anatomical evolution and the human brain

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Anatomical Evolution and The Human Brain Norman Geschwind Norman Geschwind, M.D., is Professor of Neurology, Harvard Medical School. This address was delivered at the Twenty-second National Conference of the Orton Society, Boston, 1971. Some of the work reported here was carried out under Grant NS-06209 from the National Institutes of Health to the Aphasia Research Center, Department of Neurology, Boston University School of Medicine. For many reasons it is a great pleasure and an honor for me to be in- vited to appear before you this evening. Some of the reasons are trivial, but others are perhaps of more significance. I particularly welcome the oppor- tunity to speak on an aspect of the function of the brain that was of great concern to Dr. Samuel Orton, the distinguished figure for whom this society was named and to the furtherance of whose scientific approach its member- ship has dedicated itself. It is pleasing to me that Dr. Orton considered as a major influence on his life his apprenticeship at the Boston City Hospital under the great Dr. Frank Mallory, where he received a major impetus to his interest in the pathology of the nervous system (J. L. Orton 1966). I would like to feel that those of us who are working at the Boston City Hospital over sixty years later are continuing to show the same enthusiasm for looking into the events within the brain itself which will help to elucidate its normal and abnormal mechanisms. Indeed this interest in neurological mechanisms per- vaded Dr. Orton's thinking throughout his writings, even into the 1940's when the application of neurological thinking to disorders of behavior was not merely unpopular but often received with open hostility. I wish to address myself to the problem of how the human brain has, in the course of evolution, become specialized for language. It seems prob- able that language capacity, whatever it may be, did not appear suddenly, but that like most other evolutionary acquisitions it represents the sum of a long series of gradual changes. It seems possible, and indeed likely, that forerunners of this ability are present in animals lower in the phylogenetic scale. But this should not lead us away from the obvious and important fact that, whatever the linguistic capacities of lower animals may be, the young of the human species is distinguished by almost universal and rapid acquisition of a remarkable degree of language skills in the absence of spe- cial training.

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Page 1: Anatomical evolution and the human brain

Anatomical Evolution and The Human Brain

Norman Geschwind

Norman Geschwind, M.D., is Professor of Neurology, Harvard Medical School. This address was delivered at the Twenty-second National Conference of the Orton Society, Boston, 1971. Some of the work reported here was carried out under Grant NS-06209 from the National Institutes of Health to the Aphasia Research Center, Department of Neurology, Boston University School of Medicine.

For many reasons it is a great pleasure and an honor for me to be in- vited to appear before you this evening. Some of the reasons are trivial, but others are perhaps of more significance. I particularly welcome the oppor- tunity to speak on an aspect of the function of the brain that was of great concern to Dr. Samuel Orton, the distinguished figure for whom this society was named and to the furtherance of whose scientific approach its member- ship has dedicated itself.

It is pleasing to me that Dr. Orton considered as a major influence on

his life his apprenticeship at the Boston City Hospital under the great Dr. Frank Mallory, where he received a major impetus to his interest in the pathology of the nervous system (J. L. Orton 1966). I would like to feel that those of us who are working at the Boston City Hospital over sixty years later are continuing to show the same enthusiasm for looking into the events within the brain itself which will help to elucidate its normal and abnormal mechanisms. Indeed this interest in neurological mechanisms per- vaded Dr. Orton's thinking throughout his writings, even into the 1940's when the application of neurological thinking to disorders of behavior was not merely unpopular but often received with open hostility.

I wish to address myself to the problem of how the human brain has, in the course of evolution, become specialized for language. It seems prob- able that language capacity, whatever it may be, did not appear suddenly, but that like most other evolutionary acquisitions it represents the sum of a long series of gradual changes. It seems possible, and indeed likely, that forerunners of this ability are present in animals lower in the phylogenetic scale. But this should not lead us away from the obvious and important fact that, whatever the linguistic capacities of lower animals may be, the young of the human species is distinguished by almost universal and rapid acquisition of a remarkable degree of language skills in the absence of spe- cial training.

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B U L L E T I N O F T H E O R T O N SOCIETY

It is an odd fact that there is no accepted definition of the term "lan- guage." If we substitute the word "'communication" we diminish rather than increase precision. Communication exists at all levels in the animal king-

dom, and to employ this term is simply to refuse to face the problem of the distinctiveness of the skills so specially developed in man. To speak of "symbolic" systems is to introduce a term of much greater vagueness on which there is little agreement even among philosophers. Despite the lack of an adequate definition there is a remarkable degree of agreement even among naive observers as to what language is. To define language only as the kind of complex grammatical system normally found among adult speak- ers is by a stroke of the pen to eliminate all hope of understanding the

essential nature of what may lie almost hidden in the elaborately developed linguistic superstructure developed over many thousands of years.

I have suggested elsewhere that the essence of language is the remark- ably simple ability to learn to associate stimuli to two non-limbic sensory modalities (Geschwind 1964, 1965). It may be argued that this ability must exist in lower animals, but in fact the evidence for this view is weak. A monkey can certainly learn to choose between stimuli, but what he chooses is the stimulus which has important limbic consequences, i.e., he learns the stimulus which is followed by satisfaction of elementary needs, such as hunger or thirst, or which arouses fear or aggression. All of these elementary activities have their representation in the limbic system, an evolutionarily

ancient system at the core of the brain. Anatomically the visual, auditory, and bodily sensory systems have important connections to the limbic systems.

On the other hand in lower animals there are no direct connections be- tween the association cortexes for the visual, auditory, and somesthetic sys- tems. If connections between such systems are indeed essential for the de- velopment of language, how does man solve this problem? Various authors, including Orton (1925) himself, have in the past stressed the remarkable development of the angular gyrus region, an area which develops late in childhood. As I have pointed out (1964, 1965) this enormous area lies be- tween the association areas of the visual and auditory systems, i.e., it is ideally situated so as to act as a mediator between these sensory modalities. I would like to stress here that the significance of the angular gyrus region is not that it develops as a mediator between speech areas, but that the areas lying adjacent to it become language areas because of the new possibilities of de- veloping associations with other modalities.

At the time I advanced the concept several years ago that the angular

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Page 3: Anatomical evolution and the human brain

E V O L U T I O N AND T H E BRAIN

gyrus region was serving to make language possible by permitting the forma- tion of associations between non-limbic sensory modalities, it seemed likely that so remarkable a development in the human brain must have some fore- runner in the brains of lower animals. Developments in the past few years have clearly indicated that this is probably the case. My colleague, Dr. Deepak Pandya has devoted most of the past several years to a detailed map- ping of the connections of the cerebral cortex in the monkey, first under the direction of Henricus Kuypers, later independently or with other associates. Among the most important results of these studies have been, first, the defini- tion of areas of cortex receiving input from multiple sensory association areas. There are in fact, at least two such major areas in the cortex, one in the frontal lobe, the other comprising both the inferior parietal region and the superior temporal sulcus, It is only the latter, however, which seems to be most suited for the formation of intermodal associations, since it not only receives from but also sends connections to each of the sensory association

cortexes (Pandya and Kuyper 1969; Jones and Powell 1970). This area is apparently the forerunner of the great angular gyrus region of man. The discovery of a forerunner in the animal is of great importance since it may open to experimental investigation exactly what the properties are of those capacities which underlie language. Another important contribution has been the clarification of connections from cortex to the limbic system, which should help to create further understanding of the learning process in general (Van

Hoesen et al. 1972). Let me turn now to another aspect of the brain which may perhaps

represent an even more unique endowment of the human nervous system. It is a property which, as far as is known at the present time, appears in the mammalian series only in man, and which is of major concern not only for the study of acquired disorders of the higher functions, but also for the inti- mate understanding of the disorders of development of these functions, which is the major concern of most of you. I am referring to cerebral dominance.

After the first World War there appeared a strong tendency to forget the great tradition of anatomical studies which established most of our present knowledge in this field. The study of cerebral dominance suffered in much the same way as other aspects of the higher functions. It became the rule to assert that the remarkable functional asymmetry of the brain could not be accounted for by any obvious difference in anatomical structure. Even today it is quite common to hear it said that the two cerebral hemispheres of man are mirror images, and the differences in function between the two sides must

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B U L L E T I N O F T H E O R T O N SOCIETY

be the result either of very subtle anatomical differences or of as yet undis- covered physiological differences.

Several years ago I began to review the literature on the question of the possible basis of cerebral dominance. I began, as most people would, by reading some review articles on the topic. I found at first what was generally expected, that there was a general tendency to deny the existence of significant anatomical differences between the hemispheres. I furthermore found that the work of those authors who had asserted the existence of anatomical differences was uniformly dismissed as unconvincing, biased, based on selective cases, or as showing differences which were too small to account for, the remarkable functional asymmetries of the brain.

By a chance accident, however, it became obvious that these judgements of the older literature did not accurately reflect the true caliber of the work represented in it. I happened one day to come across an article by the German neurologist Richard Arwed Pfeifer (1936) in which he described what ap- peared to be dramatic differences between the left and right hemispheres. I soon found that a whole array of great pioneers such as Heschl, Flechsig, and Cunningham, all careful and distinguished investigators, had described such differences.

It seemed that the time was ripe to reinvestigate this problem, and in association with Dr. Walter Levitsky I began the study of the asymmetries of the brain (Geschwind and Levitsky 1968).

Pfeifer's work gave us the clue, although it did not give us the full answer. We found his criteria difficult to apply and his work lacked statistical validation. It did, however, point us to the correct area, the upper surface of the temporal lobe of the brain.

Through the kindness of Dr. Turner McLardy and Dr. Jose Segarra, 100 normal adult human brains were made available to us. They were prepared by separating the hemispheres. Following this a cut was made with a broad- bladed knife along the axis of the Sylvian fissure, thus exposing the upper surface of the temporal lobe.

When we look at the upper surface of the temporal lobe we observe the following structures. Running diagonally we see Heschl's gyrus which con- tains the primary auditory cortex. Behind Heschl's gyrus and the posterior border of the Sylvian fossa lies an area which is called the planum temporale. This area on the left side is the continuation on the upper surface of the

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Page 5: Anatomical evolution and the human brain

E V O L U T I O N AND T H E BRAIN

TEMPORAL POLE LEFT y~~"-~¢ ~~'~x RIGHT

PLANUM ~ t ~ ~ ) / ~ PLANUM TEMPORALE--~,__~ ~ ~ ~ t L ~ . J / TEMPORALE

OCCIPITAL POLE Fig. 1. Diagrammatic sketch of horizontal section through the human

brain so directed as to expose the upper surface of the temporal lobe. The planum temporale (shaded) lies behind Heschl's gyrus, containing the primary auditory cortex, and forms the posterior portion of the upper surface of the temporal lobe. Note the larger size of the planum on the left.

temporal lobe of Wernicke's area, one of the major regions involved in lan- guage. We found great differences between the planum temporale on the left and the corresponding region on the right. Let us now consider these differ- ences.

Typically we find that the planum temporale is larger on the left-hand side. There are two factors leading to the greater size of this region. In the first place the left Sylvian fissure is longer than the right Sylvian fissure. As a result the posterior border of the left Sylvian fossa is angled backwards much more sharply than the one on the right. The second effect, although less marked, is the tendency for the sulcus of Heschl to be angled forward more sharply on the left. As a result of both of these tendencies the left planum temporale is longer than its fellow on the opposite side. Furthermore, the left planum tends to have a well-defined triangular shape, while the one on the right is often more elliptical in appearance.

Now let us consider the statistical summary of these findings. The planum temporal is larger on the left in 65% of cases, larger on the right in 11% of cases, and approximately equal in 24%. The right-left difference is significant at better than the .001 level.

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Page 6: Anatomical evolution and the human brain

BULLETIN OF THE ORTON SOCIETY

Another way to look at these data is to consider the absolute data. The left planum averages 3 .6+ 1.0 cm in length, while the right planum averages 2.7-+-1.2 cm in length. In other words the left planum temporale is nearly a full centimeter longer than its fellow on the right. In relative terms it is one- third larger. In many individual cases it may be several times larger.

These differences are so large that no microscope is needed to observe them. They are readily visible to the naked eye and require nothing more than an ordinary ruler if one wishes to gather quantitative data. We must also remember that no such differences are known in any other mammal as far as we know man is the only mammalian species showing such right-left asymmetries.

Now let us return once more to the nature of the area which is larger on the left side. Let me remind you again that this cortical zone is part of Wernicke's area, one of the major speech regions. Its importance for human language functions was first pointed out by Carl Wernicke, who was, in my opinion, the most important figure in the history of the study of aphasia (Geschwind 1967).

Since our original publication our data have been confirmed by a later study carried out by Wada (1969) in Vancouver. He added one further important observation. He studied not only the brains of adults but also the brains of new-borns and fetuses. He found that the right-left asymmetries described by Levitsky and myself were present in these infant brains with a similar statistical distribution. In other words these asymmetries are inborn and not acquired as the result of experience.

It certainly seems reasonable to hypothesize that this great anatomical difference accounts for the dominance of the left hemisphere in language in most people. If this is the case then it is reasonable to hope that as the basis for cerebral dominance becomes dearer, it may be possible to explore further its mechanisms at a physiological level.

References

Geschwind, N. 1964. The development of the brain and the evolution of language. Monograph Series on Languages and Linguistics 17: 155-169, Georgetown Press, Georgetown.

• 1965. Disconnexion syndromes in animals and man. Brain 88: 237-294, 585-644.

- - . 1967. Wernicke's contribution to the study of aphasia. Cortex 3: 449-463. Geschwind, N. and Levitsky, W. 1968. Human brain: right-left asymmetries in tem-

poral speech region. Science 161: 186-187. Jones, E. G., and Powell, T. P. S. 1970. An anatomical study of converging sensory

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EVOLUTION AND THE BRAIN

pathways within cerebral cortex of the monkey. Brain 93: 793-820. Orton, J. L. (compiler). 1966. "Word-Blindness" in School Children and Other

Papers on Strephosymbolia (Specific Language Disability--Dyslexia) 1925-1946, by Samuel Torrey Orton, M.D. Pomfret, Connecticut: The Ortoa Society.

Orton, S. T. 1925. "Word-Blindness" in school children. Archives of Neurology and Psychiatry 14: 581-615.

Pandya, D. N. and Kuypers, H. G. J. M. 1969. Cortico-cortical connections in the rhesus monkey. Brain Research 13: 13-36.

Pfeifer, R. A. 1936. Pathologie der Horstrahlung und der corticalen Horsph~ire. In Handbuch der Neurologie, ed. O. Bumke and O. Foerster, Vol. 6, pp. 533-625.

Van Hoesen, G. W., Pandya, D. N., and Butters, N. 1972. Cortical afferents to the entorhinal cortex of rhesus monkey. Science, in press.

Wada, J. 1969. Presentation at 9th International Congress of Neuro!ogy, New York.

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