Brain Research

The study of language and the brain has a long history, beginning with reports in the 1800s of language impairment aphasia, meaning a loss or impairment of the power to use or comprehend words, usually resulting from brain damage. These early cases demonstrated that damage to different areas of the left hemisphere of the brain produced different types of language deficit (see Figure 1). Lesions to the temporal lobe, specifically Wernicke's area, compromise the ability to understand language and the ability to speak clearly. Lesions to the frontal cortex, Broca's area, prevent a person from producing speech. For example, a person with a lesion in this area has the ability to understand language, but words are not properly formed, and speech is slow and slurred.

Initially, these correlations were established postmortem. However, with the advent of the computed tomography (CT) scan in the 1970s, it became possible to examine brain damage in living patients and to explore changes in brain structure as a patient recovered.

During the past 15 years, there has been a marked increase in the availability of brain imaging techniques to language scientists. These techniques have made it possible, for the first time, to study brain activity correlated with language learning and processing in unimpaired subjects and in some cases to examine this activity within a short time frame. Two of these techniques have been put to relatively greater use than others, and these will be the focus of this entry.

With respect to bilingualism, imaging studies permit systematic exploration of ideas stemming from earlier reports; these indicated that seizure disorders, stroke, and other injuries, along with localized electrical stimulation during brain surgery, can affect one language and leave others unaffected in a bilingual or multilingual individual. Further, imaging studies open a new window into the perennial questions of how the age of learning a second language (L2) and degree of achieved fluency influence the way the L2 is represented in the person's mind and brain. One must be cautious when interpreting results that explore the localization of language in bilin-guals, since there is some variation (particularly in localization and lateralization) even across monolin-guals. However, mounting evidence shows that second languages learned relatively late, or not learned fluently, are physically represented in somewhat different regions of the brain than the first language (LI) is and are processed in a different time frame.

Figure 1 Left Hemisphere of Brain, With Approximate Locations of Broca's Area and Wernicke's Area