Transient and phase-locked evoked magnetic fields in response to periodic acoustic signals.

Using whole-head MEG, time course and hemispheric lateralization effects of phase-locked brain responses to complex periodic acoustic signals (stimulus frequency 13, 22, 40, 67, or 111 Hz) were determined based on a dipole analysis approach. Apart from systematic rate-induced changes in amplitude and shape of the transient evoked magnetic fields (M50, M100), phase-locked brain activity emerged, being more pronounced over the right as compared to the left hemisphere. Furthermore, this MEG component showed a consistent phase angle across subjects, indicating active synchronization mechanisms within auditory cortex that operate upon afferent input. Conceivably, these early side-differences in periodicity encoding contribute to or even snowball into hemispheric lateralization effects of higher-order aspects of central-auditory processing such as melody perception.