Scientists have long debated why babies babble. Some maintain that a seven-month old's 'ooos' and 'aaas' just pop out when she opens and closes her mouth. Others suggest that babbling shows a baby's growing sensitivity to the patterns and rhythms of human speech. A new experiment, reported today in Nature, lends credence to the second idea. Laura Ann Petitto of Dartmouth College and her colleagues studied the hand movements of hearing infants born to profoundly deaf parents and found that these children seem to 'babble' with their hands.

To measure the gestures of their tiny subjects with precision, Petitto's group used optical sensors that tracked the trajectories of light-emitting diodes placed on the babies' hands. In this way, they studied three children of deaf parents and three children of hearing parents. The babies, who were all deemed to be at the same level of development, came in for examinations at six, ten and 12 months old. The researchers discovered that hearing babies exposed to sign language displayed two distinct patterns of hand movements, only one of which they shared with the other children.

Indeed, the infants of signing parents exclusively produced low-frequency hand motions. The other babies only made high-frequency gestures. Also, the sign-exposed babies made the low-frequency motions within a restricted space in front of their bodiesan area that corresponded with the space in which signs are made. "This dramatic distinction between the two types of hand movements indicates that babies find it important and can make use of the rhythmic patterns underlying human language," Petitto says. Next they plan to explore the sing-song rhythms parents use to communicate with their infants.