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Relationship between otoadmittance and threshold measurements in a TTS paradigm with phonation Andrews, Virginia Anathalie Taylor

Abstract

This investigation studies the role of the middle ear muscles in the TTS reduction that occurs when phonation accompanies exposure to a high intensity low frequency pure tone. Changes in acoustic admittance (taken as a measure of middle ear muscle activity) were compared with changes in TTS, recorded under similar experimental conditions. The TTS paradigm consisted of measuring subjects' hearing thresholds before and after 5 minute exposure to a 500 Hz, 117.5 dB SPL tone, accompanied or not by phonation (humming). The paradigm was repeated with threshold measurement being replaced by otoadmittance measurement; in this case admittance changes were recorded before, during, and after the fatigue exposure. The results show that TTS from the exposure tone with phonation was significantly less than TTS from the exposure tone with no phonation. The effect of phonation on TTS was most significant at early post-exposure times. No significant TTS differences between males and females were found. Changes in the two admittance components at the beginning and at the end of exposure were significantly larger when phonation accompanied the exposure than when not. This finding suggests that more middle ear muscle activity occurs when phonation accompanies exposure than when no phonation is performed. Most admittance measurements did not correlate significantly with any of the TTS measurements. The only significant correlations indicated that the smaller the middle ear muscle activity resulting from the fatigue exposure alone, the larger the amount of protection provided by phonation, as measured by differences between TTS values at early postexposure times between the two conditions. This finding suggests that most individuals may have middle ear muscles that contract weakly in response to intense acoustic stimulation alone but that these muscles contract significantly when phonation accompanies the acoustic stimulation. Thus, phonation provides considerable protection of the ear from the 500 Hz fatigue tone, as shown by the reduced TTS when phonation accompanies exposure. The results also suggest that the middle ear muscles are a major factor in reduced TTS with phonation but other mechanisms such as inefficient stapes vibration and attentional factors may also be involved. More research is necessary to determine the exact role each mechanism plays in the reduction of TTS with phonation.

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