Covariation of binaural, concurrently-measured spontaneous otoacoustic emissions

Bilateral Spontaneous Otoacoustic Emissions Show Coupling between Active Oscillators in the Two Ears

Spontaneous otoacoustic emissions (SOAEs) are weak sounds that emanate from the ears of tetrapods in the absence of acoustic stimulation. These emissions are an epiphenomenon of the inner ear's active process, which enhances the auditory system's sensitivity to weak sounds, but their mechanism of production remains a matter of debate. We recorded SOAEs simultaneously from the two ears of the tokay gecko and found that binaural emissions could be strongly correlated: some emissions occurred at the same frequency in both ears and were highly synchronized. Suppression of the emissions in one ear often changed the amplitude or shifted the frequency of emissions in the other. Decreasing the frequency of emissions from one ear by lowering its temperature usually reduced the frequency of the contralateral emissions. To understand the relationship between binaural SOAEs, we developed a mathematical model of the eardrums as noisy nonlinear oscillators coupled by the air within an animal's mouth. By according with the model, the results indicate that some SOAEs are generated bilaterally through acoustic coupling across the oral cavity. The model predicts that sound localization through the acoustic coupling between ears is influenced by the active processes of both ears.

Synchronized spontaneous otoacoustic emissions analyzed in a time-frequency domain

A synchronized spontaneous otoacoustic emission paradigm was used to measure the response in time intervals of 80 ms following a click stimulus. The responses obtained were decomposed into basic waveforms by means of adaptive approximations using a matching pursuit algorithm. High-resolution time-frequency distributions of signal energy were calculated and showed three types of component: (1) purely evoked of duration less than 5 ms, (2) longer lasting and decaying, with exponentially decreasing amplitude, and (3) long lasting and stable. The distributions of the frequencies of components of different durations were similar, with most components falling within the 1-2 kHz interval. It is shown that the presence of long-lasting components may influence the estimation of the latency of evoked emissions, especially at higher frequencies where the evoked part has a very short duration.

Effect of sleep stages on synchronized spontaneous otoacoustic emissions in pre-term neonates

OBJECTIVES

In infants, auditory tests are mainly performed during sleep, since they spend most of their time asleep, and because quiet is required for the duration of the recording session to obtain a precise and reliable response. The aim of this study was to investigate the effect of sleep stages on synchronized spontaneous otoacoustic emissions (sSOAEs) in pre-term neonates at the age where the sleep states begin to be well established and auditory screening can be performed in a neonatology unit before discharge.

METHODS

Synchronized SOAEs were repeatedly recorded during a polygraphic sleep recording using the Otodynamic ILO88 system in 10 pre-term neonates at 36 weeks post-conception.

RESULTS

Variations of sSOAE peak numbers occurred in each subject during the recording session. There was no clear relation between sSOAE peak number fluctuations and the different sleep stages.

CONCLUSIONS

The sSOAE variations appeared to be closely related to experimental conditions, i.e. the mean background noise level. sSOAEs with the highest amplitude were always recorded; however, those with the smallest amplitude were the first to disappear from the recordings with higher background noise.

Accurate binaural mirroring of spontaneous otoacoustic emissions suggests influence of time-locking in medial efferents

Spontaneous otoacoustic emissions (SOAEs) of nearly identical acoustic frequency in both ears are a common observation, but it is unknown if this binaural mirroring effect is random, artefactual, genetic, developmental, or of other origin. The available raw data of all human SOAE surveys were pooled, and the intervals of all possible binaural emission pairs (N = 9555) were listed according to size on the Cent-scale (1 Cent = 1/100 semitone = 1/1200 octave). Statistical analysis showed (1) a slight broad-band mirroring in the 0-100 Cent range (P < 0.05), and (2) a strong narrow-band mirroring (NBM) in the 0-20 Cent range (P < 0.001). Negative results in a detailed SOAE cluster detection program excluded experimental artefacts as causes of NBM. Analysis of the large subgroup of twin data excluded genetic and intrauterine developmental causes. Systemic developmental causes are unrealistic, as 20 Cent corresponds to only approximately 80 microm on the cochlear map. Analysis of infant data indicated that the effect may be introduced after birth by secondary factors. Interaural crosstalk was examined but had to be rejected. It is suggested that bilaterally spreading period information in the medial olivocochlear system influences outer hair cells of the same best frequency in both ears very similarly. Evidence concerning possible effects on electromotility is discussed, and experimental tests are proposed.

The influence of disappearing and reappearing spontaneous otoacoustic emissions on one subject's threshold microstructure

The effects of a consistently disappearing and reappearing spontaneous otoacoustic emission (SOAE) at around 2280 Hz on microstructure for pure tones of varying durations in a 33 year-old woman with normal hearing was studied. The SOAE began to appear after 10-15 min in a quiet test room and increased in level by up to 22 dB over a 30-40-min period. The SOAE was measured every 12 to 15 min. Between measurements, the subject performed a signal detection task for pure tones with total durations varying from 20 to 320 ms. The signal frequencies were within a +/- 30-Hz range relative to the SOAE frequency. For signal durations of 40-320 ms, there was a local dip at the target SOAE frequency when it was either not detectable or its level was lower than -14 dB SPL. Subjective threshold levels were as much as 12 dB better than those obtained when the SOAE was -6 dB SPL or greater. The results suggest that a region of the cochlea with high sensitivity and instability can be put into self-oscillation producing an SOAE, possibly by a change of efferent activity. Hearing threshold is affected possibly due to adaptation or masking.

Reliability of transient-evoked otoacoustic emissions

OBJECTIVE

This investigation addressed four factors affecting transient-evoked otoacoustic emission (TEOAE) reliability: 1) The effect of evoking-stimulus level, 2) the effect of analyzing bandwidth, 3) the effect of slight-mild hearing loss, and 4) the effect of variability in the stimulus spectrum.

DESIGN

TEOAEs at 80, 74, 68, and 62 dB pSPL evoking-stimulus levels were measured in 25 ears spanning a range of hearing levels from normal to mild hearing loss for a minimum of 10 test sessions. Reliability was assessed for 1/6-, 1/3-, 1/2-, and 1-octave analyzing bandwidths.

RESULTS

Evoking-stimulus level, hearing loss, and center frequency did not significantly affect reliability. With decreasing analyzing bandwidth, reliability decreased. Intrasubject test-retest standard deviations were 1.2 dB for a broadband analyzing bandwidth and 1.4, 1.5, 1.6, and 1.8 dB for 1-, 1/2-, 1/3-, and 1/6-octave analyzing bandwidths, respectively. Stimulus variability within narrower bandwidths was of sufficient magnitude to influence test-retest reliability, and attempts to correct for the variations in stimulus spectrum were unsuccessful. Slopes of the input-output functions differed across frequencies, with shallower slopes at higher frequencies.

CONCLUSIONS

In general, TEOAE amplitude is highly reliable. For those individuals in this study who were more variable, the variability was at low frequencies or across the entire frequency spectrum. For clinical applications, the choice of analyzing bandwidth should be based on consideration of both frequency specificity (where narrow analyzing bandwidths are optimal) and reliability (where wide analyzing bandwidths are optimal).