| The Magazine of the CALIFORNIA ACADEMY OF SCIENCES |
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| Counterpoints in science Being of Two Minds
From the California Academy of Sciences in Golden Gate Park, I can look across the concourse to the M.H. De Young Museum, which houses art treasures both modern and ancient from the Western world, Asia, and Oceania. This configuration, with the sciences on one side, the arts on the other, and connecting walkways across which people can wander at will, is a pretty good architectural representation of what scientists have discovered about the human brain during the past three decades. One hemisphere of the brain is almost always dominant over the other: the left hemisphere for right-handers, the right hemisphere for left-handers. The left hemisphere contains the speech centers for nearly all right-handers and for most left-handers, too. That's why damage to the left side of the brain from strokes or trauma usually causes difficulty with speech as well as paralysis of the right side. Right-brain injury causes left-sided paralysis but less speech impairment. The left brain processes information in a verbal, linear, logical manner. The right, non-speaking part of the brain responds more emotionally and more globally–we might say, more artistically. This specialization of the two cerebral hemispheres has accompanied the evolution of the large human brain during the past five million years. The earliest known human fossil skulls going back more than three million years, the australopithecines of Africa, have indentations on the left indicating enlargement of Broca's and Wernicke's areas, which are the speech centers of modern humans. These markers suggest that hominids with chimpanzee-sized brains were already beginning to develop some language skills. Other primates, the monkeys and our closest relatives, the chimpanzees, do not express the same kind of "sidedness" in their brains as we do. However, at a conference I attended in South Africa this past June, Ralph Holloway, a Columbia University anthropologist specializing in brain evolution, maintained that chimpanzee brains also show some rudimentary development of the language centers, which may accord with their ability to communicate by sign language and computer icons. The two hemispheres of the brain are connected by a thick neural cable called the corpus callosum. In humans, about 250 million nerve fibers pass through this cable, and billions of messages go back and forth between right and left all the time. In chimpanzees, whose brain is only one-third the size of ours, the corpus callosum is much smaller, and in monkeys it is smaller still. Primitive mammals such as the platypus, and marsupials like the kangaroo, don't even have a corpus callosum and get along fine without it. Neurosurgeons sometimes cut the corpus callosum to help patients with severe epilepsy. We don't know why this surgery reduces seizures. The patients show surprisingly little overt change in mental function, but close examination reveals some remarkable alterations in thought patterns. During the 1950s and 1960s, Roger Sperry and his associates at the California Institute of Technology carried out tests on these "split-brain" patients that demonstrated the different modes of thinking in the two cerebral hemispheres. In an intact brain, the corpus callosum transmits the left and right points of view back and forth, like the visitors walking across the concourse. When the corpus callosum is cut, and the concourse blocked, the exchange of information stops, and the right brain doesn't know what the left brain is thinking. The scientist and the artist can't talk to each other. One split-brain patient was shown a picture of a horse, projected so that it would register only on his right visual field, which is connected to the left brain, the side that has the speech centers. The patient immediately named it as a horse. When the same picture was projected on his left visual field, which is connected to the right brain, the patient denied having seen anything. The speechless right brain couldn't tell the verbal left brain what it had seen. The patient was then asked to try to draw from memory a picture of the "unseen" image. He couldn't draw it with his dominant right hand, but he could draw it with his left hand, which is controlled by the right brain. He drew a horse–and still denied having seen it. This series of events shows how art can retrieve sensations and images of which the individual is not consciously aware. Since so many of the crucial experiences in our life–birth, nursing, learning to walk–precede our acquisition of language, we can see why conscious memories of these formative events are hard to retrieve. Elaine Badgley Arnoux, a friend who is a local artist and art teacher, uses a kind of regression therapy with her adult students to try to free them from the tyranny of the left brain and tap into these preverbal memories. She has them crawl around on the floor or imagine themselves in their high chairs making art with pureed vegetables, as infants do. She has observed that her most inhibited students are highly educated scientists and physicians, and she suspects their difficulty stems from "the habit of too much logical thinking" interfering with artistic expression. One of the long-time researchers on split brains, Michael Gazzaniga of Dartmouth College, supports Elaine's suspicion. He believes that there are not just two but many modules within the brain that process sensory input and information independently. As Gazzaniga discusses in the July issue of Scientific American and in a new book, The Mind's Past (University of California Press), it's only when all the modules have sent in their reports that the conscious mind tries to make a coherent narrative. The story is put together by what Gazzaniga calls "the left brain interpreter." The interpreter does the best it can, but it doesn't have access to most of the modules that determine behavior, only the verbal one. Lacking complete knowledge, the interpreter makes up an explanation that bridges over the missing information. One split-brain patient had a picture of a chicken's foot projected on his left brain and at the same time a picture of a house surrounded by snow on his right brain. The patient was asked to select from a group of pictures the ones he associated with these images. He simultaneously pointed with his right hand to a chicken, and with his left hand to a shovel. When asked to explain his choices, he cited the chicken foot but not the snow. "Why did you pick the shovel?" he was asked. He thought for a minute and replied, "To clean up the mess the chicken makes." One job of our "logical" left brain is to explain things, even if the explanations are just-so stories like Kipling's "How the Elephant Got its Trunk." (A crocodile grabbed and stretched its nose!) There is a fine line between science and storytelling–between astronomy and astrology, for instance. Human beings have always wondered about the connections between celestial objects like the sun, moon, planets, and stars and our life here on Earth. Many cultures have identified these objects with the gods who control human affairs: Apollo the sun god, for instance, Diana the moon goddess, Mercury, Venus, and Mars. Astrology is an attempt to depersonalize these heavenly influences and "explain" human destinies by the configurations of the stars and planets ascendant at births, deaths, and other critical junctures in human lives and affairs. The persuasive power of these explanatory modes is demonstrated by the persistent popularity of astrology in modern times, even though the real science of astronomy has revealed billions of stars and galaxies not visible to the naked eye and accounted for their origins and movements by physical and mathematical principles such as gravitation, nuclear forces, and big-bang cosmology. Alas, such scientific explanations are not nearly as appealing to the average citizen as the simplistic personal advice in the astrology column of the daily newspaper. New technology in my own specialty of nuclear medicine makes it possible to watch the brain in action without taking it apart. When we look or think or carry out a mental or physical task, the part of the brain controlling these activities concentrates glucose, the main fuel of the central nervous system. The positron emission tomographic (PET) scanner takes pictures of the brain as it processes radioactive glucose. From these pictures, we're beginning to construct a detailed three-dimensional atlas of brain function. PET images of children's brains have recently confirmed my friend Elaine's intuition that left-brain control is weaker in infancy than in adulthood. Between one and three years of age, PET images actually show right hemispheric predominance: more glucose concentrates on that side. After the age of three, as the child develops visuospatial and language abilities, the dominance (and the glucose) shifts to the left. It seems that we were all potential artists before we became potential scientists! In evolution, too, art seems to have preceded science. Cave and rock paintings in Europe, Africa, and Australia go back at least 20,000 years, whereas records of scientific thinking begin with the Greeks only about 2,000 years ago. No doubt the seeds of art and science are much more ancient. Stone tools more than a million years old often reflect both a sculptor's eye for shape and the technical skill of a nascent scientist, hinting that the individuals shaping these cleavers and handaxes from basalt and flint often had not only a practical purpose in mind but an artistic sense of beauty and symmetry. The evolution of our big brain and the specialized functions of the hemispheres has given us two distinct minds in the same head. What we call reality is being constantly negotiated across the fiber-optics of the corpus callosum. Freud may be out of fashion these days, but he was right in deducing that our behavior and conscious thoughts are largely determined by unconscious processes. He would have been fascinated by split-brain research and the PET scanner. As a pioneering neurologist, he wanted to find the anatomical basis for thinking and mental illness, but methods for studying the brain a hundred years ago were inadequate for taking on this task. Freud also understood that artists are in touch with aspects of reality, especially emotional reality, that the scientist may perceive only dimly. Just as the left and right brain chat constantly through the corpus callosum, scientists and artists have a lot to learn from each other. Recognizing this complementarity, the science journal Nature has recently run a series of articles on the interplay between art and science. Two hemispheres make a whole brain and a whole world. To be well-rounded individuals and members of a well-balanced community, we need both scientific academies and art museums, with concourses full of people flowing between them.
Jerold M. Lowenstein is a professor of medicine at the University of California at San Francisco and chairman of the Department of Nuclear Medicine at California Pacific Medical Center in San Francisco. Jlowen@itsa.ucsf.edu |
Fall 1998
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