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In humans the two cerebral hemispheres of the brain are functionally specialized with the left hemisphere predominantly mediating language skills. The basis of this lateralization has been proposed to be differential localization of the linguistic, the motoric, or the symbolic properties of language. To distinguish among these possibilities, lateralization of spoken language, signed language, and nonlinguistic gesture have been compared in deaf and hearing individuals. This analysis, plus additional clinical findings, support a linguistic basis of left hemisphere specialization.

The experiment of teaching a young chimpanzee to use American sign language1 is an important advance on previous attempts to test the linguistic potential of primates. For the first time, a primate’s capacity for a language used by some humans has been clearly separated from his capacity for making the sounds of human speech. In the nature of things, this pioneer study has been made under special conditions, and (like any single study) cannot be assumed to be perfectly representative. Nevertheless, it does offer evidence of a new kind, in the light of which it is timely to reexamine the relation between human language and the signals that animals use or can learn to use.