavingascotomainitswake.Theauraandscotomaarehomonymous(inbotheyes)becausetheyareacorticalphenomenon.Visionusuallyrecoversinawaveaswell,afewminutesbehindtheslowlydriftingaura.Administrationofamigrainemedicine(aspirine+caffeine+acetaminophen)attheonsetofauramaypreventtheheadache.

FromDr.Bedell,sourceunknown

AfterimagesAfterimagesaretheresultofadaptationtolightpatterns.Theyarelocaltoparticularretinalareas,andsotheymovewiththeeye.Theyarenotusuallynoticedbecausetheeyeisconstantlymoving,changingthelocalstimulationandthuspreventingadaptation.Tryhttp://www.johnsadowski.com/big_spanish_castle.php

Fixatethecenterblackdotforaboutaminute,thenlookatthewhitespot.Youshouldseeadarkgridfloatingoverthewhitegrid.Themotionyouseeintheafterimageiscausedbyyourownfixationjitter.(fromDr.Bedell,sourceunknown).

Usingafterimagesto“tag”thefoveas,totestforAnomalousRetinalCorrespondence

Inthistest,eacheyeisflashedtoputanafterimageinthemiddleofthefovea.Withbotheyesopen,mostpeoplewillthenseethetwoafterimagesinthesamedirection(upperleftdiagram).PeoplewithAnomalousRetinalCorrespondence(ARC)donotnecessarilyseetheirtwofoveasasrepresentingthesamedirectionwhenbotheyesareopen.Eventhoughthetargetsareonthefoveasofbotheyes,theydonotperceivethemasoverlapping.AbrightmovinglightleavesatrailbehinditthatcomesfromVisualPersistenceandLightAdaptation.Inthesketchbelow,abrightgreenspotleavesbehindashortgreenstreak(fromvisiblepersistence)andbehindthatisa longlastingafterimage(fromlightadapation).Afterimageslastlongerifthebackgroundlightchanges,makingitmoreobviousthatoneregionoftheretinaisadapteddifferentlyfromanother.Movetheeyebackandforthfromabrightareatoadimarea,andtheafterimagewillkeepreappearing,thenfading.

Maxwell’sspotThemacularpigmentoverlyingthefoveaselectivelyabsorbsbluelight.Theretinalareacoveredbymacularpigmentisroughlythesamesizeasthefovealpit.Lightcomesfromthebottomandpassesthroughthispigmentbeforereachingthephotoreceptors.Undertherightviewingconditions,weseeadarkspotcenteredonthepointoffixation.Thespotishardtonoticebecauseitmoveswiththeeyeandbecauseweadapttothedifferenceincolor/brightness.

Wavelength (nm)

Leftimage:FromSnodderly1984.Crosssectionofamonkeyretinaintheregionofthefovea.Theupperimagewasmadewithgreenlightillumination,thelowerimagewithbluelight.Thedarkappearanceinthelowerimagecomesfromthemacularpigmentabsorbingthebluelight.Rightimage:Pigmentabsorbancevs.wavelength,withdatafromvariousstudies.Maxwell’sspotismosteasilyseenwithalternatingillumination.Trystaringatthegreensquareforafewsecondstoadaptyourlongandmiddlecones,thenlookattheblue.Youshouldseeadarkspotintheareaaroundyourfixationpoint.Thespotisusuallyalittlebiggerthanyourthumbatarmslength.

Maxwell’sspotissometimesusedtodiagnoseeccentricfixation.Ifapatientseesthespotoffcenterfromthepointoffixation,itindicatestheyarenotusingthecentralfoveaforfixation.AsimilartechniqueusesHaidinger’sbrushes.Theserefertoawindmillpatternthatappearswhenviewingpolarizedbluelight.Ifthewindmillisnotcenteredonthefixationpoint,itindicateseccentricfixation.