Efferent-induced shifts in synchronized-spontaneous-otoacoustic-emission magnitude and frequency.

Synchronized-spontaneous otoacoustic emissions (SSOAEs) present as slow-decaying emission energy that persists after the transient-evoked otoacoustic emission (TEOAE). SSOAEs possess high amplitudes and signal-to-noise ratios, making them potentially ideal candidates to assay the medial-olivocochlear reflex (MOCR). The current work quantified MOCR-induced changes to SSOAEs over a 36-dB stimulus level range and compared MOCR effects between TEOAE- and SSOAE-based assays. Otoacoustic emissions were evoked using band limited clicks from 52 to 88 dB peak sound pressure level (pSPL) with and without contralateral-acoustic stimulation (CAS) in 25 normal-hearing, female adults. The CAS was 50-dB sound pressure level (SPL) broadband noise and served to activate the MOCR. The number of SSOAEs increased with the stimulus level through approximately 70 dB pSPL. The presentation of CAS resulted in fewer SSOAEs. SSOAEs exhibited compressive growth and approached saturation for stimulus levels of 70 dB pSPL. The primary effects of CAS were a reduction in the SSOAE magnitude and an upward shift in the SSOAE frequency. These changes were not strongly affected by the stimulus level. Time-domain analysis of the SSOAE revealed an increase in the CAS-induced magnitude shift during the decay portion of the SSOAE. Compared to CAS-induced TEOAE magnitude shifts, SSOAE magnitude shifts were typically 2-3 dB larger. Findings support SSOAEs as a means to assay the MOCR.