LATERALIZATION OF SPEECH AND AUDITORY TEMPORAL PROCESSING

To investigate the role of temporal processing in language lateralizat ion, we monitored asymmetry of cerebral activation in human volunteers using positron emission tomography (PET). Subjects were scanned durin g passive auditory stimulation with nonverbal sounds containing rapid (40 msec) or extended (200 msec) frequency transitions. Bilateral symm etric activation was observed in the auditory cortex for slow frequenc y transitions. In contrast, left-biased asymmetry was observed in resp onse to rapid frequency transitions due to reduced response of the rig ht auditory cortex. These results provide direct evidence that auditor y processing of rapid acoustic transitions is lateralized in the human brain. Such functional asymmetry in temporal processing is likely to contribute to language lateralization from the lowest levels of cortic al processing.