Evaluating binaural hearing capabilities in individuals with sensorineural hearing loss through bilateral bone conduction stimulation

doi:10.21203/rs.3.rs-3748064/v1

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Evaluating binaural hearing capabilities in individuals with sensorineural hearing loss through bilateral bone conduction stimulation

https://doi.org/10.21203/rs.3.rs-3748064/v1

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published 21 Nov, 2024

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This study investigated the impact of bilateral bone conduction (BC) stimulation and sensorineural hearing loss on spatial release from masking, binaural intelligibility level difference, and lateralization. The study involved two groups of adults with mild to moderate sensorineural hearing loss: one group of 21 participants with symmetric hearing loss and another group of nine participants with asymmetric hearing loss. All tests were conducted through BC and air conduction (AC) headsets with non-individualized virtual positions of the sound sources and linear amplification based on individual hearing thresholds. The findings revealed a bilateral benefit for both groups of hearing-impaired individuals, with symmetric hearing loss yielding better results than asymmetric hearing loss. AC stimulation provided approximately twice the benefit in terms of dB compared to BC stimulation. A large part of this benefit originated from a favorable signal-to-noise ratio due to noise reduction from the head shadow. However, binaural processing was present in both hearing-impaired groups with bilateral BC stimulation. The ability to lateralize sounds based on interaural time delays was significantly impaired in participants with both types of hearing loss when stimulation was by BC. Despite these challenges, the study underscores the benefits of bilateral fitting of BC hearing aids, even in individuals with mild to moderate sensorineural hearing loss, whether symmetric or asymmetric.

Physical sciences/Engineering/Biomedical engineering

Biological sciences/Neuroscience/Auditory system

Binaural hearing, which involves the integration of signals from two ears, enhances our ability to understand speech in challenging environments and localize sound sources^1,2. This auditory process exploits the similarities and differences in sounds reaching each ear to gather information about the surrounding acoustic environment. The two strongest cues for binaural processing are the differences in time of arrival (interaural time delay, ITD) or phase difference for a stationary sound (interaural phase delay, IPD), and the level difference (interaural level difference, ILD)³. Utilizing these cues, the auditory system processes the sound to pinpoint the direction to the target sound source and enhance its detection⁴.

Individuals with sensorineural hearing loss, as determined with an audiogram (pure tone detection), generally exhibit poorer binaural ability compared to those with normal hearing^5,6. While factors such as age and cognitive abilities may also influence binaural processing, the integrity of the peripheral auditory system is crucial for optimal binaural function. Asymmetric sensorineural hearing loss has a more significant impact on binaural ability than symmetric hearing loss⁷.

When a sound is delivered in a sound field or via traditional earphones, the primary transmission pathway is air conduction (AC). This means that the sound travels through the ear canal and middle ear ossicles⁸. Another mode of hearing sound is through bone conduction (BC), where the sound reaches the inner ear as vibrations in the skull bone and soft tissues^9,10. Although BC sound has pathways through the ear canal and via the middle ear similar to AC sound transmission, the primary stimulation for BC sound applied at the mastoid in a healthy ear is through vibration of the skull bone^11–14.

Binaural processing that enhances hearing is present when sound is delivered bilaterally by BC^15,16. However, one argument against bilateral BC stimulation is that the sound from one stimulation position reaches both ears^17–19. This suggests that it would be sufficient to stimulate both cochleae from one position, and that binaural cues are impeded due to cross hearing^20,21. A recent study on bilateral BC stimulation found that the average low-frequency interaural time delay (at 500 Hz) was 0.2 ms and the level difference was close to 10 dB²². Even though this suggests that cross-hearing distorts binaural cues, these differences should still allow for the use of binaural information. This was confirmed in studies where subjects with normal hearing were stimulated bilaterally by BC, and speech perception in noise improved when speech and noise were spatially separated^15,16. However, these studies also showed that the improvement was less with BC stimulation compared to stimulation by earphones (AC).

Research has demonstrated that using BC hearing aids on both ears can improve speech comprehension and the ability to localize the source of a sound, compared to using a BC hearing aid on just one ear^23–26. However, these studies often involve a small and diverse group of patients with varying degrees of sensorineural hearing loss^25–27. As a result, the impact of sensorineural hearing loss on the benefits of binaural hearing when sound is delivered through BC in both ears is not well understood.

The aim of this study is to investigate the advantages of bilateral BC stimulation in terms of spatial release from masking (SRM) and binaural intelligibility level difference (BILD) among participants with symmetrical and asymmetrical sensorineural hearing loss. Additionally, the study will examine the participants’ ability to fuse sounds from both sides.

This study evaluated the threshold at which speech is understood 50% of the time in a noisy environment under three different conditions: when the speech and noise signals were both directly in front of the participant (S0N0), when the speech signal was in front while the noise was at a 45-degree angle to the side of the participant (S0N45), and when the speech signal was inverted between the two ears while the noise signal was equal at both ears (S180N0). The study also measured the precedence effect, which is the perceived location of a sound source when the time delay between the sound from the two sides varied from 0 to 50 ms.

The study included 21 participants with symmetric hearing loss and nine participants with asymmetric hearing loss. Data from subjects with normal hearing, obtained from a study by Zeitooni et al¹⁶, were used for comparison. All tests were conducted using both BC stimulation and AC stimulation through earphones.

Figure 1 shows the audiograms for all participants. The average AC PTA4 (0.5, 1, 2, 3, 4 kHz) for the right ear in the symmetric group was 35.5 dB HL and for the left ear was 36.1 dBHL (Fig. 1A). With BC stimulation, the average right ear PTA4 for this group was 34.3 dBHL and the left ear was 34.2 dBHL (Fig. 1C). In the asymmetric group, the better ear had an average AC PTA4 of 28.2 dBHL and the worse ear a PTA4 of 45.6 dBHL (Fig. 1B). With BC stimulation, the average PTA4 was 29.2 dBHL in the better ear and 40.1 dBHL in the worse ear (Fig. 1D).

The results of the speech-in-noise test, with speech and noise co-located (S0N0) and with the speech at the front while the noise is located at 45 degrees (S0N45), are presented in Fig. 2. Figure 2A displays the results for the symmetric group, while Fig. 2B shows the results for the asymmetric group. Detailed results are provided in Table 1. The signal-to-noise ratio (SNR) thresholds are shown for both AC and BC stimulation, as well as for bilateral and monaural stimulation. In the monaural situation, stimulation is applied on the side opposite to the noise in the S0N45 condition.

Table 1

The table presents the averages and standard deviations (in parenthesis) of the tests conducted on two groups with hearing losses, for both AC and BC stimulation. Data for individuals with normal hearing are derived from Zeitooni et al¹⁶.
	Symmetric				Asymmetric				Normal
	AC		BC		AC		BC		AC	BC
Test	Bilateral	Monaural	Bilateral	Monaural	Bilateral	Monaural	Bilateral	Monaural	Bilateral	Bilateral
S0N0 [dB]	-5.1 (1.5)	-3.7 (1.9)	-4.3 (1.8)	-3.5 (2.2)	-4.6 (1.8)	-2,6 (2.0)	-3.5 (1.3)	-2.6 (1.4)	-8.0 (1.4)	-8.5 (1.2)
S0N45 [dB]	-10.3 (1.9)	-6.2 (2.5)	-6.8 (2.5)	-5.5 (2.0)	-7.9 (1.4)	-4.8 (2.0)	-5.8 (1.4)	-5.8 (1.4)	-16.4 (1.8)	-12.5 (1.3)
S180N0 [dB]	-10.2 (2.4)		-9.1 (2.9)		-7.8 (2.5)		-6.1 (2.2)		-14.7 (2.3)	-13.3 (2.6)
SRM [dB]	5.2 (1.3)	2.4 (1.5)	2.5 (1.9)	2.0 (1.2)	3.3 (2.2)	2.2 (2.4)	2.2 (1.0)	3.3 (1.0)	8.4 (1.5)	4.0 (1.5)
BILD [dB]	5.1 (1.9)		4.9 (2.9)		3.2 (2.6)		2.6 (1.3)		6.8 (2.3)	4.8 (2.5)

The blue crosses in Fig. 2A and C represent results from individuals with normal hearing using AC stimulation in bilateral application tests, taken from Zeitooni et al¹⁶. The horizontal line represents the mean, and the vertical line represents ± 1 standard deviation (SD). The red crosses similarly represent results from individuals with normal hearing using bilateral BC stimulation.

The results for the symmetric group in Fig. 2A generally indicate better outcomes with AC stimulation compared to BC stimulation, better outcomes when speech and noise are separated compared to when they are co-located, and better outcomes with bilateral presentation of signals compared to monaural application. Similar trends are visible in Fig. 2B for the asymmetric group, although overall thresholds are worse for this group compared to the symmetric group. Moreover, data from individuals with normal hearing show substantially better thresholds compared to both hearing loss groups.

These observations were confirmed using a mixed-model ANOVA on the data in Fig. 2A and B. The ANOVA considered Group (symmetric, asymmetric) as a between-subjects factor and Stimulation (AC, BC), Spatial (S0N0, S0N45), and Application (binaural, monaural) as within-subjects factors. The ANOVA showed significant main effects of all within-subject factors: Stimulation [F(1, 28) = 13.077, p = .001, η² = .318], Spatial [F(1, 28) = 215.335, p < .001, η² = .885], and Application [F(1, 28) = 145,486, p < .001, η² = .713]. The main effect of Group did not reach statistical significance (p = .137). A posthoc analysis (Sidak) showed that the Spatial difference was significant for all combinations and the difference between binaural and monaural application was significant except for BC stimulation with S0N45 in the asymmetric group. The difference between AC and BC stimulation was significant for combinations with binaural application but not with monaural application.

A separate analysis was conducted on the SRM, as shown in Fig. 2C. On average, the SRM is approximately twice as high in dB for AC compared to BC stimulation: 5.2 dB versus 2.5 dB in the symmetric group, and 3.3 dB versus 2.2 dB in the asymmetric group (Table 1). This trend is also observed in the SRM from the group with normal hearing (8.4 dB versus 4.0 dB, as indicated by the blue and red crosses in Fig. 2C).

Bilateral application generally results in a better average SRM compared to monaural application, except for BC application in the asymmetric group, which shows worse results with bilateral compared to monaural application. The symmetric group had a slightly better average SRM overall than the asymmetric group.

The SRM was further examined using a mixed-model ANOVA with Group as between-subjects factor and Stimulation and Application as within-subjects factors. While the main effect of Group was not significant, all within-subject main effects and interactions were significant: Stimulation [F(1, 28) = 6.370, p = .018, η² = .185], Application [F(1, 28) = 6.773, p = .015, η² = .195], Stimulation*Group [F(1, 28) = 6.648, p = .015, η² = .195], Application*Group [F(1, 28) = 6.156, p = .019, η² = .180], Stimulation*Application [F(1, 28) = 18.577, p < .001, η² = .399]. According to the posthoc (Sidak) analysis, these significant results are primarily driven by the differences between AC and BC stimulation as well as between binaural and monaural stimulation in the symmetric group.

The BILD results are depicted in Fig. 3. Figure 3A presents the SNR thresholds for S0N0 and S180N0 with both AC and BC stimulation for the two groups with hearing loss. It is important to note that the S0N0 data are the same as those in Fig. 2. Again, the blue and red crosses represent the mean and SD for the same measurements in individuals with normal hearing. The general trend indicates improved hearing thresholds when the speech signal is inverted between the ears (S180), better average results with AC compared to BC stimulation, and better results in the symmetric group compared to asymmetric group.

Figure 3B shows the BILD, which reveals nearly identical results with AC as with BC stimulation, but approximately twice the benefit in the symmetric group compared to the asymmetric group (detailed in Table 1). Interestingly, with BC stimulation, the average BILD for the normal hearing group is the same as for the group with symmetric hearing loss.

The BILD results were analyzed with a mixed-model ANOVA with Group as between-subjects factor and Stimulation as within-subjects factor. This analysis revealed a significant effect of Group [F(1,28 = 4.579, p = .041, η² = .141] but neither Stimulation nor the interaction between Group and Stimulation were significant.

The outcomes from the precedence effect test are illustrated in Fig. 4A for low-frequency sound stimulation and in Fig. 4B for high-frequency sound stimulation. The upper panels depict the average perceived position of the sound as a function of time delay between the presentations at both ears, while the lower panels show the percentage of participants who perceived an echo of the sound (non-fused). For comparison, results from individuals with normal hearing are also included in Fig. 4.

The results with AC stimulation in individuals with normal hearing (represented by the black dashed line) follow the expected trend, moving from 0° at no time difference to full lateralization (90°) at a time difference of 0.5 to 0.8 ms, with echo perception starting at time difference of 10 to 20 ms. Both groups of participants with hearing loss follow a similar but less pronounced trend when stimulated by AC. Particularly, the high-frequency results in Fig. 4B show less lateralization effects compared to individuals with normal hearing. It is important to note that for the asymmetric group that the stimulation at the worse ear was delayed, which should result in lateralization toward the better hearing ear. However, since the sound at the worse ear was amplified with a higher gain compared to the better ear, this could have affected the perception of sound lateralization.

When stimulated by BC, the perception of position with time delay is less distinct. In individuals with normal hearing (represented by the black solid line), the lateralization effect begins at time delays between 5 and 10 ms, reaching full lateralization at the highest time delays used (40 and 50 ms), with around half of them perceiving an echo. The two groups with participants with hearing loss do not show a clear pattern. With low-frequency stimulation in Fig. 4A, the median perception in the symmetric group increases to around 30° at 0.5 ms time difference and remains around 30° with increasing interaural time differences. The asymmetric group does not show any specific trend with low-frequency stimulation and even report perception of lateralization toward the delayed side (Fig. 4A). With high-frequency stimulation (Fig. 4B), both hearing loss groups show more of a trend of increased lateralization with time delay, even though patterns vary. The median position perception is around 45° at the largest delays, and only a minority of participants report perceiving an echo.

The present study explored the advantages of using bilateral BC sound in individuals with mild to moderate sensorineural hearing loss, which could be either symmetric or asymmetric. The hearing loss range examined in this study is common for BC hearing aid users^18,28.

In most scenarios, our participants demonstrated improved speech intelligibility with bilateral stimulation compared to unilateral stimulation (Fig. 2). However, in the asymmetric group using BC stimulation, similar thresholds were achieved with both bilateral and unilateral stimulation in the S0N45 condition. This is because the noise is directed at the worse ear for the asymmetric participants, with stimulation at the better ear and a favorable SNR in the unilateral condition. Since the beneficial SNR is at the better ear, adding a worse ear with a poorer SNR is not expected to enhance speech intelligibility. However, if the noise was directed towards the better ear, bilateral stimulation would likely be beneficial. This was not tested in the current study.

In the symmetric hearing loss group, the BC spatial release from masking (SRM) is similar for both unilateral and bilateral stimulation (Fig. 2c). The SRM in the unilateral condition is also similar for AC and BC stimulation in the symmetric group, as well as with AC stimulation in the asymmetric group. The exception is the better SRM with unilateral BC stimulation in the asymmetrical group. However, the differences between the SRMs in the unilateral condition were not statistically significant. The only exception is the better SRM with unilateral BC stimulation in the asymmetric group. However, these differences were not statistically significant, suggesting that the improved speech perception in both the unilateral and bilateral conditions in the asymmetric group, and with BC stimulation in the symmetric group, can be attributed solely to the head shadow of the noise, or the better ear effect. This finding is supported by a recent study where the speech stimulation was at one ear and the noise position varied between co-located and at the opposite ear²⁹. In that study, when the noise position was at the contralateral side, the head shadow dominated the mechanism for spatial benefit.

The current study aligns with the findings in Zeitooni et al¹⁶, which reported a better ear effect in unilaterally deaf participants. In the Zeitooni et al. study, the SRM with AC stimulation in unilaterally deaf participants was 2.1 dB, while it was 2.4 dB and 2.2 dB in the symmetric and asymmetric groups in the current study, respectively. The average SRM for the unilaterally deaf participants with BC stimulation in the Zeitooni study was 1.7 dB, while the current study showed unilateral SRM with BC stimulation of 2.0 dB and 3.3 dB in the symmetric and asymmetric groups. Both studies used the same test setup and speech material, but the Zeitooni et al study provided the BC stimulation at the mastoid, while the current study gave the stimulation at a position further back on the skull where a typical implant for BC hearing aids is placed.

The current study demonstrated a significant benefit of using bilateral BC stimulation compared to unilateral stimulation. The improvement with bilateral stimulation using AC modality was approximately twice as large (in dB) compared to using BC stimulation (Fig. 2). This is consistent with previous studies using participants with normal hearing^15,16. There is also a clear decrease in binaural benefit with increasing hearing loss, where the two groups including participants with hearing loss perform worse compared to results from participants with normal hearing, independent of stimulation modality. The reduction of binaural benefit with peripheral hearing loss with AC stimulation has been communicated previously³⁰. However, a systematic investigation of the effect of peripheral hearing loss on binaural benefit with bilateral BC stimulation has not been presented. Even so, it was expected that the binaural benefit would be reduced because of peripheral hearing loss when stimulation is by BC, which is the outcome in the present study.

A noteworthy finding is that the asymmetric hearing loss group benefitted from bilateral application with AC stimulation but not with BC stimulation (Fig. 2C). Even though bilateral BC SRM averaged 2.2 dB, it was higher in the BC unilateral SRM condition (mean 3.3 dB). One issue with the asymmetric participants is that the different sides were amplified differently. The current study adopted a linear amplification rationale based on the hearing thresholds³¹. This is less of a problem for AC stimulation, where the interaural separation is large enough to prevent the amplified sound to affect the contralateral ear³². With BC stimulation, the interaural separation is typically between 0 and 10 dB^17,33,34. This means that some of the BC sound at the better ear is dominated by the amplified sound provided at the contralateral ear. Consequently, how to set amplification for bilaterally fitted BC hearing aids in individuals with asymmetric hearing loss is not clarified and would be a topic for its own investigation.

The current study examined the effect of inverting the phase of the speech signal at one ear (BILD), and found that the benefit was largely independent of the stimulation modality (Fig. 3B). In the BILD testing, there is no effect of an improved SNR at either ear since only the phase information of the speech signal is manipulated. The benefit was more pronounced in the symmetric hearing loss group compared to the asymmetric hearing loss group, but was significant for both groups and stimulation modalities. Interestingly, the benefit of inverting the speech signal with BC stimulation in the symmetric group was similar to the result with inverting the speech signal with BC stimulation in the group of normal hearing participants in the study by Zeitooni et al¹⁶. However, the Zeitooni participants reported better BILD results with AC stimulation compared to the AC generated BILD in the current study of participants with symmetric hearing loss. It should be noted that Zeitooni et al¹⁶ reported the benefit of the BILD test in unilateral deaf individuals to be 2.4 dB with BC stimulation. Therefore, even if the SNR at the stimulation position is unchanged, inverting the speech signal can result in superposition at the cochlear level, enhancing the intelligibility with BC stimulation¹⁶.

The precedence data in the current study aligns with previous results when stimulation was by BC^15,16,35 (Fig. 4). With AC stimulation, the perception is fused and directed toward the leading stimulation (dashed lines in Fig. 4). The perception is close to fully lateralized after 0.5 to 0.8 ms lag-time for the group consisting of normal hearing individuals, while the groups of hearing-impaired individuals show less consistent lateralization. When the stimulation was by BC, the lateralization did not appear until a delay of 10 ms for individuals with normal hearing and no systematic lateralization for the groups with hearing-impaired people. One possible reason for this lack of distinct lateralization with BC stimulation is the cross-head transmission with BC stimulation.

A recent study estimated the interaural time-delay of BC cross-head transmission to be approximately 0.06 ms at 250 Hz, increasing to 1.1 ms at 0.8 kHz, and decreasing to 0.6 ms at 4 kHz²². Given that the transcranial attenuation is primarily less than 10 dB within this frequency range, the impact from cross-head transmission is substantial. This can lead to confusion when trying to identify the direction of a sound when stimulation is by BC. As a result, a significant proportion of individuals listening through bilateral BC may encounter difficulties localizing a sound source.

While the current study suggests that individuals listening through bilateral BC may have difficulties localizing a sound source, previous studies have shown that individuals with bilaterally applied BC sound can exhibit localization abilities³⁶. This discrepancy may be partly due to adaptation. In the current study, participants were acutely fitted with BC sound without any adjustment period for this stimulation modality. However, individuals who have been fitted with BC hearing aids may, over time, develop better abilities to use binaural information, even if it has been distorted by cross-head BC sound transmission.

Other factors may also contribute to these differences. For instance, many of the studies using BC hearing aids in clinical patients involved patients with predominantly conductive hearing loss^{25,26,37–39}. These patients typically have minor sensorineural hearing loss, and their BC thresholds are often better than 20 dBHL. As a result, these patients are more similar to the normal hearing participants than the participants with hearing loss in the current study. Nevertheless, the BC results of lateralization and localization in individuals with normal hearing are inferior to those with AC stimulation in participants with mild to moderate hearing loss.

Another possible discrepancy between studies on localization ability with bilateral BC stimulation and the data in the current study is the use of artificial conductive hearing loss. This is typically achieved by occluding the ear canals, often with earplugs^40–42. However, this can lead to an enhancement of the ear canal sound pressure due to the occlusion effect⁴³. As a result, ear canal sound pressure enhancement may lead to a greater separation of the ears, and the stimulation becomes more akin to AC than BC at low frequencies when the ear canal pathway is enhanced compared to other pathways for BC hearing⁴⁴. A similar enhancement of the ear canal sound pressure leading to greater interaural separation was found when the BC transducer was placed close to the ear canal opening⁴⁵. Additionally, ear surgery can also increase the interaural separation, resulting in improved binaural ability⁴⁶.

A limitation of the present study is the small sample size of participants with asymmetric hearing loss. This restricts the applicability of the findings, and some of the statistically insignificant results for this group may be attributed to the limited participant numbers rather than an absence of binaural effects. The stringent inclusion criteria, which specified acceptable hearing thresholds and interaural differences, made it challenging to identify suitable participants from the database. Regrettably, the constraints of the BC transducers mean that larger threshold ranges can lead to distorted stimulation due to amplification, or inaudibility if less compensation for the hearing loss is applied. To ensure both audibility and undistorted stimulation, the restrictive inclusion criteria were necessary. Despite these limitations, the group with asymmetric hearing loss demonstrated a significant spatial benefit, although this benefit was not superior in bilateral compared to unilateral stimulation in the current setup.

The study demonstrated that individuals with mild to moderate hearing loss derive benefits from the bilateral application of BC sound compared to unilateral application. However, this binaural benefit was less pronounced compared to individuals with normal sensorineural hearing, and a symmetric hearing loss yielded a larger benefit than an asymmetric hearing loss. Moreover, even though the binaural benefit with bilateral BC stimulation was significant, it was approximately half (in dB) compared to the same benefit when stimulation was by AC. A large part of this benefit was attributed to the reduction of noise from the head shadow at the ear with a favorable SNR. However, binaural processes that improve speech intelligibility were also present, as evidenced by the enhanced perception in the BILD test. The ability to lateralize a sound to the leading side with interaural time delays was worse with BC stimulation compared to AC stimulation, and even worse when a hearing loss was present.

The study was approved by the Regional Ethical Review Board in Linköping, Sweden (2012-168-32). All participants signed an informed consent prior to the testing, and were made aware of their right to withdraw from the study at any point. All methods used follow clinical routine and the tests are conducted according to relevant guidelines and regulations.

Two groups of participants were recruited to the study. The first group comprised individuals with symmetrical mild to moderate sensorineural hearing loss. The inclusion criteria for this group were being 18 years or older, having a BC PTA4 (average of thresholds at 0.5, 1, 2, 3, and 4 kHz) between 30 and 45 dBHL, a maximum interaural BC PTA4 difference of 6 dB, and a maximum PTA4 air-bone gap of 6 dB in both ears. Potential participants were identified from the clinical database of Linköping University Hospital, Linköping, Sweden. Of those invited, 21 individuals chose to participate. These participants ranged in age from 66 to 76 years, with an average age of 72 years, and included 13 females. Their average BC PTA4 was 34.3 dBHL (SD 3.0 dB) in the right ear and 34.2 dB HL (SD 3.5 dB) in the left ear.

The second group consisted of individuals with asymmetrical hearing loss. The inclusion criteria for this group were a BC PTA4 in the better ear between 25 and 35 dBHL, an interaural BC difference between 10 and 20 dB, and a maximum PTA4 air-bone gap of 6 dB in both ears. Recruiting for this group proved more challenging, and nine individuals chose to participate. The participants in this group ranged in age from 43 to 74 years, with an average age of 68 years, and included one female. The average BC PTA4 thresholds were 29.2 dBHL (SD 4.5 dB) in the better ear and 40.1 dBHL (SD 4.8 dB) in the worse ear. Figure 1 presents the average audiograms and standard deviations for both ears in both groups.

All tests were conducted in a sound-insulated test booth. Air and bone conduction thresholds were measured using a Madsen Astera² (Natus Medical ApS, Denmark), equipped with TDH-39 earphones and a Radioear B-71 BC transducer. Computer-based speech-in-noise tests and the precedence test were implemented in Matlab (MathWorks, US), with an external 24-bit soundcard (Asus Xonar Essence One) serving as interface. Testing with AC sound was conducted using a pair of Sennheiser HDA200 audiometric headphones (Sennheiser electronic GmbH, Germany) connected to the soundcard.

The BC transducers used were two Cordelle 2 (Cochlear BAS, Sweden) devices, attached to an elastic softband with two plastic circular interfaces of approximately 15 mm diameter. The interfaces were positioned approximately 55 mm behind the ear canal opening, in line with the upper part of the pinna, mirroring the placement of a percutaneous BC hearing aid. The static force exerted by the BC transducers from the soft band was approximately 3 Newtons.

The Swedish Hagerman’s sentences⁴⁷, also known as the matrix test, was used for speech-in-noise testing. The speech was delivered by a female voice, and the noise was speech-weighted with 10% modulation. Each sentence contained five words that were semantically correct but had low predictability. Each list contained ten sentences, and two lists were used to test each condition. The aim of the test was to determine the signal to noise ratio (SNR) at which the participant could correctly repeat 50% of the words in the sentences, based on an adaptive algorithm⁴⁸.

The speech-in-noise test was conducted under three conditions: S0N0, S0N45, and S180N0. In the S0N0 condition, the speech and noise were identical at both left and right transducers. In the S180N0 condition, the speech was inverted between the two sides, while the noise remained identical at both sides. In the S0N45 condition, the speech signal was equal at both sides (appearing to originate from the front), while the noise was spatially shifted 45 degrees to one side. This was achieved by filtering the noise signal differently to the two sides based on a generic head model⁴⁹. This resulted in different noise signals at the two ears for this condition.

The study examined spatial release from masking (SRM) by comparing the SNR thresholds in two conditions: S0N0 and S0N45. These tests were conducted for two stimulation modalities (AC and BC) and two situations (bilateral and unilateral), resulting in each condition being tested four times (2 conditions x 2 situations). Binaural intelligibility level difference (BILD) was evaluated using SNR thresholds for S0N180 and S0N0. The S180N0 condition was tested only with bilateral sound application for both AC and BC stimulation. This meant that each participant underwent speech-in-noise tests ten times.

When stimulation was unilateral, only the signal to the transducer or headphone with the better SNR (opposite to the side of the noise) was used. Additionally, the opposite ear was blocked with an earplug during AC stimulation to ensure only one ear was used. For BC stimulation, the opposite ear in the unilateral condition was left open to avoid an occlusion effect⁴³. In the group with symmetric hearing loss, the noise in the S0N45 condition was directed towards the right ear, with the left ear having the better SNR. In the group with asymmetric hearing loss, the noise in the S0N45 condition was directed towards the worse ear, while the signal with the better SNR was at the better ear. Also, in the unilateral testing, the sound was provided at the better ear.

The speech level was set at 60 dB SPL for the earphones, with the noise level adaptively altered during testing. This speech level corresponds approximately to 40 dB HL, and the same level was used for the BC testing. To ensure audibility and compensate for the participants’ hearing loss, the Cambrige formulae for linear amplification was used³¹. Each participant’s audiogram for the ears separately were used to amplify the delivered sound. Consequently, for the participants in the asymmetric group, the worse ear received higher amplification of the sound than the better ear. Moreover, the same amplification was used for sounds delivered by AC and BC. For details on how the AC and BC stimulation were equalized, see Zeitooni et al¹⁶.

The study investigated the precedence effect using noise bursts of high and low frequencies, each lasting 1 second. The noise-bursts had 1 ms sine-squared rise and fall. The low-frequency noise ranged between 0.4 and 0.6 kHz, while the high-frequency noise ranged between 3.0 and 5.0 kHz. The experiment included twelve different interaural time delays, ranging from 0 to 50 ms. Participants were instructed to identify the perceived location of the sound, which could be on the left side (-90 degrees), in front (0 degrees), or on the right side (90 degrees). They indicated their perception on a computer screen by marking the location with a ruler, within a range of -90 to 90 degrees.

In the symmetric group, the delayed sound was on the left side, while in the asymmetric group, the delayed sound was at the worse ear. As a result, it was expected that the lateralization would be towards the right side for the symmetric group and toward the better ear in the asymmetric group. In the results, the better side was coded as the right side for the asymmetric group. If the sound was perceived as two separate sounds, participants were instructed to report this as an echo. The experiment involved four runs of the precedence effect test, with participants tested with low and high-frequency stimulation using both AC and BC sound.

Before the precedence effect test, participants were first tested for their hearing thresholds, followed by ten conditions of speech in noise. For practical reasons, either the BC or the AC testing was done before switching to the other stimulation modality. The order of the tests was balanced among the participants as much as possible. The total testing time for each participant ranged between 1.5 and 2 hours.

The output of the transducers was measured and verified before the testing. The audiometer was calibrated according to ISO 389 − 1⁵⁰ and ISO 389 − 3⁵¹ for measurement of hearing thresholds. The output from the headphones with the computer-based stimulation was measured on an ear simulator (IEC 60318-1⁵²) and with BC stimulation on an artificial mastoid (Brüel & Kjær type 4930) connected to an analyzer (Brüel & Kjær type 2250). Based on these measurements, the outputs of the left and right transducers were corrected to deviate at most 1 dB at one-third octave frequencies between 125 Hz and 8 kHz.

The results were analyzed using mixed-model ANOVAs with Sidak post hoc corrections, computed with IBM SPSS Statistics (ver. 29).

Acknowledgements

This work was supported by Oticon Hearing Foundation (11-1444) and Swedish Research Council (2021-05714).

Author contributions

Conception and Design: S.S., M.Z., and E.M.T. Acquisition: M.Z. Analysis, and Interpretation of Data: S.S. and M.Z. Drafting of Manuscript: S.S. Revising Manuscript and Final Approval of the Article: All Authors.

Data availability

The data presented in this study are available on request from the corresponding author.

Additional information

The authors declare no competing interests.

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Table 1. The table presents the averages and standard deviations (in parenthesis) of the tests conducted on two groups with hearing losses, for both AC and BC stimulation. Data for individuals with normal hearing are derived from Zeitooni et al¹⁶.

	Symmetric				Asymmetric				Normal
	AC		BC		AC		BC		AC	BC
Test	Bilateral	Monaural	Bilateral	Monaural	Bilateral	Monaural	Bilateral	Monaural	Bilateral	Bilateral
S0N0 [dB]	-5.1 (1.5)	-3.7 (1.9)	-4.3 (1.8)	-3.5 (2.2)	-4.6 (1.8)	-2,6 (2.0)	-3.5 (1.3)	-2.6 (1.4)	-8.0 (1.4)	-8.5 (1.2)
S0N45 [dB]	-10.3 (1.9)	-6.2 (2.5)	-6.8 (2.5)	-5.5 (2.0)	-7.9 (1.4)	-4.8 (2.0)	-5.8 (1.4)	-5.8 (1.4)	-16.4 (1.8)	-12.5 (1.3)
S180N0 [dB]	-10.2 (2.4)		-9.1 (2.9)		-7.8 (2.5)		-6.1 (2.2)		-14.7 (2.3)	-13.3 (2.6)

SRM [dB]	5.2 (1.3)	2.4 (1.5)	2.5 (1.9)	2.0 (1.2)	3.3 (2.2)	2.2 (2.4)	2.2 (1.0)	3.3 (1.0)	8.4 (1.5)	4.0 (1.5)
BILD [dB]	5.1 (1.9)		4.9 (2.9)		3.2 (2.6)		2.6 (1.3)		6.8 (2.3)	4.8 (2.5)

No competing interests reported.

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Evaluating binaural hearing capabilities in individuals with sensorineural hearing loss through bilateral bone conduction stimulation

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