Increased amplitude of distortion product emissions in the human caused by contralateral low intensity acoustic stimulation

Abstract

Distortion product otoacoustic emissions (DPOAEs) are probably based on the motile properties of the cochlear outer hair cells (OHCs), which are richly innervated by medial olivo-cochlear efferent nerves (MOCs). DPOAEs provide a stimulating model for functional exploration of the efferent system. The influences of contralateral acoustic stimulation were studied on the amplitude of ipsilateral DPOAEs at the frequency 2f1-f2. Fifteen ears of adults with normal hearing (7 women, 8 men, mean age; 23.1 +/- 4.2 years) were examined. Sound stimulation consisted of exposure to 6 successive series of contralateral pure tones at frequencies between 0.5 and 4.0 kHz and low intensities of 30 dB HL. DP grams were registered with f2 ranging from 708 to 6165 Hz. Contralateral pure tones in the frequency range above 750 Hz and with intensities of 30 dB HL increased the ipsilaterally registered DPOAE amplitudes. The results showed that this increase was frequency specific, at least for frequencies at 1 and 2 kHz. The frequency specificity was especially marked when ipsilateral f2 was near 1 kHz. Contralateral frequencies of 3 and 4 kHz induced a significant, increasing effect, including ipsilateral frequencies around f2 = 3 kHz (or 4 kHz) and also lower frequencies around f2 = 1 kHz. Our results support the concept that the observed increase in DPOAE amplitudes is mediated by the medial olivo-cochlear efferent system. The rule of OHCs as a cochlear amplifier in the presence of contralateral low-intensity sound stimulation is discussed.