Stochastic Resonance in Auditory Steady State Responses in MEG Observed as Increase in Their Power and Phase Synchrony

We measured auditory steady state responses (ASSRs) in magnetoencephalogram to the sinusoidal amplitude modulated tone superimposed with ongoing white noise of various intensities. The power and phase coherence of the ASSR were investigated and stochastic resonance (SR) was indicated by increased synchrony to the stimulus and by increase in power of the ASSR when there was a certain amount of noise compared with those measured without the noise. We also measured the hearing threshold for the noise alone and found that the optimum noise intensity for SR was slightly larger than the threshold for most of the subjects. As the power and coherence curves plotted against the noise to signal ratio varied considerably among subjects, we used the maximum values of power and phase coherence as test statistics for testing the group data.