Emergent control of manual and vocal-motor activity in relation to the development of speech.

Babbling typically precedes, resembles, and conceivably facilitates development of speech, and yet there is no accepted neurobiological characterization of babbling. Here we report a study of infants' developing control of vocal behavior in relation to manual activity performed under differing conditions of audibility. We hypothesized that babbling is associated with the onset of left-lateralized motor control, as expressed in repetitive right-handed activity, and that audibility facilitates such activity. Sixty-one normally developing infants were seen before (N = 21) or at various intervals following (N = 40) the onset of babbling. In experimental trials, audible or inaudible rattles were placed in left or right hands equally often. Analysis of manual activity revealed little shaking movement in the youngest and vocally least differentiated infants, and a sharp increase in shaking in slightly older infants who had recently begun to babble. Surprisingly, audibility only marginally enhanced shaking activity. A dextral bias was evident in the shaking of infants who had recently begun to babble, but not in younger or older infants. These and other findings suggest that the left cerebral hemisphere may be disproportionately involved in the production of repetitive vocal-motor activity as occurs in babbling.