Psy393: Cognitive Neuroscience

Transcription

1 Object recognition: Overview Psy393: Cognitive Neuroscience Prof. Anderson Department of Psychology Two visual pathways Ventral visual pathway: What Disorders: The Agnosias fmri evidence Two types of object recognition Dorsal pathway: Where action Perception II: Recognition Computational problems in object recognition What is it? Object constancy: Variability in sensory information Retinal position Viewing position Occlusion Lighting Computational problems in object recognition Where is it? Where s Waldo?? 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 Dissociation of what and where in the monkey Landmark and object discrimination task (Pohl, 1973) Parietal lobe Where Temporal lobe What

2 What pathway characteristics Anterior regions have large receptive fields Looking through a small or big window All include fovea High definition Allows positional invariance small large Ventral What pathway characteristics Complex response profile Dissimilar to V1 Not simple orientation Selectivity Hands, faces etc 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 Objects Same objects Different location 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 Lissauer (1890 s) division Apperceptive Associative Category specific agnosia Prosopagnosia Apperceptive agnosia What its not Not cortical blindness Intact visual field Not a basic deficit in processing visual information Sensation is largely intact Brightness, orientation, color, motion intact

3 What it is Apperceptive agnosia Difficulty in forming a percept (a mental impression of something based on the senses) Visual information can t be bound together No coherent percept Apperceptive agnosia: Varying degrees of perceptual problems Depends on lesion extent Deficit in copying form Can t perceive higher-order visual structure Can t integrate parts into whole Apperceptive agnosia: Higher levels of damage Problem in object constancy Retinal projection Lighting Occlusion Evidence for constancy: Lateral occipital complex (LOC) Likely locus of object constancy Reduction in fmri response w/ repetition Invariance Size, location,viewpoint, illumination, occlusion No effect of occlusion Associative agnosia What its not Can perform perceptual grouping Can form reasonable percepts What it is Failure of object recognition Difficulty in accessing semantic representations from vision psychic blindness Associative agnosia: Can copy complex objects Perceptual grouping intact

4 Associative agnosia: Can t draw objects from memory Can t name objects Not anomic Can t match by function Match by visual similarity Largely a problem in linking percepts with semantics Localization: Gradations in impairment Apperceptive Posterior Associative Anterior Anterior to posterior lesion loci Evidence for hierarchical analysis Adjacent areas of cortex likely damaged Varying degrees of perceptual/gnostic problems Always some degree of perceptual deficit Gradations rather than categorical differences Category specific agnosia: Prosopagnosia Largely specific to faces Can distinguish between faces and objects Difficulty in distinguishing between faces Facial identification Across category Within category Is there a region of the brain devoted to faces? Fusiform face area (FFA) Right middle fusiform gyrus especially responsive to faces relative to other objects FFA Are faces special? Why have an FFA? Faces are a special object class shaped by evolutionary pressures Specialized module for their recognition 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 Face identification and configural processing Face recognition depends on relationship between distinct features (nose, eyes, etc) What happens when relationships are disrupted? Face inversion effect

5 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 Difficulty remembering/perceiving inverted relative to upright faces When upright: configural processing of subtle 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? Configural processing Are prosopagnosics impaired at configural processing, not just face processing? FFA and configural encoding Greebles Train to recognize individuals Evidence of configural processing Greeble inversion effect Experts but not novices activate FFA Potentially not face specific Two systems for object recognition Two types/qualities of object vision? Dissociation and association amongst agnosic syndromes Agnosia: general object recognition Alexia: specialized for word perception/reading Prosopagnosia: specialized for face perception Independent Independent Shared Two systems for object recognition Prosopagnosia and alexia are dissociable 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 Two systems for object recognition Analytic Analysis by parts Can apply to faces Configural Holistic analysis Can apply to objects Alexia Prosopagnosia Object agnosia

6 Disorders of the dorsal pathway: Action Double dissociation Agnosia vs. optic ataxia Apperceptive Agnosia Impaired perception Intact action Appropriate reaching grasping Optic ataxia Intact perception Impaired action Eye Inappropriate saccades Hand Impaired reaching/grasping Tactile agnosia Object recognition outside of the ventral visual stream 2 types apperceptive Intact sensory discrimination Can t form tactile percepts associative (tactile asymbolia) Can form percepts Texture, temperature weight, shape Can t retrieve meaning Can be hand specific Locus of damage Somatosensory association cortex End of lecture