Signs of left-brain language.

Scientists have long noted that damage to the brain's left hemisphere dramatically impairs the ability to speak and understand language. A new line of evidence, derived from people who use sign language to communicate, challenges the notion that the left brain contains low-level functions on which language depends.

Neuropsychologists Gregory Hickok, Ursula Bellugi, and Edward S. Klima, all of the Salk Institute for Biological Studies in La Jolla, Calif., studied 13 signers with damage to the left hemisphere and 10 with damage to the right hemisphere.

Left-brain-injured signers performed much worse on language tests, but markedly better on visual and spatial tasks, than signers with right-brain injuries, the researchers report in the June 20 Nature. They conclude that efficient signing does not rely on a facility for manipulating spatial information.

Other scientists have proposed that the left hemisphere fosters language by enabling people to discern rapidly presented information. But because shifts in hand shape during signing take far longer to complete than transitions from one speech sound to the next, processing speed does not appear crucial, the Salk scientists add.

Moreover, a general disruption of muscle control cannot account for the poor sign-language performance of the left-brain-damaged volunteers, Hickok and his coworkers report.

Further work will address whether the left brain underwrites specific forms of grammatical knowledge.