Electrical recording of otoacoustic emission

Otoacoustic emissions (OAEs) could be detectable as cochlear AC potentials. Spontaneous otoacoustic emissions (SOAEs) were detected either electrically or acoustically, while evoked otoacoustic emissions (EOAEs) could be detected electrically but not acoustically.

Many lines of evidence support the hypothesis that SOAEs are produced by spontaneous mechanical oscillation within the cochlea, and perhaps motile properties of the outer hair cells. If this is the case, SOAEs, emitted acoustically in the external auditory meatus, could also be recorded electrically as cochlear AC potential. EOAE is also thought to be produced by vibration of the basilar membrane, generated by the backward-traveling waves. EOAE thus seems to be detectable electrically as cochlear AC potential. In the present study, SOAE and EOAE were recorded both acoustically and electrically in the guinea pigs to examine the correlation between electrically recorded SOAE (ER-SOAE) and acoustically recorded SOAE (AR-SOAE). In addition, a microphonics response (MPR) to tone pip was recorded to analyze the characteristics of the non-linear component and linear component of the AC responses.

(1) In 4 out of 20 guinea pigs (20%), SOAE could be detected both acoustically and electrically. (2) Electrical signals of SOAE had a better S/N ratio than acoustical signals. Generally, only some ER-SOAE could be detected acoustically. (3) Almost without exception, the prominent frequencies of multiple ER-SOAEs corresponded to the intermodulation distortion product, or harmonics. (4) ER-SOAEs were suppressed by hypoxia or intense sound exposure and reappeared upon rebreathing or discontinuation of the external tone. During recovery, prominent frequencies showed a transient downward shift in frequency. (5) SOAEs were synchronized in phase with an external tone in the spectral neighborhood of SOAE. The averaged waveform of SOAE synchronized with the external tone was the same with either acoustic or electrical signals. (6) The MPR to tone pip is composed of two components with different frequency characteristics and input/output functions. (7) The non-linear component delayed to cochlear microphonics was markedly saturated at the intensity level of 40 dB peak equivalent SPL. This component was a phase-lock response, not a frequency-locked one. (8) The non-linear component could be separated with Probst's non-linear differential extraction technique. In the MPR to a 4-kHz tone pip, high-cut filtration at 3.5 kHz produced a waveform similar to the non-linear component separated by Probst's method.