Acoustic enhancement of electrically evoked otoacoustic emissions: experiment and model results

The authors explore the effect of acoustic stimuli on electrically evoked acoustic emissions (EEAEs). A sinusoidal current of 15 μA of either 350 Hz or 750 Hz was injected into the second turn of the gerbil cochlea while acoustic signals were stepped in both sound pressure level and frequency. At each acoustic frequency, emission data without acoustic input were taken as control, and the acoustic stimuli were varied from 30 dB SPL to 80 dB SPL in 6 steps. The results show that EEAE can be enhanced up to 20 dB by presenting acoustic stimuli to the cochlea. Acoustic enhancement is highly dependent upon the condition of the cochlea. The greatest enhancement occurs for acoustic frequencies near the characteristic frequency of the electrode place. The enhancement threshold (1 dB enhancement) tuning curve is similar to a cochlea tuning curve in shape, but is more broadly tuned. This suggests that the emissions originate from a distributed cochlear region centered around the electrode place. A model based on simple assumptions gives results which are in qualitative agreement with the experimental data