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Visual agnosia or optic agnosia is the inability of the brain to 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 or optic nerve.
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 . 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 .
Careful analysis of the nature of visual agnosia has led to improved understanding of the brain's role in normal vision.
Visual agnosia is often due to damage, such as stroke, in posterior parietal lobe in the right hemisphere of 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. 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. 
The careful analysis of the nature of visual agnosia has led to improved understanding of how normal vision works.
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.
References & Bibliography
- The Cognitive Neuroscience of Vision ISBN 0631214038
- Humphreys, G. W. & Riddoch, M. J. (1987) To see but not to see: A case study of visual agnosia. London: Erlbaum.ISBN 0863770657
- Giersch, A., Humphreys, G. W., Boucart, M. & Kovacs, I. (2000) The computation of occluded contours in visual agnosia: Evidence for early computation prior to shape binding and figure-ground coding. Cognitive Neuropsychology, 17, 731-759.
- Rumiati, R. I. & Humphreys, G. W. (1997). Visual object agnosia without alexia or prosopagnosia: Arguments for separate processing mechanisms. Visual Cognition, 4, 207-218.
- Humphreys, G. W., Riddoch, M. J., Donnelly, N., Freeman, T. A. C., Boucart, M. & Muller, H. M. (1994) Intermediate visual processing and visual agnosia. In M. J. Farah & G. Ratcliff (Eds.), The Neuropsychology of High-level Vision. Hillsdale, N. J.: Erlbaum.
- Humphreys, G. W. (1994) The cognitive neuropsychology of vision: A hierarchical analysis of visual agnosia. In B. Weekes, C. Haslam, J. Ewing & U. Johns (Eds.), Cognitive Functioning in Health, Disease and Disorder. Academic Press: Sydney.
- Rumiati, R. I., Humphreys, G. W., Riddoch, M. J. & Bateman, A. (1994). Visual object agnosia without prosopagnosia or alexia: Evidence for hierarchical theories of visual recognition. In V. Bruce & G. W. Humphreys (Eds.) Object and Face Recognition. London: Erlbaum.
- Humphreys G. W. & Riddoch, M. J. (1993) Object agnosias. In C. Kennard (Ed.), Balliere's Clinical Neurology: Visual Perceptual Deficits. London: Balliere Tindall
- Humphreys, G. W., Riddoch, M. J., Quinlan, P. T., Price, C. J. & Donnelly, N. (1992) Parallel pattern processing in visual agnosia. Canadian Journal of Psychology, 46, 377-416.
- Riddoch, M. J. & Humphreys, G. W. (1988) Visual agnosia: Anatomical and functional accounts. In C. Kennard & F. Clifford-Rose (Eds.), Physiological Aspects of Clinical Neuro-ophthalmology: The Mansell-Bequest Symposium. London: Churchill-Livingstone.
- Humphreys, G. W. & Riddoch, M. J. (1987) The fractionation of agnosia. In G. W. Humphreys & M. J. Riddoch (Eds.), Visual Object Processing: A Cognitive Neuropsychological Approach. London:Erlbaum.
- Humphreys, G. W., Riddoch, M. J. & Quinlan, P. T. (1985) Interactive processes in perceptual organization: Evidence from visual agnosia. In M. I. Posner & O. S. M. Marin (Eds.), Attention & Performance XI. Hillsdale, N. J.: Erlbaum.
- Google Scholar
- Chainay, H. & Humphreys, G. W. (2001) The real object advantage in agnosia: Evidence of a role for shading and depth in object recognition. Cognitive Neuropsychology, 18, 175-191.
- Riddoch, M. J., Humphreys, G. W., Hardy, E., Blott, W. & Smith, A.(2002). Visual And Spatial Short-Term Memory In Visual Agnosia. Cognitive Neuropsychology.
- Riddoch, M. J. & Humphreys, G. W. (1987) A case of integrative visual agnosia. Brain, 110, 1431-1462.
- Bromley, J., Humphreys, G. W., Javadnia, A., Riddoch, M. J. & Ruddock, K. H. (1986) Pattern discrimination in a human subject suffering visual agnosia. Journal of Physiology, 377, 67.
- Riddoch, M. J., Humphreys, G. W., Gannon, T., Blott, W. & Jones, V. (1999). Memories are made of this: The effects of time on stored visual knowledge in a case of visual agnosia. Brain, 122, 537-559.
- ^ http://ahsmail.uwaterloo.ca/kin356/ventral/visual_agnosia.htm
- ^ http://hubel.sfasu.edu/courseinfo/SL02/visual_agnosia.htm
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