LATERALIZED HUMAN BRAIN LANGUAGE SYSTEMS DEMONSTRATED BY TASK SUBTRACTION FUNCTIONAL MAGNETIC-RESONANCE-IMAGING

Objective: To develop a procedure for noninvasive measurement of language lateralization with functional magnetic resonance imaging (MRI). Design: Functional neuroimaging using time-series echo-planar MRI. Setting: University medical center research facility. Subjects: Five healthy, right-handed, young adults. Main Outcome Measures: Number of MRI voxels in left and right hemispheres showing task-related signal increases during two contrasting auditory processing tasks. The nonlinguistic task involved processing of pure tones, while the linguistic task involved processing of single words based on semantic content. Results: The pure-tone processing task activated temporal lobe auditory areas and dorsolateral frontal regions bilaterally. Using this task as a control condition, the semantic processing task resulted in lateralized activity in distributed regions of the left hemisphere. A significant effect of task on intrahemispheric activity pattern was demonstrated in every subject. Results were reproduced in preliminary studies of test-retest reliability. Conclusions: The results demonstrate the lateralized anatomy of semantic linguistic systems in contrast to nonlinguistic auditory sensory processors and introduce a task subtraction technique adapted for functional MRI as a noninvasive measure of language lateralization.