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?
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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