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Visual Agnosia
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Visual Object Agnosia
Color Agnosia
- Most common type of visual agnosia
- Affected individuals can see objects but cannot identify them; "see but not know"
-Associative: person cannot name object but they can still draw or copy it.
-Apperceptive: perception is not intact; person cannot dopy an object.
- In a study, subject C.K. could only name 16 of 23 common objects; diagnosed with visual object agnosia
-C.K. could copy 10 geometric shapes but could not identify them; example of associate visual agnosia.
- Peripheral color sensing system intact - central color processing system abnormal
- Individuals can see and discriminate colors, but cannot name them.
- In a case study, J.T. has normal color contrast sensitivity and can decipher differences in alternating colored bars
-J.T. incorrectly identifies 46 out of 92 colors when presented digitally and on color chips.
Prosopagnosia
How Does it Work?
- Eyes take in sensory information from outside world
- Images processed through visual pathways to the visual cortex
- Dorsal vs. Ventral pathway - "Where" vs. "What"
- Inferotemperal Cortex - crucial for deciphering images (What pathway end process)
*damage to this area can cause Agnosia*
- inability to recognize human faces - even facs of celebrities, relatives, and themselves
- abnormal functioning of the fusiform gyrus "fusiform face area"
- caused by brain damage, degenerative disorders, or can be congenital
*Controversial topic: should this be classified as a separate type of agnosia?
Visual Agnosia:
What is it?
- derived from Greek term "a gnosis" =
"no knowledge"
- neurological disorder based in the
visual cortex
- inability to identify common objects or faces
-rare disorder only affecting about 200,000
people in the United States
Conclusion
History
- There is not much extensive research on this disorder.
-Although rare, visual agnosia can provide insight to the complex system of visual processing.
- 1877: Munk observed dogs with parietal-occipital lesions; dogs had no visual impairment but did not recognize familiar objects.
- 1890: Lissauer categorized "apperceptive" and "associative" forms of this disorder
-1891: Freud coined term "agnosia"