Date Published: February 16, 2018
Publisher: Public Library of Science
Author(s): W. Wiktor Jedrzejczak, Krzysztof Kochanek, Henryk Skarzynski, Edward C. Killan.
It has been reported that both click-evoked otoacoustic emissions (CEOAEs) and distortion product otoacoustic emissions (DPOAEs) have higher amplitudes in ears that possess spontaneous otoacoustic emissions (SOAEs). The general aim of the present study was to investigate whether the presence of spontaneous activity in the cochlea affected tone-burst evoked otoacoustic emissions (TBOAEs). As a benchmark, the study also measured growth functions of CEOAEs. Spontaneous activity in the cochlea was measured by the level of synchronized spontaneous otoacoustic emissions (SSOAEs), an emission evoked by a click but closely related to spontaneous otoacoustic emissions (SOAEs, which are detectable without any stimulus). Measurements were made on a group of 15 adults whose ears were categorized as either having recordable SSOAEs or no SSOAEs. In each ear, CEOAEs and TBOAEs were registered at frequencies of 0.5, 1, 2, and 4 kHz, and input/output functions were measured at 40, 50, 60, 70, and 80 dB SPL. Global and half-octave-band values of response level and latency were estimated. Our main finding was that in ears with spontaneous activity, TBOAEs had higher levels than in ears without. The difference was more apparent for global values, but were also seen with half-octave-band analysis. Input/output functions had similar growth rates for ears with and without SSOAEs. There were no significant differences in latencies between TBOAEs from ears with and without SSOAEs, although latencies tended to be longer for lower stimulus levels and lower stimulus frequencies. When TBOAE levels were compared to CEOAE levels, the latter showed greater differences between recordings from ears with and without SSOAEs. Although TBOAEs reflect activity from a more restricted cochlear region than CEOAEs, at all stimulus frequencies their behavior still depends on whether SSOAEs are present or not.
The first publication describing otoacoustic emissions (OAEs) dealt with cochlear responses evoked by clicks (i.e. click-evoked OAEs or CEOAEs) , followed shortly by descriptions of distortion product OAEs (DPOAEs) and spontaneous OAEs (SOAEs) . In the same year, tone-burst-evoked OAEs (TBOAEs) were also described . Both CEOAEs and TBOAEs are measured in a time window following a short stimulus, and are therefore classified as transient evoked OAEs (TEOAEs). OAEs can also be evoked by continuous tones and combination of tones, as in stimulus frequency OAEs (SFOAEs) and DPOAEs respectively.
OAEs were recorded from both ears of a group of 15 adults (11 females, 4 males, age 25–35). All subjects underwent visual inspection of the ear canal and tympanic membrane of both ears followed by tympanometry, pure tone audiometry, and OAE measurements. All subjects had normal middle ear function (as assessed by 226 Hz tympanometry) and hearing thresholds equal to or better than 20 dB HL for all test frequencies from 0.25 to 8 kHz. The subjects gave written informed consent prior to participation. Research procedures were approved by the Ethics Committee of the Institute of Physiology and Pathology of Hearing, Poland.
The results indicate that TBOAE levels generally depend on whether or not SSOAEs can be detected. Ears in which SSOAEs were detected had higher level TBOAEs than ears where they were absent, and this effect was evident at most of the stimulus levels used in the study. The inference is that ears with SSOAEs above the noise floor have lower TBOAE detection thresholds.
TBOAEs are significantly affected by the presence of SSOAEs, even though TBOAEs reflect activity from a narrower region of the cochlea than do CEOAEs. In ears with SSOAEs, TBOAEs show higher levels and lower detection thresholds. TBOAEs might be a better choice than CEOAEs when testing subjects with low-amplitude OAEs or in ears without SSOAEs. Interestingly, the presence of SSOAEs seems not to affect TBOAE latency. This proves that latency is quite a robust parameter and supports its use when studying evoked OAEs. TBOAEs share many of the features of CEOAEs, but they have the useful property of providing additional information in the lower frequency range, e.g. at 0.5 kHz where CEOAEs show responses only at 80 dB SPL.