Abstract
Auditory-evoked responses can be affected by the sound presented to the contralateral ear. The different contra-sound effects between noise and music stimuli on N1m responses of auditory-evoked fields and those on psychophysical response were examined in 12 and 15 subjects, respectively. In the magnetoencephalographic study, the stimulus to elicit the N1m response was a tone burst of 500 ms duration at a frequency of 250 Hz, presented at a level of 70 dB, and white noise filtered with high-pass filter at 2000 Hz and music stimuli filtered with high-pass filter at 2000 Hz were used as contralateral noise. The contralateral stimuli (noise or music) were presented in 10 dB steps from 80 dB to 30 dB. Subjects were instructed to focus their attention to the left ear and to press the response button each time they heard burst stimuli presented to the left ear. In the psychophysical study, the effects of contralateral sound presentation on the response time for detection of the probe sound of a 250 Hz tone burst presented at a level of 70 dB were examined for the same contra-noise and contra-music used in the magnetoencephalographic study. The amplitude reduction and latency delay of N1m caused by contra-music stimuli were significantly larger than those by contra-noise stimuli in bilateral hemisphere, even for low level of contra-music near the psychophysical threshold. Moreover, this larger suppressive effect induced by contra-music effects was also observed psychophysically; i.e., the change in response time for detection of the probe sound was significantly longer by adding contralateral music stimuli than by adding contra-noise stimuli. Regarding differences in effect between contra-music and contra-noise, differences in the degree of saliency may be responsible for their different abilities to disturb auditory attention to the probe sound, but further investigation is required to confirm this hypothesis.
Highlights
In the phenomenon of auditory masking, the audibility of a signal is decreased by the presence of another sound; i.e., the detection threshold for the signal is elevated and the auditory-evoked responses to the signal sound are reduced by presentation of the masker [1,2,3,4,5,6]
This previous study of Hari and Makela [4] indicated the effect of contralateral sound on the N1m amplitude obtained from the right hemisphere for one particular sound pressure level of contra-lateral sounds, but more detailed features of these contra-sound effects such as those on N1m latencies, the effects of level of contra-sound on the magnitude of contra-sound effects, and inter-hemispheric differences have not yet been fully clarified [4]
The contra-sound effects on the N1m latency and amplitude were assessed by comparing the amplitude and the latency of N1m between those obtained under each level of contralateral sound and the average of those obtained from the two control measurements just before and after each measurement condition with contralateral sound
Summary
In the phenomenon of auditory masking, the audibility of a signal is decreased by the presence of another sound (masker); i.e., the detection threshold for the signal is elevated and the auditory-evoked responses to the signal sound are reduced by presentation of the masker [1,2,3,4,5,6]. The strength of the contralateral masking effect appears to differ according to the characteristics of the contra-masking sound; i.e., the N1m response is not significantly affected by contralateral continuous white noise [4,17,18], but is significantly suppressed by speech sound, music sound, and intermittent noise [4] These results hint at the possible involvement of a factor other than a simple “masking” phenomenon in suppression of N1m caused by such as music, speech and intermittent noise, but the underlying mechanism for the different contra-effects of these stimuli remains unclear [4]. This previous study of Hari and Makela [4] indicated the effect of contralateral sound on the N1m amplitude obtained from the right hemisphere for one particular sound pressure level of contra-lateral sounds, but more detailed features of these contra-sound effects such as those on N1m latencies, the effects of level of contra-sound on the magnitude of contra-sound effects (i.e., whether or not the N1 suppression effect caused by the contralateral sound is a phenomenon that depends on the presentation level of the contralateral sound), and inter-hemispheric differences have not yet been fully clarified [4]
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