Magnetoencephalographic Study on Forward Suppression by Ipsilateral, Contralateral, and Binaural Maskers

Abstract

When two tones are presented in a short time interval, the response to the second tone is suppressed. This phenomenon is referred to as forward suppression. To address the effect of the masker laterality on forward suppression, magnetoencephalographic responses were investigated for eight subjects with normal hearing when the preceding maskers were presented ipsilaterally, contralaterally, and binaurally. We employed three masker intensity conditions: the ipsilateral-strong, left-right-balanced, and contralateral-strong conditions. Regarding the responses to the maskers without signal, the N1m amplitude evoked by the left and binaural maskers was significantly larger than that evoked by the right masker for the left-strong and left-right-balanced conditions. No significant difference was observed for the right-strong condition. Regarding the subsequent N1m amplitudes, they were attenuated by the presence of the left, binaural, and right maskers for all conditions. For the left- and right-strong conditions, the subsequent N1m amplitude in the presence of the left masker was smaller than those of the binaural and right maskers. No difference was observed between the binaural and right masker presentation. For left-right-balanced condition, the subsequent N1m amplitude decreased in the presence of the right, binaural, and left maskers in that order. If the preceding activity reflected the ability to suppress the subsequent activity, the forward suppression by the left masker would be superior to that by the right masker for the left-strong and left-right-balanced conditions. Furthermore, the forward suppression by the binaural masker would be expected to be superior to that by the left masker owing to additional afferent activity from the right ear. Thus, the current results suggest that the forward suppression by ipsilateral maskers is superior to that by contralateral maskers although both maskers evoked the N1m amplitudes to the same degree. Additional masker at the contralateral ear can attenuate the forward suppression by the ipsilateral masker.

Suppression of Subsequent N1m Amplitude When the Masker Frequency is Different from the Signal

“…One of the methods to eliminate the influence of the overlap is to subtract the response evoked by the masker without signal from that evoked by the signal with masker. 8 , 11 With regard to the merit of this analysis, there is the following risk: the responses evoked by the signal with masker are not always identical to the combination of the two responses obtained solely in the presence of the masker or signal. The evoked response in the presence of two stimuli may involve the interaction between the responses to two stimuli, and the subtraction data does not reflect the interaction.…”

“…As in the previous studies, the N1m peak amplitudes and latencies were compared at the channel where the largest N1m amplitude evoked by the signal without masker were observed in the right hemisphere. 8 , 11 The two pick-up coils of the neuromagnetometer measured the two tangential derivatives ( δ B z / δx and δ B z / δy of the field component B z ) at each position. We determined:…”