Auditory steady-state responses and word recognition scores in normal-hearing and hearing-impaired adults

Objective: The number of steady-state responses evoked by the independent amplitude and frequency modulation (IAFM) of tones has been related to the ability to discriminate speech sounds as measured by word recognition scores (WRS). In the present study IAFM stimulus parameters were adjusted to resemble the acoustic properties of everyday speech to see how well responses to these speech-modeled stimuli were related to WRS. Design: We separately measured WRS and IAFM responses at a stimulus intensity of 70 dB SPL in three groups of subjects: young normal-hearing, elderly normal-hearing, and elderly hearing-impaired. We used two series of IAFM stimuli, one with modulation frequencies near 40 Hz and the other with modulation frequencies near 80 Hz. The IAFM stimuli, consisting of four carrier frequencies each independently modulated in frequency and amplitude, could evoke up to eight separate responses in one ear. We recorded IAFM responses and WRS measurements in quiet and in the presence of speech-masking noise at 67 dB SPL or 70 dB SPL. We then evaluated the hearing-impaired subjects with and without their hearing aids to see whether an improvement in WRS would be reflected in an increased number of responses to the IAFM stimulus. Results: The correlations between WRS and the number of IAFM responses recognized as significantly different from the background were between 0.70 and 0.81 for the 40 Hz stimuli, between 0.73 and 0.82 for the 80 Hz stimuli, and between 0.76 and 0.85 for the combined assessment of 40 and 80 Hz responses. Response amplitudes at 80 Hz were smaller in the hearing-impaired than in the normal-hearing subjects. Response amplitudes for the 40 Hz stimuli varied with the state of arousal and this effect made it impossible to compare amplitudes across the different groups. Hearing aids increased both the WRS and the number of significant IAFM responses at 40 Hz and 80 Hz. Masking decreased the WRS and the number of significant responses. Conclusions: IAFM responses are significantly correlated with WRS and may provide an objective tool for examining the brain's ability to process the auditory information needed to perceive speech.