Effects of contralateral noise on 40-Hz and 80-Hz auditory steady-state responses

Objectives: The amplitude of 40-Hz auditory steady-state response (ASSR) in response to repeated click or tone pips at levels of 45 to 60 dB SL is reduced by up to 50% by the central masking mechanism if white noise at levels of 40 to 60 dB SL is applied contralaterally. However, the effect of contralateral noise on the elevation of the threshold of ASSR is unknown. The present study investigated the effects of contralateral noise on the threshold measurements of ASSR for 40- and 80-Hz amplitude modulated tones that are widely used in clinical examinations. Design: The effects of contralateral noise on the 40- and 80-Hz ASSRs for amplitude modulated tones at 500 Hz and 2000 Hz were examined in 11 healthy volunteers (10 men and 1 woman, mean age 26.1 years). Contralateral noise consisted of white noise low-pass filtered at 700 and 4000 Hz in the measurements of ASSR at a carrier frequency of 500 and 2000 Hz, respectively. Results: Contralateral noise at a level of 40 dB SL caused no significant psychophysical threshold elevation, caused significant threshold elevation of the 40-Hz ASSR (average 10 to 15 dB), and caused no significant threshold elevation of the 80-Hz ASSR. Conclusions: The different effects on the 40- and 80-Hz ASSRs were probably related to the differences in sources because the 40-Hz ASSR contains more components from the upper auditory pathway that are affected by contralateral masking, whereas the 80-Hz ASSR contains more components from the brain stem. The present results suggest that threshold elevation of the 40-Hz ASSR may occur during clinical measurements using binaural presentation of sounds and cause a possible discrepancy between psychophysical threshold and 40-Hz ASSR measurements.