Detectability of amplitude-modulation following response at different carrier frequencies.

To assess the detectability of amplitude-modulation following response (AMFR) elicited at different carrier (CFs) and modulation frequencies (MFs) during sleep, as well as the usefulness of AMFR in evoked response audiometry, AMFR was examined in 10 adults with normal hearing while sleeping. The stimulus was a 50 dBnHL sinusoidally amplitude-modulated (SAM) tone with a modulation depth of 95%. The MF of the stimulus tone was varied from 20 to 120 Hz in 20 Hz steps and CFs were 500, 1000, 2000 and 4000 Hz. An SAM tone with a CF of 20,000 Hz was used for stimulation in 5 subjects to confirm that the response did not contain any artifact. Response was determined by phase spectral analysis. The component synchrony measure of AMFRs at MF of 40 Hz (40-Hz AMFR) was high at lower CFs, but 40-Hz AMFRs at higher CFs were unreliable. The detectability of 80-Hz AMFR was high for all CFs. It was confirmed that the response waveform was not contaminated by any electromagnetic artifact. Eighty-Hz AMFR detected by phase spectral analysis should thus be useful for predicting frequency-specific hearing thresholds during sleep.