EMERGENT CONTROL OF MANUAL AND VOCAL MOTOR-ACTIVITY IN RELATION TO THE DEVELOPMENT OF SPEECH

Babbling typically precedes, resembles, and conceivably facilitates de velopment of speech, and yet there is no accepted neurobiological char acterization of babbling, Here we report a study of infants' developin g control of vocal behavior in relation to manual activity performed u nder differing conditions of audibility. We hypothesized that babbling is associated with the onset of left-lateralized motor control, as ex pressed in repetitive right-handed activity, and that audibility facil itates such activity, Sixty-one normally developing infants were seen before (N = 21) or at various intervals following (N = 40) the onset o f babbling. Ln experimental trials, audible or inaudible rattles were placed in left or right hands equally often. Analysis of manual activi ty revealed little shaking movement in the youngest and vocally least differentiated infants, and a sharp increase in shaking in slightly ol der 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 i n younger or older infants. These and other findings suggest that the left cerebral hemisphere may be disproportionately involved in the pro duction of repetitive vocal-motor activity as occurs in babbling. (C) 1995 Academic Press, Inc.