Kung, S. , Wu, D. , Huang, H. & Hsieh, I.
Institute of Cognitive Neuroscience. National Central University. Jhongli City, Taiwan.
The contour of frequency-modulated (FM) sweeps is a critical acoustic component of speech and other conspecific communication signals. Evidence from human psychophysics has shown that tonal-language speakers performed better in FM sweep direction identification, suggesting that experience in a tone-language environment sharpens perception of FM contours. The current magnetoencephalographic (MEG) study investigates the sensitivity to the contour of FM sweeps, indexed by auditory-evoked magnetic fields, in human auditory cortex. In an auditory mismatch negativity (MMN) paradigm, speakers of Mandarin Chinese determined the direction of successive presentations of FM sweeps in a random-block design. Stimuli were linear rising and falling FM sweep in the frequency range of 600-900Hz presented at durations of 10, 20, 40, 80, 160, and 320ms. The results revealed significant MMN, peaking in the time window of 150-200ms after the stimulus onset, which was associated with both upward and downward FM sweeps. As the duration of the sweep increases, the MMN peak amplitude increases for upward but not for downward sweeps. These results suggest that the auditory evoked-field pattern is dependent on the rate and direction of frequency-modulated sweeps, and their implications for speech encoding and lexical distinction in tonal-language processing will be discussed.