Cortical activity during speech recognition was examined using optical topography (OT), a recently developed non-invasive technique. To assess relative changes in hemoglobin oxygenation, local changes in near-infrared light absorption were measured simultaneously from 44 points in both hemispheres. A dichotic listening paradigm was used in this experiment, in which target stimuli and non-target stimuli were presented to different ears. Subjects were asked to track targets and to press a button when targets shifted from one ear to the other. We compared three tasks: (i) a control task, in which a tone was used as the target; (ii) a repeat task, in which the target was one repeated sentence; (iii) a story task, in which the targets were continuous sentences of a story. The activity for the story task, compared with the repeat task, was localized in the left superior temporal cortex. Relative to the control task, we observed in this region a larger increase in oxyhemoglobin concentration and a decrease in deoxyhemoglobin concentration in the story task than those in the repeat task. These results suggest that the activity in the left temporal association area reflects the load of auditory, memory, and language information processing. Copyright (C) 1999 Elsevier Science B.V.
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