The Language of the Brain: Deaf Stroke Victims Have Provided Surprising Clues to the Role of the Left Side of the Brain in Sign Language
Bower, Bruce, Science News
The Language of the Brain
For much of her life, Sarah M. was anaccomplished artist, skilled in painting and the precise brush strokes required to decorate eggshells and ceramics with elaborate designs. The delicate, gentle-looking woman, now 71 years old, was born deaf and communicates with the hand symbols and motions of sign language.
Tragically, her mastery of paint andcanvas was blotted out by a stroke that caused massive damage to the right side of her brain. A few discouraging attempts at painting and drawing consisted of haphazard lines and disorganized figures. The right-brain damage disturbed her left-eye perception, and the left side of her drawings was often left blank. Artistic ability, quite literally, abandoned her.
However, to the astonishment of scientistsat the Salk Institute for Biological Sciences in San Diego, Sarah M. continues to use sign language flawlessly and in total disregard of the assumption by many scientists that the brain's right hemisphere controls all visual and spatial tasks. Furthermore, her comprehension of American Sign Language, in which a complex grammar is conveyed through hand and arm motions, is good.
A stark contrast is provided by 38-year-oldGail D., also deaf from birth and fluent in sign language. Much of the front part of her left brain hemisphere, including areas thought to control spoken language, was devastated by a stroke. Although no artist, she can still accurately copy simple drawings and abstract figures. But her sign language is now limited to basic nouns and verbs, with none of the shifts in movement and positioning that link signs into sentences or subtly change the meaning of individual signs.
Sarah M., Gail D. and four other deafpeople with stroke-induced damage to a cerebral hemisphere have provided neuropsychologists Howard Poizner and Ursula Bellugi and linguist Edward S. Klima with intriguing insights into the organization of language in the brain. The scientists describe their work in What The Hands Reveal About The Brain (Bradford Books/MIT Press, 1987).
Three signers with right-hemispheredamage, including Sarah M., have few problems with sign language or written English, report the researchers, but show severe impairment on visual and spatial tasks outside the realm of language. Examples are the ability to assemble colored blocks into preset patterns and to draw and describe with signs the layout of objects in a familiar room.
The three signers with left-side damagedo well on these tasks, but each displays a different pattern of sign language breakdown. Gail D. is reduced to the labored production of simple words; Paul D., who has a smaller lesion, is more adept at sign language, but his communication often becomes a "word salad,' with overly complex sentences and bizarre word substitutions; and Karen L., with damage to the parietal region next to the frontal lobe, engages in animated and grammatically correct signed conversations but often fails to specify whom or what she is referring to and has difficulty understanding the signed communication of others.
In each case, problems in written Englishmirror sign-language deficits.
Although the sample is small andsimilar cases are rare, the investigators suggest that the left brain hemisphere has an "innate predisposition' for language, regardless of the capacity to hear or talk. Sign language disturbances in subjects with left-brain lesions closely match those observed in hearing individuals with damage in the same areas, they note.
Further support comes from the case ofa hearing signer whose ability to identify a series of objects in both English and sign language vanished when her left hemisphere was chemically anesthetized, but had no such problems after parts of her right hemisphere were surgically removed (SN: 8/2/86, p.70).
"We're not necessarily saying thatevery aspect of language is regulated by the left hemisphere,' says Poizner. …