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.

Pandemonium Model

Interactive Activation model