Left brain may serve as language director.

The left half, or hemisphere, of the brain possesses unique features that enable humans, at least right-handed ones, to understand language, according to a report in the March 6 Science.

The new findings, which apply to both spoken and signed language, may help resolve the current debate over the nature of the left hemisphere's role in language, assert neuropsychologist David P. Corina of the University of Southern California in Los Angeles and his colleagues at the Salk Institute for Biological Studies in La Jolla, Calif.

Some investigators, including Corina's group, theorize that the left hemisphere deals specifically with the grammar and syntax of human language; others argue that the left side governs signed and spoken language by controlling the muscles employed in speaking and making signs, including such nonlinguistic gestures as waving good-bye; and a third proposal holds that the left hemisphere directs the expression and comprehension of symbols in general, not just those found in language.

In one study, Corina and his co-workers recruited 16 right-handed, hearing adults fluent in the use of American Sign Language. Each participant repeated a list of common, one-handed signs and English words, presented on videotape and audiotape, respectively. Signs and words appeared at a rate of one per second. During two trials, volunteers tapped a telegraph key as quickly as possible, alternating right and left index fingers. A computer hooked up to the telegraph key recorded the number of taps in each 30-second trial. The researchers also measured each volunteer's "baseline" tapping rate for each hand in the absence of a competing task.

Repetition of both words and signs caused a significantly greater drop in right-handed tapping than left-handed tapping. This suggests greater involvement of the left hemisphere, which regulates movement on the right side of the body, in both spoken and manual language, the researchers argue. Left-brain responsibility for handling two attention-demanding tasks at once inevtiably disrupts performance on at least one of the tasks, they say.

The investigators repeated the experiment with 48 right-handed, hearing adults who spoke English but had no experience with any sign language. Participants repeated common words and two types of manual gestures--symbolic gestures, such as waving good-bye or giving the thumbs-up sign, and arbitrary gestures with no meaning. Only word repetition produced a marked drop in right-handed tapping, the scientists note.

A third experiment consisted of 12 right-handed, deaf adults fluent in American Sign Language. Each repeated a list of common signs, symbolic gestures and arbitrary gestures. Only the repetition of sign language generated substantially fewer right-handed taps.

The discovery that only tasks with linguistic meaning disrupt right-handed tapping indicates that the left hemisphere handles specific characteristics of language, rather than muscle movements or symbolic abilities involved in language, Corina's group contends.

Further support for this theory comes from the researchers' observations of W.L., a deaf individual who suffered stroke-induced left-hemisphere damage. Like other right-handed signers with left-brain injuries, the patient lost virtually all ability to use sign language (SN: 7/18/87, p.40). However, W.L. spontaneously substituted pantomime gestures for signs, providing evidence that the left hemisphere offers specific directions for language use.

Laboratory and clinical studies of the left hemisphere's role in language focus on right-handers because left-handers display less predictable hemispheric specializations for a variety of abilities. Both sides of the brain may play a part in regulating language use and comprehension among some left-handed persons, studies suggest.