LEFT VS. RIGHT: BRAIN FUNCTION TIED TO HORMONE IN THE WOMB
SCIENTISTS are tuning into the internal dialogue carried on between the right and left hemispheres of the brain, and their findings are offering clues about the underlying causes of mental deficits such as dyslexia and mental gifts such as mathematical genius.
Each of the brain's hemispheres has, in a sense, its own personality, the researchers are discovering. They are also finding that widespread notions about the right and left hemispheres are largely wrong.
The location in the brain of various capacities matters so much to neuroscientists because it is crucial to understanding the link between the brain and thinking. But the differences also point toward explanations for puzzles that seem far removed from the mind, including the high incidence in left-handed people of migraines, allergies and auto-immune disorders such as rheumatoid arthritis.
Left-handed people present researchers with a special challenge to sorting out how the brain is organized, particularly in determining what role each hemisphere plays. Left-handers display quite a different pattern of brain organization than right-handers. For example, in right-handed people the brain center that controls speech is located in the left hemisphere; in left-handers the speech center may be on either the right or the left half of the brain - or spread over both.
Handedness, researchers have found, is a matter of degree. About one person in 10 is fully left-handed. But as many as 20 percent of nonleft-handers are somewhat ambidextrous, so that, for example, they may write with one hand and strike a match with the other. These people, many of whom may consider themselves right-handers, may have brains with some measure of the anomalous organization found in outright left-handers.
Many researchers have been intrigued by a new, daring theory about left-handedness and the brain that at first seemed farfetched but has gained credibility as evidence mounts. The theory was proposed by Norman Geschwind, who until his death last year was an eminent neurologist at Harvard Medical School.
Dr. Geschwind's theory, parts of which were published posthumously, proposes that the unusual brain organization found among left-handers signifies alterations that occurred in the developing brain of the fetus. These changes affect the anatomy and organization of the cerebral hemispheres, and have an important impact on the immune system.
Earlier this year, in a special series of articles in the Archives of Neurology, Dr. Geschwind and Albert Galaburda, a neurologist at Harvard Medical School, set out theory and evidence. They propose that variations during pregnancy in the level of testosterone, a male sex hormone, alter the way the fetal brain takes shape by changing the locations to which cells migrate. The levels of testosterone can be altered by such factors as the amount of stress the mother undergoes.
''The male brain matures later than that of the female, and the left hemisphere matures later than the right,'' Dr. Geschwind and Dr. Galaburda note. For that reason, they contend, the left hemisphere in men is especially vulnerable to irregularities.
Should testosterone changes result in the left hemisphere becoming less developed, the right hemisphere may become correspondingly more highly developed, according to the theory. This enrichment of the right hemisphere allows it to become the site of the centers for language and handedness, activities that the less-developed left hemisphere cannot support so well.
The result is a person who is more likely to be left-handed, since the right hemisphere controls the left half of the body. That also explains, the theory says, why left-handers, unlike right-handers, have more variation in where the brain centers are located for such things as speech - and why left-handers are more prone to problems like dyslexia and stuttering that depend on the operation of these brain centers.
''Probably most, if not all, individuals with developmental learning disorders have anomalous dominance,'' even though most of them are not totally left-handed, according to an article by Dr. Geschwind.
Architects and Math Prodigies
The variations in brain organization among left-handers are not all bad, however. While some parts of the brain are relatively underdeveloped, others can be more highly developed, notably that for spatial tasks. One sign of this, the theory notes, is the greater number of left-handers among architects and mathematically gifted children.
The effect of testosterone on brain development is more pronounced for boys than for girls, which, according to the theory, accounts for the higher incidence among males not only of left-handedness but also of such problems as dyslexia and stuttering.
The same hormonal influences that shape brain development also can affect the developing immune system, according to the theory. This results in a higher rate of certain medical problems.
In a study of 3,000 people, 280 of whom were recruited from among the customers of a specialty shop for left-handers in London, Dr. Geschwind found the links his theory predicted between left-handedness, learning disorders and disease. The study, done with Peter Behan, a neurologist at the University of Glasgow, is reported in ''Cerebral Dominance,'' edited by Dr. Geschwind and recently published by Harvard University Press.
Left-handers, the study showed, were two-and-a-half times more likely than right-handers to suffer from such auto-immune disorders as rheumatoid arthritis and ulcerative colitis, and 10 times more likely to have learning disabilities. They also reported migraine headaches twice as often and allergies 11 times more often than did right-handers. Higher Rate of Learning Problems Other investigators have recently confirmed the relationship between left-handedness, learning disabilities and immune diseases. Dr. Marcel Kinsbourne, director of the division of behavioral neurology at the Eunice Kennedy Shriver reserach center in Boston, studied more than 1,500 children and their families. Some of the children were enrolled in a school for the learning disabled and the rest were from regular schools.
''The left-handers had a much higher rate of learning problems than did the right-handers, as well as a higher rate of immune diseases,'' Dr. Kinsbourne said. ''What's more, those right-handers with a near relative who was left-handed also had a high rate of immune disorders, which suggests a genetic factor is at play.''
A specific genetic link between severe reading disabilities and immune disorders has been discovered by Bruce Pennington, a psychiatrist at the University of Colorado Medical School. Dr. Pennington studied genetic markers in nearly 100 members of eight families in which there was a three-generation history of reading disabilities.
''There was a very high rate of auto-immune diseases among these families,'' Dr. Pennington said in an interview. ''We found that both dyslexia and the susceptibility to immune disorders were linked to chromosome 15.''
''No one expected this finding; neurologists and immunologists both think this is the craziest result in the world,'' Dr. Pennington said. ''Geschwind's is the only theory that explains how all this might hang together.''
''Dr. Geschwind's theory is a very provocative, though tentative, suggestion,'' said Michael Gazzaniga, a neuropsychologist at Cornell Medical College. ''It's very intelligent and worthwhile, and more people ought to do the research that is needed to test it.''
One factor that may allow for greater variation among left-handers in how their brain is organized is that, compared to right-handers, they have a larger corpus callosum - the bundle of tissues that connects the right and left halves of the brain. According to a report last month in Science, the corpus callosum of left-handers is, on average, about 10 percent larger than that of right-handers, a difference estimated to represent as many as 25 million nerve fibers.
''The left-handers' large corpus calossum seems to allow for greter communication between the halves of the brain, and a more diffuse spread of the brain centers that control such mental functions as speaking, comprehending spoken language, and the perception of faces and of melodies,'' according to Sandra Witelson, a psychologist at McMaster University in Hamilton, Ontario, who did the research.
Dr. Witelson's discovery is the fruit of a unique research program in which cancer patients volunteer to undergo an exhaustive, 30-hour battery of neurological tests, and consent to a brain autopsy after they die in which more than 100 anatomical features are studied.
''Right-handers have a sharp division of labor between the hemispheres, with centers for linear mental activity on the left hemisphere, and those for spatial activity on the right,'' Dr. Witelson said in an interview. ''In left-handers these brain centers are more spread across the hemispheres, with the greater size of their corpus callosum presumably allowing better communication.''
This spread, clinical evidence has long suggested, gives many left-handers an advantage in recovering from certain kinds of brain damage, such as stroke.
''Left-handers can show little loss from brain damage when it is in a center that is spread across both hemispheres,'' said Dr. Witelson in an interview. ''A right-hander, on the other hand, is more likely to suffer destruction from the same damage to a given brain center, such as for speech, because the center is localized in only one hemisphere and is more easily knocked out.''
Some investigators suspect that the corpus callosum, and its transfer of information from one hemisphere to the other, may play a key role in problems such as stuttering and dyslexia. Richard Davidson, a psychologist at the University of Wisconsin, reported in a recent issue of the journal Brain and Language that dyslexics show some signs of problems in the transference of information between the hemispheres.
The new findings are challenging widespread ideas about the brain's hemispheres. Researchers observe that the most often repeated ideas about the hemispheres are misguided.
''The popular cant that the right brain is better than the left'' at seeing the big picture all at once is unfounded, according to Dr. Gazzaniga, a leading researcher in the field. ''In fact, the left hemisphere is more responsive and more active than the right during most every mental activity,'' he said.
''The left hemisphere is also better at complicated spatial tasks, for which the right hemisphere was supposedly superior, Dr. Gazzaniga said. ''For example, the right hemisphere is unable to generate the total structure of a complex image. If you try to compare a dog and cat in your mind, the left hemisphere is better than the right at it.''
One sharp distinction between the hemispheres that has held up is that in right-handers the left brain is dominant when it comes to taking in information that is sequential - such as language - or when timing is crucial, such as in the rhythm of music or Morse code. The right half is dominant in simple spatial tasks, such as recognizing faces, or in perceiving patterns, such as a musical melody or recognizing an object by touching it.
But most mental processes coordinate the hemispheres so that both are involved. For example, the idea that the left half of the brain is the seat of logical thought, while the right half governs intuitive perception, is seen as naive by those who study the brain's hemispheres. Both halves of the brain, they say, are interwoven in mental activities as complex as logical thinking and intuition.
And while the right hemisphere has been touted as offering a much-needed antidote to the problems supposedly created by the too-logical left hemisphere, new data suggest that, in some ways, the left hemisphere is far more positive than the right. According to an article by Dr. Davidson in a recent issue of Psychophysiology, depressed people have a higher level of activation in the front portion of the right hemisphere than do people who are not depressed.
''The frontal region of the right hemisphere is the seat of negative emotions, such as disgust and sadness, while the left is where happiness is localized,'' Dr. Davidson said.
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