Effects of tone repetition on auditory evoked neuromagnetic fields

Objective: To investigate the impact of stimulus repetition on neuromagnetic auditory evoked fields (AEF) in the context of permanently varying auditory stimulation and uniform stimulation. Methods: Left-hemispheric AEF of 16 healthy subjects were recorded on a 31-channel system. In one condition subjects were stimulated with trains of 5 identical tones, in another condition the pitch of the stimuli varied from 800 to 1250 Hz in 50 Hz steps and every 10th stimulus was followed by an identical stimulus. Evoked cortical activity was estimated by calculation of the signal:noise ratio and by dipole reconstruction. Results: If the stimuli were permanently varied, tone repetition elicited a neuromagnetic mismatch negativity (MMNm) in about half of the subjects, while the AEF component N100m was not significantly affected. If trains of uniform stimuli were applied, a significant decrease of the N100m dipole moment was observed from the first to the second stimulation only. The N100m dipole location in mediolateral direction, however, varied at least between the first 3 stimulus positions. Conclusions: An MMNm can be elicited by tone repetition even at interstimulus intervals extending the so-called temporal window of integration. The data on repeated stimulation with uniform tones reveal a dissociation between effects on N100m dipole moment and location.