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Deficits of vision. What do visual deficits tell us about the structure of the visual system?. Serial vs. Parallel processing. One significant question that faced vision researchers for a long time was how the cortex processed vision.
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Deficits of vision What do visual deficits tell us about the structure of the visual system?
Serial vs. Parallel processing • One significant question that faced vision researchers for a long time was how the cortex processed vision. • Was visual information processed strictly hierarchically (serially)? • Or was it processed in parallel, with different features being processed by different brain areas?
How can we tell? • One way we can tell is to look at the types of visual deficits that brain damage patients experience. • Comprehensive deficits across multiple features would seem to signal serial, hierarchic processing. • Selective deficits of specific features would imply parallel, feature-specific processing.
Achromatopsia • Patients with achromatopsia are unable to see color • They fail at tasks requiring discriminations of similar objects based on color • Can still discriminate dissimilar colors, but more in the way you can discriminate such “colors” in a black & white movie • Almost universally exists in patients with a lesion encompassing area V4
Akinetopsia • Akinetopsia is an inability to perceive motion. • Patients report it’s like looking at the world under a strobe light. • Patients with damage to area MT experience this disorder (Newsome & Pare, 1988)
Post-occipital vision • Two streams: • Dorsal (where): Projects into the parietal lobe and areas responsible for attention • Ventral (what): Projects into the temporal lobe and areas responsible for object recognition.
Agnosias • Loss of object recognition abilities without attendant loss of perceptual ability, usually correlated with damage to the left temporal lobe. • Form: Inability to recognize whole objects. Recognition of object parts is relatively preserved • Simultagnosia: Can recognize individual elements, but not an entire scene • Associative: Can describe objects and recognize their functions, but cannot identify the objects • Color: An inability to recognize and name colors. This is different from achromatopsia. • Prosopagnosia: An inability to recognize faces; correlated with damage to the fusiform gyrus. • Aperceptive: Unable to process faces at all • Associative: Can make same/different judgments, but cannot recognize.
Object recognition • The different types of agnosia give us insights into how we recognize objects. • How does each different agnosia highlight a different facet of object recognition?
What makes object recognition hard? • How often do you see a given object exactly the same way more than once? • Orientation • Lighting • Obstructions • Distance • And what about recognizing novel instances of familiar object categories?
How do we do it? • View-dependent theories posit that we separately recognize objects from each of the various perspectives we see it from. • These are essentially bottom-up theories of perception. We match perceptual input to one of a large number of stored representations of objects. • View-invariant theories claim that we only hve one representation of each object. • These are top-down theories of perception. We extract basic features of the object and use these to narrow down the possible categories for the object. • This then feeds back down to the perceptual layer, similar to the winner-take-all feedback among the feature-detectors of V1.
