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Visual agnosia is a medical condition in which the afflicted person can see an object set before him or her but cannot understand what the object is or what it is used for. In other words, the person lacks object recognition. People afflicted with this condition do not have any structural or neurological damage to the eye, nor is there any erosion in their intelligence that would account for the failure of object recognition. The underlying problem with this condition resides in the processing of the visual information within the brain. There are two types of visual agnosia, apperceptive agnosia and associative agnosia.
Apperceptive agnosia, referred to also as visual space agnosia, is an inability to recognize an object and differentiate between two similar objects. For example, if a shoe and a mitten are placed before someone with apperceptive agnosia, the patient would not be able to identify either object. He would also be unable to perceive that the two objects are very different. People beset with this condition cannot copy a picture, play a matching game, or even complete a comparison test. This form of agnosia is most often caused by anoxia, or prolonged depleted oxygen levels, stroke, or carbon monoxide poisoning.
There are three parts to associative visual agnosia. First, people with this type of agnosia can perceive an object using touch, or they will understand the object if it is described to them. In other words, they can identify the object using memory or any sense other than vision.
Second, people with this condition can match like objects but will not understand the function of the objects or even know their names. Finally, a patient with associative agnosia retains enough visual memory to be able to copy a picture that she has seen. The causes of associative visual agnosia are highly variable, with lesions in many different areas of the brain resulting in this condition.
Visual agnosia was brought before the public in the Oliver Sacks book The Man Who Mistook His Wife For A Hat. Part of this book describes a man with visual agnosia who is highly functional in his professional and personal life despite his inability to recognize objects. This demonstrates that people who suffer from this condition are often able to compensate by relying on other senses, in particular their sense of touch and smell. Clearly, object recognition is a complex function that relies heavily on the eyes and brain, but it must also require the input of all the other senses in order for the brain to make a final decision. As of 2011, there is no therapy to cure this affliction.