Visual agnosia is the inability of the
brainto make sense of or make use of some part of otherwise normal visual stimulus and is typified by the inability to recognize familiar objects or faces. This is distinct from blindness, which is a lack of sensory input to the brain due to damage to the eye optic nerve, or primary visual systems in the brain such as the optic radiations or primary visual cortex. Visual agnosia is often due to damage, such as stroke, in posterior occipital and/or temporal lobe(s) in the brain.
The specific symptoms can vary depending on the cause of the
agnosia. Some sufferers are unable to copy drawings but are able to manipulate objects with good dexterity. [ [http://ahsmail.uwaterloo.ca/kin356/ventral/visual_agnosia.htm Visual Agnosia - A Disorder of the Ventral Stream] , Student Web Pages, Department of Applied Health Sciences, University of Waterloo] Commonly, patients can describe objects in their visual field in great detail, including such aspects as color, texture and shape but are unable to recognize them. Similarly, patients can often describe familiar objects from memory despite their visual problems. [ Candace N. Palmer. [http://hubel.sfasu.edu/courseinfo/SL02/visual_agnosia.htm An Examination of Visual Agnosia] , Stephen F. Austin State University]
Careful analysis of the nature of visual agnosia has led to improved understanding of the brain's role in normal vision.
Any of the following may be considered symptoms of visual agnosia:
Prosopagnosia(failure to recognize particular faces of people)
*Inability to identify common objects
copydrawings of objects
The two major types of visual agnosia are apperceptive and associative visual
agnosia. Failure in high-level object recognition despite normal vision is apperceptive visual agnosia. [cite journal |author=Shelton PA, Bowers D, Duara R, Heilman KM |title=Apperceptive visual agnosia: a case study |journal=Brain Cogn. |volume=25 |issue=1 |pages=1-23 |year=1996] Associative visual agnosias are categorized by inability to identify objects due to impaired access to stored semanticinformation about the objects. [cite journal |author=Anaki D, Kaufman Y, Freedman M, Moscovitch M |title=Associative (prosop)agnosia without (apparent) perceptual deficits: a case-study |journal=Neuropsychologia. |volume=48 |issue=8 |pages=1658-71 |year=2007] Patients with apperceptive visual agnosias cannot draw or copy drawings, whereas patients with associative visual agnosias appear to retain the neural circuits necessary for object recognition without being aware of this ability. Associative visual agnosia is likely the result of disruption of connections between visual perceptionand verbalsystems. [ Carlson, Neil R. “Analysis of Visual Information: Role of the Visual Association Cortex". Physiology of Behavior, 9. Boston, Mass., USA: Pearson Education, Inc., 2007. ISBN 0-205-46724-5.]
Visual agnosia in popular culture
* A famous report on this condition is the title essay of
Oliver Sacks' book, " The Man Who Mistook His Wife for a Hat".
* The patient in the "
House" episode " Adverse Events" suffered from agnosia.
* Farah, Martha J. "The Cognitive Neuroscience of Vision". Fundamentals of Cognitive Neuroscience, 3. Malden, Mass., USA:
Blackwell Publishers, 2000. ISBN 0-631-21403-8.
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