Effects of Presentation Level on Spatial Hearing with and without Bone-Conduction Amplification in Congenital Unilateral Aural Atresia

Abstract

Objective:

This study assessed the effect of ipsilateral bone-conduction amplification on spatial hearing abilities in subjects with congenital unilateral aural atresia (CUAA).

Patients:

Twelve patients with unilateral conductive hearing loss secondary to CUAA and normal hearing in the contralateral ear were tested. Most (75%) had limited experience with a bone-conduction hearing aid (BCHA).

Intervention:

Performance was evaluated with and without a BCHA fitted acutely on a softband.

Main Outcome Measures:

Spatial hearing abilities were evaluated in two paradigms. Spatial release from masking (SRM) was evaluated by comparing masked sentence recognition with a target and two speech maskers either co-located at 0° or with the maskers separated at +90° and −90°. Sound source localization was evaluated in a 180° arc of loudspeakers on the horizontal plane. Performance was evaluated at 50 and 75 dB SPL, and results were compared for patients tested with and without a BCHA.

Results:

Group level results indicate similar SRM in the aided and unaided conditions at both presentation levels. Localization at 50 dB SPL was similar aided and unaided, but at 75 dB SPL the root mean square error was lower unaided than aided (17.2° vs 41.3°; p=0.010).

Conclusions:

Use of a BCHA in patients with CUAA may interfere with auditory cues required for sound source localization when the signal level is intense enough to overcome the patient’s conductive hearing loss. These findings have potential clinical implications in fitting of BCHAs to support optimal spatial hearing in patients with CUAA.

INTRODUCTION

Patients with unilateral conductive hearing loss can experience poorer speech discrimination in noise, sound source localization, and academic performance when compared to their normal-hearing (NH) peers(1–4). For children with congenital unilateral aural atresia (CUAA), there may be long-term consequences associated with early auditory deprivation, such as a disruption in cortical organization and the ability to use binaural cues(5–7). These cues provide spatial awareness necessary for many everyday tasks, including sound source localization and speech recognition in noise. However, some patients with CUAA have unaided localization abilities approaching those of NH listeners, which could reflect use of monaural cues in the unaffected ear(8) or limited access to binaural cues, when sounds are intense enough to overcome the conductive loss(9). Management and timing of intervention with bone-conduction hearing aids (BCHAs) remains controversial in this population, with limited and conflicting literature on spatial hearing outcomes with amplification(10).

The aim of this study was to assess the effect of a BCHA on spatial hearing in subjects with CUAA. Spatial hearing was assessed via localization on the horizontal plane and spatial release from masking (SRM), which is the speech recognition benefit gained from spatially separating maskers from the target speech. Subjects were tested using the same hardware and protocols as Thompson et al.(9), which compared spatial hearing abilities between NH listeners and subjects with CUAA who were tested unaided at 50 dB SPL (generally inaudible in the atretic ear) and 75 dB SPL (providing some audibility in the atretic ear). Subjects with CUAA performed more poorly overall, but the effect of CUAA was smaller at 75 dB SPL than 50 dB SPL. This result suggests that patients with CUAA may use binaural difference cues when sounds are intense enough to overcome the conductive loss. If this is the case, then wearing a BCHA could mask binaural cues and degrade performance under these conditions. This study tested the hypothesis that spatial hearing abilities at high presentation levels for subjects with CUAA can be degraded by use of a BCHA compared to unaided results reported by Thompson et al.(9).

METHODS

Subjects

Subjects were children and young adults with CUAA. Unaided pure-tone detection thresholds were measured at octave intervals from 250 to 8000 Hz. Inclusion criteria were: 1) unaided air-conduction thresholds ≤20 dB HL in the unaffected ear, ≥40 dB HL in the affected ear, and masked bone-conduction ≤20 dB HL bilaterally, and 2) negative history of ear canal reconstruction. Procedures were approved by the Institutional Review Board (#86–0059), and informed consent and assent was obtained prior to enrollment.

Table 1 shows demographic information for all subjects with CUAA, including those tested aided (n=10) and unaided (n=6). Four subjects provided both aided and unaided data, with a mean of 18 months between test sessions; given this long delay, data from these time points were treated as independent for the purpose of analysis. Subject ages ranged from 7.5 to 22 years (mean=12.6 yrs, SD=5.1 yrs). Only 3 subjects consistently used a BCHA. T-tests indicated no significant differences between air-conduction thresholds for the unaided and aided cohorts (p≥0.08). Figure 1 plots mean thresholds for the unaffected (solid line) and affected (dashed line) ears.

Table 1:

Demographic information on subjects with congenital unilateral aural atresia. Subjects are ordered by age.

Subject	Sex	Age (years)	Side	BC-PTA (dB HL)	AC-PTA (dB HL)	Previous treatment	Device use‡
S1a	F	7.5	L	6.25	66.25	Softband BAHA	Sporadic
S2ab	F	7.6	R	8.75	53.75	-	Non-user
S3a	M	8.6	L	7.5	60	Softband BAHA	Non-user
S4a	F	9	L	12.5	70	Softband BAHA	Non-user
S5b	M	9.8	R	6.67	57.5	-	Non-user
S6a	M	9.8	L	8.75	67.5	-	Non-user
S7ab	M	10.2	R	8.75	58.75	-	Non-user
S8ab	M	13.9	L	11.25	67.5	-	Non-user
S9a	F	14.5	R	12.5	78.75	Percutaneous BAHA	User
S10b	F	17.2	L	8.33	71.25	Percutaneous BAHA	Sporadic
S11ab	F	21.2	L	11.25	70	Percutaneous BAHA	User
S12a	M	22	L	13.75	70	Percutaneous BAHA	User

^‡.Frequency of device use as classified by Nelissen et al. (2015)

^a.Tested in the unaided condition as reported by Thompson et al. (2020)

^b.Tested in the aided condition with a bone-conduction device on a softband

BC-PTA: Bone-Conduction Pure-Tone Average (500, 1000, 2000 and 4000 Hz)

AC-PTA: Air-Conduction Pure-Tone Average (500, 1000, 2000 and 4000 Hz)

BAHA: Bone-Anchored Hearing Aid

FIG 1.

Air-conduction thresholds for the unaffected (solid line) and affected, atretic ear (dashed line) for all subjects. Symbols and error bars signify the mean and standard deviation.

Assessment of Spatial Hearing

For aided testing, a BAHA BP100 processor (Cochlear Bone-Anchored Solutions, Mölnlycke, Sweden) on a softband was fit according to manufacturer specifications using in-situ bone-conduction thresholds. Directional features and noise reduction were disabled. The hardware and test protocols used in the present study have been previously described(9). Briefly, testing was completed in a double-walled sound booth, and subjects sat in the center of 11 evenly spaced loudspeakers, spanning +90° to −90° on the horizontal plane.

SRM was evaluated using Pediatric AzBio sentences(11); the target was an adult female, and the masker was two female talkers reading excepts from Jack and the Beanstalk(12). Target sentences were presented from 0°, and maskers were either co-located at 0° or spatially separated, with one voice presented from +90° and the other from −90°. The speech reception threshold (SRT) associated with 50% correct was determined for each condition based on responses to 40 sentences. SRM was quantified as the difference in SRTs between the co-located and spatially separated conditions.

Localization abilities were also assessed in the loudspeaker arc. The stimulus was a 200-ms burst of pink noise, bandpass filtered 126–6000 Hz (48-dB/octave). Each run consisted of 33 trials, with the stimulus presented to a randomly-selected loudspeaker. The subject was asked to identify the sound source. Localization was quantified as RMS error in degrees.

Both SRM and localization tasks were completed at nominal levels of 50 and 75 dB SPL. To minimize use of monaural level cues for localization, a random level rove of ±5 dB was applied. Aided performance was compared to previously published data for unaided CUAA subjects(9).

RESULTS

Spatial Release from Masking

Figure 2 (top panel) plots SRM for each group as a function of presentation level. A two-way repeated-measures ANOVA (repeated presentation levels) comparing SRM for subjects with CUAA in the unaided versus aided condition revealed a significant main effect of presentation level (F(1,14)=6.53, p=0.023) but not listening condition (F(1,14)=0.34, p=0.578), with no significant interaction (F(1,14)=0.15, p=0.713). As observed previously, the difference between children with CUAA and NH was smaller at 75 than 50 dB SPL. Wearing a BCHA did not significantly affect SRM.

FIG 2.

Spatial release from masking (SRM, top panel) and localization error quantified as RMS error in degrees (bottom panel) as a function of presentation level. Subjects with congenital unilateral aural atresia (CUAA) were tested unaided and aided with a BCHA; age-matched normal-hearing (NH, indicated with stars) listeners are included for reference. Unaided data from subjects with CUAA and NH were previously reported by Thompson et al. (2020). Symbol shape indicates the results for individual subjects using the identifiers from Table 1. Horizontal lines indicate the median, boxes span the 25^th to 75^th percentiles, and vertical lines span the 10^th to 90^th percentiles.

Localization

Figure 2 (bottom panel) plots RMS localization error for each group as a function of presentation level. A log transformation was applied to normalize error variance prior to analysis. A two-way repeated-measures ANOVA comparing localization for subjects with CUAA in the unaided versus aided conditions revealed a significant main effect of presentation level (F(1,14)=24.58, p<0.001), but not listening condition (F(1,14)=2.55, p=0.132), and a significant interaction (F(1,14)=10.69, p=0.006). T-tests with Bonferroni correction indicate significantly poorer localization with than without the BCHA at 75 dB SPL (p=0.010), but no difference at 50 dB SPL (p>0.999). Compared to subjects with NH, the median RMS error at 75 dB SPL for children with CUAA was elevated by a factor of 2.8 in the unaided condition and 6.7 in the aided condition.

DISCUSSION

With limited literature on outcomes following audiologic rehabilitation for CUAA, there is clearly a need to better understand factors affecting spatial hearing abilities with a BCHA. Previous research has shown that patients with CUAA may have access to some binaural difference cues in the unaided condition when the stimulus presentation level is high enough to provide audibility(9). The present study evaluated effects of presentation level on spatial hearing with a BCHA. Patients with CUAA in this dataset experienced similar SRM in the aided and unaided conditions; however, localization abilities were disrupted with a BCHA for some listening conditions and listeners.

It is difficult to compare present findings to prior studies due to differences in methodology. However, most have either shown a benefit or no effect of a BCHA for localization, with the latter generally explained by relatively good unaided performance – presumably due to use of monaural cues(8,13). In contrast, Kunst et al.(2) demonstrated highly variable localization outcomes with a BCHA, and similar to our study, some users experienced a decrement in performance. It is important to note that most studies tested subjects with greater listening experience with a BCHA than the present cohort; poor aided localization performance at 75 dB SPL might be overcome with continued device use. It is also possible that earlier intervention with a BCHA could affect aided performance. Another difference across studies is that subjects in this dataset were generally non-users, while prior work noted non-use as a reason for exclusion(8). Though some reports indicate high satisfaction with a BCHA(14), others have shown that a large proportion of patients reject a BCHA(15). Our findings could offer one reason for poor device satisfaction. Anecdotally, the subject who did not experience a decline in aided localization at 75 dB SPL (S11) reported consistent use and high satisfaction with the BCHA. Further studies are needed to better understand the relationship between device use, age at intervention, and spatial hearing abilities.

Discrepant findings between SRM and localization in the present dataset should be interpreted in light of the cues necessary to perform each task. SRM is thought to reflect the ability to use interaural difference cues to perceptually segregate the target from masker. Poorer aided performance at 75 dB SPL on localization but not SRM could indicate that binaural cues are sufficient for segregation, but the combination of direct and amplified sound does not correspond to the listener’s internal map of auditory space.

These results have potential clinical implications for fitting BCHAs in this population. If patients with CUAA are able to access binaural cues when sounds are intense enough to overcome the conductive loss, attempts could be made to avoid disrupting these cues when providing amplification with a BCHA. It is possible that reducing gain for louder stimuli (e.g., greater amplitude compression) could allow access to binaural cues at high stimulus levels while still providing ear-specific audibility. Future work is needed to optimize fitting parameters of BCHAs for patients with CUAA.