Using functional magnetic resonance imaging (fMRI), the researchers discovered patterns of brain activity in bilingual people who learned ASL before puberty differed from those who learned it after puberty.
The findings are reported in this month's issue of the journal Nature Neuroscience. They indicate there are regions in the brain's right hemisphere that are activated when children who learned ASL before puberty are reading sign language. The brains of children who learned ASL after puberty show significantly less right hemisphere activity when they are doing the same activity.
There is widespread acceptance among neuroscientists that there is a critical period for first language acquisition, and that children who are not exposed to any language before puberty, or perhaps sooner, are unable to fully acquire and use the principles of language. There also is evidence of similar critical periods for acquiring a second language.
"We know that late learners of ASL, while they are very fluent, never will be fully fluent like native, or early, learners of ASL," said David Corina, a University of Washington associate professor of psychology and a co-author of the study. Corina is fluent in ASL.
"One aspect of ASL that is difficult for late learners is verb signs of motion. You see some subtle errors in their use of these verbs, just as you might detect subtle grammatical differences when listening to bilingual users of a spoken language when they are not using their native tongue."
Other members of the research team are Aaron Newman, a University of Oregon doctoral student; Helen Neville, University of Oregon psychology professor; Daphne Bavelier, assistant professor of brain and cognitive sciences at the University of Rochester, and Peter Jezzard, a physicist at John Radcliffe Hospital in England.
The new study builds on earlier work by this research team showing that right hemisphere activity, along with activation in the left hemisphere, is necessary for processing ASL. The left hemisphere activity has long been associated with the processing of spoken languages.
"One area of the brain that is the signature, or specific, to signers if they learned ASL as a native signer, is the right angular gyrus," Corina said.
It is located at the juncture of the temporal and parietal lobes. Activation of the left angular gyrus has been associated with reading English and other spoken languages for many years. The new study shows consistent activation of the right angular gyrus among native signers and some, but not consistent, activation of that brain region among late signers.
The study involved 27 bilingual subjects. Sixteen were hearing persons born to deaf parents. They learned ASL and English from birth as native languages. The remaining 11 were the late learners who had English as their native language and learned ASL after puberty, in early adulthood. All of the subjects watched a screen while their brains were imaged using fMRI and were asked to read written English sentences and meaningless strings of consonants. They also were shown and asked to read ASL sentences and meaningless gestures that were similar to real ASL signs.
"This work is important because we want to understand the neural systems underlying language," said Corina. "We want to know if they are malleable or fixed and the degree to which they may vary in different languages. We now know there is activation in the right hemisphere when native signers view ASL, and to see that this is dependent on early exposure suggests there are specific times when neural systems for language may be particularly sensitive to change."
He added that the research has implications for early education of all children because it stressed the need for early language exposure at critical times in development. And now, it is equally important in education for the deaf to ensure linguistic competency in ASL.
The National Institute of Deafness and Communicative Disorders, the National Sciences and Engineering Council of Canada, the Charles A. Dana Foundation and a University of Oregon post-graduate scholarship funded the research.