Effects of unilateral eye closure on middle ear muscle contractions.

Middle ear muscle contractions (MEMCs) are most commonly considered a response to high-level acoustic stimuli. However, MEMCs have also been observed in the absence of sound, either as a response to somatosensory stimulation or in concert with other motor activity. The relationship between MEMCs and non-acoustic sources is unclear. This study examined associations between measures of voluntary unilateral eye closure and impedance-based measures indicative of middle ear muscle activity while controlling for demographic and clinical factors in a large group of participants (N=190) with present clinical acoustic reflexes and no evidence of auditory dysfunction. Participants were instructed to voluntarily close the eye ipsilateral to the ear canal containing a detection probe at three levels of effort. Orbicularis oculi muscle activity was measured using surface electromyography. Middle ear muscle activity was inferred from changes in total energy reflected in the ear canal using a filtered (0.2 to 8 kHz) click train. Results revealed that middle ear muscle activity was positively associated with eye muscle activity. MEMC occurrence rates for eye closure observed in this study were generally higher than previously published rates for high-level brief acoustic stimuli in the same participant pool suggesting that motor activity may be a more reliable elicitor of MEMCs than acoustic stimuli. These results suggest motor activity can serve as a confounding factor for auditory exposure studies as well as complicate the interpretation of any impulsive noise damage risk criteria that assume MEMCs serve as a consistent, uniform protective factor. The mechanism linking eye and middle ear muscle activity is not understood and is an avenue for future research.