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A Hero of the Brain

Cerebral Dominance: The Biological Foundations

edited by Norman Geschwind, edited by Albert M. Galaburda
Harvard University Press, 232 pp., $27.50

Dyslexia: Current Status and Future Directions

edited by Frank Hopkins Duffy, edited by Norman Geschwind
Little, Brown, 223 pp., $32.50

Nineteenth-century neurology was dominated by two opposing schools of thought. Early in the century the Austrian neuroanatomist Franz Gall and his disciples claimed that, to those practiced in the art, an examination of bumps on a person’s head revealed talents and psychological characteristics; traits of character, he held, were controlled by specific regions of the brain. Gall had a fashionable success in France, but was ridiculed by the leading neurologist of the day, M.J.P. Flourens, who had performed experiments on birds’ brains. At the height of his fame in the 1840s, when he defeated Victor Hugo for membership in the French Academy, Flourens believed that he had conclusively demonstrated that activities such as walking and flying were not dependent on any particular region of the brain. The brain functions as a whole, he argued, and it was impossible to predict the specific effects of any form of damage.

In 1861 the French neuroanatomist Paul Broca demonstrated that damage to a specific region on the left side of the cerebral cortex caused severe language problems, such as an inability to speak fluently. This was the first serious challenge to Flourens and the “holistic” school. Subsequently, the German neurologist Carl Wernicke found another region on the left side of the brain that apparently controlled different aspects of language, including the ability to understand speech. Wernicke argued that the region on the left side of the brain that had been discovered by Broca was somehow responsible for translating language formulated in the brain into the mechanical movements of the vocal cords, the tongue, and the mouth. A band of fibers called the arcuate fasciculus connected Broca’s region to the same region that Wernicke himself had discovered; and Wernicke believed that the region he had discovered was responsible for the recognition, or sorting, of speech as distinct from other sounds. Clinicians soon found that such “localization” of brain functions explained many other patterns of neurological disorders in addition to language. In 1884, for example, a patient with epilepsy and partial paralysis had a brain tumor removed in the first such operation in medical history. The neurological symptoms enabled the surgeon to locate the exact position of the tumor.

During the early years of the twentieth century, however, the belief that psychological behavior derived from separate mental faculties, each controlled by different centers in the brain, became increasingly unpopular; most neurologists and psychologists considered this an implausible view of human psychology. In 1927 a neurology textbook noted:

Neurologists have been prone, even up to the present time, to fall into the error of attempting to find specific centers for particular mental functions or faculties. But the evidence at present available gives small promise of success in the search for such centers. It is, in fact, theoretically improbable that such discoveries will ever be made, for psychology today recognizes no such mosaic of discrete mental faculties as would be implied by such a doctrine.1

For many neurologists, the anatomical evidence also failed to support the nineteenth-century localization doctrine. Specific functions, they argued, were attributed to regions in the brain that were not well defined anatomically; and the patterns of brain damage found on post-mortem examination proved to be more variable than the localization arguments had predicted. Perhaps most dramatic were the claims of the Harvard psychologist Karl Lashley, who poked holes in rat brains. As Flourens had argued a century earlier, Lashley claimed that the rats’ neurological problems were a function of the amount of brain damage, not the specific sites of damage. While Lashley did not claim that his rat experiments were relevant to an understanding of human brain function, other people thought that what was true of rats was true of human beings as well. By the 1950s the localization argument appeared dead. The standard American medical textbook commented in the 1958 edition:

Knowledge of the location of speech functions has come almost exclusively from study of human beings who have succumbed to local brain diseases. From the available information it seems almost certain that the whole language mechanism is not divisible into a number of parts, each depending on a certain fixed group of neurones. Instead, speech must be regarded as a sensorimotor process roughly localized…in the left cerebral hemisphere, and the more complex elaborations of speech probably depend on the entire cerebrum.2

Today, however, most psychologists and neurologists would argue that speech and other brain functions can indeed be divided “into a number of parts.” Studies in information processing and artificial intelligence have plausibly suggested how the brain processes information as a series of discrete subtasks; and the same studies do much to explain the symptoms that puzzled and confused neurologists in the first half of this century. Yet well before these arguments had begun to take shape in the 1970s, Norman Geschwind, who until his death in November 1984 was a professor of neurology at Harvard Medical School, had started a thorough reexamination of the classical neurological writings that had first suggested the arguments for localization. Geschwind vindicated the localization approach that had been dismissed by neurologists and psychologists between the 1920s and 1950s. His work is the culmination of the classical tradition of Broca and Wernicke and helped to set the stage for new approaches to the brain.

In 1961 Geschwind came upon a rarely mentioned study by the French neurologist Jules Joseph Dejerine, published in 1892. It was this work that led to his reexamination of other forgotten, dismissed, and even misquoted writings, and to the publication in 1965 of his own monograph, largely a defense of the classical approach, “Disconnexion Syndromes in Animals and Man.” (The essay appears in Selected Papers on Language and the Brain.) Dejerine had described the case of a very intelligent businessman who on awakening one morning found that he could no longer read. When looking straight ahead he could not see the color of objects on his right (known as the right visual field), though he had no trouble seeing the color of objects on his left. His speech and comprehension were perfectly normal. He could write but he was unable to read what he had written. Yet he had no difficulty reading and writing numbers. And he could copy written material without understanding what he was copying.

Though he could not read visually, he could read when he traced the outlines of letters with his fingers. He had no trouble naming objects, including pictures of complicated scientific instruments. “There was no evidence,” Geschwind wrote, “of any general intellectual disturbance since the patient continued during his illness to operate a highly successful business, to gamble at cards successfully, and to learn vocal and instrumental parts of operas by ear since he could no longer read music.”

These symptoms can be explained as follows: normally, the two sides, or hemispheres, of the brain communicate directly through the band of fibers called the corpus callosum. The nerves that carry information from the eyes to the brain cross; information from the right visual field goes to the left hemisphere of the brain and information from the left visual field goes to the right hemisphere. The man’s color blindness in the right visual field meant that the visual areas of his left hemisphere were partly damaged. This damage, however, was extensive enough to destroy the fibers carrying visual information from both hemispheres to the language centers discovered by Broca and Wernicke. The patient’s ability to see words without understanding them meant that the words and sentences he saw never reached the language centers in the left hemisphere. His color blindness in the right visual field provided the clue to the site of the damage in the left hemisphere. Information presented in tactile form, however, went directly to the language centers to be “read.”3

Geschwind’s study of classical neurological cases such as Dejerine’s as well as his own clinical work provided the basis for his 1965 monograph on the disconnection syndrome—the clinical consequences of the destruction of fibers that link functional units of the brain. Important, too, was Roger Sperry’s report in the 1950s that the severing of the corpus callosum in animal brains caused behavioral changes. In his paper Geschwind concluded that, in both higher animals and human beings, sensory information—from sight, sound, smell, and touch—is initially processed in the primary sensory areas of the brain. The information is then relayed to neighboring brain regions known as association areas. In higher animals, but not in human beings, the information then goes from the association areas to the limbic system of the brain—a structure that activates emotional responses such as “fight, flight, and sexual approach.” The sight of a snake will therefore cause a monkey to flee. If for some reason the connections between the visual areas of the monkey’s brain and the limbic system are broken, or disconnected, the monkey will fail to respond when seeing a snake. However, this disconnection will not prevent it from fleeing should it touch the snake, assuming, of course, that the touch connections are intact. For subhuman mammals, each form of sensory information has relatively direct connections to the limbic system, permitting “recognition” and consequent limbic reaction using visual, tactile, or auditory information. There is little mixing of sensory information in animal brains.

In human brains this is not true: information received through the senses passes from the primary sensory regions to the association areas, as in the higher animals, but the limbic system is circumvented. Instead, nerve fibers pass sensory information to a secondary association area (which includes the language centers)—“the association area of association areas.” This frees human beings from domination by limbic system responses. Instead of the direct connections between auditory or visual information and limbic responses that are characteristic of animals, human beings have powerful associations between visual and auditory sensations as well as between the tactile and the auditory, the tactile and the visual, etc. All these associations take place in the secondary association areas. Geschwind called them “cross-modal” associations. As he wrote: “In sub-human forms the only readily established sensory-sensory associations are those between a non-limbic (i.e. visual, tactile or auditory) stimulus and a limbic stimulus [fight, flight, or sexual response]. It is only in man that associations between two non-limbic stimuli are readily formed.4

These associations, Geschwind argued, give human beings the capacity for speech. “The ability to acquire speech,” he wrote, “has as a prerequisite the ability to form cross-modal associations.” In his 1965 paper, he claimed that language was a consequence of the association of two or more kinds of sensory information, for example, the association of the spoken word “dog” with the visual image of a dog. A disconnection between the visual cortex and the limbic system in the monkey explains why it fails to respond to the sight of a snake (though it certainly sees the snake); the disconnection between the visual cortex and the language centers of Broca and Wernicke in Dejerine’s patient destroyed his ability to read—though he could see the written words, he could not read them with his eyes. But he could still “read” by using his sense of touch, which was not disconnected from the language centers.

  1. 1

    Charles Judson Herrick, Introduction to Neurology (W.B. Sanders, 1927; first edition 1915), p. 338.

  2. 2

    T.R. Harrison, Principles of Internal Medicine (McGraw-Hill, 1958), p. 368.

  3. 3

    Geschwind appears to have misread Dejerine’s paper and therefore gave a different analysis of the case in his disconnection syndrome paper. But the basic idea that attracted him, that visual information fails to reach the language centers, remains the same as in the above description. For a discussion of these discrepancies and further clinical evidence see A.R. Damasio and H. Damasio, “The Anatomic Basis of Pure Alexia,” Neurology (December 1983), pp. 1573–1582.

    Thefirst known record of a case like Dejerine’s was in 1673. A similar but considerably more complicated case is found in The Man with the Shattered World by A.R. Luria, the great Russian neuro-psychologist. Here is his patient’s description of his discovery that he could no longer read following a war injury:

    I went into the hall to look for a bathroom I’d been told was next door. I went up to the room and looked at the sign on the door. But no matter how long I stared at it and examined the letters, I couldn’t read a thing. Some peculiar foreign letters were printed there—what bothered me most was that they weren’t Russian. When a patient passed by, I pointed to the sign and asked him what it was. “It’s the men’s room,” he replied. “What’s the matter with you can’t you read?”

    He eventually learned to decipher written material “letter by letter, syllable by syllable, word by word.” He had virtually no difficulty learning to write normally.

  4. 4

    At a memorial symposium on Geschwind’s work at Harvard on May 30, 1985, M. Mishkin noted that recent work indicates that Geschwind may have slightly underestimated the amount of cross-modal association in higher animals.

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