Processing of dynamic aspects of speech and non-speech stimuli: a whole-head magnetoencephalography study

Clinical and experimental data indicate higher proficiency of the left hemisphere in encoding dynamic acoustic events such as rapid formant transitions (30-40 ms) that distinguish consonant-vowel syllables such as /ba/ or /da/. In order to further elucidate the underlying neurophysiological mechanisms, discrimination of /bi/-like formant transitions of variable duration (18, 36, 54, or 72 ms) from a steady-state /i/-like vowel was investigated by means of whole-head magnetoencephalography (MEG) both during visual distraction and selective attention. Voiced speech-like as well as unvoiced non-speech stimuli, matched for spectral envelope, served as test materials. Based on an oddball design, magnetic mismatch fields (MMF) were determined during an early (170-210 ms) and a late (230-290 ms) time window. Selective attention toward the deviant events resulted in enhanced MMFs particularly within the left hemisphere, indicating attention-dependent left-lateralized processing of dynamic auditory events across both the speech and non-speech domains. Perceptual discrimination improved along with transient lengthening. Accordingly, early MMF was, as a rule, enlarged in case of longer as compared to shorter transients. The 36-ms transitions yielded attention- and voicing-dependent deviations from the linear regression of MMF strength on transition duration. Considering the predominance of 30- to 40-ms formant transients across the world's languages, these findings indicate an adaptation or predisposition of the human perceptual system to the spectral/temporal characteristics of prototypical speech sounds. Signal voicing had no significant main effect on MMF strength despite superior perceptual performance in case of voiced as compared to voiceless target stimuli.