Source: LAURA ANN PETITTO

Scientists have long been interested how the deaf process signed languages in the brain. Understanding that activity could shed light on whether the brain harbors specialized structures for decoding linguistic patterns in general¿regardless of how they are conveyed. In fact, a new study published in Tuesday's issue of the Proceedings of the National Academy of Science suggests that the brain does have such wiring, challenging the idea that speech and sound are vital for human language. Laura Ann Petitto and Robert Zatorre of McGill University and their colleagues base their conclusions on a series of experiments using positron emission tomography (PET) brain scans of 11 profoundly deaf people and 10 hearing people.

Earlier work had demonstrated that deaf people processing signed sentences used mostly their left hemispheres, just as hearing people parsing spoken language did. But the new study found that in addition, both groups rely on identical brain structures for similar tasks. Petitto and Zatorre's team discovered that deaf people searching for and retrieving signs showed greater activity in the left inferior frontal cortex¿the same region that lights up in PET scans when hearing people rack their brains for words. And when the deaf subjects in the study processed meaningless grammatical hand movements, they showed greater blood flow in the planum temporale (see image). In hearing individuals, this brain region handles the spoken equivalent¿incoming meaningless syllables (such as ba, ta, da and pa) from which all words are made.

The researchers were particularly stunned by this activity in the planum temporale among the deaf. For more than 100 years, scientists have thought that the tissues there¿which receive neural projections from the ears¿exclusively processed sounds, and nothing else. The puzzling finding has led Petitto and Zatorre to propose that perhaps areas of the brain once viewed as devoted to sounds actually contain different types of cells capable of responding to the patterns of natural language in any form. "Such neural specialization for aspects of language patterning appears to be neurally unmodifiable in so far as languages with radically different sensory modalities such as speech and sign are processed at similar brain sites," the authors write, "while, at the same time, the neural pathways for expressing and perceiving natural language appear to be neurally highly modifiable."