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.

Identification of neonatal hearing impairment: experimental protocol and database management.

OBJECTIVE: The purposes of this article are to describe the overall protocol for the Identification of Neonatal Hearing Impairment (INHI) project and to describe the management of the data collected as part of this project. A well-defined protocol and database management techniques were needed to ensure that data were 1) collected accurately and in the same way across sites; 2) maintained in a database that could be used to provide feedback to individual sites regarding enrollment and the extent to which the protocol was complete on individual subjects; and 3) available to answer project questions. This article describes techniques that were used to meet these needs. DESIGN: This study was a prospective, randomized study that was designed to evaluate auditory brain stem responses, transient evoked otoacoustic emissions, and distortion product otoacoustic emissions as hearing-screening tools, and to relate neonatal test findings to hearing status, defined by visual reinforcement audiometry at 8 to 12 mo of age. Measures of middle-ear function also were obtained at some sites as part of the neonatal test battery. In addition, other clinical and demographic data were gathered to determine the extent to which factors, other than auditory status, influenced test behavior. Three groups were evaluated: neonatal intensive care unit (NICU) infants (those who spent 3 or more days in a NICU), well babies with risk factors for hearing loss, and well babies without risk factors. Six centers participated in the trial. The testers for the project included audiologists, technicians, audiology graduate students, and medical research staff. The same computerized neonatal test program was applied at each center. This program generated the neonatal test database automatically. Clinical and demographic data were collected by means of concise data collection forms and were entered into a database at each site. After the neonatal test, subjects from the NICU and at-risk well babies were evaluated with visual reinforcement audiometry starting at 8 to 12 mo of age. All data were electronically transmitted to the core site where they were merged into one overall database. This database was exercised to provide feedback and to identify discrepancies throughout the course of the study. In its final form, it served as the database on which all analyses were performed. RESULTS AND CONCLUSION: The protocol was a departure from typical hearing screening procedures in terms of 1) its regimented application of three screening measures; 2) the detailed information that was obtained regarding subject clinical and demographic factors; and 3) its application of the same procedures across six centers having diverse geographic location and subject demographics. A learning curve for successfully executing the study protocols was observed. Throughout the study, monthly reports were generated to monitor subject enrollment, check for data completeness, and to perform data integrity checks. In combination with monthly data reports and checks that occurred throughout the progression of the study, miscellaneous data audits were performed to check accuracy of neonatal testing programs and to cross-check information entered in the clinical and demographic database. The data management techniques used in this project helped to ensure the quality of the data collection process and also allowed for detailed analyses once data were collected. This was particularly important because it enabled us to evaluate not only the performance of individual measures as screening tools, but also permitted an evaluation of the influence of other variables on screening test results.

Identification of neonatal hearing impairment: distortion product otoacoustic emissions during the perinatal period.

OBJECTIVES: 1) To describe distortion product otoacoustic emission (DPOAE) levels, noise levels and signal to noise ratios (SNRs) for a wide range of frequencies and two stimulus levels in neonates and infants. 2) To describe the relations between these DPOAE measurements and age, test environment, baby state, and test time. DESIGN: DPOAEs were measured in 2348 well babies without risk indicators, 353 well babies with at least one risk indicator, and 4478 graduates of neonatal intensive care units (NICUs). DPOAE and noise levels were measured at f2 frequencies of 1.0, 1.5, 2.0, 3.0, and 4.0 kHz, and for primary levels (L1/L2) of 65/50 dB SPL and 75/75 dB SPL. Measurement-based stopping rules were used such that a test did not terminate unless the response was at least 3 dB above the mean noise floor + 2 SDs (SNR) for at least four of five test frequencies. The test would terminate, however, if these criteria were not met after 360 sec. Baby state, test environment, and other test factors were captured at the time of each test. RESULTS: DPOAE levels, noise levels and SNRs were similar for well babies without risk indicators, well babies with risk indicators, and NICU graduates. There was a tendency for larger responses at f2 frequencies of 1.5 and 2.0 Hz, compared with 3.0 and 4.0 kHz; however, the noise levels systematically decreased as frequency increased, resulting in the most favorable SNRs at 3.0 and 4.0 kHz. Response levels were least and noise levels highest for an f2 frequency of 1.0 kHz. In addition, test time to achieve automatic stopping criteria was greatest for 1.0 kHz. With the exception of "active/alert" and "crying" babies, baby state had little influence on DPOAE measurements. Additionally, test environment had little impact on these measurements, at least for the environments in which babies were tested in this study. However, the lowest SNRs were observed for infants who were tested in functioning isolettes. Finally, there were some subtle age affects on DPOAE levels, with the infants born most prematurely producing the smallest responses, regardless of age at the time of test. CONCLUSIONS: DPOAE measurements in neonates and infants result in robust responses in the vast majority of ears for f2 frequencies of at least 2.0, 3.0 and 4.0 kHz. SNRs decrease as frequency decreases, making the measurements less reliable at 1.0 kHz. When considered along with test time, there may be little justification for including an f2 frequency at 1.0 kHz in newborn screening programs. It would appear that DPOAEs result in reliable measurements when tests are conducted in the environments in which babies typically are found. Finally, these data suggest that babies can be tested in those states of arousal that are most commonly encountered in the perinatal period.

Identification of neonatal hearing impairment: characteristics of infants in the neonatal intensive care unit and well-baby nursery.

OBJECTIVE: The objective of this study was to describe the demographic data, medical status, and incidence of risk factors for hearing impairment in the neonatal intensive care unit (NICU) and well-baby populations in a multicenter prospective study designed to assess neonatal hearing impairment and to evaluate factors that might affect neonatal hearing test performance. DESIGN: This was a prospective multicenter study funded by the National Institutes of Health-National Institute on Deafness and Other Communication Disorders to evaluate the effectiveness of auditory brain stem response, transient evoked otoacoustic emissions, and distortion product otoacoustic emissions for newborn hearing screening. Research staff at each site obtained informed consent and detailed demographic and medical data, including information on established risk factors for hearing loss on 4478 high-risk infants cared for in the NICU, 2348 infants from the well-baby nurseries with no risk factor, and 353 infants from the well-baby nurseries with risk factors. For follow-up purposes the sample was divided further to include a subgroup called selects. Selects were either infants from the well-baby nursery who had an established risk factor for hearing impairment (N = 353) or did not pass the neonatal hearing screen protocol (N = 80). In this study, we focus on the distribution of infants by nursery and risk factors only. Particular effort was made to enroll infants with risk factors for hearing loss in both the NICU and well-baby nurseries. Descriptive analyses are used to describe characteristics of this sample. RESULTS: All 10 of the risk factors established by the Joint Committee on Infant Hearing in 1994 were identified in the NICU population. The four most common were ototoxic medications (44.4%), very low birth weight (17.8%), assisted ventilation > 5 days (16.4%), and low Apgar scores at 1 or 5 min (13.9%). In contrast, only six risk factors were present in the well-baby nurseries: family history (6.6%), craniofacial abnormalities (3.4%), low Apgar scores (2.8%), syndromes (0.5%), ototoxic medications (0.2%), and congenital infection (0.1%). CONCLUSION: These descriptive risk factor data reflect both the newborn populations at the study sites and the bias for enrolling infants at risk for hearing loss. The high-risk NICU sample reflects the characteristics typically found in graduates of the NICU. The data summarized in this study will be used to assess the relationships between risk factor and hearing test outcome.

Identification of neonatal hearing impairment: transient evoked otoacoustic emissions during the perinatal period.

OBJECTIVES: 1) To describe transient evoked otoacoustic emission (TEOAE) levels, noise levels and signal to noise ratios (SNRs) for a range of frequency bands in three groups of neonates who were tested as a part of the Identification of Neonatal Hearing Impairment multi-center consortium project. 2) To describe the relations between these TEOAE measurements and age, test environment, baby state, and test time. DESIGN: TEOAEs were measured in 4478 graduates of neonatal intensive care units (NICUs), 353 well babies with at least one risk indicator, and 2348 well babies without risk factors. TEOAE and noise levels were measured for frequency bands centered at 1.0, 1.5, 2.0, 3.0, and 4.0 kHz for a click stimulus level of 80 dB SPL. For those ears not meeting "passing" stopping criteria at 80 dB pSPL, a level of 86 dB pSPL was included. Measurement-based stopping rules were used such that a test did not terminate unless the response revealed a criterion SNR in four out of five frequency bands or no response occurred after a preset number of averages. Baby state, test environment, and other test factors were captured at the time of test. RESULTS: TEOAE levels, noise levels and SNRs were similar for NICU graduates, well babies with risk factors and well babies without risk factors. There were no consistent differences in response quality as a function of test environment, i.e., private room, unit, open crib, nonworking isolette, or working isolette. Noise level varied little across risk group, test environment, or infant state other than crying, suggesting that the primary source of noise in TEOAE measurements is infant noise. The most significant effect on response quality was center frequency. Responses were difficult to measure in the half-octave band centered at 1.0 kHz, compared with higher frequencies. Reliable responses were measured routinely at frequencies of 1.5 kHz and higher. CONCLUSIONS: TEOAEs are easily measured in both NICU graduates and well babies with and without risk factors for hearing loss in a wide variety of test environments. Given the difficulties encountered in making reliable measurements for a frequency band centered at 1.0 kHz, its inclusion in a screening program may not be justified.

Identification of neonatal hearing impairment: auditory brain stem responses in the perinatal period.

OBJECTIVES: 1) To describe the auditory brain stem response (ABR) measurement system and optimized methods used for study of newborn hearing screening. 2) To determine how recording and infant factors related to the screening, using well-defined, specific ABR outcome measures. DESIGN: Seven thousand one hundred seventy-nine infants, 4478 from the neonatal intensive care unit (NICU) and the remaining from the well-baby nursery, were evaluated with an automated ABR protocol in each ear. Two channel recordings were obtained (vertex to mastoid or channel A and vertex to nape of neck or channel B) in response to click stimuli of 30 and 69 dB nHL in all infants as well as 50 dB nHL in infants who did not meet criteria for response at 30 dB. Criteria for response included F(SP) > or =3.1 and a tester-judgment of response. Criteria could be met in the first or repeat test with a maximum of 6144 accepted sweeps per test. RESULTS: More than 99% of infants could complete the ABR protocol. More than 90% of NICU and well-baby nursery infants "passed" given the strict criteria for response, whereas 86% of those with high risk factors met criterion for ABR response detection. The number of infants who did not meet ABR response criteria in one or both ears was systematically related to stimulus level with the largest group not meeting criteria at 30 dB followed by 50 and 69 dB nHL. Meeting criteria on the ABR was positively correlated with the amplitude of wave V, with low noise and low electrode impedance. Factors that predicted how many sweeps would be needed to reach criterion F(SP) included noise level of the test site, state of the baby (for example, quiet sleep versus crying), recording noise, electrode impedance and response latency. Channel A (vertex to mastoid) reached criterion more often than channel B (vertex to nape of neck) due to higher noise in channel B. Average total test time for 30 dB nHL screening in both ears was under 8 minutes. Well babies with risk factors took slightly longer to evaluate than other groups with this automated ABR procedure and have higher noise levels. CONCLUSIONS: ABR implemented with an automated detection algorithm using a 30 dB nHL click stimulus is reliable technique for rapid assessment of auditory status in newborns. Factors other than hearing loss that influenced the test result include infant state, electrode location and impedance, testing site, and infant risk status. Even so, ABRs were reliably recorded in the vast majority of babies under circumstances in which most babies are found in the perinatal period.

Identification of neonatal hearing impairment: recruitment and follow-up.

OBJECTIVE: The purpose of this study is to describe the recruitment and retention strategies as well as the sample demographics for families with infants completing the neonatal examination and returning for follow-up. These data are compared to those infants inactivated from the study. DESIGN: This study was a prospective, randomized clinical study. All infants who were confined to the neonatal intensive care unit (NICU) and well babies with at least one risk indicator were targeted for behavioral audiometric follow-up testing. In addition, infants without risk factors from the well-baby nursery, but who failed a newborn test, were also followed. Several variables were evaluated to determine those factors, if any, that might predict which families returned for follow-up testing. RESULTS: Recruitment was achieved as per study design with 4911 high-risk infants and 2348 well-baby nursery infants (without risk indicators for hearing) enrolled. Of the 4911 high-risk infants enrolled, 64% were successfully recruited into the follow-up portion of the study. This was less than the projected rate of 80%. Factors predicting noncompliance with the study protocol for follow-up were predominantly sociodemographic and included nonwhite race, no insurance, substance abuse, young maternal age, more than two children at home, and late onset of prenatal care. CONCLUSIONS: Factors related to low socioeconomic status and increased social risk were the strongest predictors of poor study protocol compliance. Despite retention challenges, 64% of the targeted, high-risk infants subsequently returned for the 8-to 12-mo behavioral hearing assessment protocols for validation purposes.

Identification of neonatal hearing impairment: hearing status at 8 to 12 months corrected age using a visual reinforcement audiometry protocol.

OBJECTIVES: 1) To describe the hearing status of the at-risk infants in the National Institutes of Health-Identification of Neonatal Hearing Impairment study sample at 8 to 12 mo corrected age (chronologic age adjusted for prematurity). 2) To describe the visual reinforcement audiometry (VRA) protocol that was used to obtain monaural behavioral data for the sample. DESIGN: All neonatal intensive care unit infants and well babies with risk factors (including well babies who failed neonatal tests) were targeted for follow-up behavioral evaluation once they had reached 8 mo corrected age. Three thousand one hundred and thirty-four (64.4%) of the 4868 surviving infants returned for at least one behavioral hearing evaluation, which employed a well-defined VRA protocol. VRA thresholds or minimum response levels (MRLs) were determined for speech and pure tones of 1.0, 2.0, and 4.0 kHz for each ear using insert earphones. RESULTS: More than 95% of the infants were reliably tested with the VRA protocol; 90% provided complete tests (four MRLs for both ears). Ninety-four percent of the at-risk infants were found to have normal hearing sensitivity (MRLs of 20 dB HL) at 1.0, 2.0, and 4.0 kHz in both ears. Of the infants, 2.2% had bilateral hearing impairment, and 3.4% had impairment in one ear only. More than 80% of the impaired ears had losses of mild-to-moderate degree. CONCLUSIONS: This may be the largest study to attempt to follow all at-risk infants with behavioral audiometric testing, regardless of screening outcome, in an effort to validate the results of auditory brain stem response, distortion product otoacoustic emission, and transient evoked otoacoustic emission testing in the newborn period. It is one of only a few studies to report hearing status of infants at 1 yr of age, using VRA on a clinical population. Successful testing of more than 95% of the infants who returned for the VRA follow-up documents the feasibility of obtaining monaural behavioral data in this population.

Identification of neonatal hearing impairment: infants with hearing loss.

OBJECTIVE: This article describes the audiologic findings and medical status of infants who were found to have hearing loss, detected as part of the Identification of Neonatal Hearing Impairment (INHI) project. In addition, the neonatal and maternal health variables for the group of infants who could not be tested with visual reinforcement audiometry (VRA) due to developmental and visual disability are presented. DESIGN: The overall goal of the INHI project was to evaluate the test performance of auditory brain stem response and evoked otoacoustic emission (OAE) tests given in the newborn period. These tools were evaluated on the basis of the infants' hearing when tested behaviorally with VRA at 8 to 12 mo corrected age. The neonatal test results, VRA results, medical history information and a record of intercurrent events occurring between the neonatal period and the time of VRA were collated and reviewed. The purpose of this article is to review the characteristics of those infants who were found to have hearing loss. RESULTS: Of 2995 infants who had VRA tests judged to be of good or fair reliability, 168 had a finding of hearing loss for at least one ear, an incidence of 5.6%. Sixty-six infants had bilateral losses, an incidence of 2%, and 22 infants had bilateral hearing losses in the moderate to profound range, an incidence 0.7%. The prevalence of middle ear problems was greater than 50% among these infants with hearing loss. From the larger group of 168 infants with hearing loss, a group of 56 infants (86 ears) was chosen as those with a low probability that the hearing loss was due to transient middle ear pathology and was more likely hearing loss of a permanent nature. These were the infants used for the analyses of neonatal test performance (Norton et al., 2000). In this selected group there were 30 infants with bilateral impairment of at least mild degree, which is an incidence of 1%. There were approximately equal numbers of ears in the mild, moderate, severe and profound range of hearing loss. Risk factors associated with hearing loss were reviewed for the total sample of infants tested with VRA and for those infants with hearing loss. A history of treatment with aminoglycosides was the risk factor most often reported in the entire sample; however, there was no difference in prevalence of this risk factor for the normal-hearing and hearing-impaired groups. The risk factor associated with the highest incidence of hearing loss was stigmata of syndromes associated with sensorineural hearing loss and other neurosensory disorders. Sixty-seven infants who returned for follow-up could not be tested with VRA due to severe developmental delay or visual disability. Many of these infants had medical histories indicating the sequelae of extreme prematurity and/or very low birthweight. CONCLUSIONS: Most of the hearing losses found in this study were mild and, based on clinical history and tympanometry tests, many of the mild and some of the moderate impairments may have been acquired in early infancy due to middle ear effusion. In the group of infants used for determination of neonatal test performance there were approximately equal numbers of mild, moderate, severe and profound losses. Only a small percentage of infants with a conventional risk indicator for hearing loss actually had a hearing loss, and there were a significant number of infants with hearing loss who did not have a risk indicator. These findings support the need for an early identification program based on universal neonatal hearing screening rather than by targeted testing of those with risk indicators.

The varieties of auditory neuropathy.

Auditory neuropathy (AN) was initially described as impairment of auditory neural function, with preserved cochlear hair cell function. In this report, 67 patients with audiological and neurophysiological criteria for hearing loss due to auditory neuropathy are described. Reviewing this large body of patients, AN appears to consist of a number of varieties, with different etiologies and sites affected. All varieties share a relatively spared receptor function, and an impaired neural response, with diminished ability to follow fast temporal changes in the stimulus, but different varieties in this general scheme can be distinguished. Analyses of the clinical features indicate that auditory neuropathies vary in several measures including age of onset, presence of peripheral neuropathy, etiology, and behavioral and physiological measures of auditory function. The sites affected along the peripheral auditory pathway may include dysfunction of the outer hair cells, the synapse between hair cell and auditory nerve, and the auditory nerve fibers, with myelin as well as axonal impairments contributing to the disorder.

Cochlear implantation of auditory neuropathy.

Auditory neuropathy (AN) is a hearing disorder that presents with a grossly abnormal or absent neural response as measured by evoked potentials in the presence of normal outer hair cell function evidenced by present otoacoustic emissions or cochlear microphonics. Rehabilitation for patients with AN is challenging due to abnormal temporal encoding at the auditory nerve leading to severely impaired speech perception. Although patients with AN may demonstrate improvement in thresholds with amplification, temporal encoding dysfunction, and consequently speech perception degradation, is not alleviated by amplification. Another issue is the heterogeneity of the AN population in terms of audiologic and neurologic findings, in addition to uncertain etiology and pathophysiology. For children with prelingual onset of AN, development of auditory and oral communication skills is particularly compromised. All children with hearing loss in the severe-to-profound range who do not benefit from conventional amplification can be considered candidates for a cochlear implant (CI). This paper presents a case study of a child with AN who received a CI. Whereas no synchronous neural response auditory brainstem response could be elicited to acoustic stimuli, an electrically evoked auditory nerve action potential was evident following implantation, suggesting restoration to some degree of neural synchrony. Significant improvement in speech perception was found post-CI. Recommendation to implant all patients with AN would be premature, but these findings suggest that electrical stimulation in some cases of auditory neuropathy can be a viable option.

Distortion product otoacoustic emission suppression in subjects with auditory neuropathy.

OBJECTIVE: The objective of this experiment was to address: 1) whether normal efferent system function is required for normal cochlear tuning as measured by distortion product otoacoustic emission (DPOAE) suppression in humans and 2) whether cochlear function, assessed by DPOAE suppression tuning, is normal in a small group of patients with auditory neuropathy. DESIGN: DPOAE suppression tuning curves (STCs) are similar to other physiologic measures of tuning. They are generated by evoking a DPOAE with two simultaneously presented pure tones and then suppressing the distortion product with a third tone of varying frequency and level. In this study, DPOAE STCs were generated with f2 frequencies of 1500, 3000, and 6000 Hz in 15 normal-hearing adults and four subjects with documented auditory neuropathy. Tuning curve width, slope and tip characteristics, as well as rate of suppression growth were measured in each group. Contralateral suppression of otoacoustic emissions (OAEs) was also recorded as an index of medial efferent function. RESULTS: Results show that the four subjects with auditory neuropathy lacked efferent suppression of OAEs. However, these four subjects showed normal estimates of cochlear tuning as measured by DPOAE suppression results. CONCLUSIONS: This finding suggests that normal efferent system function is not required at the time of test for normal DPOAE suppression tuning. It also suggests that cochlear function as evaluated by detailed measures of DPOAE suppression, is normal in these "typical" patients with auditory neuropathy.

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.

The case for early identification of hearing loss in children. Auditory system development, experimental auditory deprivation, and development of speech perception and hearing.

Human infants spend the first year of life learning about their environment through experience. Although it is not visible to observers, infants with hearing are learning to process speech and understand language and are quite linguistically sophisticated by 1 year of age. At this same time, the neurons in the auditory brain stem are maturing, and billions of major neural connections are being formed. During this time, the auditory brain stem and thalamus are just beginning to connect to the auditory cortex. When sensory input to the auditory nervous system is interrupted, especially during early development, the morphology and functional properties of neurons in the central auditory system can break down. In some instances, these deleterious effects of lack of sound input can be ameliorated by reintroduction of stimulation, but critical periods may exist for intervention. Hearing loss in newborn infants can go undetected until as late as 2 years of age without specialized testing. When hearing loss is detected in the newborn period, infants can benefit from amplification (hearing aids) and intervention to facilitate speech and language development. All evidence regarding neural development supports such early intervention for maximum development of communication ability and hearing in infants.

Gender distinctions and lateral asymmetry in the low-level auditory brainstem response of the human neonate.

Threshold measures of auditory brainstem response (ABR) were generated in 72 full-term newborn infants in response to clicks and tone burst stimuli between 500 and 8000 Hz as detailed in a previous study. These results were further analyzed for differences in response related to ear (lateral asymmetry) and subject gender. Thresholds obtained in male infants were significantly lower than those of females (P = 0.0485). The greatest differences in threshold between male and female infants occurs in the right ear (7.45 dB) as opposed to the left ear (1.56 dB). Both male and female infants have significantly larger wave V amplitude elicited from the right ear than the left (P = 0.0002) using low-level stimuli. Also, as has been noted in adults, female infants have larger amplitude ABRs than males (P = 0.0018), but amplitude differences across gender are significant only in the right ear (ear by gender interaction P = 0.0278). Results of this study indicate that gender differences and lateral asymmetry in auditory function are not a result of gender bias for or unbalanced auditory trauma, but a biologically significant phenomenon that is present at birth. The argument is made that superior right ear performance may be part of cerebral laterality in auditory function.

Auditory threshold sensitivity of the human neonate as measured by the auditory brainstem response.

The absolute auditory sensitivity of the human newborn infant was investigated using auditory brainstem response thresholds (ABR). ABRs were elicited with clicks and tone-bursts of 0.5, 1.5, 4.0 and 8.0 kHz, embedded in notched noise, in healthy, full-term human neonates and young adults with known, normal-hearing sensitivity. Stimuli were calibrated using a probe microphone positioned near the tympanic membrane in the ear canal of each subject to control for differences in resonance characteristics of infant and adult ear canals. ABR thresholds were also characterized relative to group psychophysical thresholds (nHL) and relative to individual psychophysical threshold or sensation level (SL) for the adult subjects. Infant ABR thresholds measured in p.e. SPL for all stimuli are elevated by to 3-25 dB relative to adult thresholds. Threshold elevation is greatest for the high-frequency stimuli. Result are consistent with neural immaturity for high-frequency stimuli in the auditory system of human neonates.

The development of cochlear frequency resolution in the human auditory system.

OBJECTIVE: The objectives of this study were: 1) to evaluate the maturity of cochlear frequency resolution in human neonates, and 2) to further elucidate the differential time course for development of frequency resolution at the cochlear and auditory-neural levels of the auditory system. DESIGN: This paper describes a relatively new technique using distortion product otoacoustic emision (DPOAE) suppression to study cochlear tuning. DPOAE suppression tuning curves (STCs) were generated in 15 normal-hearing adults and 26 healthy, term-born neonates at 1500, 3000, and 6000 Hz. The 2f1-f2 DPOAE was measured in all subjects with primary tones of 65 and 50 dB SPL (L1 > L2) and a 1.22 f2/f1 frequency ratio. Initially, an unsuppressed DPOAE was recorded. After this, a suppressor tone was introduced, and its level varied until DPOAE amplitude was reduced by 6 dB. By plotting the suppressor level required to achieve criterion amplitude reduction by suppressor frequency (for many tones), a DPOAE STC was generated. DPOAE STC shape, width, slope, and tip characteristics were analyzed for both adults and neonates. RESULTS: General shape and appearance of DPOAE STCs were comparable for adults and neonates, as was STC tip frequency and level. Statistical analyses of tuning-curve width (Q) and slope (dB/octave) failed to show age effects, further confirming the similarity between adults and neonates. DPOAE STCs were stable, show minimal intra- and intersubject variability, and closely resemble and behave like physiologic measures of tuning from the VIIIth nerve. CONCLUSIONS: Results suggest that: 1) cochlear tuning and related active processes are basically mature by term birth in the human auditory system, 2) tuning immaturities reported in infants as old as 6 mo of age probably involve auditory-neural immaturities, and 3) suppression of the 2f1-f2 DPOAE seems, to provide an indirect measure of cochlear frequency resolution in humans.

Hearing threshold as measured by auditory brain stem response in human neonates.

OBJECTIVES: This article evaluates the concept of auditory threshold and discusses the limitations of assessing threshold in human neonates. The advantages and limitations of assessing neonatal threshold by means of auditory brain stem response (ABR) are discussed, and data from several studies of newborn ABR threshold are compared. The authors report data from their own study designed to compare adult and neonatal ABR threshold using tonal stimuli. EXPERIMENTAL DESIGN: Several studies are compared. Data from the authors are ABR thresholds for tone bursts of 0.5, 1.5, 4, and 8 kHz, determined from 2-channel recordings in full-term neonates and adults. Stimuli were calibrated in SPL by means of a probe microphone inserted into the ear canal along with the insert transducer of each subject. RESULTS: All studies find a degree of threshold elevation in neonates relative to adult threshold. Neonatal ABR thresholds from our laboratory for stimuli from 500 through 8000 Hz are elevated relative to adult thresholds by 5 to 25 dB. Threshold elevation in our data and in other studies has found that neonatal ABR thresholds to high-frequency stimuli show the largest elevation relative to adults and low-frequency stimuli the most mature. CONCLUSIONS: Thresholds of neonates, as measured by the ABR, are immature especially for high-frequency stimuli. Proper stimulus calibrations, which removes the influence of ear canal resonance, are important for comparisons of data across age groups. Developmental differences in the conductive mechanism and neural immaturity are the most harmonious explanations for elevation of neonatal ABR thresholds.

Distortion product otoacoustic emission suppression tuning curves in human adults and neonates.

Distortion product otoacoustic emission (DPOAE) iso-suppression tuning curves (STC) were generated in 15 normal-hearing adults and 16 healthy term-born neonates for three f2 frequencies. The 2f1-f2 DPOAE was elicited using f2/f1 = 1.2, LI = 1.2, LI = 65 and L2 = 50 dB SPL. A suppressor tone was presented at frequencies ranging from 1 octave below to 1/4 octave above f2 and varied in level until DPOAE amplitude was reduced by 6 dB. The suppressor level required for 6 dB suppression was plotted as function of suppressor frequency to generate a DPOAE STC. Forward-masked psychoacoustic tuning curves (PTC) were obtained for three of the adult subjects. Results indicate that DPOAE STCs are stable and show minimal inter- and intra-subject variability. The tip of the STC is consistently centered around the f2 region and STCs are similar in shape, width (Q10) and slope to VIIIth-nerve TCs. PTCs and STCs measured in the same subject showed similar trends, although PTCs had narrower width and steeper slope. Neonatal STCs were recorded at 3000 and 6000 Hz only and were comparable in shape, width and slope to adult STCs. Results suggest: (1) suppression of the 2f1-f2 DPOAE may provide an indirect measure of cochlear frequency resolution in humans and (2) cochlear tuning, and associated active processes in the cochlea, are mature by term birth for at least mid- and high-frequencies. These results provide significant impetus for continued study of DPOAE suppression as a means of evaluating cochlear frequency resolution in humans.

Filtering and spectral characteristics of averaged auditory brain-stem response and background noise in infants.

Filtering of electrode-recorded activity before averaging is used in evoked-potential measurements to reduce background noise under the assumption that unwanted spectral components will be suppressed without substantially altering neural activity. Desired filter characteristics depend on signal and noise spectra and filter choice can affect the validity and efficiency of ABR measurement. Spectral characteristics of the human auditory brain-stem response (ABR) change with subject age and state and with the level and spectrum of the stimulus. Brain-stem evoked potentials from infants comprise greater low-frequency spectral components than do adult ABRs. Physiologic noise can also change with age of the subject. This study compares the effects of 30- and 100-Hz high-pass filters on ABR amplitude and response Fsp in full-term infants. Stimuli used to elicit ABRs are moderate- and low-level clicks and 500-Hz tone bursts. Spectral characteristics of the averaged ABR and the no-stimulus background noise from full-term infants are also characterized. Results indicate that (1) energy in the infant ABR is concentrated below 100 Hz and (2) a high-pass recording filter of 30 Hz reveals a larger-amplitude ABR and enhances the overall signal-to-noise ratio as measured by Fsp as compared to a 100-Hz high pass.