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Object recognition: Overview Psy393: Cognitive Neuroscience Two visual pathways Ventral visual pathway: “What” Disorders: The Agnosias fMRI evidence Two types of object recognition Prof. Anderson Department of Psychology 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? ? Retinal position Viewing position Occlusion Lighting Dissociation of what and where in the monkey Two visual cortical pathways These problems to be solved are reflected in the organization of the visual system Ventral “What” pathway Inferior longitudinal fasciculus Dorsal “Where” pathway Superior longitudinal fasciculus Landmark and object discrimination task (Pohl, 1973) Parietal lobe Where Temporal lobe What 1 “What” pathway characteristics Ventral “What” pathway characteristics Anterior regions have large receptive fields Looking through a small or big window All include fovea Complex response profile Dissimilar to V1 Not simple orientation Selectivity small 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 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 Not a basic deficit in processing visual information Lissauer (1890’s) division Apperceptive Associative Different location Intact visual field Sensation is largely intact Brightness, orientation, color, motion intact Category specific agnosia Prosopagnosia 2 Apperceptive agnosia: Behaviour Apperceptive agnosia Difficulty in forming a “percept” (a mental impression of something based on the senses) Visual information can’t be bound together Higher levels of damage Problem in object constancy Retinal projection Lighting Occlusion 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 Can copy complex objects 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 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 What it is 3 Associative agnosia: Behaviour Can’t draw objects from memory Can’t name objects Not anomic Localization: Gradations in impairment Apperceptive Associative Can’t match by function Match by visual similarity Evidence for hierarchical analysis Adjacent areas of cortex likely damaged Largely a problem in linking percepts with semantics Largely specific to faces Can distinguish between faces and objects Difficulty in distinguishing between faces 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 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? Why have an FFA? Faces are a special object class shaped by evolutionary pressures Specialized module for their recognition 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 4 Face identification and configural processing 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 When upright: configural processing of subtle relations between features When inverted: local processing of features “Greebles” Train to recognize individuals Evidence of configural processing Two types/qualities of object vision? Dissociation and association amongst agnosic syndromes “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 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 Potentially not face specific Two systems for object recognition Prosopagnosia and alexia are dissociable 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 Analytic Analysis by parts Can apply to faces Configural Holistic analysis Can apply to objects Prosopagnosia Object agnosia 5 Disorders of the dorsal pathway: Action Double dissociation Apperceptive Agnosia Agnosia vs. optic ataxia Object recognition outside of the ventral visual stream 2 types Impaired perception Intact action Tactile agnosia Appropriate reaching grasping Optic ataxia Eye Hand Intact sensory discrimination Can’t form tactile percepts “associative” (tactile asymbolia) Can form percepts Can’t retrieve meaning Intact perception Impaired action “apperceptive” Inappropriate saccades Impaired reaching/grasping Texture, temperature weight, shape Can be hand specific Locus of damage Somatosensory association cortex End of lecture 6
