Effect of Mental Fatigue on Medial Olivocochlear Reflex

Vulnerability to noise-induced hearing loss is highly variable. One of the factors for the variability is the medial olivocochlear reflex (MOCR), which controls acoustic input to the auditory system and protects it from acoustic overexposure. We investigate whether the MOCR is affected by mental fatigue. The MOCR was evaluated non-invasively using otoacoustic emissions (sounds generated by the inner ear). Mental fatigue was induced by a calculation task consisting of single-digit addition and subtraction, and the induced fatigue was assessed by a critical flicker frequency (CFF) test. A trial consisted of the calculation task session (20 minutes) and the measurements of CFF and MOCR strength (10 minutes). The trial was repeated four times continuously. A questionnaire conducted before and after the entire experimental process showed that subjective fatigue was increased by the four times repetition of the calculation task. Both of CFF and MOCR strength significantly decreased with the repetition of the task. In addition, the individual patterns of the task-induced changes in CFF and MOCR strength correlated positively, i.e. MOCR strength increased with increasing mental fatigue. These results indicate that mental fatigue affects the inner ear protection provided by the MOCR and consequently the risk for noise-induced hearing loss.