Nov. 9, 1998 LOS ANGELES -- A unique opportunity to map and test the human brain has yielded new insights into two areas involved in producing and processing of language.
Speaking here today (Nov. 8) at the annual meeting of the Society for Neuroscience, David Corina, an assistant professor of psychology at the University of Washington, reported on the roles of two brain regions called Broca's area and the supramarginal gyrus. The findings came from a rare case, a deaf person called S.T. who uses American Sign Language. S.T. underwent a procedure called an awake cortical stimulation mapping, which allows assessment of language and motor functions at specific sites in the brain's left hemisphere.
Corina, a fluent signer, and an interdisciplinary team of UW researchers tested the subject and found that electrical stimulation of Broca's area and the supramarginal gyrus created repeated but different kinds of errors in S.T.'s ability to name objects. When Broca's area, which is located in the frontal lobe, was stimulated, S.T. had difficulty making clear hand shapes and specific movements associated with signs. Nonetheless, these sloppy signs resembled the target sign. Corina likens these errors to "mumbling" made in spoken languages. The subject made no effort to self-correct these lax or imperfect signs.
Stimulation of the supramarginal gyrus, a small area in the parietal lobe, produced different kind of signing error. With stimulation, S.T. mixed up word meanings and word forms. For example, when shown a picture of a pig and asked to make the sign for it, S.T. made the sign for farm. The two signs are very similar in hand shape, movement and spatial location to the sign pig in American Sign Language, and would be distinct to skilled signer. Comparable errors in English might be saying oyster instead of lobster or plane instead of train. This type of error suggests that the supramarginal gyrus may be an area of the brain important in the selection and combining of word meanings with word forms.
Another interesting difference in these language errors was that with stimulation to Broca's area, S.T. made no effort to self-correct his imprecise signing. However, with stimulation to the supramarginal gyrus, he would make successive attempt to produce the correct sign (for oyster he would sign "loyster," then "lobster"). This suggests that stimulation of Broca's area was effecting only the final output of a correctly selected word, while supramarginal gyrus stimulation was effecting the compiling of the word forms, according to Corina.
Neuroscientists have long established that a region in the left hemisphere plays a role in language function. In the past decade it also has become evident that left hemis- phere specialization for language extends to deaf people who use sign languages, as well as for those who speak. However, scientists are just beginning to understand the particular contributions specific regions within the left hemisphere play in language processing.
The data Corina reported on came from an awake cortical stimulation mapping performed on S.T., a 50-year-old man who was suffering from epileptic brain seizures. The mapping was done prior to an operation called a temporal lobectomy which reduces severe seizures. The mapping procedure helps the neurosurgeon plan this delicate operation. In the mapping, a small electrical current is applied to the exposed cortex of the brain of an awake patient. During electrical stimulation, the patient is asked to name objects and imitate actions. This procedure is often used on speaking people undergoing the brain surgery, but Corina's report and an in press paper, are the first detailed accounts of its effects on a deaf signer.
"Although Broca's area has gotten considerable attention, its precise role in language behavior remains controversial," said Corina. "One controversy is whether Broca's areas is specialized just for speech or language in general. We have now been able to identify that Broca's area is involved in language production, not just speech production but any language spoken or signed. This is the best evidence that it is responsible for language independent of whether that language is expressed through the hands or the voice."
Corina noted the surprising finding that stimulation to areas next Broca's area resulted in movements of the mouth and lips, but not the hands.
"This study also strongly suggests that the supramarginal gyrus plays a critical role in blending semantic and phonetic information," he added, citing the word cat as an example. Cat has semantic features, being a "little furry critter that goes meow." It also has phonemic elements which correspond to the sound which make up the word cat --/k/ and /at/ in English -- and the hand shape and movements for a sign in American Sign Language. The supramarginal gyrus may be pulling together this kind of information, according to Corina.
"Some people have wondered if the human brain has specialized areas of language production and processing," he said. "This work provides new evidence in favor of specialized areas of the brain which are unique to language processing and production. People also have asked if there is a so-called language organ. Our work suggests that there is a whole network of areas responsible for speech and language. Broca's area and the supramarginal gyrus are just two pieces of that network."
The mapping procedure on S.T. lasted about 90 minutes prior to his surgery and was conducted while he was under a local anesthetic. The researchers tested a number of different left hemisphere sites for motor and language impairment by having S.T. do several tasks. He was shown pictures of 49 objects, such as a bird, chair, pig, bed and table, and asked to give the sign for each under normal conditions and while being electrically stimulated at each site. He also was asked to imitate signs and complex arm gestures. Only six sites showed any motor impairment and just two, Broca's area and the supramarginal gyrus, exhibited consistent impairments to language processing or production.
Corina said basic research such as this is important because science is very interested in being able to provide people with improved communications skills. "One way to improve communications is to discover all the sub parts that are involved in language. To do this, we need to understand where and what portions of the brain are involved so we can develop better interventions to assist people," he said.
"This work also has a practical application to help deaf people who, like hearing people, have seizures. The medical community needs to be aware that it can use the same standardized mapping procedures used on hearing people to identify language areas to ensure better post-operative outcomes on deaf patients. People need to realize that sign languages are real and naturally occurring human languages."
Other members of the UW research team that mapped S.T.'s brain included George Ojemann and Carl Dodrill, professors of neurological surgery; James Brinkley, research associate professor of biological structure; Susan McBurney, doctoral student in linguistics, and Kevin Hinshaw, doctoral student in computer science and engineering.
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