HANOVER, N.H. - Baby babbling, universally uttered by healthy hearing babies when they are about seven months old, is thought to mark the developmental moment when a young child embarks on the road to spoken language. Now, new insight into why this behavior occurs can be found in the hands of hearing babies as they acquire a natural signed language.
According to a study published in the September 6, 2001, issue of Nature, lead author Laura Ann Petitto, a Professor in the Department of Education and in the Department of Psychological and Brain Sciences at Dartmouth College, suggests a fresh approach to how babies begin the remarkable process of acquiring language.
The results of the study, titled "Language Rhythms in Babies' Hand Movements," support the idea that babies are born with sensitivity to highly specific rhythmic patterns naturally found in languages. It is so powerful that a baby can find and produce the rhythms of language even without vocal input from parents. The study further suggests the tantalizing idea that a baby's perception of the rhythmic pattern is a key mechanism that launches the process of human language acquisition.
Petitto conducted the study with students and colleagues at McGill University (Montreal, Canada): Siobhan Holowka, Lauren Sergio (now an assistant professor at York University) and Professor David Ostry (also with Haskins Laboratories in New Haven, Conn.). They examined the hand movements of hearing babies born to profoundly deaf parents and compared them to the hand movements of hearing babies with hearing parents.
"Hearing babies with signing deaf parents make a special kind of movement with their hands, with a specific rhythmic pattern, that is distinct from the other hand movements," said Petitto. "We figured out that this kind of rhythmic movement was linguistic. In fact, it was babbling, but with their hands."
Petitto and her colleagues studied six hearing babies: three received no significant exposure to spoken language, only signed language from profoundly deaf parents, and three were exposed to spoken language. The two groups were equal in all developmental respects, with the only difference found in the form of the language input they received (by hands or by mouth).
This discovery points to the centrality of the rhythmic patterns that underlie human language. Parents and educators can exploit this natural proclivity in children and aid the language learning process. The sing-song way in which delighted parents speak to their baby, and the playful rhyming games common to nursery rhymes at home and in school, could be more important for a child's developing brain than previously imagined, and they provide an important tool for the young child to discover the grammar and structure of his or her native language.
The group used an optoelectronic position-tracking system that recorded three-dimensional information of the babies' hands when they were approximately six, ten and 12 months old. While the children played in 60-minute sessions, sensors tracked the location and path of light emitting diodes (LEDs) on each of their hands. Then, computer programs calculated the speed and cycles of hand movements over time.
Independently, videotapes were made and viewed only after the measurements were calculated. Because Petitto's team used this precise method of analysis, they were able to overcome the subjective problems associated with solely viewing videotapes of babies' hands that have plagued research on this topic for decades.
The findings revealed that hearing babies exposed to signed language produce two types of hand activity, while the hearing babies exposed to spoken language produce primarily one type. Sign-exposed babies displayed a low-frequency rhythmic activity, where the hands move in undulating bursts of about one complete cycle per second, in addition to another type of high-frequency rhythmic activity, where the hands moved in undulating bursts of around two and a half complete cycles per second. Speech-exposed babies displayed high-frequency activity nearly exclusively.
Surprisingly, only the sign-exposed babies produced low-frequency hand activity largely within a tightly restricted space in front of their bodies, corresponding to the location where signs must occur in natural signed languages. Speech-exposed babies produced most of their high-frequency hand activity outside of this crucial linguistic location.
"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," said Petitto.
Next, the Petitto team will investigate the physical properties of the sing-song rhythms that parents use to communicate with their babies. The researchers want to learn just how fine-tuned a human baby's sensitivity is to the rhythmic patterns of language their parents use.