Frequency shift of individual spontaneous otoacoustic emissions in preterm infants

Characterizing spontaneous otoacoustic emissions across the human lifespan

This study characterizes 1571 archival and newly acquired spontaneous otoacoustic emissions (SOAEs) from 632 human subjects with ages ranging from premature birth through the seventh decade of life. Automated detection and Lorentzian modeling were applied to identify SOAEs and characterize SOAE features throughout the human lifespan. Results confirm higher-level, higher-frequency, and more numerous SOAEs from neonates compared to young adults. Approximately 85% of newborns have measurable SOAEs as compared to 51%-68% for young adults. Newborn SOAEs are also an average of 5 to 6 dB higher in level than those from young-adult ears. These age differences may reflect immature ear-canal acoustics and/or the pristine condition of the neonatal cochlea. In addition, newborns as a group showed broader SOAE bandwidth and increased frequency jitter, possibly due to higher intracochlear noise; additionally, 22% of newborn SOAEs had a different, non-Lorentzian spectral shape. Aging effects were also observed: 40% of elderly ears had SOAEs, and these were greatly reduced in level, likely due to lower power gain in the aging cochlea. For all ages, SOAE bandwidths decreased with frequency in a way that mirrors the frequency dependence of stimulus-frequency otoacoustic emission delays as predicted by the standing-wave model of SOAE generation.

Frequency shifts with age in click-evoked otoacoustic emissions of preterm infants

A previous study [Brienesse et al. (1997). Pediatr. Res. 42, 478-483] demonstrated a positive shift with increasing postmenstrual age (PMA) in the frequencies of synchronized spontaneous otoacoustic emissions (SSOAEs) in preterm infants. We used a mixed model approach to describe a shift with PMA in the spectra of click-evoked otoacoustic emissions (CEOAEs) measured in a group of 22 preterm infants. The rate in shift in CEOAE spectral components was found to be frequency dependent, with a mean estimate of 10 Hz/week for frequencies around 2 kHz and 30 Hz/week for frequencies around 4.25 kHz. This rate decreased with increasing PMA. Because SSOAEs are often part of the CEOAE response, a comparison was made between the shifts in SSOAEs and CEOAEs in a sub-group of 16 preterm infants. The results indicate that the shifts found for both types of OAE are similar, which supports a common mechanism for this change in OAE-characteristic. At present it is not clear to what extent developmental processes in the cochlea and the middle ear can account for these frequency shifts in the spectra of CEOAEs and SSOAEs during the preterm period.

Long-term stability of spontaneous otoacoustic emissions

Spontaneous otoacoustic emissions (SOAEs) were measured longitudinally for durations up to 19.5 years. Initial ages of the subjects ranged from 6 to 41 years. The most compelling finding was a decrease in frequency of all emissions in all subjects, which was approximately linear in %/year and averaged 0.25%/year. SOAE levels also tended to decrease with age, a trend that was significant, but not consistent across emissions, either within or across subjects. Levels of individual SOAEs might decrease, increase, or remain relatively constant with age. Several types of frequency/level instabilities were noted in which some SOAEs within an ear interacted such that their levels were negatively correlated. These instabilities often persisted for many years. SOAEs were also measured in two females over the course of their pregnancies. No changes in SOAE levels or frequencies were seen, that were larger than have been reported in females over a menstrual cycle, suggesting that levels of female gonadal hormones do not have a significant direct effect on SOAE frequencies or levels.

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.

On the spectral periodicity of transient-evoked otoacoustic emissions from normal and damaged cochleas

The spectral quasi-periodicity of transient-evoked otoacoustic emissions (TEOAE) is well acknowledged since Zwicker described a preferred spacing of 0.4 bark between consecutive peaks in the spectrum of otoacoustic emissions from normal ears. While there is scarce evidence of any anatomical reason for this regularity, several functional models of the cochlea have predicted that the structure of emission spectra reflects important characteristics of cochlear filters. In an attempt to check such predictions, the average regularity of TEOAE spectra was studied in three groups of human subjects, normally hearing adults, healthy neonates, and adults suffering from noise-induced hearing loss. Significant differences in emission periodicities were found. Around 1 kHz, the preferred spacing was close to 130 Hz in normally hearing adult ears and neonates. In contrast, no clear periodicity was found in the group of damaged ears, even though they had clinically normal pure-tone audiometry below 2 kHz. Around 4 kHz, the preferred spacing was close to 240 Hz in normal adults and neonates, whereas TEOAEs were absent in many impaired ears. A phenomenological model assuming that TEOAEs stem from the responses of a slightly disarrayed bank of highly tuned filters predicts that the filter width would be the same in healthy young adults and neonates. In contrast, ears suffering from high-frequency hearing loss could exhibit early damaged filters. The proposed method might provide an objective assessment of parameters otherwise difficult to evaluate, especially in neonatal cochleas.

Development of human cochlear active mechanism asymmetry: involvement of the medial olivocochlear system?

To study the functional development of the medial olivocochlear system, transient-evoked otoacoustic emission suppression experiments were conducted in 73 ears of 38 pre-term and 11 full-term neonates. The continuous contralateral stimulation was a broad band white noise, presented at 70 dB SPL. Efferent suppression was determined by subtracting the without-contralateral stimulation condition from the with-contralateral stimulation condition. Across this population, a mean suppression effect of contralateral stimulation on transient-evoked otoacoustic emissions was found, with most of the suppression effect observed after 8 ms. The amount of suppression is linearly, positively correlated with the conceptional age. In the subgroup of bilaterally tested neonates, the suppression of transient-evoked otoacoustic emissions is similar in the right ear and the left ear in subjects whose conceptional age is less than 36 weeks and significantly higher in the right ear than in the left ear in older neonates. This last observation was seen at frequencies where transient-evoked otoacoustic emission amplitudes became higher in the right ear than in the left ear as the conceptional age increased, a finding already reported in adults. This study shows that the functional adult pattern of the medial efferent system, probably involved in the detection of signals in noise such as speech sounds, seems to appear gradually in neonates and represents one of the several arguments in favor of functional auditory lateralization in humans, with a right ear advantage.

Long-term and short-term variations in amplitude and frequency of spontaneous otoacoustic emissions in pre-term infants

In pre-term infants, spontaneous otoacoustic emission (SOAE) frequencies show an upward shift with time. The present study aimed to monitor the SOAE amplitude variation during this frequency shift. A long-term observation of 87 SOAE frequencies from 18 pre-term infants yielded a positive frequency shift of 0.72 per cent per week, which was not accompanied by a simultaneous amplitude shift, as the mean variations in SOAE amplitude were practically zero. Furthermore, there was no relationship between the short-term SOAE amplitude variation and the infant's post-conceptional age. Only the absolute amount of SOAE amplitude variation seemed to grow with time. Comparison with induced variations in SOAE frequency argues against a middle ear influence on the SOAE frequency shift. In our view, the absence of any amplitude shift during the upward SOAE frequency shift further suggests cochlear development during the last period of gestation.