LIMBIC INFLUENCES ON HUMAN SPEECH *

The history of the modern study of human communicative disorders began in the latter half of the last century with the studies of Broca,' of Wemicke.2 and of other neurologists. Their discovery and description of critical areas of neocortex located in a dominant hemisphere set the context of discussion and theory for the next one hundred years. Studies and experiments on human speech have, to a large extent, been confined to these areas of the neocortex and to their relationship to human propositional speech. Apparently forgotten for many years has been the comparative neurology of vocalization and vocal communication. It is known now that animal vocalization is generated and supported by tissue lying exclusively, or nearly so, within the limbic system. But, at the time of the discoveries of Broca and Wernicke. this was not known. The limbic system was not yet defined, and techniques for studying the subcortical and basal parts of the brain were a number of years away. Therefore, most workers assumed that the neural apparatus supporting animal vocalization was related to the human speech areas, that the latter developed or evolved out of the former, and that human speech was itself a highly developed form of animal vocalization. However, evidence gathered in the past fifteen years has suggested strongly that animal vocal communication is limbic in origin334 and that human speech arose from new If this is true, and if human speech did not evolve out of animal vocalization but represents an independent development, it then becomes important to inquire whether the limbic system, essential for vocal communication in subhuman species, has lost this function in man or whether it still plays a significant role in human communication.