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 6.