Frequency-channel interactions of the auditory steady-state responses at different levels of the auditory pathways

This study evaluated the effect of 3 different modulation rates and 3 different rate separations on the interactions (response amplitudes) of multiple ASSRs. Responses from 12 normal-hearing subjects were examined using three amplitude-modulation rates (14,40 and 80 Hz) and four conditions: one tone (1000 Hz), two tones (1000 and 2000 Hz, presented monotically or dichotically) and four tones (500, 1000,2000 and 4000 Hz, presented monotically). Within the multiple-tone conditions, rate separation between the amplitudemodulated tones was varied from 2 to 6 Hz. A 1000-Hz tone served as the baseline condition and interactions between tones were measured as a function of the change in response amplitude from this baseline. Results indicate separation between modulation rates had no effect on the interactions between responses. Both modulation rate and condition had significant effects on ASSR interactions. In general, interactions became greater as the number of stimuli increased from 1 to 4 tones. However, each modulation rate had a different pattern of interactions. The response amplitudes for all modulation rates were significantly decreased in the four-tone condition. The two-tone dichotic condition amplitudes were decreased from the baseline for the 40-Hz ASSRs, but not for the 14- and 80-Hz ASSR amplitudes. Furthermore, the 40-Hz and 80- Hz two-tone monotic condition response amplitudes were decreased from the baseline, but not the 14-Hz ASSR amplitudes. Results from relative efficiency calculations indicate that at high intensities (80 dB SPL), ASSRs to multiple tones are not more efficient than ASSRs to single tones for any modulation rate range. The different patterns suggest that the 14-, 40- and 80-Hz ASSRs are generated in different areas along the auditory pathways. These results may be helpful in determining the usefulness of multiple ASSRs for diagnostic testing at high intensity levels and for testing auditory functioning at different levels in the auditory pathways. [Research supported by BCMSF, NSERC and CIHR.Medicine, Faculty ofAudiology and Speech Sciences, School ofGraduat