Special talents and defects

I. THE PHYSIOLOGICAL MECHANISM OF MENTAL LIFE

Psychologists no longer question that the product of mind, which we call behavior, by which mind is judged, is in some way intimately connected with the sensitivity and integrity of the nervous system. The proof of this has often been set forth, and will merely be taken for granted here. Any organ or substance which reacts upon this sensitivity or integrity may then indirectly influence mental life in certain respects. For instance, the glandular system of the body, especially that part of it which comprises the glands of internal secretion, may affect behavior by affecting the growth or function of the nervous system. Drugs may influence mental processes, because they act upon the neurones. However, all present knowledge points to the conclusion that if the nervous tissue could be isolated from such influences, mental life would be immune from their effects. Mental life is but indirectly subject to such influences, in so far as nervous tissue is affected in a particular manner by them.

II. ATTEMPTED LOCALIZATION OF MENTAL FUNCTIONS

When it was thought that such supposed entities as “the reason,” “the will,” “the memory,” and “the imagination” would be identified as mental functions, it was also supposed that a definite location for each might be found in the brain. As investigators were compelled to change their concept of a mental function, and to define mental functions in terms of observable performance, they still sought to discover whether or not each performance might be referred to a definite set of neurones. This question of brain localization constitutes a current topic of research. So little information can be given as yet upon the subject that it is, perhaps, unwarranted to consider it at all in this volume, where the chief interest does not center in the controverted theories of neurology.

Much of the proof for the statement first made in this chapter, that the nervous system is the physiological mechanism of mental life, has been adduced through study of neuropathology. Persons impaired in a given manner in their nervous tissue, show behavior characteristically altered. Moreover, given alterations in behavior can be produced experimentally in animals, by altering the connections in the nervous system, and by no other means. Through these observations it has been possible to assign certain functions to parts of the physiological mechanism.

In the case of man, both by observation and experiment, “the nervous structure below the hemispheres of the cerebrum has been excluded from the possibility of acting as the immediate physical basis of mental states.”^[9] The higher mental processes, which involve the possibility of speaking, calculating, and responding by learned reactions to complex situations, have their correlate in the cortex (the agglomeration of neurones in the cerebral hemispheres). Physiological psychologists therefore investigate the cortex, in their search for the particular neurone-patterns or areas involved in particular intellectual performances.

The problems of brain localization have, therefore, been approached through the study of the alterations in performance, which accompany alterations in given areas of the cortex. Alterations in restricted areas of brain tissue, in human beings, are brought about chiefly by obstruction of a blood vessel, hemorrhage, tumor, and laceration or depression through injury to the skull.

One of the early observations, bearing upon topics considered in the subsequent chapters of this volume, was that by Broca. Broca described two cases of pathological impairment in a limited convolutional region of the left cerebral hemisphere, in which the use of words was lost, without loss of intelligence as expressed in other ways. Broca therefore suggested “articulate language” to be a function connected with the part of the brain to which the impairment had been restricted.

A large number of similar observations have been reported since Broca’s publication, describing cases of selective loss of some linguistic function, especially in connection with paralysis of limbs. The localization of articulate language, as a special ability, in Broca’s area, is still, however, debated by those most competent to discuss the matter, and no positive statement is at present warranted. Head, one of the foremost among modern students of neurology, has recently advanced the theory that special disturbances of articulate language (aphasia, alexia, agraphia, aphemia) are due to disturbances of those psychic processes whereby symbolic association is accomplished,—whereby men learn to imbue symbols with meaning. Von Monakow interpreted the array of data existing in 1914 to show that all gnostic functions (intellectual performances) pertain to the cortex as a whole, and not to any center or centers in the brain. He held that no case of aphasia permanently remains, unless there is at the same time diffuse cortical degeneration. Ladd and Woodworth, writing in 1911, concluded that “there is good evidence that the Broca region is the most vulnerable part of the cortex, as regards the motor coördination of speech,” but that “the entire cerebrum would seem to be, of necessity, involved in man’s linguistic attainments and uses.”

III. THEORY OF CONGENITAL LESION OR ATROPHY CRITICIZED

Reasoning from analogy with cases where a function of language is lost selectively, through organic disease or impairment of brain structure, it was thought by those who first described innate special disabilities, as in reading or spelling, that such defects must be due to congenital brain lesions or atrophies. Neurological research has never verified this supposition. No cases showing innate disability to be correlated with any peculiarities of restricted areas in the cortex have ever been recorded. Tilney and Riley, summarizing critically the data of neurology in 1921, cite no cases considered to afford authentic evidence of localized lesions or defects, as the basis of congenital difficulty in reading, spelling, music, or other functions with which the present treatise is concerned.

The theory of innate lesion or atrophy of a cortical area, to account for disability in a special mental function, seems unscientific for other reasons, aside from the fact that it has never been objectively verified by actual observation of a structural defect. One of these reasons is that a theory, formulated to take care of the neural basis of specialized disabilities, must take care of specialized gifts, as well. Cases where a generally stupid child is innately gifted with special ability to master the mechanics of reading, for example, are no doubt as frequent as cases where a generally capable child learns them with difficulty. The theory of specialized lesions or other faults of structure might cover disabilities, but would it cover special talents as well?

Still another consideration prevents us from regarding the theory of localized brain defects as masterly. This is the fact referred to in our preliminary discussion, that every single mental function, which yields to measurement, is found to be distributed among human beings according to a probability curve. (See Figure 1, page 8.) The functions which we herein consider are not exceptions to this principle. Performance in reading, spelling, arithmetic, drawing, music, and so forth, shows children or adults, chosen at random, to be distributed in the given form. Those who have exceptional talents or defects in the function fall within the symmetrical surface of this curve, at its opposite extremes. Nowhere is there a point of demarkation, denoting absolute lack of the trait in a group falling below that point, as there would be if a number of individuals suffering from lesions were introduced into the distribution. We may fairly demand of a theory which undertakes the explanation of the most extreme deviations, the explanation of the deviations of lesser magnitude, as well. The curve obtained by test approximates that form which mathematicians tell us appears when an infinite number of factors act together in an infinite number of ways, the extreme deviations occurring inevitably, by chance. A theory introducing the adventitious circumstance of lesion or atrophy is thus superfluous to the explanation of the extreme unfortunate deviations. To admit it would violate the rule of scientific method known as the law of parsimony, for we do not need it in order to explain the facts.

IV. RESTITUTION OF FUNCTION WITHOUT REGENERATION OF STRUCTURE IN INJURED BRAINS

Fully as important as any of the facts mentioned above, for criticism of the theory that special deficiencies are due to localized defects in brain structure, are the experiments with reëducation of those who have suffered loss of an ability. Persons who have lost the power to read, or write, or speak after destruction of brain tissue, may learn to perform these functions again, without regeneration of the area impaired.

If the neurones destroyed, and no others, were the special mechanisms rendering possible the functions lost, how would restitution of function be possible, without repair of the destroyed tissues?

V. ATTEMPTS TO ESTABLISH A NEURAL BASIS FOR THE “TWO FACTOR THEORY” AND “THE TWO LEVEL THEORY”

In prosecuting their researches from the psychological point of view, by the method of testing performance, Spearman, Moore, Thomson, and other investigators referred to in the preceding chapter, did not neglect the attempt to reconcile their findings with a possible neural basis.

Spearman wrote: “The theory of ‘two factors’ just delineated, though primarily of psychological origin, has shown itself capable of translation into terms of cerebral physiology.” The “specific factors” Spearman would identify with some “particular cortical region, or other neural characteristic, coördinated to the particular performance in question.” The “general factor” is derived from the fact that all neurones of the cortex arise from the same heredity, and must resemble each other, as “the hair in one region of a person’s scalp normally resembles that on the other regions” (a somewhat precarious analogy); also, from the fact that all parts of the brain are nourished by the same blood supply; and from the supposition that “each momentary focus of cortical activity receives continual support from energy liberated by the entire cortex (or some still wider neural area).”

Thomson said: “Let us suppose that the mind, in carrying out any activity such as a mental test, has two levels at which it can operate. The elements of activity at the lower level are entirely specific; but those at the higher level are such that they may come into play in more than one kind of activity, in more than one mental test.... The difference between the levels may be physiological, as between cortex and spinal cord, or it may be the difference between conscious and non-conscious, or what not. The theory may later be reduced to a less harsh dichotomy and there may be gradations from the one level to the other.”

These attempts to find a neural basis for the “Two Factor Theory” and the “Two Level Theory” are obviously not very complete.

VI. PRESENT STATUS OF THE PROBLEM

The conclusion is that at present experimental neurology has nothing secure to offer by way of establishing the neural basis of the special talents and defects, which we wish to consider. We must suppose that in some way unknown they are connected with neural activity, but localization of each function in a restricted area of the brain structure has never been established.

The deviations in performance are almost certainly biological, and not pathological. Each mental function is by original nature possible in some degree to every person, the degrees of potentiality being of enormous range, and distributed among members of the species according to a frequency curve. The form of this curve indicates that the determinants of aptitude are approximately infinite in possibility of combination. The extremes of deviation from the typical result of these determinants acting together, are, as stated, very widely separated, as in any game of chance combining many factors, but they nevertheless have limits, which are knowable. The determinants exist chiefly (perhaps exclusively) in the germ-plasm, from which human organisms spring, and which carries inheritance from countless combinations of ancestry for persons now alive. It is neither necessary nor plausible to introduce a theory of brain lesion or atrophy to explain the extreme minus deviations, leaving the equally extreme plus deviations thus unexplained.

The sum total of a child’s standings on these curves, in the multitude of mental functions which are possible to human beings, constitutes his psychograph or mentality. The physiological aspects of this inheritance may ultimately be found in brain chemistry, or in the discovery of some principle of physics at present unknown. It may be an inheritance of function, rather than of structure. We do not know.

The present status of the problems indicated in this chapter may be recapitulated in the words of Ladd and Woodworth: “The analysis of mental functions into their elements, in a manner suitable for physiological use, has scarcely been begun.”

REFERENCES

Broca, P. P.—Sur le siège de la faculté du langage articulé, avec deux observations d’aphémie; V. Masson et Fils, Paris, 1861.

Franz, S. I.—“Cerebral-Mental Relations”; Psychological Review, 1921.

Head, H.—“Aphasia and Kindred Disorders of Speech”; Brain, 1920.

Head, H.—“Release of Function in the Nervous System”; Proceedings of the Royal Society, Series B, 1921.

Head, H.—“Disorders of Symbolic Thinking Due to Local Lesions of the Brain”; British Journal of Psychology, 1921.

Ladd, G., and Woodworth, R. S.—Physiological Psychology; Charles Scribner’s Sons, New York, 1911.

Lashley, K. S.—“Studies of Cerebral Functions in Learning”; Psycho-Biology, 1920.

Monakow, C. von—Die Lokalisation im Grosshirn; Bergmann, Wiesbaden, 1914.

Tilney, F., and Riley, H. A.—The Form and Functions of the Central Nervous System; Hoeber, New York, 1921.