Growth of Auditory Reflexes with Stimulus Level

In the human auditory system, the brainstem can modify how sound is processed in the inner ear (cochlea). The medial olivocochlear efferent reflex (MOCR) is one such inhibitory neural response originating in the brainstem. When activated, this reflex reduces amplification in the cochlea, which is believed to improve hearing in background noise. Previous research has shown that the reflex reduces amplification in the inner ear by different amounts for soft vs. loud acoustic stimuli. We hypothesized that these varying levels of reduction are equivalent to a constant reduction of stimulus input level. To measure these level changes, we used otoacoustic emissions, which are soft sounds emitted from the cochlea that can be measured with a small microphone placed in the ear canal. Otoacoustic emission amplitudes obtained with MOCR activated included a constant reduction of stimulus input; however, they also showed an additional shift. The findings suggested that previous research should be revisited using the new measurements.