Magnetic brain responses to second formant transitions in dependency of transition time, signal voicing, and attention

Abstract

Using an oddball paradigm, magnetoencephalographic (MEG) recordings were performed during perception of synthetic /i/-like signals (frequent stimuli) and interspersed /bi/ or /yi/-like deviants with an initial second formant (F2) transition of 20, 40, 60, or 80 ms. The experiment was run under two different signal conditions: formant-synthesized voiced stimuli or spectrally and loudness-matched unvoiced cognates generated by filtered white noise. Listeners’ attention was directed either to the infrequent stimuli or to a moving geometric pattern (visual distraction task). Left- and right-hemispheric M50, M100, and MMNm dipole strengths were derived from the MEG measurements. The results show a strong bilateral effect of signal voicing on the M50 and M100 responses, irrespective of F2 transition time, being enhanced with attention directed to the acoustic signal. The deviant stimuli evoked a significant mismatch reaction which under directed attention was lateralized to the left hemisphere. The strength of the MMNm increased across transition time, but exhibited a nonlinearity at 40 ms in terms of an interaction with signal voicing. In conclusion, these results suggest signal-related bilateral preprocessing during the M50 and M100 responses and attention-dependent lateralization of mismatch reactions to speech-relevant acoustic features.