Investigation of the Function of the Olivocochlear Efferent Nerve Pathway in Humans

Abstract

The cochlea transduces sound into neural information. An important part of this transduction is an active process whereby the passive resonant characteristics are enhanced, leading to greater sensitivity and frequency selectivity. A population of sensory cells, called the outer hair cells, are the primary effectors of the active process, and can alter their length and stiffness dynamically. The outer hair cells are also innervated by the efferent nerve pathway descending from the region of the brainstem surrounding the medial portion of the superior olivary complex, and called the medial olivocochlear pathway. Previous research has demonstrated that activation of this pathway is associated with changes in cochlear micromechanics, and changes in the output measured at the auditory nerve. The pathway innervating one ear can be stimulated by presentation of sound to the contralateral ear. The roles of these pathways in hearing are not understood. The present research has investigated the perceptual effects of different levels of efferent activation in human subjects. Adaptive psychophysical procedures were used to investigate the change in monaural detectability of signals (broadband, high frequency, and low frequency noise; and 1-kHz tones, 7-kHz tones, and 100-µs clicks embedded in background noise) under a range of contralateral noise stimulation (from none to 80 dB SPL). Additionally, transient otoacoustic emissions were measured under the same range of contralateral stimuli, to confirm cochlear physiological changes. Auditory evoked potentials in response to a click, embedded in noise, were recorded from scalp electrodes, in order to confirm that the output of the cochlea was altered. A decrease in the detectability of all the noise stimuli, and click and 1-kHz tone signals embedded in background noise with increasing efferent activation was observed. The function relating change in detectability of the 7- kHz tone in noise to level of contralateral stimulation was U-shaped, suggesting that detectability was improved by middle levels of contralateral stimulation. Otoacoustic emission amplitudes were reduced by contralateral noise at the same levels at which the changes in detectability occurred. The amplitude and latency of the first wave of the auditory brainstem response were reduced with increasing olivocochlear pathway activation. It was reasoned that these effects resulted from suppression of the function of active processes within the cochlea by the activity of the olivocochlear pathway. In all cases, this produced a monotonic decrease in detectability of the signals, except for the 7-kHz tone in noise. The explanation for this finding was that the 7-kHz tone would have been more strongly masked by the broadband noise than either the 1-kHz tone or the click because of the upward spread of masking. Thus, suppression of activity in response to the noise would have improved the detectability of the tone under middle levels of efferent activity.