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Psy280: PerceptionPsy280: Perception

Prof. AndersonProf. AndersonDepartment of PsychologyDepartment of Psychology

Week 4Week 4

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Lets enter the brain, shall we?Lets enter the brain, shall we?

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Visual maps: Multiple neuralVisual maps: Multiple neuralrepresentations of realityrepresentations of reality

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Neural convergence & divergence:Neural convergence & divergence:Complexity & parallel processingComplexity & parallel processing

Eye

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Eye to CNS�� Contralateral projectionsContralateral projections

�� RVFRVF——> left hemisphere> left hemisphere�� LVFLVF——>right hemisphere>right hemisphere

�� Multiple pathwaysMultiple pathways�� Retino-geniculate-striateRetino-geniculate-striate

pathwaypathway�� Conscious visionConscious vision

�� Retino-collicular-pulvinarRetino-collicular-pulvinarpathwaypathway�� Vision for actionVision for action�� Eye movementsEye movements�� UnconsciousUnconscious

�� Retino-SuprachiasmaticRetino-Suprachiasmaticnucleusnucleus

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Retino-geniculate-striate pathRetino-geniculate-striate path�� Vision for perceptionVision for perception

�� Conscious visionConscious vision

�� Cortical blindness: HemianopiaCortical blindness: Hemianopia�� Retina intact: Brain blindRetina intact: Brain blind

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Retino-collicular pathRetino-collicular path

�� ““BlindsightBlindsight””: Nonconscious sight: Nonconscious sight�� Visual cortical lesionVisual cortical lesion

�� Yet, above chance localizationYet, above chance localization�� Intact eye movementsIntact eye movements

�� Conclude:Conclude:�� Spared retino-collicular pathSpared retino-collicular path

�� Different Different ““kindkind”” of vision of vision

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Cortical blindness:Cortical blindness:Fear blindsightFear blindsight

XMorris et al., 2001

•Examine patient w/ cortical blindness

• Examine amygdala response to fear evoking events inthe absence of cortex/awareness

L R

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Intact/Seen Lesion/Blind

Cortical blindness:Cortical blindness:Fear blindsightFear blindsight

• Stimuli presented to intact and blind hemifield

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Intact Blind

Cortical blindness: FearCortical blindness: Fearblindsightblindsight

• Amygdala discrimination of fear in blind field

• Depends on retino-collicular pathway to amygdala

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RetinaRetina——>Suprachiasmatic>Suprachiasmaticnucleusnucleus

�� Other forms of nonconscious visionOther forms of nonconscious vision�� Non-rod, non-cone, melatonin basedNon-rod, non-cone, melatonin based

photoreceptorsphotoreceptors�� Regulation of circadian behaviorRegulation of circadian behavior

�� Humans lacking rods and conesHumans lacking rods and cones�� Sleep/wake cycles influence by lightSleep/wake cycles influence by light

�� Supported by connection between retina andSupported by connection between retina andbiological clock in the SCNbiological clock in the SCN

�� Conclude: Many types of Conclude: Many types of ““visionvision””

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Retino-geniculate pathway� Organization of LGN: Retinal origin

� Laminar structure

� Temporal/Nasal adjacent (Same VF)

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Retino-geniculate pathway:Parallel processing III

� Organization of LGN: Retinotopy� Adjacent regions in retina project

to adjacent regions of LGN

� 6 representations of retina in register

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Retino-geniculate pathway� LGN: Physiology

� Similar to retinal ganglion cells

� Center-surround

� LGN: Anatomy

� Not all retinal maps the same� Parvocellular (P-cells)

� Small cells

� Top 4 layers

� Largely Cone-based

� Magnocellular (M-cells)� Large cells

� Bottom 2 layers

� Largely rod-based

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Organization of visual cortices:Divide & Conquer!

�� BifurcationsBifurcations�� LGN LGN ——> V1> V1

�� 2 divisions2 divisions�� M & PM & P

�� V1 V1 ——> extrastriate> extrastriate�� Even greater divergenceEven greater divergence�� Maintain M & P originMaintain M & P origin

�� Differ in features (Parallel)Differ in features (Parallel)& complexity (Hierarchical)& complexity (Hierarchical)

Parvo

Magno

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M & P pathways:M & P pathways:Functional significanceFunctional significance

�� Do M & P pathways representDo M & P pathways representdifferent modes of perception?different modes of perception?

�� P-cells: colour and luminanceP-cells: colour and luminance�� Colour, form, textureColour, form, texture

�� M-cells: luminanceM-cells: luminance�� MotionMotion

�� Isoluminance studiesIsoluminance studies�� Robust motion when definedRobust motion when defined

by luminanceby luminance�� Decreased motion when defined byDecreased motion when defined by

color (isoluminant)color (isoluminant)

�� Lesions of M & PLesions of M & P

Motion

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Primary visual cortex:Striate cortex/V1/Area 17

� First cortical synapse in vision: Calcarine sulcus

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Striate cortex (V1): Retinotopy

�� 6 LGN maps 6 LGN maps ——>>1 striate map1 striate map

�� Up is downUp is down�� Right is leftRight is left�� Fovea is posteriorFovea is posterior

�� Map is distortedMap is distorted�� Cortical magnificationCortical magnification�� Like motor homunculusLike motor homunculus�� Fovea > peripheryFovea > periphery

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Neuropsychological evidence:Neuropsychological evidence:RetinotopyRetinotopy

�� Visual field deficits: ScotomasVisual field deficits: Scotomas�� Cortical blindnessCortical blindness

�� Hemianopia distinguishes between peripheral (retinal) andHemianopia distinguishes between peripheral (retinal) andcentral (cortical) blindnesscentral (cortical) blindness

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Striate cortex (V1):Eye and orientation selectivity

� Ocular dominance columns� Selective for each eye

� Orientation selectivity� Increase in complexity

relative to LGN

� “simple” cells

� Orientation detectors for

each region of retinal space

All from same portion of space

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10 min break!10 min break!

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How do we create orientationHow do we create orientationselectivity?selectivity?

�� Combine center surround cells to createCombine center surround cells to create““simplesimple”” cells cells

+-

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Does V1 have to representDoes V1 have to representevery orientation?every orientation?

�� NopeNope�� Distributed orDistributed or

coarse codingcoarse coding�� Orientation isOrientation is

coded bycoded bymagnitude ofmagnitude ofresponse acrossresponse acrossmultiple cellsmultiple cells

Orientation “tuning curve”

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Cortical magnification:Cortical magnification:The oblique effectThe oblique effect

�� Not just fovea versus peripheryNot just fovea versus periphery�� More cells/cortical space devoted to horizontalMore cells/cortical space devoted to horizontal

and vertical lines relative to obliqueand vertical lines relative to oblique

�� Physiology=Physiology=perceptionperception

Perception Brain signal: fMRI

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Orientation selectivity:Orientation selectivity:Nature or Nurture?Nature or Nurture?

�� Selective rearing during early developmentSelective rearing during early development�� Vertical or horizontal orientation blindnessVertical or horizontal orientation blindness

�� Reflects visual plasticity in V1Reflects visual plasticity in V1

Verticaldeprivation

Horizontaldeprivation

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Definitions: Visual angle andDefinitions: Visual angle andspatial frequencyspatial frequency

�� Angle of retinal projectionAngle of retinal projection�� Equal visual angle, same retinal sizeEqual visual angle, same retinal size

�� Spatial frequencySpatial frequency�� Number of light dark alternations (cycles) perNumber of light dark alternations (cycles) per

degree of visual angledegree of visual angle

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Spatial frequency channelsSpatial frequency channels

�� Visual system might decompose stimuli intoVisual system might decompose stimuli intodifferent scales (spatial frequencies)different scales (spatial frequencies)�� Small changes, big changes in fluctuation inSmall changes, big changes in fluctuation in

luminance over spaceluminance over space

Low f High f

Low High

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Fourier analysisFourier analysis

�� Can decompose eventsCan decompose eventsinto constituent spatialinto constituent spatialfrequency (SF)frequency (SF)componentscomponents

�� High SF captures edgesHigh SF captures edges

�� Descriptive or doesDescriptive or doesvisual system may usevisual system may usesimilar procedure ?similar procedure ?

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Simple cells and spatialSimple cells and spatialfrequencyfrequency

�� Tuning curves ofTuning curves ofsimple cellssimple cells

�� MaximallyMaximallyresponsive toresponsive todifferent SFdifferent SF

�� Suggests V1 doesSuggests V1 doesSF analysisSF analysis

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SF orientation coding andSF orientation coding andperceptionperception

�� Are these cells important for perception?Are these cells important for perception?�� Selective adaptationSelective adaptation

�� Exercise neurons until weak responseExercise neurons until weak response�� Like fatigued musclesLike fatigued muscles�� Examine contrast sensitivity at diff orientationsExamine contrast sensitivity at diff orientations

Dec

reas

e in

con

tras

tse

nsiti

vity

Orientation of grating40 20 0 20 40

test30°

Adapt0°

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Contrast sensitivity functionContrast sensitivity function

�� Adaptation results in selective fatigueAdaptation results in selective fatigue�� Reveals that CSF is the result of multiple SFReveals that CSF is the result of multiple SF

channelschannels

CSF

Con

tras

t sen

sitiv

ity

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SF adaptation and perceptionSF adaptation and perception�� Phenomenal and neural correlatesPhenomenal and neural correlates�� Perception determined by multiple SF channelsPerception determined by multiple SF channels�� Coarse coding across channelsCoarse coding across channels

�� Also: Perception differs for identical stimulusAlso: Perception differs for identical stimulus�� Perception parallels V1 response, not retinal imagePerception parallels V1 response, not retinal image

a

b

Neural response

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Beyond V1:Beyond V1:Higher order extra-striate corticesHigher order extra-striate cortices

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Higher order visual cortex:Higher order visual cortex:Extrastriate cortexExtrastriate cortex

�� CytoarchitectureCytoarchitecture�� Distinct from V1Distinct from V1

�� Functional correlatesFunctional correlates�� More complex features:More complex features:�� Motion, MT/V5Motion, MT/V5

�� Direction and speedDirection and speedSelectiveSelective

�� ColourColour�� DepthDepth�� FormForm

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EndEnd