Exploration of the Relationship between Cochlear Gain Reduction and Speech-in-Noise Performance.

Listening to speech in noisy environments is difficult for listeners with normal hearing, but a task that is often accomplished successfully. The mechanisms that underlie this ability are not well understood. One proposed mechanism, the medial olivocochlear reflex (MOCR), is explored in this dissertation. The MOCR is a bilateral reflex between the brainstem and cochlea that reduces cochlear gain in response to preceding sound. The magnitude of ipsilateral cochlear gain reduction is explored in the present study at 1, 2, and 4 kHz using forward masking techniques, in an effort to evaluate the magnitude of cochlear gain reduction at a range of frequencies important for speech perception. Gain reduction estimates were not significantly different at 2 and 4 kHz using two forward masking measurements. Gain reduction estimates at 1 kHz were significantly higher than those at 2 and 4 kHz. The relationship between psychoacoustic and physiologic measures of cochlear gain reduction was also explored, as well as how these measures relate to speech-in-noise performance at +3, 0, and -3 dB signal-to-noise ratios. A significant relationship between the psychoacoustic measure and speech-in-noise performance at -3 dB signal-to-noise ratio was found; individuals with larger gain reduction estimates at 2 kHz had poorer performance on the speech-in-noise task. The utility of cochlear gain reduction for speech perception may depend on the signal-to-noise ratio