Differential impact of periodic and aperiodic speech-like acoustic signals on magnetic M50/M100 fields

Voiced and unvoiced sounds, characterized by a periodic or aperiodic acoustic structure, respectively, represent two basic information-bearing elements of the speech signal. Using whole-head magnetencephalography (MEG), magnetic fields (M50/M100) in response to synthetic vowel-like as well as noise-like signals matched in spectral envelope were recorded in 20 subjects. Aperiodic events gave rise to increased M50 concomitant with reduced M100 activity as compared to their periodic cognates. Attention toward the auditory channel enhanced the effects of signal periodicity. These data provide first evidence that speech-relevant acoustic features differentially affect evoked magnetic fields as early as the M50 component. Conceivably, the M50 field reflects an ongoing monitoring process whereas the M100 component is bound to more specific operations such as detection of signal periodicity. NeuroReport 11:4017-4020 (C) 2000 Lippincott Williams & Wilkins.