Effects of Age on Contralateral Suppression of Distortion Product Otoacoustic Emissions in Human Listeners with Normal Hearing

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Abstract
The auditory efferent system presumably plays a role in enhancing signals in noise and, in particular, speech perception in background noise. This study measured the age-related changes of the medial olivocochlear (MOC) system by comparing distortion product otoacoustic emissions (DPOAEs) with and without contralateral white noise stimulation. Otoacoustic emissions were typically reduced in level (magnitude) when white noise was presented to the contralateral ear. This contralateral suppression (CS) is attributed to activation of the MOC system, which has an inhibitory effect on the outer hair cell (OHC) system. By studying CS on cochlear output in human listeners of different ages, it is possible to describe aging effects on the MOC system. Human subjects were young adult, middle aged and old (n = 10/group). All subjects had normal hearing and middle-ear function based upon standard audiometric criteria. The present study recorded 2f1–f2 DPOAE-grams in response to moderate primary tones (L1 = 75, L2 = 65 dB SPL), from 1 to 6.3 kHz. The principal findings were that DPOAE levels were smaller in the old compared to the young group and that CS declined with age for the middle-aged and old groups. In addition, CS in the 1- to 2-kHz range was greater than in the 4- to 6-kHz range for all ages, but especially for the old group. These findings suggest that a functional decline of the MOC system with age precedes OHC degeneration. Moreover, the MOC system maintains better function in the 1- to 2-kHz range than in the 4- to 6-kHz range as a function of age.