Critical Time For Learning Sign Language

There is widespread acceptance 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 and nuances of language. There also is evidence of similar critical periods for acquiring a second language. It now appears that this same rule applies for the acquisition of Sign language (SL) as well.

Neuroscientists examining the brain activity of people who learned to speak American Sign Language (ASL) at different times in their lives have found the first evidence that there is a critical period for acquiring a non-verbal language, just as there is for spoken languages. This research has major implications for early education of all children because it stresses 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 Sign Language.

Using functional magnetic resonance imaging, the researchers discovered that patterns of brain activity in bilingual people who learned There is widespread acceptance 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 and nuances of language. There also is evidence of similar critical periods for acquiring a second language. It now appears that this same rule applies for the acquisition of ASL as well. ASL before puberty differed from those who learned it after puberty. 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.

David Corina, co-author of the study comments: "We know that late learners of SL, while they are very fluent, never will be fully fluent like native, or early, learners of SL." 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 SL. The left hemisphere activity has long been associated with the processing of spoken languages. Specifically, one area of the brain that is the signature, or specific, to signers if they learned SL as a native signer, is the right angular gyrus. This 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.

These findings are reported in the March 2002 issue of the journal Nature Neuroscience. They indicate there are regions in the brain's right hemisphere that are activated when children who learned 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.