Effective Masking Levels for Bone Conduction Auditory Brainstem Response Stimuli in Infants and Adults with Normal Hearing.

OBJECTIVE: The purpose of the present study was to investigate effective masking levels (EMLs) for bone conduction (BC) auditory brainstem response (ABR) testing in infants and adults. Early hearing detection and intervention programs aim to limit delays in identifying ear-specific type/degree of hearing loss in infants using the ABR. Ear-specific assessment poses challenges as sound delivered to one ear can travel across the skull and activate the contralateral cochlea. Wave V amplitude and latency measures ipsilateral and contralateral to the bone oscillator can be compared to isolate the test cochlea in some cases; however, when these findings are equivocal, clinical masking is required. This study aims to determine EMLs for ABRs elicited to 500- and 2000-Hz BC stimuli for normal-hearing infants (0 to 18 months) and adults. DESIGN: Participants were 21 adults (18 to 54 years) and 24 infants (5 to 47 weeks) with normal hearing. BC 500- and 2000-Hz brief tonal stimuli at intensities approximating normal levels were presented via a B-71 oscillator (infants: 20 dB nHL at 500 Hz and 30 dB nHL at 2000 Hz; adults: 500 and 2000 Hz at 20 and 30 dB nHL, respectively). White noise masking was presented binaurally via ER-3A earphones (22 to 82 dB SPL; 10-dB steps). The lowest level of masking to eliminate a BC response was deemed the EML. RESULTS: For stimuli presented at 20 dB nHL, adult mean (1 SD) EMLs for 500 and 2000 Hz were 65 (9) and 53 (6) dB SPL, respectively. Mean EMLs for infants were 80 (6) dB SPL for 500 Hz at 20 dB nHL and 64 (9) dB SPL for 2000 Hz at 30 dB nHL. Compared to adults, infants required approximately 13 dB more masking at 500 Hz but a similar amount of masking at 2000 Hz. Infants required 26 dB more masking at 500 versus 2000 Hz, whereas, adults required only 12 dB more masking. CONCLUSIONS: Maximum binaural EMLs for infant BC responses elicited to 500 Hz at 20 dB nHL are 82 dB SPL, and for 2000 Hz at 30 and 40 dB nHL, respectively, are 72 and 82 dB SPL. Monaural masking levels for the nontest ear (assuming 10 dB of interaural attenuation) recommended clinically are as follows: (1) 500 Hz: 72 and 82 dB SPL at 20 and 30 dB nHL, respectively; and (2) 2000 Hz: 62, 72, and 82 dB SPL at 30, 40, and 50 dB nHL, respectively. Unsafe levels of white noise would be needed to effectively mask at greater stimulus levels.

Telehealth-enabled auditory brainstem response testing for infants living in rural communities: the British Columbia Early Hearing Program experience.

Objective: The present study investigated a telehealth-enabled auditory-brainstem-response (TH-ABR) programme provided by the British Columbia Early Hearing Program (BCEHP) to families in a remote northern area who face barriers such as travel distance and access to audiologists trained in ABR testing. Objectives were to: (i) outline the design/implementation of a TH-ABR programme, (ii) summarise equipment/procedures, and (iii) report on results for the TH-ABR programme (cost/time effectiveness, testing accuracy/efficiency, and caregiver satisfaction). Design: TH-ABR implementation was described and TH-ABR results were compared to behavioural follow-up findings to evaluate TH-ABR test accuracy. Caregivers were invited to complete TH-ABR satisfaction surveys following their appointment(s). Study sample: One hundred and two infants (mean age: 2.3 months) were assessed via TH-ABR at four points-of-care; 41/66 caregivers completed surveys. Results: The TH-ABR programme was suitable, sustainable and scalable. After 29 TH-ABR events, the service was cost neutral to BCEHP ($91,250 averted after 102 TH-ABRs). Fifty infants were identified with hearing loss and TH-ABR accuracy and efficiency were comparable to face-to-face assessments. Parent survey results indicated a high level of satisfaction with the TH-ABR experience. Conclusions: TH-ABR is efficient, accurate, valued by parents, optimises availability to audiology resources, builds local service capacity, and reduces costs for northern BC communities.

The Effect of Signal to Noise Ratio on Cortical Auditory-Evoked Potentials Elicited to Speech Stimuli in Infants and Adults With Normal Hearing.

OBJECTIVES: Identification and discrimination of speech sounds in noisy environments is challenging for adults and even more so for infants and children. Behavioral studies consistently report maturational differences in the influence that signal to noise ratio (SNR) and masker type have on speech processing; however, few studies have investigated the neural mechanisms underlying these differences at the level of the auditory cortex. In the present study, we investigated the effect of different SNRs on speech-evoked cortical auditory-evoked potentials (CAEPs) in infants and adults with normal hearing. DESIGN: A total of 10 adults (mean age 24.1 years) and 15 infants (mean age 30.7 weeks), all with normal hearing, were included in the data analyses. CAEPs were evoked to /m/ and /t/ speech stimuli (duration: 79 ms) presented at 75 dB SPL in the sound field with a jittered interstimulus interval of 1000-1200 ms. Each of the stimuli were presented in quiet and in the presence of white noise (SNRs of 10, 15, and 20 dB). Amplitude and latency measures were compared for P1, N1, and P2 for adults and for the large positivity (P) and following negativity (N: N250 and/or N450) for infants elicited in quiet and across SNR conditions. RESULTS: Infant P-N responses to /t/ showed no statistically significant amplitude and latency effects across SNR conditions; in contrast, infant CAEPs to /m/ were greatly reduced in amplitude and delayed in latency. Responses were more frequently absent for SNRs of 20 dB or less. Adult P1-N1-P2 responses were present for all SNRs for /t/ and most SNRs for /m/ (two adults had no responses to /m/ for SNR 10); significant effects of SNR were found for P1, N1, and P2 amplitude and latencies. CONCLUSIONS: The findings of the present study support that SNR effects on CAEP amplitudes and latencies in infants cannot be generalized across different types of speech stimuli and cannot be predicted from adult data. These findings also suggest that factors other than energetic masking are contributing to the immaturities in the SNR effects for infants. How these CAEP findings relate to an infant's capacity to process speech-in-noise perceptually has yet to be established; however, we can be confident that the presence of CAEPs to a speech stimulus in noise means that the stimulus is detected at the level of the auditory cortex. The absence of a response should be interpreted with caution as further studies are needed to investigate a range of different speech stimuli and SNRs, in conjunction with behavioral measures, to confirm that infant CAEPs do indeed reflect functional auditory capacity to process speech stimuli in noise.

Maturation of bone-conduction transcranial and forehead attenuation using a measure of sound pressure in the ear canal.

OBJECTIVE: Although it is understood that bone-conduction (BC) hearing is different between infants and adults, few studies have attempted to explain why these differences exist. The main objective in this study was to better understand how properties of the developing skull contribute to the maturation of BC sensitivity through an indirect measurement of BC attenuation across the skull. DESIGN: Estimation of transcranial and forehead attenuation of pure-tone BC stimuli was conducted using sound pressure in the ear canal for a transducer placed on the skull ipsi- and contralateral to the probe ear and at the forehead. STUDY SAMPLE: Seventy-six individuals participated in the study, including 59 infants and children (1 month-7 years) and 17 adults. RESULTS: BC attenuation was greatest for young infants, and decreased throughout maturation. Attenuation from the forehead to the ipsilateral temporal bone was also greater compared to the transcranial measures for infants and children older than 10 months. CONCLUSIONS: These results provide evidence that physical maturation of the skull contributes to infant-adult differences in BC attenuation. Clinicians may consider these results, in combination with previous studies using physiological measures, when fitting infants and young children with bone-anchored hearing systems.

Infant Cortical Auditory Evoked Potentials to Lateralized Noise Shifts Produced by Changes in Interaural Time Difference.

OBJECTIVES: Newborns reliably orient to sound location soon after birth; by age 1 month this orienting disappears until after age 4 months. It has been suggested that orienting by the newborn reflects subcortical-mediated reflexes, which are suppressed by age 1 month; reappearance of orienting then occurs after age 4 months with maturation of cortical mechanisms of sound localization. In the present study, we assess auditory lateralization in young infants (and adults) by recording slow cortical auditory evoked potentials to lateralization shifts in dichotic noise produced by changes in interaural time difference (ITD). DESIGN: Fifteen normal infants aged under 4 months (mean = 10.7 weeks) had cortical auditory evoked potentials assessed in response to (1) diotic "onset" noise bursts (0 msec ITD) and (2) shifts in continuous lateralized noise (75 dB SPL) produced by ITD shifts of 0.5, 0.8, 1, 2, 4, and 8 msec. Shifts alternated between ears occurred every 2 sec. Stimuli were presented using insert earphones; infants slept during recordings. For comparison, similar recordings were obtained in 11 normal-hearing, awake, adults. Additionally, "control" recordings to the ITD-shift stimuli presented to only one ear were obtained in the adults. RESULTS: Similar to previous research, adults showed clear N1-P2 responses to the lateralization shifts (ITD 0.5 to 2.0 msec). Responses decreased for longer ITD shifts, with no adult responses to the 8-msec ITD shift. N1 latencies to ITD-shift stimuli were 28 to 34 msec longer than to the onset stimuli. No responses were seen in the control conditions when ITD-shift stimuli were presented to only one ear (confirming the binaural nature of the ITD-shift responses). All infants showed P2 responses to one or more of the ITD-shift stimuli up to ±1 msec; compared with adults, infants showed larger amplitude decreases and fewer responses to longer ITD-shift stimuli. As was seen with the adult responses, infant response (P2) latencies to ITD shifts were longer compared with their responses to the onset stimuli; however, these increases, 32 to 78 msec, were significantly longer than those seen in the adults. CONCLUSIONS: Young infants (even as young as 5 weeks) show clear evidence of auditory cortical responsivity to lateralization shifts produced by changes in the ITD of continuous noise, indicating that they have the capacity to process binaural ITD timing cues well before the age of 4 months. Further research is required to determine whether the larger latency increase in infants for ITD-shift stimuli (relative to the onset stimuli) and the greater effect of longer ITD shifts on response presence and amplitude in infants reflects immaturity of lateralization processing and/or reduced responses recorded during sleep. Slow cortical auditory evoked potentials elicited to lateralization shifts in dichotic noise provide a method to investigate binaural hearing processes in young children with normal or impaired hearing.

Maturation of Mechanical Impedance of the Skin-Covered Skull: Implications for Soft Band Bone-Anchored Hearing Systems Fitted in Infants and Young Children.

OBJECTIVES: Little is known about the maturational changes in the mechanical properties of the skull and how they might contribute to infant-adult differences in bone conduction hearing sensitivity. The objective of this study was to investigate the mechanical impedance of the skin-covered skull for different skull positions and contact forces for groups of infants, young children, and adults. These findings provide a better understanding of how changes in mechanical impedance might contribute to developmental changes in bone conduction hearing, and might provide insight into how fitting and output verification protocols for bone-anchored hearing systems (BAHS) could be adapted for infants and young children. DESIGN: Seventy-seven individuals participated in the study, including 63 infants and children (ages 1 month to 7 years) and 11 adults. Mechanical impedance magnitude for the forehead and temporal bone was collected for contact forces of 2, 4, and 5.4 N using an impedance head, a BAHS transducer, and a specially designed holding device. Mechanical impedance magnitude was determined across frequency using a stepped sine sweep from 100 to 10,000 Hz, and divided into low- and high-frequency sets for analysis. RESULTS: Mechanical impedance magnitude was lowest for the youngest infants and increased throughout maturation in the low frequencies. For high frequencies, the youngest infants had the highest impedance, but only for a temporal bone placement. Impedance increased with increasing contact force for low frequencies for each age group and for both skull positions. The effect of placement was significant for high frequencies for each contact force and for each age group, except for the youngest infants. CONCLUSIONS: Our findings show that mechanical impedance properties change systematically up to 7 years old. The significant age-related differences in mechanical impedance suggest that infant-adult differences in bone conduction thresholds may be related, at least in part, to properties of the immature skull and overlying skin and tissues. These results have important implications for fitting the soft band BAHS on infants and young children. For example, verification of output force form a BAHS on a coupler designed with adult values may not be appropriate for infants. This may also hold true for transducer calibration when assessing bone conduction hearing thresholds in infants for different skull locations. The results have two additional clinical implications for fitting soft band BAHSs. First, parents should be counseled to maintain sufficient and consistent tightness so that the output from the BAHS does not change as the child moves around during everyday activities. Second, placement of a BAHS on the forehead versus the temporal bone results in changes in mechanical impedance which may contribute to a decrease in signal level at the cochlea as it has been previously demonstrated that bone conduction thresholds are poorer at the forehead compared with a temporal placement.

Physiological Measures and Their Potential for the Assessment of Pediatric Hearing Needs.

PURPOSE: The purpose of this article is to discuss how cortical auditory evoked potentials might be used to assess speech perception capacity in infants, including acoustic change complex data collected in our laboratory. This article is a summary of a paper presented at the HEaring Across the Lifespan (HEAL) Conference held June 5-7, 2014, in Cernobbio, Italy. METHOD: Highlights from data collected in infants and the main issues needing investigation for clinical application are presented. CONCLUSIONS: Preliminary studies show promising results for the acoustic change complex and confirm that further inquiry into its clinical application is warranted. The presence of an onset response can be used clinically to confirm that auditory information has reached the cortex; however, the absence of a response cannot be interpreted with confidence.

Elicitation of the Acoustic Change Complex to Long-Duration Speech Stimuli in Four-Month-Old Infants.

The acoustic change complex (ACC) is an auditory-evoked potential elicited to changes within an ongoing stimulus that indicates discrimination at the level of the auditory cortex. Only a few studies to date have attempted to record ACCs in young infants. The purpose of the present study was to investigate the elicitation of ACCs to long-duration speech stimuli in English-learning 4-month-old infants. ACCs were elicited to consonant contrasts made up of two concatenated speech tokens. The stimuli included native dental-dental /dada/ and dental-labial /daba/ contrasts and a nonnative Hindi dental-retroflex /daDa/ contrast. Each consonant-vowel speech token was 410 ms in duration. Slow cortical responses were recorded to the onset of the stimulus and to the acoustic change from /da/ to either /ba/ or /Da/ within the stimulus with significantly prolonged latencies compared with adults. ACCs were reliably elicited for all stimulus conditions with more robust morphology compared with our previous findings using stimuli that were shorter in duration. The P1 amplitudes elicited to the acoustic change in /daba/ and /daDa/ were significantly larger compared to /dada/ supporting that the brain discriminated between the speech tokens. These findings provide further evidence for the use of ACCs as an index of discrimination ability.

An investigation into the clinical utility of ipsilateral/contralateral asymmetries in bone-conduction auditory steady-state responses.

OBJECTIVE: To determine whether ipsilateral/contralateral asymmetries in the bone-conduction (BC) ASSR are robust enough in infants to be used clinically to isolate the test ear. DESIGN: Retrospective investigation of three two-channel BC ASSR datasets. SUBJECTS: Forty-eight adults (mean age 26.7 years), 49 infants (mean age 29.6 weeks). METHODS: BC ASSR stimuli were presented as amplitude/frequency modulated sinusoidal tones with carrier frequencies of 500, 1000, 2000, and 4000 Hz (-10 to 45 dB HL). RESULTS: Infants showed greater differences in ipsilateral/contralateral mean amplitudes and phase delays for all experimental conditions compared to adults. Ninety percent of infants had ipsilateral/contralateral asymmetries at 500 and 4000 Hz (20-35 dB HL) using an "amplitude or phase delay" criterion, and at 4000 Hz (20-25 dB HL) using an "amplitude and phase delay" criterion. CONCLUSIONS: As ipsilateral/contralateral asymmetries are not consistently present for 1000- and 2000-Hz BC ASSRs in infants, clinical masking would be needed at these frequencies to isolate the test ear. For 500- and 4000-Hz BC ASSRs, the accuracy of using these asymmetries requires clinical confirmation in a group of infants with hearing loss.

Comparisons of auditory steady state response and behavioral air conduction and bone conduction thresholds for infants and adults with normal hearing.

OBJECTIVE: To improve understanding of normal responses in infants by comparing air conduction (AC) and bone conduction (BC) auditory thresholds using both the auditory steady state response (ASSR) and behavioral testing methods in normal-hearing infants (6 to 18 months of age) and adults. At present, there are no correction factors available for estimating BC behavioral thresholds from BC ASSR thresholds, which is a barrier to clinical implementation of the ASSR. In addition, previous studies have reported infant-adult differences in AC and BC sensitivity, which suggest a "maturational" air-bone gap (ABG) that is not attributable to a conductive pathology; no study has yet compared AC and BC thresholds for either ASSR or behavioral methods in the same individuals. The objectives of the present study are: (1) to compare BC thresholds between methods and provide the initial step toward positing correction factors to predict BC behavioral thresholds, (2) to directly compare AC and BC thresholds to provide an accurate estimate of the maturational ABG, (3) to determine preliminary normal levels for BC and AC ASSRs to exponentially amplitude modulated stimuli, and (4) to investigate infant-adult differences in AC and BC thresholds using ASSRs and behavioral assessment tools. DESIGN: Participants were 23 infants (6.5 to 19.0 months of age) and 12 adults (17 to 50 years of age) with normal hearing. Thresholds were estimated at 500, 1000, 2000, and 4000 Hz using air- and bone-conducted stimuli for ASSRs and behavioral testing. The ASSR stimuli were exponential envelope modulated (amplitude modulation [AM]) at modulation frequencies of 78, 85, 93, and 101 Hz for 500, 1000, 2000, and 4000 Hz, respectively, presented simultaneously. Frequency-modulated (warble tone) stimuli were used for behavioral testing for both infants and adults, respectively. All stimuli were calibrated in dB HL. Thresholds were compared across frequency and between stimulus presentation modes, between age groups and assessment method. Normal levels for AC and BC ASSRs to AM stimuli were also calculated. RESULTS: The findings indicated that BC thresholds were, on average, 7 to 16 dB poorer for ASSR compared with visual reinforcement audiometry (VRA), but varied widely across infants. For infants, mean ABGs of 14 to 17 dB were found for low-frequency ASSR thresholds but mean ABGs for VRA thresholds were less than 10 dB. The preliminary normal levels for ASSR AM stimuli at 500, 1000, 2000, and 4000 Hz, respectively, were: (i) AC: 30, 30, 20, and 20 dB HL, and (ii) BC: 20, 20, 30, and 30 dB HL. There was a tendency for infant and adult ASSR thresholds to differ for BC, but not for AC. Behavioral thresholds for AC and BC were similar between infants and adults and across frequency. CONCLUSIONS: Infant-adult and AC-BC threshold differences are greater for ASSRs compared with behavioral measures. The results support the presence of a clinically significant maturational ABG in the low frequencies for infant ASSRs but not for VRA. The findings also show a significant offset between BC ASSR and BC VRA thresholds and large intersubject variability.

Effective masking levels for 500 and 2000 Hz bone conduction auditory steady state responses in infants and adults with normal hearing.

OBJECTIVES: Few studies have investigated effective masking levels (EMLs) needed to isolate the test ear for bone conduction assessments in infants. The objective of this study was to determine EMLs for 500 and 2000 Hz bone conduction auditory steady state responses (ASSRs) to amplitude (AM)/frequency-modulated (FM) stimuli for infants and adults with normal hearing. Maturational factors that contribute to infant-adult differences in EMLs will also be investigated. The present study and previously published 1000 and 4000 Hz EML data will be compared to investigate EML across four frequencies. These findings will provide a starting point for implementing clinical masking for infant bone conduction testing using physiological measures. DESIGN: Participants were 15 infants (7 to 35 weeks) and 15 adults (21 to 56 years) with normal hearing. Bone-conducted single ASSR stimuli (research MASTER) were 100% AM and 25% FM at 85 and 101 Hz for 500 and 2000 Hz carrier frequencies, respectively. They were presented at 25 and 35 dB HL for 500 Hz and at 35 and 45 dB HL for 2000 Hz for both infants and adults (approximately 10 and 20 dB SL at each frequency for infants). Air-conducted narrowband maskers were presented to both ears simultaneously. Real-ear to coupler differences were measured to account for differences in the sound pressure developed in infant and adult ear canals as a result of ear-canal size. Data analyses were conducted for mean EMLs across frequency (500 to 4000 Hz) and between age groups. Masked and unmasked ASSR amplitudes were compared for 500 and 2000 Hz. RESULTS: Both infants and adults required much more masking (25 to 33 dB) to eliminate responses at 500 compared with 2000 Hz. On average, infants required 16 dB more masking at 500 Hz and similar amounts of masking at 2000 Hz compared with adults. When adjusted for ear-canal size and bone conduction sensitivity, the pattern of results did not change. Across all four frequencies, infants showed a systematic decrease in mean EMLs with an increase in frequency; all pair-wise comparisons were significant except 2000 versus 4000 Hz. Adults showed smaller frequency-dependent changes in EML (only significantly greater for 500 versus 2000 Hz and 4000 Hz). When ear-canal size and bone conduction sensitivity were taken into account, only 500 Hz required more masking than other frequencies in infants; there were no significant frequency-dependent trends for adults, although the greater EMLs at 1000 versus 2000 Hz and 4000 Hz approached significance. Unmasked and masked amplitudes tended to be larger for 2000 Hz but not for 500 Hz when comparing infants with adults. CONCLUSIONS: EMLs appropriate for infants for bone conduction ASSRs elicited to AM/FM stimuli are considerably higher at 500 compared with 2000 Hz. Infants also need more masking at 500 Hz compared with adults but the same amount of masking at 2000 Hz. Comparisons across four frequencies reveal a systematic decrease in EML with an increase in frequency in infants, which is not apparent in adults. Recommended EMLs for AM/FM bone-conducted ASSR stimuli presented at 35 dB HL for 500, 1000, 2000, and 4000 Hz, respectively, are: (1) infants: 81, 68, 59, and 45 dB SPL, and (2) adults: 66, 63, 59, and 55 dB SPL.

Cochlear implantation for a child with cochlear nerve deficiency: parental perspectives explored through narrative.

OBJECTIVE: The objective of this study was to explore, from the parents' perspectives, decision-making regarding a cochlear implant (CI) for their child when a favourable outcome is less likely because of abnormal neurophysiology. DESIGN: The primary research method of this single case study was qualitative interviewing drawing on a narrative approach to elicit the parents' perspectives about their experiences over time. Each parent was interviewed separately, but thematic analyses were undertaken both within and across interviews in order to identify overlaps and differences. STUDY SAMPLE: Participants included the parents of a five-year old child with severe-profound hearing loss, cochlear nerve deficiency, and bilateral common cavities who had received a CI at the age of 18 months. RESULTS: Four themes were identified across the four narrative stages that emerged from the parents' accounts of their experiences regarding their daughter's CI. Themes included hope and despair, questioning professionals' motivations, does deafness need a cure, and bringing the child into the family. Although these themes emerged from both parents' accounts, each parent expressed different perspectives and insights within them. CONCLUSIONS: Findings highlight the central place of parental needs and perspectives in decision-making regarding a CI, particularly in the context of uncertain outcomes.

Does the ACC have potential as an index of early speech discrimination ability? A preliminary study in 4-month-old infants with normal hearing.

OBJECTIVE: The acoustic change complex (ACC), an auditory evoked potential (AEP) comprises overlapping slow cortical responses, which reflects discrimination capacity in the absence of attention, has not yet been recorded in infants. Because the ACC is a large response, it may be useful as an index of discrimination for infants at both the individual and group level. This is an advantage compared with mismatch negativity, another AEP that reflects discrimination of a change in stimulus, because mismatch negativity is based on difference waves and is most sensitive to group effects. The two objectives of this study were to determine whether: (1) the ACC can be elicited to a change in the English consonants /da/ and /ba/ in young infants and adults whose native language is English, and (2) the ACC can also be elicited to changes in Hindi consonant contrasts reflecting the predicted patterns of discrimination for young infants reported in previous studies. DESIGN: Participants were six adults and twenty-five 4-month-old infants whose native language was English, and were at low risk for hearing loss. Stimuli were concatenated consonant pairs comprised from a dental /da/, plus either /ba/, Hindi retroflex /Da/, a second /da/ or a silent period (i.e., /dada/, /daba/, /daDa/ and /da_/). It was predicted that adults would show the largest ACC to /daba/, similar responses to /dada/ and /daDa/, and no ACC to /da_/, whereas, it was predicted that infants would show a similar ACC to /daba/ and /daDa/, a smaller ACC to /dada/ and no ACC to /da_/. The stimuli were a total of 564 msec in duration and were presented at 86 dB peak SPL with an interstimulus interval of 2200 msec. At least 100 accepted trials for each participant were required in the final waveform to be included in the study. Individual peak amplitudes and latencies were measured for the P1, N1, P2, and N2 components of the response to the initial /da/ and the ACC. Grand mean waveforms were averaged for each stimulus condition. RESULTS: ACCs were elicited in adults to /dada/, /daba/, and /daDa/ with a trend toward a larger grand mean ACC for /daba/ compared with the other stimulus conditions. For infants, cortical responses to /da_/ resembled the adult P1-N1-P2 complex in morphology but had much longer latencies; /daba/ was the only stimulus that consistently elicited ACCs in infants. The ACC to /daba/ had a more distinct and less variable morphology compared with both /dada/ and /daDa/, which might reflect that the infants detected a greater change from /da/ to /ba/ than from /da/ to either /da/ or /Da/. It may also be the case that the ACC could not be detected for these other stimuli because the stimulus duration and interstimulus intervals used in this study were not optimal for eliciting ACCs for a range of stimuli. The pattern of speech discrimination, as reflected by the ACC, only loosely parallels the behavioral discrimination patterns reported in previous studies. CONCLUSIONS: These preliminary findings show that it is possible to record an ACC in young infants and provide a starting point for further investigation of the infant ACC and its utility as an index of discrimination.

Effective masking levels for bone conduction auditory steady state responses in infants and adults with normal hearing.

OBJECTIVE: To obtain ear-specific bone conduction thresholds, masking of the nontest ear is often required. Bone conduction masking has not been formally investigated for infants assessed physiologically. The objective of this study was to determine effective masking levels (EMLs) for auditory steady state responses (ASSRs) elicited by bone-conducted stimuli in a group of normal-hearing infants and adults. DESIGN: Participants were 15 infants younger than 6 mo and 15 adults, all with normal hearing. EML was defined as the lowest level of a binaural air-conducted masker that resulted in absent bone conduction ASSRs. Stimuli were single bone-conducted tones that were 100% amplitude modulated and 25% frequency modulated at 85 and 101 for 1000 and 4000 Hz, respectively. The stimuli were calibrated in dB HL (ANSI S3.6-1996) and expressed in dB HL or dB SL (dB relative to mean bone conduction ASSR thresholds reported in a previous study). The maskers were 1 and 4 kHz narrowband noise generated by a clinical audiometer. Unmasked and masked ASSRs were obtained for each participant. Real ear-to-coupler differences (RECDs) were also obtained for each participant and were used to convert masker dB SPL measured in the coupler to dB SPL in the individual ear canal. RESULTS: Infant EMLs for ASSRs elicited to bone-conducted stimuli in dB HL were 6 to 7 dB higher and 8 to 10 dB lower for 1000 and 4000 Hz, respectively, compared with adults. When masker was adjusted for RECDs, infant EMLs were 12 dB higher at 1000 Hz and similar at 4000 Hz compared with adults. When the stimulus levels were corrected for the mean differences in ASSR bone conduction thresholds between infants and adults and the masker levels adjusted for RECDs, infants had lower EMLs at 1000 Hz and equal EMLs at 4000 Hz, in comparison to adults. Frequency- and level-dependent effects on ASSR amplitude due to masking were found and differed between infants and adults. CONCLUSIONS: Our findings indicate that there are frequency- and level-dependent infant-adult differences in EMLs for bone conduction ASSRs and confirm that a 1000 Hz stimulus is 12 dB more effective for infants compared with adults. The following infant preliminary masking levels for bone conduction stimuli are recommended: (i) 1000 Hz: 48 and 58 dB SPL at 15 and 25 dB HL, respectively, and (ii) 4000 Hz: 40 and 45 dB SPL at 25 and 35 dB HL, respectively.

Effective masking levels for bone-conducted amplitude- and frequency-modulated tones in adults with normal hearing: A behavioural study.

OBJECTIVES: (1) to estimate the amount of masking needed to eliminate perceptual responses to 1000- and 4000-Hz bone-conducted mixed amplitude- (AM) and frequency-modulated (FM) tonal stimuli in adults with normal hearing, and (2) to compare these findings to recently reported effective masking levels (EMLs) for bone-conducted 80-Hz ASSRs. DESIGN: Stimuli were bone-conducted single sinusoidal tones with carrier frequencies of 1000 and 4000 Hz (Mixed modulation (MM): 100% AM & 25% FM at 85-101 Hz) presented to the temporal bone at 15-45 dB HL for 1000 Hz and 25-35 dB HL for 4000 Hz. Air-conducted 1- and 4-Hz narrow-band noise maskers were presented to both ears simultaneously using ER-3A insert earphones. EMLs for each of the stimuli were determined behaviourally. STUDY SAMPLE: Seventeen adults (mean age: 27.6 years) with normal hearing participated. RESULTS: Overall, EMLs were 10-17 dB higher for perceptual responses compared to ASSRs for 1000 and 4000 Hz. Linear regression analyses revealed that behavioural and ASSR EMLs were not significantly correlated for most of the stimuli presented except for 1000-Hz presented at 45 dB HL (r =.64, p = .013). CONCLUSIONS: EMLs are frequency- and testing method-dependent for bone-conducted MM tonal stimuli for normal-hearing adults.

Maturation of the occlusion effect: a bone conduction auditory steady state response study in infants and adults with normal hearing.

OBJECTIVE: The aim of this study was to investigate the maturational time course of the occlusion effect in infants with normal hearing. The objectives were (i) to investigate the occlusion effect in a larger group of young infants, (ii) to determine whether the occlusion effect is seen in bone conduction auditory steady state responses (ASSRs) for older infants, and (iii) to investigate the mechanisms that underlie bone conduction hearing in unoccluded and occluded ears in infants by measuring sound pressure in the ear canal. DESIGN: Experiments 1A and 1B: The SPL in the ear canal to 500, 1000, and 2000 Hz bone-conducted pure tones were compared in 22 young infants (0-7 mo), 10 older infants (10-22 mo), and 34 adults, all with normal hearing, for unoccluded and occluded ears. Experiment 2: Bone conduction behavioral thresholds in 17 adults were compared for unoccluded and occluded ears at 500, 1000, 2000, and 4000 Hz. Experiment 3: Bone conduction ASSR thresholds and amplitudes were compared in 22 young infants, 10 older infants, and 20 adults for an unoccluded and occluded test ear. Stimuli were bone-conducted amplitude/frequency-modulated tones presented simultaneously at 500, 1000, 2000, and 4000 Hz. RESULTS: There were significant increases in sound pressure in the ear canal for stimuli presented at 40 dB HL when ears were occluded at 500 and 1000 Hz for all age groups. Infants showed the largest increases in SPL at 500 and 1000 Hz (5-8 dB > adults). Young infants showed no significant decreases in ASSR thresholds (2-6 dB) and amplitudes (0-10 nV) across frequency with occlusion; however, a significant number of infants had an occlusion effect at 500 Hz. Older infants showed a nonsignificant decrease in ASSR thresholds with occlusion (8 dB), a significant increase in ASSR amplitudes at 1000 Hz (6-21 nV), and a significant number of infants with an occlusion effect at 1000 Hz. Adult behavioral thresholds decreased significantly when ears were occluded at 500 and 1000 Hz; for ASSRs, thresholds also decreased (6-7 dB) and amplitudes increased (3-11 nV) at both 500 and 1000 Hz, but the mean trends and statistical findings were not in agreement in all cases. A significant number of adult subjects had an occlusion effect at 500 and 1000 Hz for both behavioral and ASSR thresholds. CONCLUSIONS: Our findings suggest that the occlusion effect for ASSR thresholds in young infants is small but emerging at 500 Hz but negligible at 1000 Hz and that the occlusion effect in older infants is emerging at both 500 and 1000 Hz. The clinical implications of these findings are that it is appropriate to conduct bone conduction testing on young infants without compensating for an occlusion effect; however, for older infants, it is prudent to remove insert earphones during bone conduction testing. For both young and older infants, occluding the ear canal increases the sound pressure near the tympanic membrane; however, this pathway appears to contribute less to bone conduction hearing when ears are occluded compared with adults as measured by ASSRs.

Behavioral bone-conduction thresholds for infants with normal hearing.

BACKGROUND: Bone-conduction thresholds have been used in audiologic assessments of both infants and adults to differentiate between conductive and sensorineural hearing losses. However, air- and bone-conduction thresholds estimated for infants with normal hearing using physiological measures have identified an "air-bone gap" in the low frequencies that does not result from conductive hearing impairment but, rather, from maturational differences in sensitivity. This maturational air-bone gap appears to be present up to at least 2 yr of age. Because most infants older than 6 mo of age are clinically assessed behaviorally, rather than physiologically, it is necessary to determine whether a similar maturational air-bone gap is present for behavioral air- and bone-conduction thresholds. PURPOSE: The purpose of this study was to estimate behavioral bone-conduction thresholds for infants using a standard clinical visual reinforcement audiometry (VRA) protocol to determine whether frequency-dependent maturational patterns exist as previously reported for physiological bone-conduction thresholds. RESEARCH DESIGN: Behavioral bone-conduction minimum response levels were estimated at 500, 1000, 2000, and 4000 Hz using VRA for each participant. STUDY SAMPLE: Young (7-15 mo; N = 17) and older (18-30 mo; N = 20) groups of infants were assessed. All infants were screened and considered to be at low risk for hearing loss. DATA COLLECTION AND ANALYSIS: Preliminary "normal levels" were determined by calculating the 90th percentile for responses present as a cumulative percentage. Mean bone-conduction thresholds were compared and analyzed using a mixed-model analysis of variance across frequency and age group. Linear regression analysis was also performed to assess the effect of age on bone-conduction thresholds. RESULTS: Results of this study indicate that, when measured behaviorally, infants under 30 mo of age show frequency-dependent bone-conduction thresholds whereby their responses at 500 and 1000 Hz are significantly better than those at 2000 and 4000 Hz. However, thresholds obtained from the younger group of infants (mean age of 10.6 mo) were not significantly different from those obtained from the older group of infants (mean age of 23.0 mo) at any frequency. CONCLUSIONS: The findings of the present study are similar to the results obtained from previous physiological studies. Compared to previously documented air-conduction thresholds of infants using similar VRA techniques, a maturational air-bone gap is observed in the low frequencies. Therefore, differences between infant and adult bone-conduction thresholds persist until at least 30 mo of age. As a result, different "normal levels" should be used when assessing bone-conduction hearing sensitivity of infants using behavioral methods.

Normal ipsilateral/contralateral asymmetries in infant multiple auditory steady-state responses to air- and bone-conduction stimuli.

OBJECTIVES: Two-channel recordings of infants' air- and bone-conduction auditory brainstem responses to brief tones show ipsilateral and contralateral (to the stimulated ear) asymmetries which may be used to isolate which cochlea is the primary contributor to the response. The objective of this study was to determine whether similar ipsilateral/contralateral asymmetries are also present in the air- and bone-conduction "brainstem" (77 to 101 Hz) auditory steady-state responses (ASSRs) of infants. DESIGN: Two-channel ASSRs were recorded in infants (2 to 11 mo) and adults (18 to 40 yr) with normal hearing. Multiple stimuli (carrier frequencies: 500 to 4000 Hz; amplitude/frequency modulated) were presented using a B-71 oscillator on the temporal bone or an ER3-A insert earphone. Bone-conduction ASSR amplitudes, phase delays, and thresholds were obtained for the electroencephalographic (EEG) channels ipsilateral and contralateral to the oscillator temporal-bone placement. Bone-conduction ASSRs were also obtained to the stimulus presented to the opposite temporal bone (at 40 dB HL only). Air-conduction ASSR amplitudes and phase delays were obtained at 60 dB HL in each ear for the EEG channels ipsilateral and contralateral to the transducer. RESULTS: Infants showed more ipsilateral/contralateral asymmetries in both air- and bone-conduction ASSRs compared with adults. Mean bone-conduction ASSR thresholds in infants were 13 to 15 dB higher (i.e., poorer) in the contralateral EEG channel compared with the ipsilateral EEG channel for 500 to 4000 Hz. In adults, there were no large differences (i.e., within 1 dB) between ipsilateral and contralateral ASSR thresholds. Based on ipsilateral/ contralateral threshold differences in infants, interaural attenuation for bone-conducted stimuli was estimated to be at least 10 to 30 dB for most infants. In contrast, most adults showed little interaural attenuation for bone-conducted stimuli. ASSR amplitudes are larger and phase delays are shorter in the ipsilateral EEG channel. For infants, the difference in air-conduction ASSR amplitude between EEG channels was twice that observed for adults. Infants also had greater ASSR amplitude differences between EEG channels for bone-conduction stimuli compared with adults, but the difference was less than that seen for air-conduction stimuli. For air-conduction stimuli, infants had significantly longer phase delays in the contralateral EEG channel compared with the ipsilateral EEG channel. Adults showed no significant differences in air-conduction ASSR phase delay between EEG channels. For bone-conduction stimuli, both infants and adults had significantly longer phase delays in the contralateral EEG channel compared with the ipsilateral EEG channel; the differences in ASSR phase delays between EEG channels were much smaller in infants compared with adults and fewer adults had absent responses in the contralateral EEG channels compared with infants (12% versus 34%). When the transducers were switched to the opposite ear/mastoid, the infant and adult ipsilateral/contralateral asymmetries also switched. CONCLUSIONS: Ipsilateral/contralateral asymmetries in air- and bone-conduction ASSRs are clearly present more often and are larger in infants compared with adults. Our findings also suggest that most infants have at least 10 to 30 dB of interaural attenuation to bone-conducted stimuli. These asymmetries in the bone-conduction ASSR have potential as a clinical tool for isolating the cochlea that is contributing to the response in infants.

Maturation of bone conduction multiple auditory steady-state responses.

The objective of this study was to compare bone-conduction (BC) auditory steady-state responses (ASSR) for infants and adults with normal hearing to investigate the time course of maturation of BC hearing sensitivity. Bone-conduction multiple ASSRs were recorded in 0-11-month-old (n=35), and 12-24-month-old infants (n=13), and adults (n=18). Low-frequency BC ASSR thresholds increased with age, whereas, high-frequency ASSR thresholds were unaffected by age except for a slight improvement at 2000 Hz. Compared to adults, BC ASSR amplitudes for young infants were larger for low frequencies, whereas, their amplitudes were smaller or similar for high frequencies. Compared to adults, young infants are much more sensitive to low-frequency BC stimuli, and probably more sensitive to high-frequency BC stimuli; these differences between infants and adults persist until at least two years of age. Different 'normal levels' for infants of different ages must be used and are proposed in this study.

Effects of bone oscillator coupling method, placement location, and occlusion on bone-conduction auditory steady-state responses in infants.

OBJECTIVE: The aim of these experiments was to investigate procedures used when estimating bone-conduction thresholds in infants. The objectives were: (i) to investigate the variability in force applied using two common bone-oscillator coupling methods and to determine whether coupling method affects threshold estimation, (ii) to examine effects of bone-oscillator placement on bone-conduction ASSR thresholds, and (iii) to determine whether the occlusion effect is present in infants by comparing bone-conduction ASSR thresholds for unoccluded and occluded ears. DESIGN: Experiment 1A: The variability in the amount of force applied to the bone oscillator by trained assistants (n = 4) for elastic-band and hand-held coupling methods was measured. Experiment 1B: Bone-conduction behavioral thresholds in 10 adults were compared for two coupling methods. Experiment 1C: ASSR thresholds and amplitudes to multiple bone-conduction stimuli were compared in 10 infants (mean age: 17 wk) using two coupling methods. Experiment 2: Bone-conduction ASSR thresholds and amplitudes were compared for temporal, mastoid and forehead oscillator placements in 15 preterm infants (mean age: 35 wk postconceptual age (PCA)). Experiment 3: Bone-conduction ASSR thresholds, amplitudes and phase delays were compared in 13 infants (mean age: 15 wk) for an unoccluded and occluded test ear. All infants that participated had passed a hearing screening test. RESULTS: Experiment 1A: Coupling method did not significantly affect the variability in force applied to the oscillator. Experiment 1B: There were no differences in adult bone-conduction behavioural thresholds between coupling methods. Experiment 1C: There was no significant difference between oscillator coupling method or significant frequency x coupling method interaction for ASSR thresholds or amplitudes in the young infants tested. However, there was a nonsignificant 9-dB better threshold at 4000 Hz for the elastic-band method. Experiment 2: Mean bone-conduction ASSR thresholds for the preterm infants were not significantly different for the temporal and mastoid placements. Mean ASSR thresholds for the forehead placement were significantly higher compared to the other two placements (12-18 dB higher on average). Mean ASSR amplitudes were significantly larger for the temporal and mastoid placements compared to the forehead placement. Experiment 3: There was no difference in mean ASSR thresholds, amplitudes or phase delays for the unoccluded versus occluded conditions. CONCLUSIONS: Trained assistants can apply an appropriate amount of force to the bone oscillator using either the elastic-band or hand-held method. Coupling method has no significant effect on estimation of bone-conduction thresholds; therefore, either may be used clinically provided assistants are appropriately trained. For preterm infants, there are no differences in ASSRs when the oscillator is positioned at the temporal or mastoid placement. However, thresholds are higher and amplitudes are smaller for the forehead placement, consequently, a forehead placement should be avoided for clinical testing. There does not appear to be a significant occlusion effect in young infants; therefore, it may be possible to do bone-conduction testing with ears unoccluded or occluded without applying a correction factor, although further research is needed to confirm this finding.