Deaf babies of deaf parents babble with their hands in the same rhythmic, repetitive fashion that hearing infants babble vocally, a new study has found.
The deaf babies, who presumably watch their parents use sign language at home, start their manual babbles before they are 10 months old, the same age that hearing children begin stringing together sounds into word-like units.
And just as hearing babies experiment with a few key noises like "dadadada" or "babababa," so deaf infants use several motions over and over, including one gesture that looks like "OK" and another that resembles a hand symbol for the numeral 1.
The gestures of the deaf children do not have real meaning, any more than babble has meaning, but they are far more systematic and deliberate than are the random finger flutters and fist clenches of hearing babies.
The motions seem to be the deaf babies' fledgling attempts to master language, said Dr. Laura Ann Petitto, a psychologist at McGill University in Montreal. She is the principal author of the new report, which is appearing today in the journal Science.
The new research strongly suggests that the brain has an innate capacity to learn language in a particular, stepwise fashion, by stringing together units into what eventually become meaningful words, Dr. Petitto said.
The brain will progress from one stage to another regardless of whether language is conveyed through speaking, hand-signing or presumably any other method of communication, she added.
The results contradict a widespread assumption among linguists that the maturation of the vocal cords affects language development among infants.
For centuries, people thought that speech is language, and language is speech," she said.
"There's been a whole complicated notion that the structure of the motor apparatus and the unfolding of the mouth muscles actually influenced the structure and development of language."
But in showing that deaf babies babble with their hands in a manner that has all the basic elements of vocal babbling, she said: "We've decoupled language from speech. We've torn them apart."
Other researchers in the field of early language development praised the new study as significant and extremely well designed.
"I think this is important work," said Dr. Richard P. Meier, assistant professor of linguistics and psychology at the University of Texas, Austin. "It's been suggested that all children pass through a regular sequence of milestones in speech acquisition, from simple cooing early on, to structured babbling at 8 months, to the first word at about 12 months. This work gives us a new dimension of how language matures."
Dr. Petitto and a graduate student, Paula F. Marentette, videotaped five infants at ages 10, 12 and 14 months.
Two of the infants were deaf children of deaf parents who use American Sign Language to communicate, while the other three were hearing offspring of hearing adults.
The researchers analyzed every hand gesture of the infants and compared the two groups.
They found that the hearing children made many hand gestures but that the gestures never became organized or repetitive.
By comparison, the deaf babies soon began showing evidence of using about 13 different hand motions over and over again.
Nearly all of them were actual elements of American Sign Language: gestures that do not in themselves mean anything, ** but have the potential to indicate something when added to other gestures.
Sign languages are structured much like any spoken language, Dr. Petitto said.
Distinct gestures and hand shapes are the equivalent of syllables, and thus must be presented in a series to assume any sense.
She believes that the deaf parents noticed the nascent efforts of their children to communicate through signs and began reinforcing the gestures, just as normal parents talk back to and reinforce their babbling infants by turning the babble into words, for example, by saying "dadadada . . . Daddy."