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.”