Few tasks in science are so daunting as attempts to unravel the workings of the human mind; the range of mental processes seems almost without limit. Neurobiologists have therefore often sought clues about normal functioning by observing the specific changes that result from brain damage. One of the most striking of these mental disabilities is anosognosia--derived from the Greek for "loss of knowledge"--which was first described by French neurologist M.J. Babinski in 1914. Anosognosia has long fascinated cognitive scientists because it suggests that perception and self-awareness may be distinct functions carried out by separate parts of the brain.
Anosognosia usually occurs among people who have suffered a stroke in the right hemisphere of their brains that has made them unable to use their left arms or legs. Such persons are said to be hemiplegic. A small proportion of these hemiplegics display a remarkable symptom: they cannot perceive that they are paralyzed. They insist that their paralyzed limbs are functioning normally, even immediately after failing to perform some simple task that requires their use.
Now two researchers at the University of California at San Diego's Center for Research on Brain and Cognition have shown that in some patients the effects of anosognosia are more profound than previously recognized. Some anosognosics not only are unaware of their own paralysis, they are also unable to perceive paralysis in others. This finding seems to indicate that our awareness of ourselves is inextricably entwined with our awareness of others.
The experiment, described in the August 8 issue of Nature by Vilayanur S. Ramachandran and Diane C. Rogers-Ramachandran, a husband-and-wife team, involved three women, two 77 years old and one 78 years old. All were paralyzed on their left sides and hence confined to wheelchairs, as a result of strokes.
Ramachandran claims that the women were all mentally lucid, able to converse fluently and to perform simple arithmetic operations. The researchers asked them a series of questions to test the nature of their anosognosia: Can you walk? Can you use both hands? Can you use your right hand? Can you use your left hand? Are both hands equally strong? The three women answered all these questions affirmatively, even when they were sitting in front of a mirror that belied their answers.
The U.C.S.D. scientists then asked the women to observe another person exhibiting left hemiplegia. (In one case, the researchers presented a healthy male student who pretended to be paralyzed.) The investigators first established that this person, whom they called the "stooge," was unable to respond to commands to move his left arm. They then asked the original subjects whether the stooge was moving his arm properly or was paralyzed.
One woman, seemingly surprised at the question, answered that "of course he is paralyzed; he is not moving his arm." The other two women, however, answered without hesitation that the man was "OK" and was "moving his arm up and down." The awareness of anosognosic patients, Ramachandran comments, "is completely warped to accommodate the strange new sensory world they inhabit."
Previous workers have proposed that each individual's brain has a specific neural region that contains a schema, or representation, of the individual's body. According to this model, anosognosia occurs when brain damage prevents new sensory data from being integrated into the schema. The schema hypothesis might also explain the "phantom limb" phenomena that often occur after spinal injury or amputation, which involve tactile rather than visual hallucinations about the body.
The Ramachandrans speculate that this same region of the brain may contain representations of other persons' bodies as well. (The fact that one of patients perceived the stooge's paralysis correctly suggests, however, that the regions do not completely overlap.) Experiments on monkeys, described two years ago in Science (Michael S. A. Graziano et al., Vol. 266, pages 1054-1057; November 11, 1994), support this hypothesis. The researchers discovered that the neural cells in the monkeys' frontal lobes that fire when the animals perform certain functions also show activity when they watch other monkeys perform the same function. This work suggests a new twist on the old mind-body problem; our bodies are, in a sense, self-projected constructs, an integral part of our minds.