Assessment of Contralateral Efferent Effects in Human Via ECochG
Efferent projections from the brainstem to the inner ear are well-described anatomically and physiologically but their precise function remains debated. The medial olivocochlear (MOC) system and its reflex, the MOCR, have been particularly well studied. In animals, anatomical and physiological data are fine-grained and extensive and suggest an important role for the MOCR in anti-masking e.g. to improve the detection of tones in background noise. Extensive behavioral studies in human support this role, but direct linking of behavioral paradigms to the MOCR is challenging because of the difficulty in obtaining appropriate human neural measures. We developed a new approach in which mass potentials were recorded near the cochlea of normal hearing and awake human volunteers to increase the signal-to-noise (SNR) ratio, and examined whether broadband noise to the contralateral ear elicited MOCR anti-masking effects as reported in animals. Probing the mass potential to the onset of brief tones at 4 and 6 kHz, convincing anti-masking or suppressive effects consistent with the MOCR were not detected. We then changed the recording technique to examine the neural phase-locked contribution to the mass potential in response to long, low-frequency tones, and found that contralateral sound suppressed neural responses in a systematic and progressive manner. We followed up with psychophysical experiments in which we found that contralateral noise elevated detection threshold for tones up to 4 kHz. Our study provides a new way to study efferent effects in the human peripheral auditory system and shows that contralateral efferent effects are biased towards low frequencies.