Human middle-ear muscles rarely contract in anticipation of acoustic impulses: Implications for hearing risk assessments.

The current study addressed the existence of an anticipatory middle-ear muscle contraction (MEMC) as a protective mechanism found in recent damage-risk criteria for impulse noise exposure. Specifically, the experiments reported here tested instances when an exposed individual was aware of and could anticipate the arrival of an acoustic impulse. In order to detect MEMCs in human subjects, a laser-Doppler vibrometer (LDV) was used to measure tympanic membrane (TM) motion in response to a probe tone. Here we directly measured the time course and relative magnitude changes of TM velocity in response to an acoustic reflex-eliciting (i.e. MEMC eliciting) impulse in 59 subjects with clinically assessable MEMCs. After verifying the presence of the MEMC, we used a classical conditioning paradigm pairing reflex-eliciting acoustic impulses (unconditioned stimulus, UCS) with various preceding stimuli (conditioned stimulus, CS). Changes in the time-course of the MEMC following conditioning were considered evidence of MEMC conditioning, and any indication of an MEMC prior to the onset of the acoustic elicitor was considered an anticipatory response. Nine subjects did not produce a MEMC measurable via LDV. For those subjects with an observable MEMC (n = 50), 48 subjects (96%) did not show evidence of an anticipatory response after conditioning, whereas only 2 subjects (4%) did. These findings reveal that MEMCs are not readily conditioned in most individuals, suggesting that anticipatory MEMCs are not prevalent within the general population. The prevalence of anticipatory MEMCs does not appear to be sufficient to justify inclusion as a protective mechanism in auditory injury risk assessments.