Speculating about the evolution of self-awareness is relatively easy; finding evidence for it is hard. But if Daniel Povinelli’s clambering hypothesis is right--if there is indeed a connection between our ancestors’ sophisticated tree-negotiating skills and our current capacity for self- awareness--then one might expect to see that development reflected in the anatomy of the human brain. For now, incontestable physical evidence of the linkage isn’t even close to being available, but neuroscientists are finding that both motor and cognitive tasks may in fact be processed in the same part of the brain. The site of the action consists of two fist-size clumps of tissue at the base of the brain called the cerebellum, from the Latin for little brain.
Until recently, researchers thought the cerebellum’s role was solely to regulate the speed, intensity, and direction of movement. But lately there has been a revolution in thinking about the cerebellum, led by Henrietta and Alan Leiner--two rather unlikely revolutionaries. Both are retired scientists in their eighties, and both were trained in computer science. But back in the 1960s, Henrietta Leiner decided to go to medical school. She was particularly interested in studying the brain. At that time, she recalls, it took a roomful of computers to do what the human brain can do in an instant.
While dissecting a human brain in a neuroanatomy class, she began to wonder about the function of a thick cable of fibers running through the cerebellum and up into the cerebral cortex. The received neurological wisdom said that the cerebral cortex--the thin, deeply fissured top layer of the brain, which constitutes its higher regions--sent signals through this cable to the cerebellum; the cerebellum processed and coordinated these signals, then fed them back up to the cerebral motor cortex, the region of the brain that controls movement.
What puzzled Leiner was the thickness of the cable. Not only was the cable far bigger in humans than in, say, monkeys, but it seemed outsize even relative to other human structures. It had some 40 million nerve fibers, 40 times as many as are found in the optic tract, which carries the stupendous amount of visual information the human brain receives. Moreover, those 40 million fibers were presumably coming from regions all over the brain. If the cerebellum was involved only in movement, Leiner wondered, why did it need so much diverse information?
She began asking more questions. And she learned that over the course of evolution, as the cerebral cortex expanded in apes and humans, so too did the cerebellum. She discovered that a small structure within the cerebellum called the dentate nucleus--the last processing stop in the cerebellum before signals are sent back upstairs--had also become relatively larger in apes and humans. Finally, she found that the most evolutionarily recent part of the dentate nucleus--called the neodentate-- is present only in humans.
Putting all this together, Leiner began to suspect that the cerebellum might play a role not just in movement but in cognition--that is, in those processes, such as language, by which humans come to know, and make judgments about, themselves and the world around them. The expansion of the dentate nucleus, she reasoned, could have been driven by a need to process the massive amounts of information the cerebral cortex was transmitting. The neodentate, she reasoned further, was found only in the one animal that we know for sure has exceptional cognitive skills. Besides, she thought, if you were going to look for a part of the brain that could allow for high-speed information processing between different brain regions, you could hardly do better than the cerebellum: although it takes up only a tenth of the brain’s volume, it contains at least half the brain’s neurons. I thought, ‘What a terrific computer!’ Leiner recalls. ‘Now, where does it send its results?’ If the cerebellum was indeed involved in activities other than movement, its information would be sent to other areas besides the motor cortex. At the time, however, no such anatomical pathway had been identified.
Neurologist Robert Dow of Good Samaritan Medical Center in Portland, Oregon, was the first to provide some clinical support for Leiner’s ideas, in 1986. He tested a patient with cerebellar damage and found--to his surprise--problems in subtle cognitive functions, such as planning.
Since that finding, several other studies have implicated the cerebellum in nonmotor skills. Among the first was a report of cerebellar activity in word-selection tasks; it was followed by a report of poor performance on similar tasks by a patient with cerebellar damage. In yet another study, researchers asked a normal subject to put rings of different sizes on a pole. If the subject slipped the rings on randomly, the cerebellum showed normal activity. But when the subject had to put the rings on in order from small to large, the cerebellum’s activity increased. Most recently, a study detected cerebellar involvement when a subject was asked to judge whether two small, irregularly shaped balls--which he could feel but not see--were the same shape.
Meanwhile, neurologist Jeremy Schmahmann of Harvard Medical School has found that patients with cerebellar injury are often unable to make accurately proportioned line drawings of simple objects. He has also observed a transient but palpable flatness of emotion in such patients. All of this leads him to speculate that the cerebellum regulates a host of complex cognitive tasks, one of which, he suspects, may be the coordination of the physical cues through which nonverbal communication occurs.
In the past, says Leiner, the cerebellum was thought to receive signals from the cerebral cortex and send back signals through the dentate nucleus only to the motor cortex. The mistake was in assuming the signals went only to the motor cortex. A recent anatomical finding has finally proved that point. Peter Strick and his colleagues at the Veterans Affairs Medical Center in Syracuse, New York, have traced a pathway in monkeys from the cerebellum back to parts of the brain that are involved in memory, attention, space perception, body positioning, and spatial guidance--all regions that lie outside the motor cortex.
Intriguingly, these diverse findings coincide with one neuroscientist’s theory about the origins of autism. Since 1985, Eric Courchesne of the University of California at San Diego has been proposing that autism--a disorder that might well be characterized as an abnormal sense of self--is linked to cerebellar deficits. His own brain-imaging studies have found smaller cerebellums in autistic children; other researchers have found fewer cerebellar neurons. Although Courchesne’s theory of autism is directed at humans, it is appealing to speculate that the cerebellum might somehow be involved in the skills that underlie self- awareness in the great apes. Cerebellar impairments will be an impediment to social language and other kinds of knowledge, Courchesne explains. And they may affect body-image knowledge as well.
Courchesne has also linked cerebellar damage in humans to an impaired ability to shift attention quickly from one task to another. Like Leiner and Schmahmann, he sees the cerebellum as filtering and integrating the stream of incoming sensory information in ways that permit swift, complex decision making--a skill that, in the end, calls to mind the trait Povinelli thinks distinguishes some seemingly thoughtful apes from their tree-swinging kin.