Psy393: Cognitive Neuroscience - Department of Psychology [PDF]

Object recognition: Overview

Psy393: Cognitive Neuroscience

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Two visual pathways „

Ventral visual pathway: “What” Disorders: The Agnosias fMRI evidence „ Two types of object recognition „ „

Prof. Anderson Department of Psychology

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Dorsal pathway: “Where” “action”

Perception II: Recognition

Computational problems in object recognition „ „

What is it? Object constancy: Variability in sensory information „ „ „ „

Computational problems in object recognition „

Where is it? Where’s Waldo?

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Retinal position Viewing position Occlusion Lighting

Dissociation of what and where in the monkey

Two visual cortical pathways „

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These problems to be solved are reflected in the organization of the visual system Ventral “What” pathway „

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Inferior longitudinal fasciculus

Dorsal “Where” pathway Superior longitudinal fasciculus „

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Landmark and object discrimination task (Pohl, 1973)

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Parietal lobe „

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Where

Temporal lobe „

What

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“What” pathway characteristics „

Ventral “What” pathway characteristics

Anterior regions have large receptive fields Looking through a small or big window „ All include fovea

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Complex response profile Dissimilar to V1 Not simple orientation „ Selectivity

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small

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High definition

„ Hands, faces etc

Allows positional invariance large

Dorsal “where” pathway characteristics „

Have large receptive fields Minority (40%) foveal „ Majority are extrafoveal: Periphery „ Rods—> magnocellular—>dorsal pathway „

Neuroimaging evidence for “what” and “where: „ „

Attend to change in objects or locations Same objects Objects „

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Occipito-temporal

Locations „

Posterior parietal

Disorders of the ventral visual pathway „ „

Agnosia: “without knowledge” Visual agnosia: vision w/out knowledge „ „

Modality specific: Restricted to vision Not a memory disorder Item can be recognized through other modalities „ Touch, sound, smell

Apperceptive agnosia „

What its not „

Not cortical blindness

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Not a basic deficit in processing visual information

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Lissauer (1890’s) division „ „

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Apperceptive Associative

Different location

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Intact visual field

Sensation is largely intact Brightness, orientation, color, motion intact

Category specific agnosia „

Prosopagnosia

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Apperceptive agnosia: Behaviour

Apperceptive agnosia „

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Difficulty in forming a “percept” (a mental impression of something based on the senses)

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Visual information can’t be bound together „

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Higher levels of damage Problem in object constancy Retinal projection Lighting „ Occlusion „ „

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What its not „

Evidence for constancy: Lateral occipital complex (LOC) „ Likely locus of object constancy „ „

Reduction in fMRI response w/ repetition Invariance „

Associative agnosia: Behaviour

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Can copy complex objects

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Perceptual grouping intact

Can form reasonable “percepts”

What it is Failure of object recognition „ Difficulty in accessing semantic representations from vision „ “psychic blindness” „

Size, location,viewpoint, illumination, occlusion No effect of occlusion

Can perform perceptual grouping „

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Depends on lesion extent

Deficit in copying form „ Can’t perceive higher-order visual structure „ Can’t integrate parts into whole „

Associative agnosia „

Varying degrees of perceptual problems

No coherent percept

Apperceptive agnosia: Behaviour „

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What it is

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Associative agnosia: Behaviour „ „

Can’t draw objects from memory Can’t name objects „

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Not anomic

Localization: Gradations in impairment „

Apperceptive

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Associative

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Can’t match by function „ Match by visual similarity

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Evidence for hierarchical analysis Adjacent areas of cortex likely damaged „

Largely a problem in linking percepts with semantics

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Largely specific to faces Can distinguish between faces and objects Difficulty in distinguishing between faces „

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Always some degree of perceptual deficit Gradations rather than categorical differences

Is there a region of the brain devoted to faces? „ „

Facial identification Across category

Varying degrees of perceptual/gnostic problems „

Category specific agnosia: Prosopagnosia „

Anterior Anterior to posterior lesion loci

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Posterior

Fusiform face area (FFA) Right middle fusiform gyrus especially responsive to faces relative to other objects

Within category

FFA

Face identification and configural processing

Are faces special? „ „ „

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Why have an FFA? Faces are a special object class shaped by evolutionary pressures Specialized module for their recognition

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Face recognition depends on relationship between distinct features (nose, eyes, etc) What happens when relationships are disrupted? Face inversion effect

Or within-category (subordinate level) discrimination? „ „

Depends upon special quality of object processing Can extend to other objects that require this type of special processing

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Face identification and configural processing „

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Face recognition depends on relationship between distinct features (nose, eyes, etc) What happens when relationships are disrupted?

What does the face inversion effect tell us? „

Face inversion effect „

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When upright: configural processing of subtle relations between features When inverted: local processing of features „

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“Greebles” Train to recognize individuals „ Evidence of configural processing „

Two types/qualities of object vision? Dissociation and association amongst agnosic syndromes „

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“Greeble inversion effect”

Impaired configural/holistic processing Intact analytic/local processing A deficit in configural rather than face processing?

Two systems for object recognition

Configural processing Are prosopagnosics impaired at configural processing, not just face processing? FFA and configural encoding

Don’t notice configural violations

Prosopagnosics perform equivalently to controls on inverted faces „

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Difficulty remembering/perceiving inverted relative to upright faces

Agnosia: general object recognition Alexia: specialized for word perception/reading Prosopagnosia: specialized for face perception

Independent

Experts but not novices activate FFA „

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Potentially not face specific

Two systems for object recognition „

Prosopagnosia and alexia are dissociable „ „

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Independent Shared

But, rarely occur in isolation Associated with object agnosia, but not always

When both present „ „

Not a single case w/out object agnosia Share common process needed for object recognition

Alexia

Two systems for object recognition

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Analytic „ „

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Analysis by parts Can apply to faces

Configural „ „

Holistic analysis Can apply to objects

Prosopagnosia Object agnosia

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Disorders of the dorsal pathway: Action „

Double dissociation

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Apperceptive Agnosia

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Agnosia vs. optic ataxia

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Object recognition outside of the ventral visual stream 2 types „

Impaired perception Intact action „

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Tactile agnosia „

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Appropriate reaching grasping

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Optic ataxia „ „

Eye

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Hand

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Intact sensory discrimination Can’t form tactile percepts

“associative” (tactile asymbolia) „

Can form percepts

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Can’t retrieve meaning

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Intact perception Impaired action „

“apperceptive”

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„ Inappropriate saccades Impaired reaching/grasping

Texture, temperature weight, shape

Can be hand specific

Locus of damage „ Somatosensory association cortex

End of lecture

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