Milner, A.D., Perrett, D.I., Johnston, R.S., Benson, P.J., Jordan, T.R., Heeley, D.W., Bettucci, D., Mortara, F., Mutani, R., Terazzi, E., and Davidson, D.L.W.
Patients with good vision and without intellectual impairment may nevertheless have severe difficulty recognizing objects. This condition is called visual agnosia. Previously a number of patients have been studied who, at first glance, appeared to have agnosia but upon investigation were found to have more fundamental deficits as well, including the inability to distinguish common geometric shapes. Although the term is considered potentially misleading by some neurologists, this condition has been called visual form agnosia. The authors present the case of a patient with visual form agnosia, and their investigations reveal previously unsuspected complexities of perceptual abnormalities in such patients. The patient was 34 years old at the time of suffering carbon monoxide poisoning from a faulty propane heater. Among many visual deficits, the patient could not distinguish a square from a rectangle. When presented with a test pattern, such as a square, the patient could not distinguish which of four presented figures, a square, trapezoid, parallelogram, or diamond matched it. The patient also had some difficulty distinguishing lines on the basis of their orientation, yet it was clear that the patient had no difficulty walking around objects in a room, or picking up a pencil from a desk regardless of its orientation. Although the patient was unable to perform tasks such as grouping objects by shape similarity, she could effectively use visual information for tasks such as reaching and could even read some words. The authors propose that such a syndrome might be the result of the output from some cells of the lateral geniculate nucleus (LGN), but not others, being disrupted. The lateral geniculate nucleus is the first stop of visual signals from the eyes on their way to the visual cortex. The LGN has both magnocellular (large-cell) and parvicellular (small-cell) components. The authors suggest that the selective loss of the magnocellular stream of visual information might result in a selective loss of visual information similar to the syndrome of visual form agnosia seen in their patient. (Consumer Summary produced by Reliance Medical Information, Inc.)