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Visual agnosia is the inability of the brain to recognize or understand visual stimuli. An individual with visual agnosia has otherwise normal visual functioning and can see, but is unable to interpret or recognize what he or she is seeing. There is also no deficit in general mental ability (Rothi, Mack, & Heilman 1986). A simple test can be run to identify if this is indeed the issue. These tests usually consist of showing the person pictures of common inanimate objects, famous people, and famous places. When there is a generally low ability to recognize the photographs, visual agnosia is likely; and it will be more evident what subtype the individual has depending on which pictures are easier or more difficult to identify. Visual agnosia is often due to bilateral damage in the 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. 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.
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. Associative visual agnosias are categorized by inability to identify objects due to impaired access to stored semantic information about the objects. 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 perception and verbal systems.
Known clinical variants of visual agnosia include prosopagnosia (inability to recognize faces), pure word blindness (inability to recognize words), agnosias for colors (inability to differentiate colors), agnosias for the environment (inability to recognize landmarks etc.) and simultanagosia (inability to sort out multiple objects in a visual scene).
Categories and subtypes of visual agnosia
The two main categories of visual agnosia are:
- Apperceptive visual agnosia, an impaired recognition of objects. Individuals who have apperceptive visual agnosia would never make a complete, conscious percept.
- Associative visual agnosia, an impaired ability to recognize how to use an object, after the object has been correctly identified. The percept is created, but it would have no meaning for individuals who have an associative visual agnosia.
- Subtypes of Associative Visual Agnosia
- Achromatopsia, an impaired recognition of color.
- Prosopagnosia, an impaired recognition of human faces. These patients know that they are looking at a face, but cannot recognize whose, even if it is someone they know well.
- Orientation Agnosia: can recognize objects, but have lost knowledge of orientation.
- Pantomime Agnosia: inability to understand pantomimes (gestures). It appears that the inferior cortical visual cortex is critical in recognizing pantomimes.
While most cases of visual agnosia are seen in older adults who have had extensive brain damage, there are also cases of young children with less brain damage during developmental years acquiring the symptoms. Commonly, visual agnosia presents as an inability to recognize an object by its shape, in the absence of other explanations, such as vision problems. This is most easily seen in prosopagnosia, where the individual can clearly see the faces of those people nearby, but is unable to draw the mental connections and recognize the person. Another common manifestation of visual agnosia is a difficulty to identify objects that look similar in shape, difficulty with line drawings that lack much detail, and recognizing objects that are shown from less common views.
Visual agnosia occurs after damage to the visual association cortex. This occurs even when no damage has been done to the eyes or optic tract that leads visual information into the brain. Visual agnosia is caused by damage to parts of the ventral stream of vision. The ventral stream is also known as the "what pathway of vision" because this pathway allows the individual to identify they objects they see. The other stream is the dorsal stream or "where/how" pathway; and this pathway remains undisturbed, allowing individuals with visual agnosia to show relatively normal visually guided behavior. Damage to specific areas of the ventral stream impair the ability to recognize certain categories of visual information.
Visual agnosia occurs due to damage in the association areas of the visual cortex. The lateral occipital complex appears to respond to many different types of objects. Prosopagnosia (inability to recognize faces) is due to damage of the fusiform face area (FFA). Studies with functional imaging show that there is a certain area specialized for facial recognition, known as the fusiform face area, in the fusiform gyrus of the temporal lobe. However, this area is not exclusive to faces; recognition of other objects of expertise are also processed in this area. The extrastriate body cortex (EBA) was found to be activated by photographs, silhouettes, or stick drawings of human bodies. The parahippocampal place area (PPA) of the limbic cortex has been found to be activated by the sight of scenes and backgrounds. Cerebral achromatopsia (the inability to discriminate between different hues) is caused by damage to the V8 area of the visual association cortex. The left hemisphere seems to play a critical role in recognizing the meaning of common objects.
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.
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- Google Scholar
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- 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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