Effect of amplitude modulation of background noise on auditory-evoked magnetic fields

Although it is known that amplitude modulation of a background masker improves signal detection, its effects on supra-threshold sounds are still unclear. In this study, the effects of amplitude modulation of a background masker on supra-threshold sounds were evaluated using a whole-head magnetoencephalography system. We compared the auditory-evoked magnetic fields recorded with a 500-Hz tone-burst of supra-threshold level with either random speech-spectrum noise (Unmodulated noise) or amplitude-modulated noise (Modulated noise) in 10 young normal-hearing subjects. The sounds were presented binauraly. The strengths of the N100m in the two noise conditions were examined. The masking effect of Modulated noise on supra-threshold sound was stronger than that of Unmodulated noise, a finding opposite to that on peri-threshold sound. The growth rate of the N100m strengths in the Modulated noise condition was less than that in the Unmodulated noise condition in the left hemisphere, whereas they were not different in the right hemisphere, suggesting that the left hemisphere was more susceptible to the envelope fluctuation.