A neuromagnetic analysis of the mechanism for generating auditory evoked fields

Averaged and non-averaged neuromagnetic responses to repetitive transients of sound were analyzed in the frequency and time domains. It was found that ongoing oscillations between 3 and 16 Hz are relevant to the generation of auditory evoked fields (AEF). First, phases of the oscillation were locked at around the timings of N100m, but randomly distributed from trial to trial at the other timings. Secondly, amplitudes of the oscillation were larger on average at around the timings of N100m than at the other timings, although the amplitudes fluctuated from trial to trial with almost the same standard deviations throughout the periods of observation. Thirdly, spatial distributions of the oscillatory activities were often reduced to an equivalent current dipole in the auditory cortex at around the timings of N100m, but seldom reduced at the other timings. An explanation of these results would be to suppose several oscillators within the cortex whose phases are locked at around the timings of N100m, but fluctuated randomly at the other timings.