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Chapter 8

PSYC 303 Chapter Notes - Chapter 8: Photometer


Department
Psychology
Course Code
PSYC 303
Professor
Thomas Spalek
Chapter
8

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Photometer:measureslightintensitylevels
● 241distinctlevels
Colorimeter:measureswavelengthsoflight
● 672
Lightintensityandwavelengthscombinetocreate8184differentdiscriminablecolors.
ContourPerception
Contouristhefundamentalbuildingblockforanyvisualpattern.Therearefourfundamental
characteristicsoflightthatareimportanttovision:intensity,wavelength,andthedistribution
oflightoverspaceandtime.
Contours:“edges”;suddenchangesinlightintensityacrossspace
Luminance/Firstordercontour:regionintheretinalimagewherethelightintensity(or
wavelengthcomposition)changesabruptly
● eg.edgeofblackboard,moonagainstdarksky,silhouetteofaperson
● blacklineonwhitepaperhas2contours:whitetoblackandblacktowhite
Shape:regionsofrelativelyuniformbrightness
Emergent/subjective/secondordercontour:constructedbytheperceptualsystemfrom
thefirstordercontoursthatarepresent
canoccurinmanyways
Ganzfeld:Germanfor“wholefield”;avisualfieldthatcontainsnoabruptluminancechanges
andthusnocontours
Blankout:afeelingthatanobserver“can’tsee”afterprolongedviewing,butnotattributed
toanylossintheabilitytodetectchangesinilluminanceiftheydooccur
Snowblindness:akindofnaturalblankout,causedbythelackofcontourintheretinal
imagewhenthevisualfieldcontainsalotofsnowandice
● affectsaround3%ofclimbersonsnowcoveredmountains
ContourInteractions
Thereisaconceptualtrapaboutneuralfiltersthataperson’sthinkingcouldfallinto:our
perceptionofcontoursarefairlypassive,andthatneuralcircuitryassociatedwithcentral
surroundandorientationspecificcellsinthevisualsystemexaggerateanycontours.
● Exaggerationhappensbytheexcitatoryandinhibitoryresponsesintheregionofthe
visualfield
Hermanngrid:gridofblacksquares;whenjustpassivelylooking,grayspotsappearatthe
whiteintersectionsbetweensquaresbutdisappearwhentrytolookatthem

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● attheganglionlevel,whenareceptivefieldthatiscenterexcitatoryalignswithan
intersection,itwillhavealargernetamountofinhibitioninitssurround
● thatis,theganglioncellswillregisterlesslightattheintersectionsthatatthevertical
andhorizontalregions
● donotoccurwheninfocusbecauseganglioncellsaresmallestatthefoveaandget
largerfurtheraway
Crowding:theinabilitytorecognizeobjectsinacluster
● theexcitatoryandinhibitoryinteractionsthatenhanceacontouralsomakesitmore
difficulttoperceivethemundercertainconditions
● mostlikelytohappenwhenthespacingofthecontourinterfereswithothersnearby
Amblyopiaexanopsia:associatedwithdiminishedvisualacuity,ofteninoneeye,but
usuallywithoutanyobviousvisualpathology
● resultfromafailureoftheneuralcontourdetectionandenhancementmechanismsat
theleveloftheorganizationofsimplereceptivefield
MaskingandMetacontrast
Spatialsummation:awayofachievinganactionpotentialinaneuronwithinputfrom
multiplepresynapticcells.
● closelyneighbouringneurons/stimuli
Lateralinhibition:thecapacityofanexcitedneurontoreducetheactivityofitsneighbors.
● slightlymoredistantneurons/stimuli
Visualmasking:reductioninthevisibilityofacontourortargetthatiscausedby
presentationofasecondstimulusthatisclosetothetargetinspaceand/ortime
Simultaneousmasking:crowding;thetargetstimuliandthosethatflankatpresentatthe
sametime
Severalwaystoseparatethetargetstimulusintimefromtheonethatissupposetointeract
with:
Patternmasking:firstshowthetargetforashortperiodoftime,thenimmediately
followitwithamaskingstimulusthatconsistsofsomearrayofcontourthatoverlap
thesamepositioninspace
● Metacontrast:showthetarget,thenfollowedbyamaskingstimulusthatcontains
closelyadjacent,butnonoverlappingcontours
Interstimulusinterval/stimulusonsetasynchrony:thetimebetweenthepresentationof
thetargetandthepresentationofthemask
Backwardmasking:involvesaffectingthevisibilityofatargetthatpresentedearlier
● strongest50100msintervenesbetweentargetandmask
Ifamaskinghappenstooearlyortoolate,theamountofmaskingisreduced.

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