Functional maturation of cochlear active mechanisms and of the medial olivocochlear system in humans

Inhibitory effect of contralateral noise on transient otoacoustic emissions in infants with congenital syphilis

OBJECTIVES

Analyze the inhibitory effect of contralateral noise on transient otoacoustic emissions in infants with congenital syphilis (CS).

METHODS

Cross-sectional study, approved by the Research Ethics Committee n° 3.360.991. Infants with treated CS at birth and infants without risk indicators for hearing impairment were selected. Both groups had the waves I, III and V presence at 80 dB nHL with click BAEP and the presence of response in the nonlinear TEOAEs at 80 dB NPS bilaterally. For suppression, TEOAE were analyzed without the contralateral noise, with the linear stimulus at 60 dB SPL. The neonates who presented a response in three frequencies per ear performed the second TEOAE collection with the contralateral white noise at an intensity of 60 dB SPL. Inferential analysis were performed using the Mann-Whitney and Wilcoxon test, adopting a significance level p < 0.05.

RESULTS

The sample consisted of 30 subjects divided into two groups, the Study Group (SG), consisting of 16 infants, and the Control Group (CG), consisting of 14 infants with no risk indicators for hearing loss. No differences were observed between the groups and the inhibition values, in the SG 30.8% presented inhibition and 25% for the CG in the right ear, in the left ear it was 46.7% in the SG and 38.5% in the CG. The SG demonstrated greater inhibition in the RE for the frequency bands from 1.5 to 4 KHz.

CONCLUSIONS

The analyses adopted in this study point out that the inhibitory effect of contralateral noise on TEOAEs in infants with CS does not differ from infants without risk indicators for hearing loss.

Use of sound-elicited fetal heart rate accelerations to assess fetal hearing in the second and third trimester

OBJECTIVES

We previously reported that fetal heart rate (FHR) accelerations could be obtained after fetal sound stimulation. We examined FHR accelerations during 20-37 weeks gestational age (GA) in order to assess the optimal time for the test.

METHODS

The fetus was stimulated from the maternal abdomen with pure tone 2000 Hz, 90 dB, 5 s. Changes in the FHR before and after the sound stimulation were measured by a cardiotocometer.

RESULTS

Compared with the positive rate of FHR accelerations at 20-21 weeks GA, significant increases were recognized in 26-27, 28 to 29, 30 to 31, and 34-35 weeks GA. Comparing the positive rate of FHR accelerations between the minimal and moderate variability of FHR baseline, no significant differences were observed at 20-27 weeks GA. On the other hand, at 28-37 weeks GA, the positive rate to detect FHR accelerations due to sound stimulation was 100% in moderate FHR baseline variability.

CONCLUSION

Considering development of human fetal hearing, the method should be performed between 28 and 37 weeks GA and during moderate FHR variability corresponding to active sleep conditions. The method developed in the present study may provide a promising tool for evaluating the fetal hearing.

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.

The 1.06 frequency ratio in the cochlea: evidence and outlook for a natural musical semitone

A frequency ratio of about 1.06 often appears in cochlear mechanics, and the question naturally arises, why? The ratio is close to that of the semitone (1.059) in music, giving reason to think that this aspect of musical perception might have a cochlear basis. Here, data on synchronised spontaneous otoacoustic emissions is presented, and a clustering of ratios between 1.05 and 1.07 is found with a peak at 1.063 ± 0.005. These findings reinforce what has been found from previous sources, which are reviewed and placed alongside the present work. The review establishes that a peak in the vicinity of 1.06 has often been found in human cochlear data. Several possible cochlear models for explaining the findings are described. Irrespective of which model is selected, the fact remains that the cochlea itself appears to be the origin of a ratio remarkably close to an equal-tempered musical semitone, and this close coincidence leads to the suggestion that the inner ear may play a role in constructing a natural theory of music. The outlook for such an enterprise is surveyed.

Piezoelectric vibrator-stimulated potential and heart rate accelerations detected from the fetus

OBJECTIVES

The fetus is well known to have a substantial capacity for sound recognition in the uterine environment. The aim of this study was to develop a sound stimulus system equipped with a piezoelectric vibrator (PV), record the PV-stimulated potential (PVSP) of the fetus and monitor changes of the fetal heart rate (FHR) under PV stimulation.

METHODS

The relationship between the input voltage applied to a piezoelectric vibrator and the sound pressure generated in the uterus was calibrated based on a model of the maternal abdomen. Fourteen fetuses for the measurement of the PVSP and 22 fetuses for the measurement of the heart rate changes from low-risk pregnant women were recruited.

RESULTS

The PVSP responses were obtained in 9 out of 14 fetuses. All the tested fetuses accelerated the FHR after the 2 kHz tone stimulation at 70 dB intensity generated by PV from 32 to 37 weeks gestational age.

CONCLUSIONS

Using a newly developed sound stimulus system equipped with PV, the electric responses of a fetus recorded from electrodes placed on the mother's abdomen may be closely related to the auditory evoked response. Significant accelerations of FHR were objectively, accurately and readily obtained after the sound stimulation.

Morphological and morphometrical maturation of ventral cochlear nucleus in human foetus

Auditory impulses perceived by the hair cells of the organ of corti are relayed in the cochlear nucleus, the first relay station in the brainstem, by the cochlear nerve. The human foetus is well known to respond to sound during the last trimester of gestation. On the contrary, studies conducted in rat, cat and mouse have shown that these mammals have an immature auditory system at the time of birth. There are very few reports available regarding the morphological and functional maturation of the cochlear nucleus in human. Although the human cochlear nucleus neurons attain adult morphological characters by mid-gestation, there are hardly any studies discussing the functional maturation of the cochlear nucleus. Hence the present study was aimed at observing the morphological as well as functional maturation of the human foetal cochlear nuclei at various gestational ages. Morphological maturation was observed qualitatively while stereological estimation of the volume of well defined ventral cochlear nucleus (VCN) was calculated by the Cavalieri principle; neuronal count and density was estimated by dissector principle. The functional maturation was assessed by observing the expression of synaptophysin, a synaptic marker, at different gestational ages and by the presence of parvalbumin, a calcium binding functional neuronal marker by immunohistochemistry. Neurons showed coarse Nissl's substance and well developed cell processes and gradual increase in cell size by the 24th-30th gestational week. Synaptophysin labeling in the complete cochlear nucleus was observed at 20 weeks of gestation. Adult pattern of synaptophysin labeling was observed finally at37weeks of gestation. Earliest presence of parvalbumin expression was detected at 16 weeks of gestation and a distinct adult pattern was seen at 37 weeks of gestation. This study concluded that morphological and functional maturation of the human cochlear nuclei occurs simultaneously during mid-gestation which represents the critical period of development and continues up to term.

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.

An acoustic gap between the NICU and womb: a potential risk for compromised neuroplasticity of the auditory system in preterm infants

The intrauterine environment allows the fetus to begin hearing low-frequency sounds in a protected fashion, ensuring initial optimal development of the peripheral and central auditory system. However, the auditory nursery provided by the womb vanishes once the preterm newborn enters the high-frequency (HF) noisy environment of the neonatal intensive care unit (NICU). The present article draws a concerning line between auditory system development and HF noise in the NICU, which we argue is not necessarily conducive to fostering this development. Overexposure to HF noise during critical periods disrupts the functional organization of auditory cortical circuits. As a result, we theorize that the ability to tune out noise and extract acoustic information in a noisy environment may be impaired, leading to increased risks for a variety of auditory, language, and attention disorders. Additionally, HF noise in the NICU often masks human speech sounds, further limiting quality exposure to linguistic stimuli. Understanding the impact of the sound environment on the developing auditory system is an important first step in meeting the developmental demands of preterm newborns undergoing intensive care.

Maturation of the human medial efferent reflex revisited

Past work applying otoacoustic emissions to gauge maturational status of the medial olivocochlear (MOC) reflex in human newborns has produced mixed results. The present study revisits the question while considering the dual nature of the 2f(1) - f(2) distortion product otoacoustic emission (DPOAE) and expanding measures of medial efferent function. Subjects included premature and term-born neonates, 6-month-old infants and young adults. The MOC reflex was elicited with contralateral acoustic stimulation (CAS) while shifts in amplitude and phase of the DPOAE, and its distortion and reflection components, were monitored. Overall, CAS-elicited reductions in DPOAE level did not differ among age groups. For all ages, the MOC reflex was strongest at frequencies below 1.5 kHz, and the reflection component of the DPOAE was most affected, showing maximally reduced amplitude and shallower phase slope when contralateral noise was presented. Results suggest that the MOC reflex likely reaches maturation prior to full-term birth. However, prematurely born neonates show markedly more episodes of CAS-induced DPOAE level enhancement. This may be due to more intrusive component mixing in this age group or disruptions in the formation of the MOC pathway or synapse in the most premature neonates.

The role of early auditory development in attachment and communication

Auditory perception and learning take place during the third trimester of gestation. Fetuses and newborns who lack typical auditory experience can go on to develop typical socioemotional attachment and language, given a supportive environment. For hospitalized preterm infants in developmentally sensitive neonatal intensive care units, detrimental effects of deviant early auditory experience may be remediated by later experience, but much is unknown about the causes of language deficits of prematurity. Prenatal auditory stimulation programs that incorporate audio speakers against the maternal belly should be discouraged because of possible overstimulation effects on the developing auditory system and sleep/wake state organization.

A melodic contour repeatedly experienced by human near-term fetuses elicits a profound cardiac reaction one month after birth

BACKGROUND

Human hearing develops progressively during the last trimester of gestation. Near-term fetuses can discriminate acoustic features, such as frequencies and spectra, and process complex auditory streams. Fetal and neonatal studies show that they can remember frequently recurring sounds. However, existing data can only show retention intervals up to several days after birth.

METHODOLOGY/PRINCIPAL FINDINGS

Here we show that auditory memories can last at least six weeks. Experimental fetuses were given precisely controlled exposure to a descending piano melody twice daily during the 35(th), 36(th), and 37(th) weeks of gestation. Six weeks later we assessed the cardiac responses of 25 exposed infants and 25 naive control infants, while in quiet sleep, to the descending melody and to an ascending control piano melody. The melodies had precisely inverse contours, but similar spectra, identical duration, tempo and rhythm, thus, almost identical amplitude envelopes. All infants displayed a significant heart rate change. In exposed infants, the descending melody evoked a cardiac deceleration that was twice larger than the decelerations elicited by the ascending melody and by both melodies in control infants.

CONCLUSIONS/SIGNIFICANCE

Thus, 3-weeks of prenatal exposure to a specific melodic contour affects infants 'auditory processing' or perception, i.e., impacts the autonomic nervous system at least six weeks later, when infants are 1-month old. Our results extend the retention interval over which a prenatally acquired memory of a specific sound stream can be observed from 3-4 days to six weeks. The long-term memory for the descending melody is interpreted in terms of enduring neurophysiological tuning and its significance for the developmental psychobiology of attention and perception, including early speech perception, is discussed.

Atypical fetal voice processing in preeclamptic pregnancy

OBJECTIVE

To compare fetal heart rate (HR) response to the mother's voice in pregnancies complicated by preeclampsia with those of fetuses in uneventful, normotensive pregnancies.

METHOD

Fifty fetuses (n = 22, preeclampsia; n = 28, uneventful, normotensive pregnancies) between 32 and 40 weeks gestational age were recruited. Each fetus was presented with a 2-min no-sound baseline period followed by a 2-min voice period during which an audio recording of his/her mother reading a story was played through a loud speaker over the maternal abdomen at an average of 95 dBA followed by a 2-min no-voice offset period. HR was recorded continuously.

RESULTS

Fetuses in the preeclamptic group showed no response to the mother's voice when it was played. In comparison, fetuses in the uneventful, normotensive group responded to the mother's voice with a HR increase.

CONCLUSION

Fetuses in pregnancies complicated by preeclampsia show atypical auditory processing of the mother's voice. Such atypical responding may reflect a delay in auditory system maturation, functional elevation of sensorineural threshold, or decreased thyroid hormone.

Emissões otoacústicas e medidas de imitância acústica com tons de sonda de 226 e 1000 hz em lactentes

OBJETIVO: analisar os resultados encontrados na imitanciometria com tons de sonda de 226 e 1.000Hz em lactentes sem indicadores de risco para perda auditiva (IRPA) e correlacioná-los com as Emissões Otoacústicas Transitórias (EOAT). Além de estudar o reflexo acústico ipsilateral com tom de sonda de 226 Hz. MÉTODOS: foram avaliados 52 lactentes a termo, 26 do sexo masculino e 26 do feminino, com faixa etária de 11 a 51 dias de vida. Foram realizadas anamnese, meatoscopia, EOAT, timpanometria com tons de sonda de 226 e 1000 Hz e pesquisa do reflexo acústico ipsilateral. Na análise estatística dos dados aplicou-se: teste de Wilcoxon pareado e McNemar, testes de Mann-Whitney, qui-quadrado (ou exato de Fisher) e teste ANOVA por Kruskal-Wallis. RESULTADOS: os dados foram analisados independentes de sexo e orelha. Em relação à EOAT, 78,8% estavam presentes. As curvas timpanométricas encontradas com tom de sonda de 226 Hz foram: A (55,8%), D (40,4%), Ar (1,0%), B (2,9%); já com tom de sonda de 1000 Hz encontrou-se: A (74%), D(1%), B (14,4%) e Bloqueada (10,6%). O reflexo acústico ipsilateral mostrou-se presente em 81,7% das orelhas. CONCLUSÃO: houve relação estatisticamente significante entre as curvas timpanométricas (para os tons de sonda de 226 e 1000 Hz) e a relação sinal / ruído e a reprodutibilidade geral das EOAT. O reflexo acústico ipsilateral com tom de sonda de 226 Hz esteve presente na maioria dos lactentes sem IRPA.

Distortion product otoacoustic emission phase and component analysis in human newborns

Apical distortion product otoacoustic emissions (DPOAEs) are comprised of at least two components, as evidenced by the interference pattern of alternating maxima and minima known as fine structure. DPOAE fine structure is produced by the shifting phase relationship in the ear canal, between the generator and characteristic frequency (CF) component of the response. Each component arises from a different cochlear region and, according to theory, reflects a distinct generation mechanism. The analysis of DPOAE components and phase in newborns may provide a window into targeted aspects of cochlear physiology during development. 2f(1)-f(2) DPOAE fine structure was recorded from 15 adults and 14 newborns using a swept-tone technique. DPOAE group delay, as well as magnitude and phase of each component, was compared between age groups. Results show narrower fine structure spacing, a longer group delay (steeper phase gradient) in low frequencies, and a stronger relative contribution from the CF component in newborns. The prolonged group delay for low-frequency DPOAEs could indicate immature basilar membrane motion in the apex of the cochlea and warrants further investigation. The enhanced contribution from the CF component may have implications for clinical practice as well as for theories of cochlear maturation.

Changes in Doppler blood flow velocity in middle cerebral artery in response to airborne sound in low- and high-risk human fetuses

OBJECTIVE

To examine fetal auditory perception in low- and high-risk pregnancies in period from 27 to 31 weeks gestational age with the aim to establish diagnostic parameters in prenatal detection of the degree of hearing development in a fetus.

METHODS

Method of prenatal hearing screening was applied on 80 women divided in two groups: Control group (C=22), consisted of pregnant women with low-risk pregnancies, and Experimental group (E=58), consisted of pregnant women with high-risk pregnancies (pregnancies with diagnosis of: preterm delivery, hypertension and/or intrauterine growth restriction (IUGR), diabetes). PHS was applied in period from 27 to 31 weeks gestational age. Brain circulation changes in fetal middle cerebral artery (MCA) caused by defined sound stimulus, as the indicator of fetal auditory reactions, were registered on Doppler ultrasound apparatus. After visualization of MCA, a sound stimulus was delivered. The stimulus consisted of one defined sound which is a digitally produced sound with the intensity of 90 dB, frequency range of 1500-4500 Hz, and duration of 0.2s (click) and it was presented only once. Measurements in observed artery were taken before (baseline) and after defined sound stimulation.

RESULTS

Results showed that the absolute and relative difference in Pulsatility index (baseline and after sound stimulation) were greater for the high-risk group compared to the low-risk group (absolute difference: mean=0.36 vs mean=0.36) (relative difference: mean = ∼ 18% vs mean = ∼ 12%). When the low-risk group and the three high-risk group mean pairs were compared using multiple t-test, the diabetic group differed from the low-risk and two other high-risk groups; the low-risk and the two other high-risk groups did not differ from each other. Fetuses from pregnancies with diagnosis of diabetes demonstrated the most expressive reactibility and significantly higher absolute and relative changes of Pi values (absolute difference: mean=0.54, relative difference: mean=25.49%).

CONCLUSION

The values of Pulsatility index (Pi) registered by PHS in low- and high-risk pregnancies may be used as differential and diagnostic parameters in fetal auditory perception examination. Fetuses from pregnancies with diagnosis of diabetes demonstrated significantly higher absolute and relative changes of Pi values compared to other groups of examined fetuses.

Changes in the DP-gram during the preterm and early postnatal period

OBJECTIVES

To describe and define changes in the infant DP-gram during an age continuum from the preterm period through the first 6 mo of postnatal life. This information provides normative guidelines for audiologists or hearing screeners using DPOAEs to monitor infant hearing status.

DESIGN

In this retrospective study, 2f1 - f2 DP-grams (DPOAE level x f2) were recorded with primary tones at 65/55 dB SPL, f2/f1 = 1.2, and f2 frequencies ranging from 1500 to 9000 Hz. Results from one ear of 290 healthy infants ranging in age from 31 wks postconceptional age to 6-mo-old were examined. Data were collected using both longitudinal design (repeated tests on the same infant over time) and cross-sectional methodology (a different group of subjects representing each age category). Subjects were divided into three groups according to age and experimental design. The effects of age and frequency on DPOAE level were analyzed in the three groups separately.

RESULTS

The combined results from the three databases indicate that (1) DPOAE level increased for mid-frequencies throughout the preterm period, from 31 to 33 wks until the time period associated with term birth. This change was significant for 4500 and 6000 Hz; (2) DPOAE level decreased as f2 frequency increased. In many infants, a shallow trough was observed with peak amplitude at 1500 Hz, a reduction in response amplitude through 4500 Hz, and a second peak around 6000 Hz; (3) during the postnatal period from birth through 6 mo, DPOAE level did not change significantly as a function of age and the DP-gram was relatively flat across f2 frequency; and (4) infants showed mean DPOAE levels that were 4 to 12 dB higher than adult levels.

CONCLUSIONS

The results indicate a frequency-dependent increase in DPOAE level during the preterm period in human infants. After birth, there is little change in amplitude through 6 mo. The infant DPOAE remains larger than adult amplitude at all ages tested, as shown in other reports, well into childhood, suggesting continued changes in DPOAE level during the first decade of life. Recent research suggests that immaturities of the conductive pathway may account for infant-adult differences in DPOAE level; however, it is not yet clear whether other sources contribute.

Contralateral suppression of linear and nonlinear transient evoked otoacoustic emissions in neonates at risk for hearing loss

To investigate the transient evoked otoacoustic emissions (TEOAE) contralateral suppression in neonates at risk for hearing loss, 55 neonates at risk for hearing loss (risk group) and 72 full-term neonates not at such risk (control group) were bilaterally tested. In all neonates, the TEOAE were recorded in two stimulation modes (linear and nonlinear clicks), with and without contralateral acoustic stimulation. Findings revealed significant contralateral suppression of otoacoustic emissions in both groups, but the amount of TEOAE contralateral suppression was reduced for at risk group (p=0.001), supporting the hypothesis that medial olivocochlear bundle function is reduced in neonates at risk for hearing loss. The combination of contralateral acoustic stimulation and TEOAE enables easy and noninvasive study of auditory efferent function. However it should be emphasized that the reduction in TEOAE contralateral suppression in the risk group, statistically identified as a group effect, might not be detectable in individual cases. Further studies are needed in order to determine whether the lower amount of TEOAE contralateral suppression in neonates at risk for hearing loss represents a risk for developing auditory processing disorders.

LEARNING OUTCOMES

The reader will be introduced to the study using auditory efferent pathway activation by contralateral acoustic stimulation (CAS), resulting in the TEOAE suppression effect. The characteristics of TEOAE suppression in the neonatal population, in which it provides evidence of the reduced medial olivocochlear system function in those at risk for hearing loss, will also be addressed.

Distortion product otoacoustic emission suppression tuning and acoustic admittance in human infants: birth through 6 months

Previous work has reported non-adultlike distortion product otoacoustic emission (DPOAE) suppression in human newborns at f2=6000 Hz, indicating an immaturity in peripheral auditory function. In this study, DPOAE suppression tuning curves (STCs) were recorded as a measure of cochlear function and acoustic admittance/reflectance (YR) in the ear canal recorded as a measure of middle-ear function, in the same 20 infants at birth and through 6 months of age. DPOAE STCs changed little from birth through 6 months, showing excessively narrow and sharp tuning throughout the test period. In contrast, several middle-ear indices at corresponding frequencies shifted systematically with increasing age, although they also remained non-adultlike at 6 months. Linear correlations were conducted between YR and DPOAE suppression features. Only two correlations out of 76 were significant, and all but three YR variables accounted for <10% of the variance in DPOAE suppression tuning. The strongest correlation was noted between admittance phase at 5700 Hz and STC tip-to-tail (R=0.49). The association between middle-ear variables and DPOAE suppression may be stronger during other developmental time periods. Study of older infants and children is needed to fully define postnatal immaturity of human peripheral auditory function.

Auditory processing deficits in growth restricted fetuses affect later language development

An increased risk for language deficits in infants born growth restricted has been reported in follow-up studies for more than 20 years, suggesting a relation between fetal auditory system development and later language learning. Work with animal models indicate that there are at least two ways in which growth restriction could affect the development of auditory perception in human fetuses: a delay in myelination or conduction and an increase in sensorineural threshold. Systematic study of auditory function in growth restricted human fetuses has not been reported. However, results of studies employing low-risk fetuses delivering as healthy full-term infants demonstrate that, by late gestation, the fetus can hear, sound properties modulate behavior, and sensory information is available from both inside (e.g., maternal vascular) and outside (e.g., noise, voices, music) of the maternal body. These data provide substantive evidence that the auditory system is functioning and that environmental sounds are available for shaping neural networks and laying the foundation for language acquisition before birth. We hypothesize that fetal growth restriction affects auditory system development, resulting in atypical auditory information processing in growth restricted fetuses compared to healthy, appropriately-grown-for-gestational-age fetuses. Speech perception that lays the foundation for later language competence will differ in growth restricted compared to normally grown fetuses and be associated with later language abilities.

Early maturation of evoked otoacoustic emissions and medial olivocochlear reflex in preterm neonates

The present study was designed to investigate the early maturation of the brainstem regulation of the cochlear function in preterm neonates. Evoked otoacoustic emissions (EOAE) and their regulation via the medial olivocochlear efferent (MOC) reflex were investigated in a large population of preterm neonates and compared with full-term neonates and young babies from birth to 4 y and school-aged children. In 28-wk preterm neonates, EOAE were seen in the mid-frequency range. These responses extended both to the low (down to 1025 Hz) and high (up to 6152 Hz) frequency ranges at 38 wk of gestational age and remained stable up to 4 mo. At this stage, the amplitude of EOAE overlapped adult values. EOAE amplitudes then decreased to reach adult values at 3 y of age. Maturation of MOC efferents innervating the outer hair cells was investigated by studying the suppressive effect of contralateral sound on the EOAE amplitudes (MOC reflex). The first MOC responses were recorded in preterm neonates of 32-33 wk of gestational age, reaching adult-like values at 37 wk of gestational age. The maximum effect of MOC efferent activation occurred between 2000 and 4000 Hz. These results suggest that, in humans, MOC efferents mature in utero. Thus, testing the MOC reflex may have a clinical relevance to detect an abnormal development of the auditory pathways, particularly of a brainstem circuitry not explored through conventional testing.

Comparison of fetal behavior in low- and high-risk pregnancies

Meta-analyses were conducted on archival data of human fetal behavior to identify differential behavior among high-risk fetuses in pregnancies complicated by threatened preterm delivery, maternal hypertension or diabetes compared with low-risk fetuses in uneventful pregnancies, delivering as healthy, full-term infants. Data for a total of 493 fetuses (260 high risk, 233 low risk) from 23 weeks' gestation to term who participated in a study using a standardized protocol including observations of spontaneous and auditory-induced behavior were retrieved from our laboratory database. There were no differences in spontaneous behaviors when scored using clinical criteria for the nonstress test and biophysical profile; however, there were differences in the magnitude of the behaviors measured in the tests. Developmental differences were observed between those threatening to deliver early and the fetuses of hypertensive and diabetic mothers. The latter two groups differed little from one another but differed from low-risk fetuses in their response to auditory stimulation. We concluded that differences in behavior among high-risk groups suggest that atypical fetal behaviors may represent adaptation to condition specific insult rather than a generalized response to insult per se. The finding that high-risk fetuses showed atypical responses to auditory stimuli indicates a need to examine the relation between fetal auditory function and later language acquisition.

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

Click-evoked otoacoustic emissions (CEOAEs) from preterm infants were analyzed to characterize developmental changes of cochlear active mechanisms. Due to their strong time-varying properties, CEOAEs were studied with a time-frequency approach--the wavelet transform (WT). By means of the WT, CEOAEs were decomposed into 12 frequency bands, spanning the 0.25-6.25 kHz range. For each band, the root-mean-square (RMS) level and latency were studied as functions of both frequency and age. Because CEOAEs were averaged using the non-linear mode of acquisition, the developmental changes in observed in this study are related to the non-linear component (which is actually the most predominant component of the active cochlear response) of CEOAEs, the linear one being mostly canceled out by non-linear averaging. In our study, there was evidence that properties of CEOAE non-linear components are related to the post-conception age (PCA) in that the levels and latency of CEOAE frequency components changed until the age of about 38 weeks post-conception, whereas after 38 weeks, CEOAE features were very similar to those of term newborns. In particular, the CEOAE levels increased and latency decreased with age. The observed changes in CEOAE properties seem to reveal a development of cochlear active mechanisms, although contributions from outer and middle ear development cannot be excluded. Also, in agreement with previous physiological and behavioral findings, our results revealed that the development of CEOAE properties was not the same for all the frequencies, being greater for frequencies 4 kHz, and resembled the development of the cochlear partition, which proceeds from base to apex.

Maturation of fetal responses to music

Maturation of fetal response to music was characterized over the last trimester of pregnancy using a 5-minute piano recording of Brahms' Lullaby, played at an average of 95, 100, 105 or 110 dB (A). Within 30 seconds of the onset of the music, the youngest fetuses (28-32 weeks GA) showed a heart rate increase limited to the two highest dB levels; over gestation, the threshold level decreased and a response shift from acceleration to deceleration was observed for the lower dB levels, indicating attention to the stimulus. Over 5 minutes of music, fetuses older than 33 weeks GA showed a sustained increase in heart rate; body movement changes occurred at 35 weeks GA. These findings suggest a change in processing of complex sounds at around 33 weeks GA, with responding limited to the acoustic properties of the signal in younger fetuses but attention playing a role in older fetuses.

Asymmetric Cochlear Processing Mimics Hemispheric Specialization

Otoacoustic emissions or OAEs (reflections of cochlear energy produced during the processing of sound) were measured in response to two types of stimuli, rapid clicks and sustained tones, in each ear of neonates. OAEs were larger to tones when elicited in the left ear and to clicks when elicited in the right. This finding is similar to those of enhanced processing of tones in right auditory cortical areas and of rapid stimuli on the left, given strong crossed connections from ear to brain. These findings indicate that processing at the level of the ear may facilitate lateralization of auditory function in the brain.

Assessment of medial olivocochlear system function in pre-term and full-term newborns using a rapid test of transient otoacoustic emissions

This study was conducted to investigate maturation of the medial olivocochlear efferent system (MOCS) in pre- and full-term neonates using Quickscreen (Otodynamics Ltd) and to confirm previous findings on transient otoacoustic emission (TEOAE) suppression in neonates. MOCS maturation was investigated in 46 neonates born at the Chaim Sheba Medical Center, Tel Hashomer, Israel, using Quickscreen. All neonates were normal with no family history of general or auditory disease and no risk factors for hearing impairment. MOCS function appears gradually in human pre-term neonates and is considered to reach maturity shortly after term birth. The clinical value of MOCS testing in specific populations of newborns at risk for hearing and/or brainstem function can be legitimately raised as activation of the MOCS clearly alters cochlear output. The present results can be interpreted to support the testing of infants at risk of developing abnormal MOCS function using a commercially available rapid TEOAE measurement system.

A longitudinal study of distortion product otoacoustic emission ipsilateral suppression and input/output characteristics in human neonates

Past work has shown that distortion product otoacoustic emission (DPOAE) (2f1-f2) ipsilateral suppression and input/output (I/O) characteristics are not adult-like in prematurely born neonates [Abdala, J. Acoust. Soc. Am. 110, 1465-1476 (2001)]. These age differences are most pronounced at f2 = 6000 Hz and have been interpreted to indicate a subtle immaturity in human cochlear function prior to term birth. It is still not clear, however, whether term-born neonates are completely adult-like in cochlear function. To study this question, DPOAE suppression and I/O functions for f2 = 6000 Hz were measured in a group of prematurely born neonates at weekly intervals over a period of 7-8-weeks, a group of normal-hearing adults, and during a one-time test session in a group of term-born neonates. Results show that there was no significant change in suppression tuning, suppression growth, and various I/O characteristics across test session for premature neonates, but there was an age-group effect; even once prematurely born neonates reached the equivalence of term-like status (38-40-weeks postconceptional weeks), they continued to show narrower suppression tuning than adults, shallower suppression growth for low-frequency side suppressor tones, and an elevated amplitude saturation plateau on the I/O function. Term-born neonates showed DPOAE results that were comparable to those measured from premature neonates and unlike adult findings. These results suggest that a subtle immaturity in cochlear function persists into the postnatal period.

Contralateral suppression of distortion-product otoacoustic emissions declines with age: a comparison of findings in CBA mice with human listeners

OBJECTIVES/HYPOTHESIS

The auditory efferent system plays presumed roles in enhancing signals in noise, maintaining the cochlea for optimal acoustic signal processing, and may have a protective role in preserving auditory function in the face of ototoxic events. The objective of the study was to measure age-related changes of the medial olivocochlear efferent system in mice by comparing distortion-product otoacoustic emissions generated with and without contralateral white noise stimulation. Consistent with prior work, distortion-product otoacoustic emissions were typically reduced in magnitude when white noise was presented to the contralateral ear. This contralateral suppression is attributed to activation of the medial olivocochlear efferent system, which has an inhibitory effect on the cochlear hair cell system. By studying contralateral suppression on cochlear output in subjects of different ages, it is possible to describe aging effects on the medial olivocochlear system.

STUDY DESIGN

CBA mice were divided into three age groups: young adult, middle-aged, and old-aged animals (21, 13, and 22 animals per group, respectively), and auditory brainstem responses were obtained before distortion-product otoacoustic emission testing to assess overall hearing abilities.

METHODS

2f1-f2 distortion-product otoacoustic emission recordings were obtained from individual subjects (anesthetized with ketamine/xylazine) in each age group under two conditions: 1) in quiet and 2) in the presence of a contralaterally applied wideband noise.

RESULTS

Principal findings were that distortion-product otoacoustic emission levels decreased with age for mice in a way similar to humans, when correcting for the absolute difference in life spans. In addition, contralateral suppression declined in middle-aged and old-aged groups relative to the young adults for mice in a manner similar to humans. The contralateral suppression decline at low frequencies preceded that of the decline in distortion-product otoacoustic emissions with age.

CONCLUSION

Functional decline of the medial olivocochlear efferent system with age precedes outer hair cell degeneration. Loss of medial olivocochlear suppressive function may play a role in the development of presbycusis in both clinical cases and animal models.

Emissões otoacústicas evocadas transientes (EOAET): amplitude da resposta em recém-nascidos a termo e pré-termo

As emissões otoacústicas evocadas transientes (EOAET) têm sido a técnica mais empregada nas triagens auditivas neonatais. Nos últimos anos, uma análise mais detalhada das EOAET tem evidenciado a presença de características até então pouco exploradas do sistema auditivo periférico. OBJETIVO: O objetivo deste trabalho foi o de estudar o comportamento da amplitude de resposta das EOAET em recém-nascidos a termo e pré-termo em função dos parâmetros lados da orelha, sexo, espectro de freqüência e idade pós-concepcional. FORMA DE ESTUDO: Clínico prospectivo. MATERIAL E MÉTODO: A amostra compôs-se de 526 recém-nascidos, sendo 440 nascidos a termo e 86 nascidos pré-termo. Estes últimos foram subdivididos em um grupo com idade pós-concepcional na data do teste variando entre 31 e 36 semanas e outro grupo com idade pós-concepcional na data do teste variando entre 37 e 44 semanas. O teste das EOAET foi realizado em ambiente sem tratamento acústico com o analisador de emissões otoacústicas ILO 88 da Otodynamics, no modo "Quickscreener". RESULTADO: Ao analisarmos os resultados, observamos assimetria significante da amplitude média de resposta a favor da orelha direita, do sexo feminino, das bandas de freqüências altas e dos recém-nascidos com maior idade pós-concepcional. Estas assimetrias sugerem que o parâmetro amplitude, além de fornecer evidência da presença das EOAET e portanto da integridade do sistema auditivo periférico, também pode ser tomado como um indicador de maturação do sistema auditivo periférico em recém-nascidos.

Contralateral suppression of otoacoustic emissions in neonates

The effect of contralateral acoustic stimulation on transient evoked otoacoustic emissions (TEOAEs) was used to investigate the activity of the medial olivocochlear bundle in neonates. TEOAEs were bilaterally recorded, with and without contralateral noise, in 120 full-term neonates without risk for hearing loss. Otoacoustic emission recordings were carried out using the Otodynamic ILO88 system quickscreener non-linear mode, and the contralateral stimulation was presented at 60 dB SPL delivered by CD player. The results revealed a significant (p<0.01) contralateral suppression of TEOAE response amplitude: 2.32 dB in females and 3.28 dB in males. This study shows that the suppression effect is present in 88.5% of neonates, with a gender difference (greater suppression in males) and frequency effect, suggesting a functional maturation of the medial olivocochlear bundle in the studied population.

The pathway for the transmission of external sounds into the fetal inner ear

After at least 20 weeks gestation, the human fetus in utero is able to hear and respond to external and internal (maternal) sounds. The external sounds are attenuated by maternal tissues and fluids - higher frequencies by about 20 dB, and lower frequencies are only slightly reduced. The sounds in the amniotic fluid, which completely envelops the fetus, then reach the fetal inner ear by bone conduction. The sound pressure in the amniotic fluid induces skull vibrations which are transmitted directly into the contents of the cranial cavity (brain and CSF) and from there, presumably by fluid channels connecting them, into the cochlear fluids. A further stage of conductive attenuation is probably involved in this transmission. Since the fetus in utero receives oxygen by placental diffusion (less efficient than pulmonary diffusion), the fetal inner ear is hypoxic compared to that following birth (pulmonary oxygen diffusion). This leads to a reduction in the magnitude of the endocochlear potential, to a depression of cochlear transduction and amplification, and thus to an additional sensorineural component of threshold elevation in the fetus. Upon birth, these conductive and sensorineural attenuations are removed.

Prevalence and ultrastructural morphology of axosomatic synapses on spiral ganglion cells in humans of different ages

Axosomatic synapses were found on human spiral ganglion cells (HSGCs). Ultrastructural characterization and calculation of the prevalence of these synapses were performed by electron microscopic semi-serial sections of both type I and type II HSGCs, in specimens from subjects of ages 1 day, 14 days, 21 years and 51 years. Synapses on type I HSGCs were extremely rare. In contrast, axosomatic synapses were present on approximately 50% of type II HSGCs of a young adult. This prevalence seemed to vary by age. Thus, no synapses were found in a 1-day old neonate, few in a 14-day old, and on approximately 15% of the type II SGCs from a 51-year old specimen. The origin of the nerve fibers synapsing on the type II HSGCs could not be determined. In view of the fact that some of the fibers projected from the intraganglionic spiral bundle, which is known to contain olivocochlear efferents, these fibers may represent an efferent pathway to the spiral ganglion. However, since there was morphological evidence of more than one type of nerve fiber synapsing on type II HSGCs, other neural origins must be considered. Although the physiological function of these synapses is unknown, they may mediate pre-synaptic neural modulation of the type II HSGCs at the level of the spiral ganglion.

Maturation of human fetal responses to airborne sound in low- and high-risk fetuses

The purpose of the study was to characterize the onset and maturation of airborne sound-elicited responses in low- and high-risk preterm fetuses. In Study 1, a total of 91 low-risk fetuses at 27, 30, 33, and 36 weeks GA received three sound trials at 90, 100, 105 and 110 dB and three no-stimulus control trials. The onset of cardiac acceleration and body movement responses occurred at 30 weeks GA. Maturation of the cardiac response was observed with a decrease in threshold from 105-110 dB at 33 weeks GA to 100-105 dB at 36 weeks GA. In Study 2, the procedure was similar except that the 43 high-risk fetuses at 27, 30 and 33 weeks GA did not receive sounds at 90 dB. For the high-risk fetuses, the onset of cardiac and motor responses also occurred at 30 weeks GA. At 33 weeks GA, those high-risk fetuses subsequently born at term showed an increased magnitude of the cardiac acceleration response compared to low-risk fetuses. The results indicate that both low- and high-risk fetuses begin responding to sounds at the same gestational age. Differential responses observed over gestation in the high-risk group most likely indicate differential functional development of the auditory-response system.

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.

Maturation of medial efferent system function in humans

Otoacoustic emissions are typically reduced in amplitude when broadband noise is presented to the contralateral ear. This contralateral suppression is attributed to activation of the medial olivocochlear system, which has an inhibitory effect on outer hair-cell activity. By studying the effects of contralateral noise on cochlear output at different stages of auditory maturation in human neonates, it is possible to describe the timecourse for development of medial efferent system function in humans. The present study recorded 2 f1-f2 distortion product otoacoustic emissions (DPOAE) in human adults, term and premature neonates at three f2 frequencies: 1500, 3000, and 6000 Hz, using fixed primary tone frequency ratio (f2/f1 = 1.2) and level separation (10 dB, L1 > L2). Average DPOAE growth functions were recorded with and without contralateral broadband noise. Results indicate that contralateral suppression of DPOAEs is absent at 6000 Hz, but present at 1500 and 3000 Hz for all ages. However, DPOAE amplitude from premature neonates was not altered by noise in an adult-like manner; in this age group, DPOAE amplitude was equally likely to by suppressed or enhanced by noise presented contralaterally. Contralateral enhancement may reflect a temporary stage of immaturity in outer hair cell-medial efferent fiber synapses just prior to term birth.

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.

Wavelet analysis of click-evoked otoacoustic emissions

Time-frequency distribution methods are being widely used for the analysis of a variety of biomedical signals. Recently, they have been applied also to study otoacoustic emissions (OAE's), the active acoustic response of the hearing end organ. Click-evoked otoacoustic emissions (CEOAE's) are time-varying signals with a clear frequency dispersion along with the time axis. Analysis of CEOAE's is of considerable interest due to their close relation with cochlear mechanisms. In this paper, several basic time-frequency distribution methods are considered and compared on the basis of both simulated signals and real CEOAE's. The particular structure of CEOAE's requires a method with both a satisfactory time and frequency resolution. Results from simulations and real CEOAE's revealed that the wavelet approach is highly suitable for the analysis of such signals. Some examples of the application of the wavelet transform to CEOAE's are provided here. Applications range from the extraction of normative data from adult and neonatal OAE's to the extraction of quantitative parameters for clinical purposes.

Frequency shift of individual spontaneous otoacoustic emissions in preterm infants

In adults, spontaneous otoacoustic emissions (SOAE) have shown a considerable frequency stability. In preterm infants, however, the SOAE proved to show an apparent and consistent upward shift of frequency at increasing postconceptional age (PCA). In 25 ears of 14 preterm infants (PCA, 29.1-41.3 wk) a total of 66 SOAE frequencies were monitored, ranging from 1611 to 5774 Hz. All but one of the SOAE frequencies shifted toward higher frequency. The SOAE frequency shift rate in Hertz per week was proportionally constant relative to the SOAE frequency. The mean shift rate was 0.74 +/- 0.39%/wk. At increasing PCA, the SOAE frequency shift rate tended to slow down. A linear fit through the data predicted the SOAE frequency to stop at about 45-50-wk PCA. The frequency dependence and time course of the SOAE frequency shift strongly suggest cochlear maturation during the last period of gestation.

Electrophysiologic changes in preterm neonates: auditory brain stem response and distortion product otoacoustic emissions

The purposes of this investigation were to determine 1) if auditory peripheral maturity is present in the newborn; 2) if not, at what age maturational changes occur in the peripheral auditory system from preterm to full-term; and 3) how results of tests used to identify auditory dysfunction in neonates, such as distortion product otoacoustic emissions (DPEs) and auditory brain stem responses (ABRs), change during this period. Longitudinal DPE amplitude and ABR wave I latency measurements were obtained from a single ear of 18 preterm neonates. The DPEs were evoked at f2s of 2, 3, 4, and 5 kHz. The longitudinal data revealed that in general, DPE amplitude increased and ABR wave I latency decreased as a function of postconceptional age. These findings suggest that 1) the peripheral auditory system has not reached maturity in the preterm neonate; 2) maturational changes continue from preterm to full-term; and 3) these changes are reflected in ABR and DPE measurements.

Evidence of peripheral hearing asymmetry in humans: clinical implications

Auditory system hemispheric asymmetry in language processing is well-established, and there are many indications of lateralization as of the peripheral auditory system i.e., as of the cochlea. The left ear is more susceptible to noise damage; tinnitus is more predominant there, while spontaneous otoacoustic emissions (SOAEs) are more often found in the right ear. The present study addressed the following two questions: Does this right-ear SOAE prevalence exist as early as preterm birth? Is there any functional asymmetry in the medial olivo-cochlear (MOC) efferent system, known to modulate outer hair cell contractions? The study involved 483 preterm neonates (gestional age: 24-37 weeks) and 70 right-handed adults (age: 18-31 years). In each ear, SOAEs and evoked otoacoustic emissions (EOAEs) were recorded and analysed, and, for the adults, functional MOC system assessment was made. Results showed SOAEs and EOAE amplitude to be right-predominant and in adults a right MOC functional predominance. These results indicate peripheral auditory system lateralisation, and an early origin thereof. The MOC system being thought to play a protective role, its physiological lateralisation may be relevant to the left prevalence of tinnitus and of auditory fatigue.

Development of cochlear active mechanisms in humans differs between gender

Despite onset of function early during the third term of gestation, the human auditory system demonstrates continued maturation, thought previously to occur primarily at the neural level. The electromotile properties of outer hair cells appear to contribute substantially to hearing sensitivity and frequency selectivity and lead to the generation of otoacoustic emissions (OAEs). This report demonstrates continued development of cochlear active mechanisms (i.e. end-organ level) after onset of cochlear function, as reflected by OAEs. Significant gender differences also are reported, corresponding to recently observed intersex differences in cochlear length and precursory to gender differences observed in the adult.