The Level Effect of Contralateral Acoustic Stimulation (CAS) on the Distortion Product Otoacoustic Emission (DPOAE) Suppression

Abstract

As an objective and non-invasive method to evaluate functional state of medial olivo cochlear (MOC) system, there has been growing interest in contralateral suppression of distortion product otoacoustic emission (DPOAE). Also, MOC system can be quantified by measuring the difference DPOAE amplitude recorded in the presence of contralateral acoustic stimulation (CAS). The purpose of this study is to identify the optimal CAS level for measuring the DPOAE contralateral suppression. In order to do that, the effect of gender, direction of ear, and the CAS level of the DPOAE suppression for normal hearing adults were tested. Data were collected from the thirty six ears of the eighteen subjects with normal hearing in the both ears ranged from 21 to 26 years of age. DPOAE was tested by ILO 292 analyzer in DP-gram function. For each measurement, primary tones frequencies were ratio f2/f1 = 1.22(f1 < f2) and intensities were L1 = 65 dB SPL, L2 = 50 dB SPL. 2f1-f2 DPOAE was elicited using two pure tones and frequencies were measured in the range of 1,000 Hz to 6,727 Hz. Broad band noise was used for CAS of DPOAE. CAS was delivered through a TDH 39 headphone that was placed in the participant’s contralateral ear. CAS level was randomly presented at 50, 60, 65, 70, and 80 dB SPL. The results showed that, first, there was no significant effect of the direction of ear for DPOAE suppression (p > .05). The results showed that the DPOAE suppression was not significantly different between male and female (p > .05). However, the means and SDs of DPOAE suppression for 5 different CAS levels, 50, 60, 65, 70, and 80 dB SPL, were .66(±1.20), 1.02(±1.23), 1.47(±1.73), 1.58(±1.70), and 1.21(±1.46) dB. As the CAS levels increased, the majority of DPOAE suppression increased generally until 70 dB SPL. Taken together, the results of this study showed that CAS at different levels induced mainly DPOAE amplitude suppression. The amount of suppression increased as the CAS level was increased with the rolling phenomenon at 80 dB SPL than 70 dB SPL in CAS level. Consequently, the 70 dB SPL or more level appears to generate the acoustic reflex in normal ears, and the 70 dB SPL or less CAS level is expected to be efficient for observing decent DPOAE suppression.