Left brain reigns over sign language.
Traditional notions of brain function hold that the left hemisphere controls spoken language while the right hemisphere calls the shots regarding visual and spatial skills, including those necessary for using sign language. But converging lines of evidence, underscored by a report in the July 24 NATURE, indicate that this assumption is wrongheaded.
Rather, the ability to use and understand sign language, in which hand movements and their manipulation in space are critical to meaning, appears to be rooted in the left brain hemisphere, according to the new study, headed by Antonio Damasio of the University of Iowa College of Medicine.
"The lef hemisphere's specialization for language appears to rest more on its ability to transform labels into meanings regardless of the modality of a language [spoken or signed]," says Damasio.
His interpretation is based on the rare opportunity to study a hearing individual who is also proficient in sign language, before and after damage to a critical portion of her right hemisphere. The 27-year-old woman, who works as an interpreter and counselor at a community agency for deaf people, sought surgical treatment after drug therapy failed to quell her persistent seizures. Images of the right-handed subject's brain, taken while she listened and responded to a spoken language task, revealed increased activity in left hemisphere language centers.
The researchers then injected a barbiturate drug, sodium amobarbital, into an artery leading into her left hemisphere in order to shut down temporarily these language areas. The subject was initially unable to identify a series of objects in either English or sign language. Correct English responses appeared about 2 1/2 minutes before the first correct sign language responses accurred. During that short period, the woman was able to identify many objects verbally while simultaneously using incorrect hand shapes and movements for the same objects.
Surgical removal of parts of the right hemisphere, including the hippocampus and amygdala, was conducted and the women's seizures abated. Tests of language, memory, perception and other skills, administered 3 and 12 months after surgery, showed no change from before surgery, say the researchers. Videotaped signing interviews confirmed the subject's impression that her ability to sign and understand signing had not been dimmed.
This result is consistent with several recent studies--some directed by Ursula Bellugi of the Salk Institute for Biological Studies in La Jolla, Calif., who participated in the present experiment--of deaf individuals fluent in sign language who suffered brain damage due to strokes. Those with left hemisphere lesions had marked problems signing; lesions to the rght hemisphere created difficulty with several spatial skills, but sign language was unaffected.
In the latest cast, say the researchers, recovery from barbiturate action may have taken place gradually in the left hemisphere. "Classic" speech areas probably regained functioning first, they suggest, followed by visual and sensory regions that are crucial for sign language processing.
It is possible, note the scientists, that parts of the right hemisphere could regulate the learning of sign language. Once learned, however, the use and comprehension of signs are predominantly controlled by the left side.
"Both hemispheres are capable of motor and perceptual functions," says Damasio, "but one may be more important in the early stages of these functions while the other takes over later on. The lingering idea that the right hemisphere is involved in all visual and spatial tasks in changing."
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|Date:||Aug 2, 1986|
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