Hemispheric difference in processing tone frequency and amplitude modulations

TRANSIENT frequency and amplitude modulations (FMs,AMs) of sound are requis ite to speech recognition.:We recorded whole-head magnetoencephalographic s ignals from seven subjects to binaural 620 ms 667 Hz tones, with 3, 30, or 300 ms FMs or AMs in the beginning or middle of the tone. Responses were si gnificantly larger and earlier for FMs than AMs, for rapid than slow modula tions, and for modulations at the beginning (BEG) than in the middle (MID) of the sound. BEG :3 ms FMs elicited strongest signals in the left and MID 3 ms FMs in the right hemisphere. Fast MID modulations produced significant ly stronger responses:in-the right than left hemisphere. These differences may reflect different functions of the left and right hemisphere-in speech perception. (C) 1999 Lippincott Williams & Wilkins.