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When an Adult Adds a Language, It's One Brain, Two Systems

By SANDRA BLAKESLEE

S thousands of teen-agers who have struggled to engrave high school French on recalcitrant neurons might have guessed, a new study has found that second languages are stored differently in the human brain depending on when they are learned.Babies who learn two languages simultaneously, and apparently effortlessly, have a single brain region for generating complex speech, researchers say. But people who learn a second language in adolescence or adulthood possess two such brain regions, one for each language.






The findings, described in the current issue of the journal Nature, shed new light on some notoriously difficult questions about brain development: How does the brain organize language in infancy and how are multiple languages represented in the brain? Why do some brain regions appear immutable after childhood, while others appear flexible and malleable in adult life? Why are languages harder to learn later in life?
There have always been strong hints that the brain can use separate brain regions for first and second languages, said Dr. Michael Posner, a psychologist at the University of Oregon in Eugene. Bilingual epilepsy patients may, during seizures, lose the ability to speak one language and not another. A stroke victim can permanently lose the ability to speak French but retain English or another language. "But it's not been known how these separate language areas form in the brain," Dr. Posner said. "Are the languages fused? Do they prime one another? Is one translated by another?"
The new study shows for the first time that two languages can be mapped in common neural tissue, Dr. Posner said, adding, "It is very helpful for understanding bilingualism."
The research was carried out by Dr. Joy Hirsch, head of Memorial Sloan-Kettering Hospital's functional M.R.I. Laboratory and her graduate student, Karl Kim. Functional magnetic resonance imaging, or M.R.I., is a relatively new, noninvasive brain imaging technique that can pinpoint exactly which parts of the brain are active during cognitive tasks such as talking, seeing, waving an arm or daydreaming. Brain surgeons at the hospital are now using the technique to identify critical brain regions so that they will not do more harm than good when removing a tumor or other abnormality.
Of these critical regions, language is perhaps at the top of the list, said Dr. Philip H. Gutin, the hospital's chief of neurosurgery. Some functions such as seeing and hearing are located in both brain hemispheres, he said. When a tumor forms, surgeons can cut out tissue and not do great harm because the other side of the brain will take over. "But language is a high-rent district," Dr. Gutin said. Some high-level aspects of language tend to be found only on one side of the brain. By removing a spot of tissue smaller than an eraser, a surgeon could excise a crucial region of language production and destroy a person's ability to speak or understand English.
Moreover, language areas are never found in exactly the same spot, Dr. Gutin said. These regions are formed in childhood as language is acquired and are in slightly different spots in different people. Given that a quarter of all brain tumors occur in regions of the brain where language skills might reside, accurate imaging is a must, he said.
To explore where languages lie in the brain, Dr. Hirsch recruited 12 healthy bilingual people from New York City. Ten different languages were represented in the group. Half had learned two languages in infancy. The other half began learning a second language around age 11 and had acquired fluency by 19 after living in the country where the language was spoken.
With their heads inside the M.R.I. machine, subjects thought silently about what they had done the day before using complex sentences, first in one language, then in the other. The machine detected increases in blood flow, indicating where in the brain this thinking took place.
Aspects of language ability are distributed all over the brain, Dr. Hirsch said. But there are some high-level, executive regions that are usually localized in a certain neighborhood on the left side of the brain, but are sometimes found in the same neighborhood on the right side, or on both sides. One is Wernicke's area, a region devoted to understanding the meaning of words and the subject matter of spoken language, or semantics. Another is Broca's area, a region dedicated to the execution of speech as well as some deep grammatical aspects of language. The regions are identified by observing brain function.
None of the 12 bilinguals had two separate Wernicke's areas, Dr. Hirsch said. In an English and Spanish speaker, for instance, Spanish semantics blended with English semantics in the same area. But there were dramatic differences in Broca's areas, Dr. Hirsch said.
In people who had learned both languages in infancy, there was only one uniform Broca's region for both languages, a dot of tissue containing about 30,000 neurons. Among those who had learned a second language in adolescence, however, Broca's area seemed to be divided into two distinct areas. Only one area was activated for each language. These two areas lay close to each other but were always separate, Dr. Hirsch said, and the second language area was always about the same size as the first language area.
This implies that the brain uses different strategies for learning languages, depending on age, Dr. Hirsch said. A baby learns to talk using all faculties -- hearing, vision, touch and movement -- which may feed into hardwired circuits like Broca's area. Once cells in this region become tuned to one or more languages, they become fixed. If two languages are acquired at this time, they become intermingled.
But people who learn a second language in high school have to acquire new skills for generating the complex speech sounds of the new tongue, which may explain why a second language is harder to learn. Broca's area is already dedicated to the native tongue and so an ancillary Broca's region is created. But Wernicke's area, which handles the simpler semantic aspects of language, can overlap.