The Effect of Activating the Medial Olivocochlear Fibers on Cochlear Distortions in Humans

Abstract

The function of the medial olivocochlear (MOC) fibers has been investigated extensively in animals, and far less in humans. A possible function of the MOC efferents is protection against loud sounds. The aim of this study is to investigate a potential tool for evaluating the MOC reflex clinically in humans. Cochlear microphonic (CM) and the associated distortions were measured while activating the MOC fibers for an extended period of time. CM was recorded in 16 normal hearing young adults using 500 Hz toneburst at 80 dB nHL. Recording of CM was conducted every three minutes for a time-block of 18-minutes. Four total 18-minute time-blocks were recorded, two without contralateral broadband noise (CBBN) [condition (1)] and two with 50 dB SPL CBBN [condition (2)]. The CM responses were subjected to fast-Fourier transform to obtain the amplitude of the primary frequency (F1=500Hz), and the second (2F1=1000Hz) and the third (3F1=1500Hz) harmonics. A repeated-measures ANOVA was completed on the amplitude of F1, 2F1, and 3F1, and post-hoc analysis was utilized using LSD. There is approximately 21% increase in the F1 amplitude as a result of presenting CBBN, which is significant (p<0.01). There is a significant change in 2F1 (p<0.01) and 3F1 (p<0.01) amplitudes as a result of presenting the CBBN. The current study shows that the activation of the MOC fibers results in enhancement of the CM response. Furthermore, the results show that activation of the MOC fibers causes modulation of 2F1 and 3F1 of the CM response. The resulting changes of the CM distortions are in agreement with the proposed model of adjusting the operating point of the cochlear amplifier as a result of activating the MOC fibers. These results support the use of CM measurement as an objective measure for evaluating the MOC reflex clinically.