Cochlear maturation and otoacoustic emissions in preterm infants: a time–frequency approach

Denoising click-evoked otoacoustic emission signals by optimal shrinkage

Click-evoked otoacoustic emissions (CEOAEs) are clinically used as an objective way to infer whether cochlear functions are normal. However, because the sound pressure level of CEOAEs is typically much lower than the background noise, it usually takes hundreds, if not thousands, of repetitions to estimate the signal with sufficient accuracy. In this paper, we propose to improve the signal-to-noise ratio (SNR) of CEOAE signals within limited measurement time by optimal shrinkage (OS) in two different settings: covariance-based optimal shrinkage (cOS) and singular value decomposition-based optimal shrinkage (sOS). By simulation, the cOS consistently enhanced the SNR by 1-2 dB from a baseline method that is based on calculating the median. In real data, however, the cOS cannot enhance the SNR over 1 dB. The sOS achieved a SNR enhancement of 2-3 dB in simulation and demonstrated capability to enhance the SNR in real recordings. In addition, the level of enhancement increases as the baseline SNR decreases. An appealing property of OS is that it produces an estimate of all single trials. This property makes it possible to investigate CEOAE dynamics across a longer period of time when the cochlear conditions are not strictly stationary.

Acoustic Reflex Testing in Neonatal Hearing Screening and Subsequent Audiological Evaluation

PURPOSE

The aims of the study were to examine the acoustic reflex screening and threshold in healthy neonates and those at risk of hearing loss and to determine the effect of birth weight and gestational age on acoustic stapedial reflex (ASR).

METHOD

We assessed 18 healthy neonates (Group I) and 16 with at least 1 risk factor for hearing loss (Group II); all of them passed the transient evoked otoacoustic emission test that assessed neonatal hearing. The test battery included an acoustic reflex screening with activators of 0.5, 1, 2, and 4 kHz and broadband noise and an acoustic reflex threshold test with all of them, except for the broadband noise activator.

RESULTS

In the evaluated neonates, the main risk factors were the gestational age at birth and a low birth weight; hence, these were further analyzed. The lower the gestational age at birth and birth weight, the less likely that an acoustic reflex would be elicited by pure-tone activators. This effect was significant at the frequencies of 0.5, 1, and 2 kHz for gestational age at birth and at the frequencies of 1 and 2 kHz for birth weight. When the broadband noise stimulus was used, a response was elicited in all neonates in both groups. When the pure-tone stimulus was used, the Group II showed the highest acoustic reflex thresholds and the highest percentage of cases with an absent ASR. The ASR threshold varied from 50 to 100 dB HL in both groups. Group II presented higher mean ASR thresholds than Group I, this difference being significant at frequencies of 1, 2, and 4 kHz.

CONCLUSIONS

Birth weight and gestational age at birth were related to the elicitation of the acoustic reflex. Neonates with these risk factors for hearing impairment were less likely to exhibit the acoustic reflex and had higher thresholds.

Differential Rates of Perinatal Maturation of Human Primary and Nonprimary Auditory Cortex

Primary and nonprimary cerebral cortex mature along different timescales; however, the differences between the rates of maturation of primary and nonprimary cortex are unclear. Cortical maturation can be measured through changes in tissue microstructure detectable by diffusion magnetic resonance imaging (MRI). In this study, diffusion tensor imaging (DTI) was used to characterize the maturation of Heschl's gyrus (HG), which contains both primary auditory cortex (pAC) and nonprimary auditory cortex (nAC), in 90 preterm infants between 26 and 42 weeks postmenstrual age (PMA). The preterm infants were in different acoustical environments during their hospitalization: 46 in open ward beds and 44 in single rooms. A control group consisted of 15 term-born infants. Diffusion parameters revealed that (1) changes in cortical microstructure that accompany cortical maturation had largely already occurred in pAC by 28 weeks PMA, and (2) rapid changes were taking place in nAC between 26 and 42 weeks PMA. At term equivalent PMA, diffusion parameters for auditory cortex were different between preterm infants and term control infants, reflecting either delayed maturation or injury. No effect of room type was observed. For the preterm group, disturbed maturation of nonprimary (but not primary) auditory cortex was associated with poorer language performance at age two years.

Otoacoustic emissions in newborns evoked by 0.5kHz tone bursts

BACKGROUND

Otoacoustic emissions (OAEs) are currently widely used in newborn hearing screening programs. OAEs evoked by transients (TEOAEs) in newborns are usually characterized by large response levels at higher frequencies but lower frequencies are affected by physiological noise. The purpose of the present study was to acquire responses at lower frequencies by measuring OAEs evoked by 0.5kHz tone bursts (TBOAEs).

METHODS

Otoacoustic emissions (OAEs) were recorded from 49 newborns. Measurements were made using the ILO 292 equipment from Otodynamics. In each ear, three measurements were made: first with a standard click stimulus at 80dB pSPL (CEOAEs), a second using a 0.5kHz tone burst at 80dB pSPL (TBOAEs), and a third recording of spontaneous OAEs (SOAEs). Global and half-octave-band values of OAE signal-to-noise ratio (SNR) and response level were used to assess statistical differences between CEOAEs and 0.5kHz TBOAEs. Additionally, time-frequency (TF) analysis of signals was performed using the matching pursuit method.

RESULTS

Global levels were highest for CEOAEs. However, at low frequencies (0.7-1kHz), 0.5kHz TBOAEs had significantly higher levels and SNRs than CEOAEs. At these frequencies, SNRs of CEOAEs were usually below 0dB. At 0.5kHz there were no statistically significant differences between CEOAEs and TBOAEs. In ears with recordable SOAEs, CEOAEs and TBOAEs had higher levels and SNRs than in ears without SOAEs.

CONCLUSIONS

Use of 0.5kHz TBOAEs may be a useful addition to standard CEOAE tests in newborns. They provide information about lower frequencies, a region where CEOAEs are usually prone to noise. The presence of SOAEs affects the magnitudes of both CEOAEs and TBOAEs.

Repeatability of transient-evoked otoacoustic emissions in young adults

Background

The aim of this study was to evaluate the repeatability and variability of TEOAE characteristics in hearing screening tests performed under practical conditions on normal subjects.

Material/Methods

A group of 11 young, normal-hearing subjects aged 19–24 years was tested. They were examined otologically and audiologically prior to the tests and no ear pathologies were found. Responses were acquired with a commercially available instrument (Integrity, Vivosonic Inc.) using a standardized OAE protocol. The TEOAE tests were repeated 3 times in each subject at random intervals within 24 h. The analyzed parameters of interest were: (i) whole wave reproducibility (WWR) and; (ii) signal-to-noise ratio (SNR).

Results

WWR and SNR did not differ significantly among the 3 measurement sessions. In most cases the differences in WWR among measurements were around 1–2% and for SNRs they were 1–4 dB SNRs and were highest in the 1–2 kHz range. TEOAE-based tests can be useful tools for hearing screening.

Conclusions

The tests can give reliable results provided that adequate procedures are used and low-noise conditions are ensured. The tests are best complemented with other examinations to widen the range of ear pathologies able to be detected.

[The hearing function in the premature infants]

The present review of the literature is focused on the morphological and functional aspects of antenatal ontogenesis of the hearing system with special reference to the peculiar features of its formation during the post-natal period in the premature infants. In this context, the results of the objective psychoacoustic investigation s are considered (including those obtained by such methods as threshold audiometry, verbal and non-verbal tests, evaluation of central auditory processing of the sound information). Also presented are the results of the objective methods of hearing investigations (acoustic impedancometry, registration of optoacoustic emission and auditory evoked potentials). The underdevelopment of the hearing system in the premature infants manifests itself as the specific functional features of all its components. Their parameters are presented with reference to dynamics of their formation. In addition, the problem of central auditory processing of the disorders that are very likely to be present in the premature infants are discussed.

Emissões otoacústicas evocadas transientes em recém-nascidos a termo e pré-termo

Objetivo verificar comparativamente a amplitude das emissões otoacústicas evocadas por estímulos transientes, observando as variáveis gênero e orelha em recém-nascidos a termo e pré-termo com e sem risco para alterações auditivas. Métodos participaram deste estudo 156 recém-nascidos, de ambos os gêneros, com idade pós-concepcional de até 54 semanas, alocados em três grupos de acordo com a idade gestacional. O G1 foi composto de 83 recém-nascidos a termo e o G2 de 73 pré-termo. Este último, subdividido em G2A, com 42 recém-nascidos sem risco para alterações auditivas e G2B com 31 recém-nascidos com risco. As emissões otoacústicas transientes foram obtidas com clique não-linear, à 84 dB NPS utilizando o Echocheck ILO EOA Screener, Otodynamics. Para análise dos resultados, foram utilizados os testes estatísticos: Mann-Whitney, qui-quadrado ou exato de Fisher, ANOVA de Kruskal-Wallis e múltiplas de Dunn, teste dos postos sinalizados de Wilcoxon; sendo considerado como significante o p < 0,05. Resultado observou-se diferença significante nas amplitudes das emissões otoacústicas evocadas transientes, maior em G1 (p= 0,017) do que em G2 (p=0,048) na orelha direita e esquerda. O grupo G1 (p= 0,009) apresentou amplitude das emissões otoacústicas estatisticamente maiores que G2B na orelha direita. Conclusão o grupo a termo apresentou amplitude das emissões otoacústicas maiores do que o grupo pré-termo. Não houve diferença das emissões otoacústicas entre as variáveis gênero e orelha.

Analysis of otoacoustic emissions in neonates at term and preterm

The transient evoked otoacoustic emissions (TEOAEs) have been widely used in neonatal hearing screening.

Objective

To compare the TEOAEs in newborns at term and preterm vis-à-vis the following variables: ear side, gender, frequency spectrum and gestational age.

Method

By means of a cross-sectional cohort of 66 newborns up to the 28^th day of life (41 newborns at term and 25 premature babies), we recorded TEOAEs. All the individuals did not have risk indicators for hearing loss.

Results

There was a signal/noise ratio improvement with frequency increase. No differences were observed between genders and between the ears, but there were differences among the children born at term and preterm in the frequency bands at 3 kHz and 4 kHz.

Conclusion

The TEOAEs test is important for assessing the peripheral auditory system of newborns at term and preterm, making it possible to have responses regardless of gender and gestational age.

[Cochlea development in infants with central nervous system impairment occurring as an effect of perinatal asphyxia]

Asphyxia in neonates with inadequate blood-inner ear barrier function causes damage to the inner ear included the degeneration of outer hair cells of the organ of Corti and oedematous changes in stria vascularis. The major consequences in the central nervous system (CNS) are found to be hypoxic-ischemic encephalopathy (HIE) in term newborns, peri- (PVH) or intraventricular hemorrhages (IVH) and periventricular leucomalacia (PVL) in preterm neonates.

THE AIM OF THE STUDY

was to carry out the objective assessment of the cochlea development using CEOAEs in infants with CNS impairment occurring as an effect of perinatal asphyxia, imaged during trans-fontanel ultrasonography.

MATERIAL AND METHODS

To the investigation 36 infants with HIE, IVH or PVL were included, the control group encompassed 32 born at term, health children. In all children three times otoscopic examination and CEOAEs recordings were performed: after birth, in 3rd and in 6th month of life using Otodynamics Otoacoustic Analyser ILO-88 in Quick Screen version. Perinatal anamnesis, general pediatric status, results of trans-fontanel ultrasonography and biochemical test results were taken into account in statistical analyses.

RESULTS

The mean amplitudes of CEOAEs in the first days of life were significantly reduced in InvG comparing to control babies. In half-octave frequency bandwidth analysis significant reduction of S/N Ratio at 1,5–4 kHz in InvG was observed. 3 and 6 months later the recorded responses significantly increased, they didn't reach values of CG but no statistical differences between groups were observed.

CONCLUSION

In conclusion, the outer hair cells activity in first days of life is reduced in newborns with CNS impairment and perinatal asphyxia in anamnesis comparing to health children. The maturation of the cochlea is intensive during first 3 months of life, later only subtle changes in CEOAEs are observed.

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.

Quantitative analysis of cochlear active mechanisms in tinnitus subjects with normal hearing sensitivity: Time-frequency analysis of transient evoked otoacoustic emissions and contralateral suppression

OBJECTIVE

The aim of this study was to analyze the fine structure of transient evoked otoacoustic emissions (TEOAEs) and contralateral suppression effects in tinnitus subjects with normal hearing in order to assess whether a minor cochlear or efferent dysfunction, possibly limited in narrow cochlear regions, might play a role in tinnitus.

METHODS

TEOAEs were recorded, both in the absence and in the presence of contralateral acoustic stimulation, in 23 tinnitus patients with normal hearing sensitivity and in 31 non-tinnitus control subjects. The broad-band TEOAE recordings were analyzed by using an innovative algorithm and separated into a set of 33 narrow-band frequency components, that represent the different cochlear contributions to the whole TEOAE response. In each frequency component, three different parameters were analyzed and compared between tinnitus and non-tinnitus subjects, i.e., reproducibility, latency, and the suppression effects induced by contralateral acoustic stimulation.

RESULTS

Significantly lower reproducibility was observed in the frequency components of the tinnitus subjects compared to the controls, whereas no significant differences in latency and in suppression effects were observed between tinnitus and non-tinnitus ears.

CONCLUSIONS

The analysis of the fine structure of TEOAEs revealed that the tinnitus subjects involved in this study might, possibly, have a minor dysfunction of the cochlear active mechanisms that resulted in frequency components with lower reproducibility. Conversely, the analysis of suppression effects in the narrow-band frequency components of TEOAE indicated that the subjects involved showed no relevant damage to the efferent regulatory mechanisms that control the cochlear activity, neither through the cochlea as a whole, nor in limited cochlear regions.

Emissões otoacústicas evocadas transientes em recém-nascidos a termo e pré-termo

OBJETIVO: analisar a amplitude absoluta e nível de Response das emissões otoacústicas evocadas transientes em recém-nascidos pré-termo e a termo. MÉTODOS: participaram deste estudo 50 recém-nascidos a termo e 50 pré-termo, os quais foram submetidos à triagem auditiva neonatal por meio da pesquisa das emissões otoacústicas evocadas transientes, realizada entre 24 horas e 11 semanas de vida. Todos os recém-nascidos apresentaram presença de emissões otoacústicas evocadas por clique no teste da triagem auditiva neonatal. RESULTADOS: não houve diferença estatisticamente significante da amplitude e nível de Response entre os recém-nascidos a termo e pré-termos. Não houve correlação significante entre os fatores prematuridade, idade do recém-nascido no teste e tempo de estimulação auditiva com os resultados obtidos. CONCLUSÃO: as emissões otoacústicas evocadas transientes registradas em recém-nascidos a termos e pré-termos apresentam semelhante amplitude absoluta e nível de Response.

Use of the matching pursuit algorithm with a dictionary of asymmetric waveforms in the analysis of transient evoked otoacoustic emissions

Transiently evoked otoacoustic emissions (TEOAEs) are normally modeled as the sum of asymmetric waveforms. However, some previous studies of TEOAEs used time-frequency (TF) methods to decompose the signals into symmetric waveforms. This approach was justified mainly as a means to reduce the complexity of the calculations. The present study extended the dictionary of numeric functions to incorporate asymmetric waveforms into the analysis. The necessary calculations were carried out using an adaptive approximation algorithm based on the matching pursuit (MP) numerical technique. The classic MP dictionary uses Gabor functions and consists of waveforms described by five parameters, namely, frequency, latency, time span, amplitude, and phase. In the present investigation, a sixth parameter, the degree of asymmetry, was added in order to enhance the flexibility of this approach. The effects of expanding the available functions were evaluated by means of both simulations using synthetic signals and authentic TEOAEs. The resulting analyses showed that the contributions of asymmetric components in the OAE signal are appreciable. In short, the expanded analysis method brought about important improvements in identifying TEOAE components including the correct detection of components with long decays, which are often related to spontaneous OAE activity, the elimination of a "dark energy" effect in TF distributions, and more reliable estimates of latency-frequency relationships. The latter feature is especially important for correct estimation of latency-frequency data, which is a crucial factor in investigations of OAE-generation mechanisms.

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.

Transient evoked otoacoustic emission latency and estimates of cochlear tuning in preterm neonates

The latency of transient evoked otoacoustic emissions has been evaluated in a sample of 58 ears from 34 preterm neonates, to understand if the estimates of cochlear tuning based on the otoacoustic emission latency show signs of developmental changes. A previous study on the same otoacoustic emissions analyzed here [Tognola et al. (2005). "Cochlear maturation and otoacoustic emissions in preterm infants: A time-frequency approach," Hear. Res., 199, 71-80] reported indeed a significant change in the otoacoustic emission latency with postconception age. This last result, which would imply a significant decrease of tuning, was partially biased by the presence of spontaneous emissions. In this study, the same neonate data are reanalyzed using a novel time-frequency algorithm, less sensitive to spontaneous emissions. Asymmetry between right and left ears has been found, with the left ears showing no significant change, whereas in the right ears and in the 1.5-2.5 kHz frequency range only, a slow decrease of latency with postconception age (0.1-0.2 ms/week) was observed. The correspondent tuning estimates based on latency decrease by 0.4-0.5/week. Significant differences between neonate and adult latency were confirmed, which could be either cochlear or middle ear in nature. These findings are compared to previous studies on distortion product suppression tuning curves in preterm neonates.

Tone burst-evoked otoacoustic emissions in neonates: normative data

BACKGROUND

Tone-burst otoacoustic emissions (TBOAEs) have not been routinely studied in pediatric populations, although tone burst stimuli have greater frequency specificity compared with click sound stimuli. The present study aimed (1) to determine an appropriate stimulus level for neonatal TBOAE measurements when the stimulus center frequency was 1 kHz, (2) to explore the characteristics of 1 kHz TBOAEs in a neonatal population.

METHODS

A total of 395 normal neonates (745 ears) were recruited. The study consisted of two parts, reflecting the two study aims. Part I included 40 normal neonatal ears, and TBOAE measurement was performed at five stimulus levels in the range 60-80 dB peSPL, with 5 dB incremental steps. Part II investigated the characteristics of the 1 kHz TBOAE response in a large group of 705 neonatal ears, and provided clinical reference criteria based on these characteristics.

RESULTS

The study provided a series of reference parameters for 1 kHz TBOAE measurement in neonates. Based on the results, a suggested stimulus level and reference criteria for 1 kHz TBOAE measures with neonates were established. In addition, time-frequency analysis of the data gave new insight into the energy distribution of the neonatal TBOAE response.

CONCLUSION

TBOAE measures may be a useful method for investigating cochlear function at specific frequency ranges in neonates. However, further studies of both TBOAE time-frequency analysis and measurements in newborns are needed.

Wavelet and matching pursuit estimates of the transient-evoked otoacoustic emission latency

Different time-frequency techniques may be used to investigate the relation between latency and frequency of transient-evoked otoacoustic emissions. In this work, the optimization of these techniques and the interpretation of the experimental result are discussed. Time-frequency analysis of click-evoked otoacoustic emissions of 42 normal-hearing young subjects has been performed, using both wavelet and matching pursuit algorithms. Wavelet techniques are very effective to provide fast and reliable evaluation of the average latency of large samples of subjects. A major advantage of the matching pursuit technique, as observed by Jedrzejczak et al. [J. Acoust. Soc. Am. 115, 2148-2158 (2004)], is to provide detailed information about the time evolution of the response of single ears at selected frequencies. A hybrid matching pursuit algorithm that includes Fourier spectral information was developed, capable of speeding-up computation times and of identifying "spurious" atoms, whose latency-frequency relation is apparently anomalous. These atoms could be associated with several known phenomena, either intrinsic, such as intermodulation distortion, spontaneous emissions and multiple internal reflections, or extrinsic, such as instrumental noise, linear ringing and the acquisition window onset. A correct interpretation of these phenomena is important to get accurate estimates of the otoacoustic emission latency.

Speech processing for cochlear implants with the discrete wavelet transform: feasibility study and performance evaluation

An innovative approach is investigated for speech processing in cochlear implants (CI). Differently from the traditional filter-bank spectral analysis strategies, the proposed method analyses the speech signal by means of the discrete wavelet transform (DWT). Preliminary tests were conducted in order to compare the WT and the filter-bank analysis methods. Additionally, the intelligibility of the speech processed with the proposed WT strategy was tested on normal hearing people by means of the acoustic simulations and a comparison was made with respect to traditional CI algorithms. Results showed that the WT could be a suitable method for speech coding in CIs. The information loss was minimal and, in speech recognition tests, the WT performance was similar to traditional filter-bank strategies.

Otoacoustic emissions latency difference between full-term and preterm neonates

Transiently evoked otoacoustic emissions (TEOAEs) were recorded from full-term and preterm neonates. The responses were decomposed, by means of an adaptive approximation method, into waveforms of defined frequencies, amplitudes, latencies and time spans. Statistically significant differences in the latency values were found between the tested groups. Differences were also found in the time spans of the TEOAEs components. For the preterm neonates the contribution of long-duration components (i.e. long-time span) was higher. Those components were characterized by narrow frequency band and contrary to the short-time span components their latencies did not depend on frequency. The removal of the long-duration components, from the pool of analyzed data, decreased the latency differences between the tested groups. The results indicate that the origin of the longer latency values for preterm neonates (with a post conceptional age up to 33 weeks) in respect to full-term neonates can be attributed to the presence of long-lasting components. The correspondence, which was found between frequencies of long-duration components and the spectral peaks of spontaneous otoacoustic emissions (SOAEs), suggests that those components may be connected with SOAEs.