Entoptic Phenomena_1

Perception2016ScottStevenson
EntopticPhenomena
Visionissupposedtotellusabouttheworldbeyondourselves,butsometimeswhat
weseeiscausedbythevisualapparatusitself.Becausetheycomefrom“inside”they
arecalled“Etoptic”phenomena.Perceptionoftenfilterstheseimagesout,butwhen
theyhavesuddenonsetorbecomeannoying,patientswillcomplainaboutthem.
Manyofthesephenomenaareproducedbyshadowsfallingontheretinafrom
opaqueobjectsintheeye.
Shadowsincollimatedlightaresharp,howevernearorfartheyarefromthescreen.
Placeapinholeattheanteriorfocalpointoftheeye,andopacitieswithintheeye
willcastsharpshadows,whethertheobjectsareintheanteriororposteriorregion
oftheretina.
Featuresfromtheanteriorsegmentappearasshadowswhenviewedwithapinhole
attheanteriorfocalpointoftheeye.Asmallpinhole,alargepupil,andaverybright
backgroundwillenhancetheeffect.
Shadowsfromextendedsourceshavetwocomponents,anUmbraandaPenumbra.
Here,lightfromtheSunhitstheMoon,whichcastsashadowontheEarth.Partof
theearthisinsidetheUmbra,andseesatotaleclipse.TherestoftheEarthisinside
thePenumbraandseesapartialeclipse.
Insidetheeye,withalargebrightbackgroundlikethebluesky,thepupilactslikean
extendedsource.Objectsclosetotheretinawillgeneratedistinctshadowsbecause
theyarecloseenoughfortheumbratohittheretina.Objectsfartherdonot
generatedistinctshadows,andonlyreducetheoverallaveragelighthittingthe
retina.Withalargepupil(left)theumbraisshort,butwithasmallpupilorpinhole,
theumbraextendsandsoobjectsmaybecomevisiblethatareinsidethevitreous.
Note,objectsineyearenottoscale.
Floaters,aka“muscaevolitantes”(Latinfor“flyingbugs”)
“Floatersarelittle“cobwebs”orspecksthatfloataboutinyourfieldofvision.They
aresmall,dark,shadowyshapesthatcanlooklikespots,thread-likestrands,or
squigglylines.Theymoveasyoureyesmoveandseemtodartawaywhenyoutryto
lookatthemdirectly.Theydonotfollowyoureyemovementsprecisely,andusually
driftwhenyoureyesstopmoving.“
[“FactsaboutFloaters”https://nei.nih.gov/health/floaters/floaters]
movie“Floater_WhiteBloodCells.mp4”
https://upload.wikimedia.org/wikipedia/commons/b/b2/Floaters.png
SimulatedimageoffloatersfromtheWikimediaCommons,astheyappearagainsta
brightbluesky.Notethattheysometimesdonotappearinsharpfocus.
CausesofFloaters
Primarycauseistheliquefactionofthevitreousgelwithaging.Inparticular,asthe
vitreousshrinkswithage,itbecomesdetachedfromtheretina(PVD:Posterior
VitrealDetachment),allowingafreeflowofmaterialbetweenthevitreousgeland
theretina.Collagenfibersthatwerepartofthevitreousbecomelooseclumpsthat
movearoundastheeyemoves.
Othercausesincludematerialleftoverfromthehyaloidarterythatbreaksdown
duringthethirdtrimester,andsometimesmaterialfromtheretinaitselfthatbreaks
offduringavitrealdetachment.
AppearanceofFloaters
Whentheyareveryclosetotheretina,floaterscausesharpshadowsordiffraction
patterns.Whentheyarefartherfromtheretinatheyproduceblurryshadowsand
arelessdistinct.
Shadowscastontheretinaarenotvisibleunlesstheymove.Whenshadowsare
stationary,theretinaandbrainadapttothelowerlightlevelthereandtheshadow
isnotnoticed.
Shadowsmovewhena)theobjectmoves,orb)thedirectionoflightchanges.
Inthecaseofa),rapidmovementoftheeyecausesthevitreousbodytoshift,
displacingthefluidbetweenvitreousandretina.Clumpsofdifferentsizemoveat
differentspeeds,resultinginanappearanceofobjectsmovingacrossthevisualfield.
Astheeyesettlesdown,thevitreoustendstosink,displacingthefluidanddebris
upward.Becausetheretinalimageisinvertedrelativetogravity,thefloatersappear
tosink.Infact,theyaremovingupward.
Inthecaseofb),ifapinholeismovedacrossalargepupil,thenmanyshadowson
theretinabecomenoticeable,particularlytheonescausedbybloodvessels.
Collagenfiberclumps,“floaters”,alsobecomemorenoticeableinthiscase.
Awarenessoffloaters:
Althoughthedebrisareconstantlypresent,ourawarenessoffloatersvaries
dependingonseveralthings:
1)Iftheydon’tmovemuch,weadapttothedimmerretinalilluminationunderthe
shadow
2)Iftheyareintheperiphery,ouracuityistoolowtoseethediffractionpattern
causedbytheverysmallfibers.
3)Highlevelattentionmechanismsfilteroutthefloaterimagesbecausetheyarenot
partoftheworld“outthere”thatwearetryingtosee.Assoonassomeonementions
floaters,however,attentionturnstothemandtheymaybecomevisibleand
annoying.Thisissimilartothewayeyeblinksbecomenoticeablewhensomeone
mentionsthem.
Clinicalsignificanceoffloaters:
Asuddenincreaseinfloatersmaybeasignofinfection,inflammation(uveitis),
hemorrhaging,retinaltears,orotherinjurytotheeye.
Sufficientlydensefloaterscanobscurefovealvisionenoughtoaffectacuity,ormay
becomesoannoyingthatpatientsseektreatment.
Whenapatientcomplainsoffloaters,itisimportanttoruleoutrecentonset
significantissueslikeretinaldetachment,beforegivingassurancethattheyare
commonandbenign.
Treatmentforfloaters:
Mostcasesarenottreated.Patientsaretolditisanaturalpartofagingandthereis
nosimpleproceduretoremovethem.
Moreseverecasescanbetreatedwithvitrectomy.Removalofthevitreousbody
resolvesthefloaters,butbringswithitothercomplicationslikecataractand
possibleretinaldetachment.“Floaterectomy”isatermusedtodescribepartial
vitrectomytoresolvefloaters.
“FloaterectomyVersusConventionalParsPlanaVitrectomyForVitreousFloaters”
http://www.djo.harvard.edu/site.php?url=/physicians/oa/1004
SomedoctorsmaytryYAGlasertreatmenttovaporizethefibers,orenzymesto
breakthemup.
WhiteBloodCells:
Whitebloodcellsarelargeenoughtocompletelyfillthesmallcapillariesinthe
retina.Sincetheydisplacethesmallerredbloodcells,whichabsorblightandcasta
shadow,andsincetheyaremostlywater,theymakethebloodvesseltemporarily
clear.Inretinalimaging,whenawhitebloodcellmovesthroughweareabletosee
thephotoreceptorsbehindthecapillary.Visually,theymayappearassmallwhite
spotsmovinginthevisualfield.ThisiscalledScheerer’sphenomenon.Wikipedia
hasanicesimulationofhowtheyappearagainstabrightsky.
https://en.wikipedia.org/wiki/Blue_field_entoptic_phenomenon
https://upload.wikimedia.org/wikipedia/commons/7/70/Blue_field_entoptic_phen
omenon_animation.gif
Tounderstandwhytheyappearassmallwhitespots,watchthedemo
WBCsim.mp4.Fixatethecenterspotuntilthedarkbandfades,andthemovinggap
inthebandwillappearasawhitespot.Foramoreelaborateversionofthis,watch
thedemoTroxlerDemo3Hz.mp4.Spotsmovearoundacircle,appearingfirstas
gapsintheringthenasobjectsofoppositecolor.
PurkinjeTree:
ThePurkinjetreeisashadowofthesuperficialretinalvasculature.Itisnormally
notvisiblebecauseofadaptationandcentralinattention.Thereisalsoevidenceof
localamblyopiaforcellsshadowedbythebloodvessels.
AdamsDL,HortonJC."Shadowscastbyretinalbloodvesselsmappedinprimaryvisualcortex."
Science2002;298:572-6.
ThisfigurefromAdamsandHortonshowsthattherearefewercellsincortex
devotedtotherighteyeinlocationsthatareshadowedbybloodvessels.Dataare
fromasquirrelmonkey.
ThePurkinjetreebecomesvisiblewhenthelightsourcemoves.Thiscanbe
achievedbyviewingwithapinholepupil,whenthenaturalpupilislarge.Moving
thepinholeacrossthenaturalpinholemakestheshadowsofthevesselsmove
enoughthattheybecomevisible.
AbettermethodtovisualizethePurkinjetreeistoilluminatetheeyethroughthe
sclera.Placeapenlightagainsttheclosedlidnearthelimbusandmoveitinsmall
circles.Thiswillmaketheshadowsfallonreceptorsthatarenotusuallyshadowed,
andthetreebecomesmorevisible.Themorepinpointthelightsource,thesharper
theshadowswillbe,andthussmallerbloodvesselscanbeseen.