Influence of contralateral acoustic stimulation on the quadratic distortion product f2–f1 in humans

Contralateral acoustic stimulation is known to activate the medial olivocochlear system which is capable of modulating the amplification process in the outer hair cells of the inner ear. We investigated the influence of different levels of contralateral broadband noise on distortion product otoacoustic emissions in humans, with a particular focus on the quadratic distortion product at f2–f1. The primary stimulus frequency ratio was optimized to yield maximum f2–f1 level. While the cubic distortion product at 2f1–f2 was not significantly affected during contralateral noise stimulation, the level of f2–f1 was reduced by up to 4.8 dB on average (maximum: 10.1 dB), with significant suppression occurring for noise levels as low as 40 dB SPL. In addition, a significant phase lead was observed. Quadratic distortions are minimal at a symmetrical position of the transfer function of the cochlear amplifier. The observed sensitivity of f2–f1 to contralateral noise stimulation could hence be resulting from a shift of the operating state and/or a change in the gain of the cochlear amplification due to contralateral induced efferent modulation of the outer hair cell properties.