Acoustic Stapedius Reflex Function in Man Revisited

Abstract

The objective of this study was to examine the role of the acoustic stapedius reflex in the protection of speech recognition from the upward spread of masking arising from low-frequency background noise. Speech recognition scores were measured for nine control participants (19-34 years) and six patients with transected stapedius tendons poststapedotomy (39-57 years) as a function of the amplitude of a low-frequency masker, presented at nominal signal to noise ratios of +5 dB, -5 dB, and -15 dB. All participants had pure-tone hearing thresholds in the normal range. Continuous high-pass noise was present in all conditions to avoid ceiling effects; this reduced performance in quiet to approximately 85% for all participants. Scores were measured for soft and loud nonsense syllables (average third octave band levels of 35 and 65 dB SPL), so that a comparison of the low-frequency noise masking functions at the two levels would provide information about the effects of the reflex on speech intelligibility in noise. A third group of nine control participants (19-22 years) listened in the presence of a low-frequency masker gated to come on 1 sec before stimulus onset, to reduce the likelihood of reflex adaptation. The Speech-Intelligibility Index was used to quantify the amount of speech information available in each condition. Patients with transected tendons performed more poorly than control participants as a function of Speech-Intelligibility Index in all conditions, even at levels that were too soft for reflex activation. This could be because of postsurgical differences in sensitivity, the more advanced age of poststapedotomy group, or differences in medial olivocochlear inhibition. For loud speech, patient performance fell nearly linearly with increases in the low-frequency masker, but control participants' performance declined little as the signal to noise ratio declined from +5 to -5 dB, and then fell rapidly as the ratio declined to -15 dB. This plateau in the masking function did not occur for the patients. Masking functions obtained with the gated low-frequency masker were either highly similar or poorer to those obtained with a continuous masker, suggesting that the use of a continuous low frequency masker did not result in significant reflex adaptation. The stapedius reflex appears to offer some protection from the upward spread of masking of speech by background low-frequency noise at moderate levels, but not at high levels.