Contralateral auditory stimulation alters acoustic distortion products in humans

It is now generally accepted that otoacoustic emissions (OAE) represent the only objective and non-intrusive means of functional exploration of the active micromechanical characteristics of the outer hair cells of the organ of Corti. Previous studies showed a decrease of the transiently evoked otoacoustic emissions and spontaneous otoacoustic emissions in humans, during acoustic stimulation of the contralateral ear, and attributed this effect to the medial efferent system. Such an effect has been shown on acoustic distortion product otoacoustic distortion emissions (DPOAE) in guinea pigs, but has not been investigated for DPOAEs recorded in humans, although DPOAEs represent the easiest means of exploring active micromechanical cochlear properties both in humans and in laboratory animals. The present study sought to investigate the existence and characteristics of a contralateral auditory stimulation effect on DPOAEs recorded in humans. This study shows that contralateral broad-band noise (BBN) has a suppressive effect on DPOAEs recorded from 0.5 kHz to 5 kHz. This effect is not due to air conduction, as no change in the noise floor occurred under increasing contralateral stimulation, and as no reduction in DPOAE amplitude was obtained in subjects whose contralateral ear was sealed with a plastic ear plug. Moreover, cross-over attenuation by bone transmission has been ruled out, as no change in DPOAE amplitude was recorded in the healthy ear of total unilaterally deaf patients during acoustic stimulation of the deaf ear. The effect seen was not entirely due to the acoustic reflex, as it was found and could indeed be even greater in subjects with no acoustic reflex. Results presented here show that the contralateral BBN effect is greater at low levels of ipsilateral stimulation, which leads us to discuss the involvement of both passive and active mechanisms in DPOAE generation at high stimulation levels. The contralateral BBN effect seems to be greater in mid frequency cochlear regions. There is strong evidence that the medial efferent system is involved and that afferent and efferent inputs are, at least partly, integrated at a brainstem level in order to ensure cochlear interaction. DPOAEs provide an interesting model for functional exploration of the efferent system, since they seem to be the only type of otoacoustic emission that can be recorded in both humans and in the majority of animals, and since results are obtained in the same way from both animals and humans, which allows experimental animal models very close to the human model.