We completed three fMRI studies examining frontal-lobe activation associated with memory and language processes. Methods In study #1, we compared frontal-lobe activation between conditions where subjects either thought about the meaning of words or the appearance of words. In each of four subjects, left inferior prefrontal cortex was more activated for semantic than for perceptual processing of words. In a second experiment, the same subjects made repeated semantic judgments for the same words presented twice several minutes apart. In the same left inferior prefrontal region, there was decreased activation for the repeated relative to the initial semantic processing of a word. This decrease represents the consequence of some sort of memory created during the initial semantic processing of each word. In study #2, we replicated the above findings, and analyzed the nature of the mental process being imaged in left inferior prefrontal cortex. Left-frontal activation for semantic processing was due to semantic analysise, rather than the difficulty of the processing. The decrease in activation for repeated processing was due to repeated semantic analysis, rather than mere re-exposure to words. In study #3, the semantic/perceptual task was applied to the identification of the language-dominant hemisphere in an individual. Currently, one can only determine whether the left or right hemisphere is dominant for language by using invasive procedures that are uncomfortable for patients, involve some risk of morbidity, and are costly. fMRI offers the potential of a noninvasive, comfortable, safe, and relatively inexpensive alternative. We tested seven patients in whom lateralization of language was determined invasively, and found that ~RI semantic activation occurred in the dominant language hemisphere. Results and Discussion We were able to image an important linguistic and mnemonic function in individual brains, demonstrate fMRI usefulness to characterize a localized mental process, and show fMR1s effectivenss to reliably identify language dominance.
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