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In an excerpt from her book, Fractured Minds: A Case-Study Approach to Clinical Neuropsychology, Jenni A. Ogden, PhD discusses a patient who was thought to be blind after a traumatic brain injury, but seemed to be getting his sight back:
“[The rehabiliation therapist] had been trying to improve his sight by showing him various common objects day after day, but he seemed unable to recognize the objects unless he picked them up. Often he would describe the shape of the object in a slow, disjointed manner, but only rarely did this seem to help him recognize the object."
The patient did not go blind after his injury; instead, he had a condition called visual agnosia, which is caused by a disruption to normal brain function. Visual agnosia patients can “see" an image, but cannot process what the object is.
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Visual agnosia is caused by damage to the occipital lobe of the brain, which is the most posterior lobe of the brain and is responsible for processing visual information. Beside head trauma, the National Institute of Neurological Disorders and Stroke points out that a stroke, dementia and developmental disorders can also be causes of visual agnosia. The occipital lobe damage is bilateral, meaning it affects the occipital lobe on both the right and left hemispheres of the brain.
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Types of Visual Agnosia
Dr. Ogden notes that three types of visual agnosia can occur. The first, apperceptive visual agnosia, has symptoms where the patient cannot identify an object. For example, a patient with apperceptive visual agnosia would be unable to recreate the image of a circle. In the second type of visual agnosia, associative visual agnosia, the patient can copy a picture; however, the patient cannot define the object. A patient with associative visual agnosia could draw a circle, but would tell the observer that she does not know what the image is. With integrative visual agnosia, the third form, the patient can copy an image, but cannot put pieces together to make a whole. In Dr. Ogden's example, the patient is given a picture of a toothbrush to copy; the patient, however, draws the toothbrush in fragmented parts (not perceiving it as a “whole" image) and cannot identify the object.
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The Merck Manual on Agnosia states that clinical diagnosis requires two parts: neuropsychological testing and brain imaging. During the neuropsychological testing, the practitioner will ask the patient to identify objects, testing the patient's various senses. The brain imaging, either done with CT or MRI, can pinpoint the area of injury, and the extensiveness of the damage.
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Because visual agnosia can be very debilitating for the patient, the Merck Manual recommends occupational therapy to help the patient “learn to compensate for [her] deficits." Treatment varies and is dependent on the amount of damage to the brain.