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Infant vocal babbling has been assumed to be a speech-based phenomenon that reflects the maturation of the articulatory apparatus responsible for spoken language production. Manual babbling has now been reported to occur in deaf children exposed to signed languages from birth. The similarities between manual and vocal babbling suggest that babbling is a product of an amodal, brain-based language capacity under maturational control, in which phonetic and syllabic units are produced by the infant as a first step toward building a mature linguistic system. Contrary to prevailing accounts of the neurological basis of babbling in language ontogeny, the speech modality is not critical in babbling. Rather, babbling is tied to the abstract linguistic structure of language and to an expressive capacity capable of processing different types of signals (signed or spoken).
Petitto et al. (1991) studied this question.