Comparison of the effects of mental fatigue on medial olivocochlear reflex and cortical activity

Abstract

The medial cochlear reflex (MOCR) is assumed to protects the inner ear from acoustic overexposure. The MOCR strength assay is expected to be applied to screen for individuals at high risk for noise-induced hearing loss. However, the MOCR strength varies from one measurement to another, and it is necessary to identify the factors that cause the MOCR variations for more accurate risk assessment. It has been reported that variations in cortical cognitive function, e.g., attention and expectation, influence MOCR strength. Here we examined the effects of mental fatigue, one of the causes that degrade cognitive functions, on MOCR. The degree of mental fatigue was adjusted by varying the difficulty of a calculation task and investigated the effects on MOCR. To elucidate the mechanism of MOCR variations associated with mental fatigue, cortical activities were evaluated by measuring slow vertex response (SVR). Although MOCR strength and SVR amplitude did not change significantly after the calculation task, two measures had reverse dependence on the difficulty of the calculation task; A moderately difficult calculation task increased SVR amplitude while reduced MOCR strength. The results implicate that MOCR variations associated with mental fatigue can be attributed to cortical activity changes.