An unusual brain disorder that leaves its victims able to recognize most human-made objects but unable to differentiate among living things is helping scientists understand how the brain categorizes different kinds of information. The work may someday lead to new methods of rehabilitation for people suffering from cognitive disabilities resulting from head injury, brain infection or stroke.
The bizarre syndrome, caused by a loss of nerve cells in a part of the brain responsible for processing shapes and textures, leaves a person able to recognize that there is an animal in the room, for example, but unable to tell whether the animal is a dog, a cat or a horse. People suffering from a related syndrome can differentiate among natural objects but not among human-made objects such as plates, photographs and books.
Scientists so far have identified about a dozen individuals with category-related disorders resulting from damage to specific parts of the brain, says Antonio R. Damasio of the University of Iowa College of Medicine in Iowa City. Among the better-known examples is prosopagnosia, which leaves people unable to recognize familiar faces. Patients with such perceptual defects represent unique natural experiments providing insights into the principles by which the brain organizes information, Damasio said recently at the annual meeting of the Society of Neuroscience in Phoenix.
The observation that some individuals cannot differentiate among most human-made objects, for example, suggests that some parts of the brain may deal primarily with objects displaying straight edges--most of which are hand-made--while other parts of the brain focus on objects featuring the uneven outlines typical of animal and plants.
Informative exceptions do exist. For example, Damasio says, among people with a general inability to differentiate among animals, "I have yet to meet a patient who cannot recognize an elephant or a giraffe." And among fruits, bananas stand out as recognizable even in people lacking the ability to identify most other natural objects.
Exceptions like these may provide hints about the kinds of information the brain finds easiest to remember or process, Damasio says. Moreover, that knowledge can bring therapeutic benefits. "By knowing about the strategies the brain uses to process categories, we have a chance to develop improved re-education strategies" in brain-injured patients. Such strategies would use sensory or cognitive pathways left undamaged by the initial injury.
For example, many injuries that result in a loss of visual processing ability leave unscathed the somatosensory system responsible for such senses as touch, taste and smell--senses that can prove helpful in the re-learning process when visual processing becomes impaired. In the brain, says Damasio, "almost everything that gets done one way can be done another way."
Until recently, he concludes, people with processing disorders were considered neurotic. With the new realization that they suffer from specific brain diseases, "there's a lot of hope for rehabilitation."