Sound presented to the contralateral ear suppresses the amplitude of the 40-Hz auditory steady-state response (ASSR). The frequency characteristics of this suppression of the 40-Hz ASSR for amplitude modulated (AM) tones at 1,000 Hz (79-dB SPL) were examined in 12 healthy volunteers (10 males and 2 females, mean age 32.3 years) using contralateral AM tones (500, 1,000, 2,000, and 4,000 Hz) and 1/3 octave-band noise (500, 1,000, 2,000, and 4,000 Hz). The 40-Hz ASSR at 1,000 Hz was suppressed by a relatively wide frequency range of contralateral sound than expected from the known characteristics of psychophysical central masking by contralateral sound: the greatest suppression was obtained with 500- and 1,000-Hz sounds, but considerable suppression was also obtained with 2,000- and 4,000-Hz sounds. Substantial differences in the suppression pattern were not observed between two types of contra-suppressors; i.e., AM tones and 1/3 octave-band noise. Therefore, any sound presented to the contralateral ear, regardless of the frequency, can suppress the 40-Hz ASSR. Moreover, the different frequency characteristics of the contralateral sound effects between the psychophysical central masking and the 40-Hz ASSR would support the idea that the 40-Hz ASSR has an additive role in the processing of auditory signals to simple threshold judgment. Investigation of the type of psychophysical measurement using the AM signal showing similar suppression patterns by the presentation of contralateral sound would be helpful to reveal the functional relevance of ASSRs.
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