Susan A. Small

Artifactual responses when recording auditory steady-state responses.

Objective: The goal of this study was to investigate, in hearing-impaired participants who could not hear the stimuli, the possibility of artifactual auditory steady-state responses (ASSRs) when stimuli are presented at high intensities. Design: ASSRs to single (60 dB HL) and multiple (20 to 50 dB HL; 500 to 4000 Hz) bone-conduction stimuli as well as single 114 to 120 dB HL air-conduction stimuli, were obtained using the Rotman MASTER system, using analog-to-digital (A/D) conversion rates of 500, 1000, and 1250 Hz. Responses (p < 0.05) were considered artifactual when their numbers exceeded that expected by chance. In some conditions, we also obtained ASSRs to “alternated” stimuli (stimuli inverted and ASSRs to the two polarities averaged). A total of 17 subjects were tested. Results: Bone conduction results: 500 Hz A/D rate: Large-amplitude (43 to 1558 nV) artifactual ASSRs were seen at 40 and 50 dB HL for the 500 Hz carrier frequency. Smaller responses (28 to 53 nV) were also recorded at 20 dB HL for the 500 Hz carrier frequency. Artifactual ASSRs (17 to 62 nV) were seen at 40 dB HL and above for the 1000 Hz carrier frequency and at 50 dB HL for the 2000 Hz carrier frequency. Alternating the stimulus polarity decreased the amplitude and occurrence of these artifactual responses but did not eliminate responses for the 500 Hz carrier frequency at 40 dB HL and above. No artifactual responses were recorded for 4000 Hz stimuli for any condition. 1000 Hz A/D rate: Artifactual ASSRs (15 to 523 nV) were seen at 50 dB HL and above for the 500 Hz carrier frequency and 40 dB HL and above for the 1000 Hz carrier frequency. Artifactual responses were also obtained at 50 dB HL for a 2000 Hz carrier frequency but not at lower levels. Artifactual responses were not seen for the 4000 Hz carrier frequency. Alternating the stimulus polarity removed the responses for the 1000 and 2000 Hz carrier frequencies but did not change the results for the 500 Hz carrier frequency. 1250 Hz A/D rate: Artifactual ASSRs (16 to 220 nV) were seen at 50 dB HL and above for the 500 Hz carrier frequency and 60 dB HL and above for the 1000 Hz carrier frequency. Alternating the stimulus polarity removed the responses for the 1000 Hz carrier frequency but did not change the results for the 500 Hz carrier frequency. There were no artifactual responses at 2000 and 4000 Hz. Air conduction results: 500 Hz A/D rate: Artifactual ASSRs (49 to 153 nV) were seen for 114 to 120 dB HL stimuli for 500 and 1000 Hz carrier frequencies. Alternating the stimulus polarity removed these responses. There were no artifactual responses at 2000 and 4000 Hz. 1000 and 1250 Hz A/D rates: Artifactual ASSRs (19 to 55 nV) were seen for a 120 dB HL stimulus for a 1000 Hz carrier. Alternating the stimulus polarity removed these responses. Conclusions: High-intensity air- or bone-conduction stimuli can produce spurious ASSRs, especially for 500 and 1000 Hz carrier frequencies. High-amplitude stimulus artifact can result in energy that is aliased to exactly the modulation frequency. Choice of signal conditioning (electroencephalogram filter slope and low-pass cutoff) and processing (A/D rate) can avoid spurious responses due to aliasing. However, artifactual responses due to other causes may still occur for bone-conduction stimuli 50 dB HL and higher (and possibly for high-level air conduction). Because the phases of these spurious responses do not invert with inversion of stimulus, the possibility of nonauditory physiologic responses cannot be ruled out. The clinical implications of these results are that artifactual responses may occur for any patient for bone-conduction stimuli at levels greater than 40 dB HL and for high-intensity air-conduction stimuli used to assess patients with profound hearing loss.

Multiple auditory steady-state response thresholds to bone-conduction stimuli in young infants with normal hearing.

Objective: Multiple auditory steady-state responses (ASSRs) probably will be incorporated into the diagnostic test battery for estimating hearing thresholds in young infants in the near future. Limiting this, however, is the fact that there are no published bone-conduction ASSR threshold data for infants with normal or impaired hearing. The objective of this study was to investigate bone-conduction ASSR thresholds in infants from a Neonatal Intensive Care Unit (NICU) and in young infants with normal hearing and to compare these with adult ASSR thresholds. Design: ASSR thresholds to multiple bone-conduction stimuli (carrier frequencies: 500 to 4000 Hz; 77 to 101-Hz modulation rates; amplitude/frequency modulated; single-polarity stimulus) were obtained in two infant groups [N = 29 preterm (32 to 43 wk PCA), tested in NICU; N = 14 postterm (0 to 8 mo), tested in sound booth]. All infants had passed a hearing screening test. ASSR thresholds, amplitudes, and phase delays for preterm and postterm infants were compared with previously collected adult data. Results: Mean (±1 SD) ASSR thresholds were 16 (11), 16 (10), 37 (10), and 33 (13) dB HL for the preterm infants and 14 (13), 2 (7), 26 (6), and 22 (8) dB HL for the postterm infants at 500, 1000, 2000, and 4000 Hz, respectively. Both infant groups had significantly better thresholds for 500 and 1000 Hz compared with 2000 and 4000 Hz, in contrast to adults who have similar thresholds across frequency (22, 26, 18, and 18 dB HL). When 500- and 1000-Hz thresholds were pooled, pre- and postterm infants had better low-frequency thresholds than adults. When 2000- and 4000-Hz thresholds were pooled, pre- and postterm infants had poorer thresholds than adults. ASSR amplitudes were significantly larger for low frequencies compared with high frequencies for both infant groups, in contrast to adults, who show little difference across frequency. ASSR phase delays were later for lower frequencies compared with higher frequencies for infants and adults, except for 500 Hz in the preterm group. ASSR phase delays were later for infants compared with adults across frequency. Conclusions: Infant bone-conduction ASSR thresholds are very different from those of adults. Overall, these results indicate that low-frequency bone-conduction thresholds worsen and high-frequency bone-conduction thresholds improve with maturation. Bone-conduction ASSR threshold differences between the postterm infants and adults probably are due to skull maturation. Differences between preterm and older infants may be explained both by skull changes and a masking effect of high ambient noise levels in the NICU (and possibly to other issues due to prematurity).

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 × 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.

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.

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 2000Hz. 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.

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 [AM2]) 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 AM2 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 AM2 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.

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.

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.

Abstract 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 daughters 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.

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.