The effects of monotic and dichotic interference tones on 40 Hz auditory steady-state responses in normal-hearing adults.

Auditory steady-state responses (ASSRs) recorded with simultaneous presentation of multiple tones modulated from 77-105 Hz exhibit frequency specificity and can be acquired with monotic or dichotic stimulation. This study examined the frequency specificity and dichotic characteristics of 40 Hz ASSRs recorded with amplitude-modulated (AM) probe tones and unmodulated (UM) or AM interfering tones in 27 normal-hearing adults. The effects on ASSR amplitudes of monotically or dichotically presented interfering tones of various frequency, modulation depth, and modulation rate were studied. Significant decreases in ASSR amplitudes occurred when the UM interfering tone was monotic, higher in frequency, and approximately within an octave of the probe tone. ASSR amplitudes were also reduced when the AM interfering tone was monotic and modulated at a lower depth and was an octave above the probe tone. Probe and interfering AM tones modulated at different rates produced similar reductions in amplitude for ASSRs acquired with monotic and dichotic stimulation. The findings of this study contribute to clarifying the carrier and temporal envelope interactions between tonal stimuli. Description of the effects of these stimulus parameters on 40 Hz ASSRs can benefit clinical applications of this technique, including evaluating auditory function in adults not capable of participating in behavioral audiometric tests.