1 psy280: perception prof. anderson department of psychology week 5
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Psy280: Psy280: PerceptionPerception
Prof. AndersonProf. Anderson
Department of PsychologyDepartment of Psychology
Week 5Week 5
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What’s the meaning of life? Not What’s the meaning of life? Not exactlyexactly
Computational approach to visionComputational approach to vision David Marr (MIT)David Marr (MIT)
What is the purpose of vision?What is the purpose of vision? What are the problems it must solve?What are the problems it must solve?
Visual philosophersVisual philosophers
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Computational problems Computational problems in object recognitionin object recognition
What is it?What is it? Object constancy: Variability in sensory Object constancy: Variability in sensory
informationinformation Retinal positionRetinal position Viewing positionViewing position OcclusionOcclusion Lighting, colourLighting, colour
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Computational problems Computational problems in object recognitionin object recognition
Where is it? Where’s Waldo?Where is it? Where’s Waldo?
?
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Two visual cortical Two visual cortical pathwayspathways
These problems are reflected in the These problems are reflected in the organization of the visual systemorganization of the visual system
Ventral “What” pathwayVentral “What” pathway Inferior longitudinal fasciculusInferior longitudinal fasciculus
Dorsal “Where” pathwayDorsal “Where” pathway Superior longitudinal Superior longitudinal
fasciculusfasciculus
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Dissociation of what and Dissociation of what and where in the monkeywhere in the monkey
Landmark and object discrimination Landmark and object discrimination task (Pohl, 1973)task (Pohl, 1973)
Parietal lobeParietal lobe WhereWhere
Temporal lobeTemporal lobe WhatWhat
Landmark Object discrimination
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Neuroimaging evidence Neuroimaging evidence for “what” and “where:for “what” and “where:
Attend to change in objects or Attend to change in objects or locationslocations
ObjectsObjects Occipito-temporal Occipito-temporal
LocationsLocations Posterior parietalPosterior parietal
Same objectsDifferent location
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Ventral “what” pathwayVentral “what” pathway
V4: Isoluminant colorV4: Isoluminant color Lingual, fusiform gyrusLingual, fusiform gyrus
MT (V5): MotionMT (V5): Motion Middle temporal gyrusMiddle temporal gyrus
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Neuropsychological Neuropsychological evidence:evidence:
Color (V4) and Motion Color (V4) and Motion (MT)(MT) Fractionation of perception following Fractionation of perception following
cortical lesionscortical lesions AchromatopsiaAchromatopsia
Loss of colour visionLoss of colour vision AkinetopsiaAkinetopsia
Loss of motionLoss of motion
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Higher-order “what” Higher-order “what” pathway characteristicspathway characteristics
Complex response profileComplex response profile Dissimilar to V1Dissimilar to V1 Not simple orientation, colour, motionNot simple orientation, colour, motion SelectivitySelectivity
Hands, Hands,
faces etcfaces etc
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Disorders of higher-order Disorders of higher-order ventral visual pathwayventral visual pathway
Agnosia: “without knowledge”Agnosia: “without knowledge” Visual agnosia: vision w/out Visual agnosia: vision w/out
knowledgeknowledge Modality specific: Restricted to visionModality specific: Restricted to vision Not a memory disorderNot a memory disorder
Item can be recognized Item can be recognized through other modalitiesthrough other modalities Touch, sound, smellTouch, sound, smell
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Higher-order cortex is Higher-order cortex is highly specialized: highly specialized:
ProsopagnosiaProsopagnosia Largely specific to facesLargely specific to faces Can distinguish between faces and Can distinguish between faces and
objectsobjects Difficulty in distinguishing between Difficulty in distinguishing between
facesfaces Facial identificationFacial identification
Within categoryAcross category
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Is there a region of the Is there a region of the brain devoted to faces? brain devoted to faces?
Fusiform face area (FFA)Fusiform face area (FFA) Right middle fusiform gyrus Right middle fusiform gyrus
especially responsive to faces especially responsive to faces relative to other objectsrelative to other objects
FFA
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Neural selectivity: Neural selectivity: Evolution vs experienceEvolution vs experience
FFA and other objectsFFA and other objects ““Greebles”Greebles” Train to recognize Train to recognize individualsindividuals Experts but not novices Experts but not novices activate FFAactivate FFA
Potentially not face Potentially not face specificspecific
Reflects both evolution Reflects both evolution and experience?and experience?
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Object recognition: Object recognition: InvarianceInvariance
Recognize object Recognize object despite differences in:despite differences in: Size, orientation, Size, orientation,
viewpoint, lighting, viewpoint, lighting, colour, locationcolour, location
Inferotemporal cortex Inferotemporal cortex (IT)(IT) Anterior ventral streamAnterior ventral stream Invariance in neural Invariance in neural
responseresponse
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Evidence for constancy: Evidence for constancy: Lateral occipital complex Lateral occipital complex
(LOC)(LOC) Likely locus of object constancyLikely locus of object constancy Reduction in fMRI response w/ repetitionReduction in fMRI response w/ repetition InvarianceInvariance
Size, location,viewpoint, illumination, Size, location,viewpoint, illumination, occlusionocclusion No effect of occlusion
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How does cortex How does cortex represent all objects?represent all objects?
Specificity codingSpecificity coding Grandmother cellsGrandmother cells Every orientationEvery orientation Every colorEvery color
Distributed codingDistributed coding ““Coarse” coding across Coarse” coding across
neuronsneurons CombinatorialCombinatorial
ColorColor 3 cone types3 cone types
FormForm Shape columns in ITShape columns in IT GeonsGeons
Geons
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Ventral stream neurons Ventral stream neurons and consciousnessand consciousness
Binocular rivalryBinocular rivalry
fMRI evidence: FFA fMRI evidence: FFA and awarenessand awareness FFA turns on when FFA turns on when
aware of facesaware of faces FFA turns off when FFA turns off when
unaware of facesunaware of faces
Ventral stream Ventral stream representations representations support support consciousnessconsciousness
FFA PPA
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Dorsal pathway: ActionDorsal pathway: Action Double dissociationDouble dissociation
Agnosia vs. optic ataxiaAgnosia vs. optic ataxia Apperceptive AgnosiaApperceptive Agnosia
Ventral stream damageVentral stream damage Impaired perceptionImpaired perception Intact actionIntact action
Appropriate reachingAppropriate reaching graspinggrasping
Optic ataxiaOptic ataxia Dorsal stream damageDorsal stream damage Intact perceptionIntact perception Impaired actionImpaired action
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How does it all come How does it all come together? The binding together? The binding
problemproblem Division of labour: Division of labour: Parallel processingParallel processing Colour, shape, motion, Colour, shape, motion,
depth, locationdepth, location All in separate regionsAll in separate regions
How bound together?How bound together? Unified perceptionUnified perception Not separate featuresNot separate features
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Integration (binding) Integration (binding) across feature mapsacross feature maps
Synthesis requires attention—allows Synthesis requires attention—allows coherence across feature maps: coherence across feature maps: ObjectsObjects
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Attention is mental glueAttention is mental glue
Allows features to Allows features to stick togetherstick together
Without which, Without which, perception falls perception falls apartapart
No coherent No coherent perception of worldperception of world
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What is attention?What is attention? Everybody knows what attention is. It is Everybody knows what attention is. It is
taking possession by the mindtaking possession by the mind, in clear and , in clear and vivid form, of vivid form, of one out of what seem several one out of what seem several simultaneously possible objectssimultaneously possible objects or trains of or trains of thought. Focalization, concentration of thought. Focalization, concentration of consciousness are of its essence. consciousness are of its essence. It implies It implies withdrawal from somethings in order to withdrawal from somethings in order to deal effectively with othersdeal effectively with others, and is a , and is a condition which condition which has a real opposite in the has a real opposite in the confused, dazed scatterbrain stateconfused, dazed scatterbrain state …. ….
William James (1890)William James (1890)
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What is attention?What is attention? ““taking possession of the mind”taking possession of the mind”
Control of the focus of attentionControl of the focus of attention ““one out of what seem several simultaneously one out of what seem several simultaneously
possible objects”possible objects” Inability attending to multiple things at onceInability attending to multiple things at once
““It implies withdrawal from somethings in It implies withdrawal from somethings in order to deal effectively with othersorder to deal effectively with others”” ““Paying” attention comes with a costPaying” attention comes with a cost It has limited capacities that must be sharedIt has limited capacities that must be shared
““has a real opposite in the confused, dazed has a real opposite in the confused, dazed scatterbrain state”scatterbrain state” Attention is the glue that keeps perception togetherAttention is the glue that keeps perception together
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Pay attention!Pay attention!
QuickTime™ and aCinepak decompressor
are needed to see this picture.
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Visual experience: A Visual experience: A grand perceptual illusiongrand perceptual illusion Rich and complex?Rich and complex? Sorry folks, its an illusionSorry folks, its an illusion We don’t “see” as much as we believeWe don’t “see” as much as we believe
Don’t notice big changes in our environmentDon’t notice big changes in our environment Change blindnessChange blindness
We fill-in our experience to make it coherentWe fill-in our experience to make it coherent Little persistence of experience from one Little persistence of experience from one
moment to the nextmoment to the next Seems coherentSeems coherent
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Selective attentionSelective attention DefinitionDefinition
Process relevant and ignore irrelevantProcess relevant and ignore irrelevant Why do we need selective Why do we need selective
attention?attention? Can’t remember/processes everythingCan’t remember/processes everything
Can be independent from eye Can be independent from eye movementsmovements Helmholtz (1894)Helmholtz (1894)
Fixate eyesFixate eyes Brief flashBrief flash Can pick what to perceiveCan pick what to perceive
Perception is not fixedPerception is not fixed Not just what your eyes doNot just what your eyes do Perceptual free willPerceptual free will
Can choose what to perceiveCan choose what to perceive
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Attention: Feature Integration Attention: Feature Integration TheoryTheory
Attention needed to glue Attention needed to glue features togetherfeatures together
Feature vs. conjunction Feature vs. conjunction visual search visual search (Treisman)(Treisman) Parallel (“Pop out”)Parallel (“Pop out”)
E.g., Color, orientationE.g., Color, orientation Requires little attention Requires little attention
Serial Serial Color and orientationColor and orientation Requires focal attentionRequires focal attention Need to move attention aroundNeed to move attention around
Parallel Cereal
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Attention is required for Attention is required for binding across feature binding across feature
mapsmaps Need attentional Need attentional beambeam
Without attention?Without attention? Illusory conjunctionsIllusory conjunctions Unbound featuresUnbound features
RecombinedRecombined Illusory perceptionIllusory perception
Need attention to glue Need attention to glue features togetherfeatures together
Otherwise fall apartOtherwise fall apart
Report seeing Yellow Square and purple triangle
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Perceptual primitives:Perceptual primitives:What makes a “feature”?What makes a “feature”?
Perceptual primitivesPerceptual primitives Building blocks of perceptionBuilding blocks of perception Cortical feature mapsCortical feature maps
Luminance, orientation, color,Luminance, orientation, color,
Motion, depthMotion, depth
Higher-order objectsHigher-order objects Synthesis of primitivesSynthesis of primitives Objects defined by Objects defined by
conjunctions of primitivesconjunctions of primitives Need attention to bond into Need attention to bond into
compoundscompounds
Share primitives
Unique primitives
Molecules of perception?
Pop-out No pop-out
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Constructivism vs Gestalt Constructivism vs Gestalt approaches to perceptionapproaches to perception StructuralismStructuralism
Perception is created by adding elements togetherPerception is created by adding elements together Perception from the bottom upPerception from the bottom up Like building a house Like building a house
Gestalt approachesGestalt approaches Top-down perceptual organizationTop-down perceptual organization Perceptual inferences: Best GuessesPerceptual inferences: Best Guesses ““Different than sum of its parts”Different than sum of its parts” Not like building a houseNot like building a house
Don’t want best guess, want correct answerDon’t want best guess, want correct answer Don’t want illusory foundation or beams!Don’t want illusory foundation or beams!
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Constructivism vs Gestalt Constructivism vs Gestalt approaches to perceptionapproaches to perception Perception is Perception is
much more than much more than what is projected what is projected on retinaon retina
StructuralismStructuralism inputs determine inputs determine
perceptionperception GestaltGestalt
Active role of Active role of perceiverperceiver
Same input, many different kinds of perceptionSame input, many different kinds of perception
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Gestalt principles of Gestalt principles of perceptual organizationperceptual organization
Best guesses about Best guesses about worldworld HeuristicsHeuristics ““Rules of thumb”Rules of thumb”
Visual system is problem Visual system is problem solving for ussolving for us Visual intelligenceVisual intelligence
Takes into account Takes into account probability of occurrenceprobability of occurrence
What is likely vs notWhat is likely vs not
Likely
Unlikely
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Some of the Some of the principles/heuristicsprinciples/heuristics
SimilaritySimilarity Good figure/simplicityGood figure/simplicity
Good continuationGood continuationAll have multiple interpretations but we all tend to agree on one
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Visual “guessing”: Visual “guessing”: Shape from shadingShape from shading
Sensitivity to perceptual invarianceSensitivity to perceptual invariance Light from above, not from belowLight from above, not from below
Square recessed
Square raised
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Figure-ground Figure-ground segregationsegregation
What’s an object?What’s an object? What’s What’s
background?background?
Mental imposition Mental imposition of depth of depth OcclusionOcclusion Infer continuation of Infer continuation of
background background underneath objectunderneath object
Best guessBest guess Unlikely share same Unlikely share same
contourscontours
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Figure-ground Figure-ground segregationsegregation
Factors that influence itFactors that influence it SymmetrySymmetry
DevelopmentDevelopment BeautyBeauty
MeaningMeaning See arrows?See arrows?
EmotionEmotion Perceptual autismPerceptual autism Pair faces w/ shockPair faces w/ shock Bias perception toward Bias perception toward
vasevase
Black White
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Chicken or the egg?Chicken or the egg? Meaning processed before or after figure-Meaning processed before or after figure-
ground segregation?ground segregation?
Top-down perceptual organization over-Top-down perceptual organization over-rides initial processing rides initial processing Only aware of the final products of this processOnly aware of the final products of this process
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Substantial feedback Substantial feedback onto early visual onto early visual
processingprocessing Higher-order cortex Higher-order cortex influences lower-influences lower-order cortexorder cortex
More feedback than More feedback than feedforward feedforward connectionsconnections
Has delayed influenceHas delayed influence Neural processing is Neural processing is
dynamicdynamic Need to revise classic Need to revise classic
definition of receptive definition of receptive fieldfield
Greater # of feedback connections applies to early cortex as well
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Meaning and perceptual Meaning and perceptual primitivesprimitives
Common fateCommon fate Can perceive objects Can perceive objects
based on motionbased on motion
Higher-order Higher-order meaning influences meaning influences lower-order lower-order perceptual primitivesperceptual primitives E.g. motion E.g. motion
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Meaning influences Meaning influences perceptual primitivesperceptual primitives
Structure from motionStructure from motion Can perceive objects Can perceive objects
based on motionbased on motion
Higher-order meaning Higher-order meaning influences lower-order influences lower-order perceptual primitivesperceptual primitives E.g. motion E.g. motion
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