Auditory evoked magnetic fields in relation to iterated rippled noise.

Auditory evoked magnetic fields in relation to iterated rippled noise (IRN) were examined by magnetoencephalography (MEG). IRN was used as the sound stimulus to control the peak amplitude of the autocorrelation function of the sound. The IRN was produced by a delay-and-add algorithm applied to bandpass noise that was filtered using fourth-order Butterworth filters between 400-2200 Hz. All sound signals had the same sound pressure level. The stimulus duration was 0.5 s, with rise and fall ramps of 10 ms. Ten normal-hearing subjects took part in the study. Auditory evoked fields were recorded using a 122 channel whole-head magnetometer in a magnetically shielded room. The results showed that the peak amplitude of N1m, which was found above the left and right temporal lobes around 100 ms after the stimulus onset, increased with increase in the number of iterations of the IRN. The latency and estimated equivalent current dipole (ECD) locations of N1m did not show any systematic variation as a function of the number of iterations.