Effects of Aging, Hearing Loss, and Co-Activation on the Middle Ear Muscle Reflex and Medial Olivocochlear Reflex

Abstract The middle ear muscle reflex (MEMR) and medial olivocochlear reflex (MOCR) are increasingly studied for their role in suprathreshold auditory processing. However, recording these reflexes in humans is potentially complicated by age-related (sub)clinical hearing loss and co-activation. This study investigates the influence of age-related sensorineural and audiometric hearing loss, and how MEMR activation contaminates MOCR recordings. Three test groups were included: young normal-hearing adults, middle-aged normal- hearing adults, and middle-aged adults with hearing loss. The sensorineural auditory status was assessed using distortion-product otoacoustic emissions (DPOAEs) and envelope following responses. MEMR recordings were evaluated using wideband stimuli and MOCR was recorded at elicitor levels of 60 and 75 dB to evaluate MEMR co-activation. MEMR amplitude, but not threshold, was related to age; however, this effect was significantly reduced after correcting for DPOAE and extended high-frequency thresholds. Associations between MOCR strength and age or hearing-related measures were mixed and reliable recordings were challenging in participants with hearing loss due to poor OAE baselines. MEMR co-activation was detectable in the click response and could alter MOCR-induced suppression. These findings suggest that MEMR amplitude may be a marker of subclinical cochlear damage, while MOCR sensitivity to hearing status appears more closely tied to audiometric than subclinical sensorineural status. Clinical measurements can be improved using broadband stimuli, accounting for cochlear damage, and defining criteria for reflex co-activation.