Object recognition: Overview

„ Two visual pathways Psy393: Cognitive „ Ventral visual pathway: “What” „ Disorders: The Agnosias Neuroscience „ fMRI evidence „ Two types of object recognition Prof. Anderson „ Dorsal pathway: “Where” “action” Department of Psychology

Perception II: Recognition

Computational problems in Computational problems in object recognition object recognition „ What is it? „ Where is it? Where’s Waldo? „ Object constancy: Variability in sensory information „ Retinal position ? „ Viewing position „ Occlusion „ Lighting

Dissociation of what and Two visual cortical pathways where in the monkey „ These problems to be solved are reflected „ Landmark and object discrimination task in the organization of the visual system (Pohl, 1973) „ Ventral “What” pathway „ Inferior longitudinal fasciculus „ Parietal lobe „ Dorsal “Where” pathway „ Where „ Superior longitudinal „ Temporal lobe fasciculus „ What

1 “What” pathway Ventral “What” pathway characteristics characteristics „ Anterior regions have large receptive fields „ Complex response profile „ Looking through a small „ Dissimilar to V1 small or big window „ Not simple orientation „ All include fovea „ Selectivity „ High definition „ Hands, „ Allows positional invariance faces etc large

Dorsal “where” pathway Neuroimaging evidence for characteristics “what” and “where:

„ Have large receptive fields „ Attend to change in objects or locations „ Minority (40%) foveal „ Objects Same objects Different location „ Majority are extrafoveal: Periphery „ Occipito-temporal „ Rods—> magnocellular—>dorsal pathway „ Locations „ Posterior parietal

Disorders of the ventral visual Apperceptive agnosia pathway „ Agnosia: “without knowledge” „ Visual agnosia: vision w/out knowledge „ What its not „ Modality specific: Restricted to vision „ Not a memory disorder „ Not cortical blindness „ Item can be recognized „ Intact visual field through other modalities „ Not a basic deficit in processing visual „ Touch, sound, smell information „ Lissauer (1890’s) division „ Sensation is largely intact „ Apperceptive „ Associative „ Brightness, orientation, color, motion intact „ Category specific agnosia „ Prosopagnosia

2 Apperceptive agnosia: Apperceptive agnosia Behaviour „ Varying degrees of perceptual problems „ What it is „ Depends on lesion extent „ Difficulty in forming a “percept” (a mental „ Deficit in copying form impression of something based on the senses) „ Can’t perceive higher-order visual structure

„ Visual information can’t be bound together „ Can’t integrate parts into whole „ No coherent percept

Evidence for constancy: Apperceptive agnosia: Lateral occipital complex Behaviour (LOC) „ Higher levels of damage „ Likely locus of object constancy „ Problem in object constancy „ Reduction in fMRI response w/ repetition „ „ Retinal projection Invariance „ Size, location,viewpoint, illumination, occlusion „ Lighting No effect of occlusion „ Occlusion

Associative agnosia: Associative agnosia Behaviour „ Can copy complex objects „ What its not „ Can perform perceptual grouping „ Can form reasonable “percepts” „ What it is „ Perceptual grouping intact „ Failure of object recognition „ Difficulty in accessing semantic representations from vision „ “psychic blindness”

3 Associative agnosia: Localization: Gradations in Behaviour impairment „ Can’t draw objects from memory „ Apperceptive „ Can’t name objects „ Posterior „ Not anomic „ Associative „ Anterior „ Can’t match by function

„ Match by visual similarity Anterior to posterior lesion loci

„ Evidence for hierarchical analysis „ Adjacent areas of cortex likely damaged „ Varying degrees of perceptual/gnostic problems „ Largely a problem in linking percepts with „ Always some degree of perceptual deficit „ Gradations rather than categorical differences semantics

Category specific agnosia: Is there a region of the brain Prosopagnosia devoted to faces?

„ Largely specific to faces „ Fusiform face area (FFA) „ Can distinguish between faces and objects „ Right middle fusiform gyrus especially „ Difficulty in distinguishing between faces responsive to faces relative to other „ Facial identification objects Across category Within category

FFA

Face identification and Are faces special? configural processing

„ Why have an FFA? „ Face recognition depends on relationship „ Faces are a special object class shaped by between distinct features (nose, eyes, etc) evolutionary pressures „ What happens when relationships are „ Specialized module for their recognition 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

4 Face identification and What does the face inversion configural processing effect tell us?

„ Face recognition depends on relationship „ Face inversion effect „ Difficulty remembering/perceiving inverted relative to between distinct features (nose, eyes, etc) upright faces „ What happens when relationships are „ When upright: configural processing of subtle disrupted? relations between features „ When inverted: local processing of features „ Don’t notice configural violations „ Prosopagnosics perform equivalently to controls on inverted faces „ Impaired configural/holistic processing „ Intact analytic/local processing „ A deficit in configural rather than face processing?

Two systems for object Configural processing recognition

„ Are prosopagnosics impaired at configural „ Two types/qualities of object vision? processing, not just face processing? „ Dissociation and association amongst agnosic „ FFA and configural encoding syndromes „ “Greebles” „ Agnosia: general object recognition „ Train to recognize „ Alexia: specialized for word perception/reading individuals „ Prosopagnosia: specialized for face perception „ Evidence of configural processing „ “Greeble inversion effect” Independent Independent „ Experts but not novices Shared activate FFA „ Potentially not face specific

Two systems for object Two systems for object recognition recognition „ Analytic „ Prosopagnosia and alexia are dissociable „ Analysis by parts „ But, rarely occur in isolation „ Can apply to faces „ Associated with object agnosia, but not always „ Configural „ When both present „ Holistic analysis „ Not a single case w/out object agnosia „ Can apply to objects „ Share common process needed for object recognition

Alexia Prosopagnosia Object agnosia

5 Disorders of the dorsal pathway: Action Tactile agnosia „ Object recognition outside of the ventral visual „ Double dissociation stream „ Agnosia vs. optic ataxia „ 2 types „ Apperceptive Agnosia „ “apperceptive” „ Impaired perception „ Intact sensory discrimination „ Intact action „ Can’t form tactile percepts „ Appropriate reaching „ “associative” (tactile asymbolia) „ grasping „ Can form percepts „ Optic ataxia „ Texture, temperature weight, shape „ Intact perception „ Can’t retrieve meaning „ Impaired action „ Can be hand specific „ Eye „ Locus of damage „ Inappropriate saccades „ Somatosensory association „ Hand „ Impaired reaching/grasping cortex

End of lecture

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