Entoptic_Phenomena_2

Phosphenes:
Phosphenesareperceivedflashesoflightthatarisebecauseofinternalactivityin
theretinaorhighervisualsystem.Phosphenescanbecausedbymechanicalaction
ontheretina,asoccursduringvitreousdetachment.
Moore’slightningstreaksareflashesoflightthatresemblelightningbolts.They
usuallyappearinthetemporalvisualfield,andarefairlycommoninmiddleageand
olderpatientsastheyexperiencePosteriorVitrealDetachment(PVD).Thevitreous
putstractionontheretinaandthemechanicalactioncausesthecellstodischarge.
FlickPhosphenesareproducedbyrapid,large,saccadicmovementsoftheeye.
Theyaremorecommonlynoticedbyolderpatients,suggestingthatthepartiallyor
fullydetachedvitreousmaybetuggingand/orbumpingontheretina.Theyare
mostnoticeablewhendarkadapted.
PressurePhosphenesareproducedbydirectmechanicalstimulationoftheeye.
Closeyoureye,turnyoureyetowardthenose,andpressonthetemporalside
throughyourlid.Youwillseeaspotappearinyournasalvisualfield.
Theplotontheleftshowsa
correlationbetweenthepressure
requiredtoproduceaphosphene
andtheintraocularpressure
measuredwithGoldmann
tonometry.Inthephoto,asimple
pressuresensorispushedagainst
theeyeuntilaphospheneisseen.
FromFresco,Ophthalmology,1998
ElectricalPhosphenesareproducedbyelectricalstimulationoftheretinaor
cortex.ThesearesometimesnoticedbysubjectsbeingfittedwithEOG
(EloctroOculoGram)electrodeswhentheimpedenceisbeingtested.Examinersrun
asmallamountofcurrentthroughtheelectrodestomakesurethecontactisgood,
andthiscurrentstimulatestheretina.
CorticalPhosphenesarelikewiseproducedbyelectricalstimulationofthecortex.
AnewtechniquecalledTranscranialMagneticStimulationisgaininginterestasa
toodforstudyingcorticalfunction,andthissometimesproducesflashesoflight.
Someneurologistshaveexperimentedwithimplantedelectrodearraysonthevisual
cortexinordertoproducephosphenesinblindpatients.Stimulationelectrode
arraysimplantedintheeyearecurrentlyunderdevelopmentaswell.Themost
advancedoftheseistheARGUSIIfromSecondSight
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,theimportantthingaboutPhosphenesisthattheyare
perceivedaslight,andcanbeusedtorecognizeforms.Thisisanexampleof
Müller’slawofspecificnerveenergies.Activationoftheopticnervefibersis
perceivedaslightbecausethosenervesareassociatedwithvision.
Migraineauras:
Migrainesufferersoftenexperienceasensoryphenomenonpriorto,orin
conjunctionwithasevereheadache.Thesephenomenacanaffectanyofthesenses,
butvisualmigraineaurasarefairlycommon.Theaurashavebeencalled
“fortificationillusions”becausetheyappearascriss-crossedlinesforminga“fence”
thatmovesacrossthevisualfield,leavingascotomainitswake.Theauraand
scotomaarehomonymous(inbotheyes)becausetheyareacorticalphenomenon.
Visionusuallyrecoversinawaveaswell,afewminutesbehindtheslowlydrifting
aura.Administrationofamigrainemedicine(aspirine+caffeine+acetaminophen)
attheonsetofauramaypreventtheheadache.
FromDr.Bedell,sourceunknown
Afterimages
Afterimagesaretheresultofadaptationtolightpatterns.Theyarelocalto
particularretinalareas,andsotheymovewiththeeye.Theyarenotusuallynoticed
becausetheeyeisconstantlymoving,changingthelocalstimulationandthus
preventingadaptation.Tryhttp://www.johnsadowski.com/big_spanish_castle.php
Fixatethecenterblackdotforaboutaminute,thenlookatthewhitespot.You
shouldseeadarkgridfloatingoverthewhitegrid.Themotionyouseeinthe
afterimageiscausedbyyourownfixationjitter.(fromDr.Bedell,sourceunknown).
Usingafterimagesto“tag”thefoveas,totestforAnomalousRetinalCorrespondence
Inthistest,eacheyeisflashedtoputanafterimageinthemiddleofthefovea.With
botheyesopen,mostpeoplewillthenseethetwoafterimagesinthesamedirection
(upperleftdiagram).PeoplewithAnomalousRetinalCorrespondence(ARC)donot
necessarilyseetheirtwofoveasasrepresentingthesamedirectionwhenbotheyes
areopen.Eventhoughthetargetsareonthefoveasofbotheyes,theydonot
perceivethemasoverlapping.
AbrightmovinglightleavesatrailbehinditthatcomesfromVisualPersistence
andLightAdaptation.Inthesketchbelow,abrightgreenspotleavesbehinda
shortgreenstreak(fromvisiblepersistence)andbehindthatisa
long
lastingafterimage(fromlightadapation).
Afterimageslastlongerifthebackgroundlightchanges,makingitmoreobviousthat
oneregionoftheretinaisadapteddifferentlyfromanother.Movetheeyebackand
forthfromabrightareatoadimarea,andtheafterimagewillkeepreappearing,
thenfading.
Maxwell’sspot
Themacularpigmentoverlyingthefoveaselectivelyabsorbsbluelight.Theretinal
areacoveredbymacularpigmentisroughlythesamesizeasthefovealpit.Light
comesfromthebottomandpassesthroughthispigmentbeforereachingthe
photoreceptors.Undertherightviewingconditions,weseeadarkspotcenteredon
thepointoffixation.Thespotishardtonoticebecauseitmoveswiththeeyeand
becauseweadapttothedifferenceincolor/brightness.
Wavelength (nm)
Leftimage:FromSnodderly1984.Crosssectionofamonkeyretinaintheregionof
thefovea.Theupperimagewasmadewithgreenlightillumination,thelowerimage
withbluelight.Thedarkappearanceinthelowerimagecomesfromthemacular
pigmentabsorbingthebluelight.
Rightimage:Pigmentabsorbancevs.wavelength,withdatafromvariousstudies.
Maxwell’sspotismosteasilyseenwithalternatingillumination.Trystaringatthe
greensquareforafewsecondstoadaptyourlongandmiddlecones,thenlookatthe
blue.Youshouldseeadarkspotintheareaaroundyourfixationpoint.Thespotis
usuallyalittlebiggerthanyourthumbatarmslength.
Maxwell’sspotissometimesusedtodiagnoseeccentricfixation.Ifapatientsees
thespotoffcenterfromthepointoffixation,itindicatestheyarenotusingthe
centralfoveaforfixation.
AsimilartechniqueusesHaidinger’sbrushes.Theserefertoawindmillpattern
thatappearswhenviewingpolarizedbluelight.Ifthewindmillisnotcenteredon
thefixationpoint,itindicateseccentricfixation.