Speech and language functions are of fundamental human significance, both in social interaction and in private intellectual life. When they are disturbed as a consequence of brain disease, the functional loss exceeds in many ways all others in gravity—even blindness, deafness, and paralysis. The neurologist is concerned with all derangements of speech and language, including those of reading and writing because they are almost invariably manifestations of disease of the brain.
Viewed broadly, language is the means of symbolic representation of objects, actions, and events and, therefore, the mirror of all higher mental activity. The internal manipulation of these symbols constitutes thinking and their retention is the substance of memory. In a narrower context, language is the means whereby patients communicate their complaints and problems to the physician and at the same time, the medium for all delicate interpersonal transactions. Consequently, any disease process that interferes with speech or the understanding of spoken words touches the very core of the physician–patient relationship. Finally, the study of language disorders and the development of language (see Chap. 27) serve to illuminate the relationship between psychologic functions and the anatomy and physiology of the brain.
It has been remarked that as human beings, we owe our commanding position in the animal world to two faculties: first, the ability to develop and employ verbal symbols as a background for our own ideation and as a means of transmitting thoughts, by spoken and written word, and second, the remarkable facility in the use of our hands. One curious and provocative fact is that both language and manual dexterity (as well as praxis) have evolved in relation to particular aggregates of neurons and pathways in one cerebral hemisphere (the dominant one). This is a departure from most other localized neurophysiologic activities, which are organized according to a contralateral or bilateral and symmetrical plan. The dominance of one hemisphere, usually the left, emerges in brain development together with speech and the preference for the right hand, especially its use for writing. It follows that a lack of development or loss of cerebral dominance as a result of disease deranges both these traits, causing aphasia and apraxia.
There is abundant evidence that higher animals are able to communicate with one another by vocalization and gesture. However, the content of their communication is their feeling or reaction of the moment. This emotional language, as it is called, was studied by Charles Darwin, who noted that it undergoes increasing differentiation in the animal kingdom. Only in the chimpanzee do the first semblances of propositional language become recognizable. Indeed, there are distinct differences between the human and chimp versions of a gene called FOXP2, which has been linked to the ability to produce language, as noted in Chap. 27 (also see Balter). Another genetic influence on language has been found by Somerville and colleagues, who studied ...