Understanding Binaural Hearing Loss: Levels, Impact, and Treatment
Binaural hearing, or binaural processing, refers to the brain’s ability to integrate auditory information from both ears to improve speech understanding in challenging environments and localize sound sources. This process utilizes the similarities and differences in sounds received by each ear to gather information about the surrounding acoustic environment.
The two primary cues for binaural processing are the differences in time of arrival (interaural time difference, ITD) or phase (interaural phase difference, IPD) for stationary sounds, and the difference in sound level (interaural level difference, ILD). By using these cues, the auditory system can accurately determine the direction and distance of a target sound source, thereby improving its detection.
In contrast, bilateral hearing refers to the detection of sound by both ears, regardless of whether the auditory information is processed jointly by the brain. Although both ears may detect sound in bilateral hearing, there may be minimal or no integration of auditory information between them. Thus, bilateral hearing involves the simple detection of sound with both ears, whereas binaural hearing enables sound source localization and noise suppression through integrated processing.
Impact of Hearing Loss on Binaural Abilities
Research consistently shows that individuals with sensorineural hearing loss, as measured by audiograms (pure-tone detection), generally exhibit reduced binaural abilities compared to those with normal hearing. These deficits manifest in difficulties with sound localization and poorer speech perception in complex listening environments. While factors such as age and cognitive function can also influence binaural processing, the integrity of the peripheral auditory system plays a key role in ensuring optimal binaural function.
Asymmetric sensorineural hearing loss has a greater impact on binaural abilities than symmetric hearing loss. Asymmetric hearing loss can distort binaural cues, leading to significant disruptions in binaural hearing. Although individuals with asymmetric hearing loss may have similar speech understanding in noise as individuals with symmetric hearing loss when both speech and noise is collocated in front of them, their ability to benefit from spatial separation of speech and noise is reduced compared to those with symmetric hearing loss. Additionally, sound source localization is significantly worse in individuals with asymmetric hearing loss compared to those with symmetric hearing loss.
Air Conduction (AC) vs. Bone Conduction (BC)
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 ossicles. Another mode of hearing sound is through bone conduction (BC), where the sound reaches the inner ear via vibrations in the skull bone and soft tissues. 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.
Binaural processing, which enhances hearing, is also observed when sound is delivered bilaterally by BC. This has been demonstrated through spatial benefits in speech-in-noise tests, where bilateral BC stimulation improved performance, both in experimental and clinical settings. Additionally, the ability to localize sound sources further supports the presence of binaural processing with bilateral BC sound. However, it is important to note that while these studies have shown binaural benefits with BC, the advantages are typically less pronounced compared to bilateral sound delivered through AC.
One argument against bilateral BC stimulation is that the sound from one stimulation position reaches both ears. This suggests that it would be sufficient to stimulate both cochleae from one position, and that binaural cues are impeded due to cross hearing. 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 dB. 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. However, these studies also showed that the improvement was less with BC stimulation compared to stimulation by earphones (AC).
Previous studies have shown that using BC hearing aids on both ears can improve speech comprehension and sound localization, compared to using a BC hearing aid on only one ear. However, these studies often involve small, heterogenous groups of patients with varying degrees of sensorineural hearing loss. As a result, the combined impact of sensorineural hearing loss and cross-head transmission, present with BC stimulation, on the binaural benefits of hearing through bilateral BC devices is not fully understood.
Both sensorineural hearing loss and cross-head sound transmission can interfere with binaural hearing benefits. Therefore, it remains unclear at what level of hearing loss the advantages of binaural processing become negligible when sound is delivered bilaterally via BC. Furthermore, asymmetric hearing loss presents an even greater challenge to binaural integration compared to symmetric hearing loss, but the extent of asymmetry that still allows for meaningful binaural benefit, considering cross-head transmission, is currently unknown.
The Importance of Understanding Binaural Hearing Loss
Understanding this relationship is critical for determining candidacy for bilateral BC hearing aid use. The aim of this study is to investigate how varying degrees of symmetric and asymmetric sensorineural hearing loss affect binaural benefits when sound is delivered bilaterally via BC devices. This knowledge will help optimize the application of bilateral BC hearing aids and improve patient outcomes.
In this study, binaural processing was evaluated using three different methods. The first method, spatial release from masking (SRM), assessed the benefit of spatially separating speech and noise sources. SRM is influenced by two mechanisms: the better-ear effect and binaural unmasking. The better-ear effect refers to the improvement in the signal-to-noise ratio (SNR) at one ear due to the relative positioning of speech and noise sources. To isolate the better-ear effect, the SRM was measured under both binaural and monaural conditions, with monaural stimulation at the ear with the more favorable SNR.
To evaluate the binaural unmasking in speech-in-noise, binaural intelligibility level difference (BILD) was measured. In this test, the speech signal was either presented equally or phase-inverted between the two ears, while the noise remained identical at both ears. Since the SNR at both ears was the same, any improvement in speech perception was attributed solely to binaural processing.
Lastly, the precedence effect test was used to assess the ability to fuse two sounds into a single auditory image. In this test, two sounds of approximately equal loudness were presented to the two ears with a short interaural time delay. The fused sound image was perceived on the side of the leading sound and remains there until the delay became long enough for the listener to perceive two distinct sounds (an echo).
The aim of this study is to investigate the advantages of bilateral BC stimulation in terms of SRM and BILD among participants with symmetrical and asymmetrical sensorineural hearing loss. Additionally, the study will examine the participants’ ability to fuse sounds from both sides.
Study Results: Evaluating Speech Understanding in Noise
This study evaluated the threshold at which speech was 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 phase-inverted between the two ears while the noise signal was equal at both ears (S180N0). The study also measured the precedence effect, which was 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 al. were included for comparison. All tests were conducted using both BC stimulation and AC stimulation through earphones.
Figure 1 shows the audiograms for all participants. In the asymmetric group, the better ear is designated as the right ear, and the worse ear as the left ear. The average AC PTA4 (0.5, 1, 2, 4 kHz) for the right ear in the symmetric group was 35.5 dB HL and for the left ear was 36.1 dB HL (Fig. 1a). With BC stimulation, the average right ear PTA4 for this group was 34.3 dB HL and the left ear was 34.2 dB HL (Fig. 1c). In the asymmetric group, the better ear had an average AC PTA4 of 28.2 dB HL and the worse ear a PTA4 of 45.6 dB HL (Fig. 1b). With BC stimulation, the average PTA4 was 29.2 dB HL in the better ear and 40.1 dB HL in the worse ear (Fig. 1d).
Fig. 1 Average hearing thresholds for the two participant groups. Panels (a) and (c) present the symmetric group with AC and BC stimulation, respectively. Panels (b) and (d) present the asymmetric group with AC and BC stimulation, respectively. In the symmetric group, the right ear is represented by red lines and markers (circles and left-pointing triangles), while the left ear is represented by blue lines and markers (crosses and right-pointing triangles). In the asymmetric group, the better ear is represented by red lines and markers, and the worse ear is represented by blue lines and markers. The vertical lines denote 1 standard deviation.
The results of the speech-in-noise test, both with speech and noise co-located (S0N0) and with speech at the front and noise positioned at 45 degrees (S0N45), are shown in Fig. 2. Figure 2a displays the outcomes with AC stimulation, while Fig. 2b shows the results with BC stimulation. Individual data points are displayed, with blue circles representing participants with normal hearing (data from Zeitooni et al.), red squares representing participants with symmetric hearing loss, and black crosses representing participants with asymmetric hearing loss. Averages are indicated by horizontal lines in Fig. 2, and the mean values along with standard deviations of all conditions are provided in Table 1. SNR thresholds are shown for both bilateral and unilateral stimulation in the two hearing loss groups. In the unilateral condition, stimulation was applied on the side with the more favorable SNR in the S0N45 condition, and to the better ear for participants with asymmetric hearing loss.
Fig. 2 SNR thresholds at 50% correct word recognition for (a) AC stimulation and (b) BC stimulation. (c) displays the spatial release from masking (SRM). Participants with normal hearing are represented by blue circles, those with symmetric hearing loss by red squares, and those with asymmetric hearing loss by black crosses. The horizontal lines indicate the mean values.
The general results presented in Fig. 2a-b indicate better outcomes for the normal hearing group compared to the hearing loss groups, followed by the symmetric hearing loss group, with the worst outcomes observed in the asymmetric hearing loss group. In general, bilateral stimulation yielded better results than unilateral stimulation, sound and noise separation (S0N45) led to better outcomes than collocated sound and noise (S0N0), and AC stimulation performed better than BC stimulation. These trends were analyzed using mixed-model ANOVAs.
Since the normal hearing group was not tested unilaterally, it was not possible to include all conditions in a single ANOVA. Therefore, the first ANOVA assessed the Group (normal, symmetric hearing loss, asymmetric hearing loss) as between-subjects factor, and Mode (AC, BC) and Spatial configuration (S0N0, S0N45) as within-subjects factors. All main effects were significant: Group [F(2,54) = 116.977, p < .001, η2 = 0.812], Mode [F(1,54) = 77.955, p < .001, η2 = 0.591], and Spatial [F(1,54) = 572.182, p < .001, η2 = 0.914]. Significant interactions were found for Group x Spatial [F(2,54) = 40.627, p < .001, η2 = 0.601], Mode x Spatial [F(1,54) = 85.655, p < .001, η2 = 0.613], Mode x Spatial x Group [F(2,54) = 16.936, p < .001, η2 = 0.288], while the interaction between Mode x Group was not significant (p = .489).
Posthoc analysis (Sidak) revealed that the normal hearing group had significantly better scores than both hearing-impaired groups (p < .001), though the two hearing-impaired groups did not significantly differ from each other (p = .078). The normal hearing group showed significantly better results with both AC and BC stimulation (p < .001). The symmetric hearing loss group approached significantly better outcomes compared to the asymmetric hearing loss group with AC stimulation (p = .051), but no significant difference was observed with BC stimulation (p = .35). Across all groups, results were significantly better with AC stimulation compared to BC stimulation (p < .001), and across both spatial configurations (S0N0, p = .014; S0N45, p < .001). No significant difference was found between the two hearing-impaired groups for the S0N0 condition (p = .55), but the symmetric hearing loss group performed significantly better than the asymmetric hearing loss group in the S0N45 condition (p = .025). All other pairwise comparisons between Group and Spatial configuration were significant (p < .001). Overall, the S0N45 condition resulted in better outcomes than the S0N0 for both AC and BC stimulation (p < .001).
These findings were primarily driven by the superior performance of the normal hearing group compared to the hearing-impaired groups, the better performance of AC stimulation compared to BC stimulation, and the improved outcomes with spatially separated sound and noise (S0N45) compared to collocated sound and noise (S0N0).
| Condition | Normal Hearing (Zeitooni et al.) | Symmetric Hearing Loss | Asymmetric Hearing Loss |
|---|---|---|---|
| AC Stimulation | Data from Zeitooni et al. | Values for Symmetric HL | Values for Asymmetric HL |
| BC Stimulation | Data from Zeitooni et al. | Values for Symmetric HL | Values for Asymmetric HL |
The second ANOVA included only the two hearing-impaired groups as the between-subjects factor (Group), with Stimulation (bilateral, unilateral) added as an additional within-subjects factor. Since the other factors (Mode and Spatial configurati...
Additional Considerations for Hearing Loss Treatment
Individuals often experience different levels of hearing loss in each ear. And patients in this situation frequently ask us, “Can’t I just treat my really bad ear for hearing loss? Won’t that be improvement enough?”
While we at NW Ear Institute sometimes see patients with hearing loss in only one ear (also known as unilateral hearing loss), typically the factors that led to the impairment have affected both ears - just to a different degree. Sounds collected by your left ear are initially processed by the right side of the brain, while sounds collected by your right ear are initially processed by the left side of the brain. After they are received, the two halves of your brain work together to organize the signals into recognizable words and sounds. Similarly, using more of your brain to focus on the sound you want to hear is tremendously important in overcoming one of the primary complaints of individuals with hearing loss: hearing among background noise. Also, a person wearing two hearing aids generally needs less amplification than someone wearing only one.
In less common cases in which there is a total hearing loss in one ear (also known as profound unilateral hearing loss or single-sided deafness), there are medical therapies that may help to re-create some of the effects of binaural hearing. These include bone-conduction systems (also known as bone-anchored hearing aids, or BAHA devices) that can help transmit vibrations from the non-hearing ear to the functioning ear.
Hearing loss is a puzzle that our professionals love to solve, and it is based on your individual experiences, lifestyle, and severity of impairment. There is no one-size-fits-all treatment method for hearing loss - it’s based on the sounds that you can’t hear, which vary greatly, and the sounds that you want to be able to hear.
Health Implications of Untreated Hearing Loss
Research has established a relationship between hearing loss and dementia. There is strong evidence that hearing loss accelerates brain-tissue atrophy, particularly in areas of the brain that auditory nerves would stimulate but can’t because they aren’t receiving a signal (due to a hearing loss). These areas of the brain are also related to memory and speech. Individuals with a mild hearing loss are three times as likely to fall down as those without, and the likelihood of falls increases as degree of hearing loss increases.
Preventing and Managing Hearing Loss
Since hearing loss is cumulative, hearing loss begins as an infant and continues throughout life. Most individuals don’t begin to experience symptoms until their late 20s or early 30s, and by age 45 a yearly hearing check becomes of greater importance.
Unfortunately, many forms of hearing loss are permanent because there is no cure.
Protecting your hearing from noise levels greater than 85 decibels at work and during leisurely activities will greatly reduce your chances of noise-induced hearing loss.
Though it is difficult to say what genetic factors predispose individuals to hearing loss, there seems to be a connection.
See your physician immediately; sudden hearing loss is considered a medical emergency. Sudden hearing loss typically resolves on its own within two weeks, but it might not - meaning your hearing might be gone for good.
Understanding Hearing Loss Terminology
If words like pinna, cochlea and sensorineural aren’t part of your daily language - you aren’t alone. The amount of medical vocabulary surrounding hearing can make your head spin. Add in a hearing loss, and you aren’t sure if your audiologist said bilateral, binaural, or bimodal.
Hearing loss can affect one or both ears. When it is only in one ear, it is referred to as unilateral hearing loss. Bilateral means two sides, so bilateral hearing loss describes loss in both ears.
There are two main types of hearing loss: sensorineural and conductive. Sensorineural hearing loss is a permanent form of loss due to damage of the sensory cells of the inner ear, called the cochlea, or the auditory nerve. This type of hearing loss can be caused by noise exposure, genetics, aging, or many other factors.
Conductive hearing loss occurs when sound is obstructed along the pathway from the outer ear, known as the pinna, to the inner ear. Common causes of conductive hearing loss include wax blockage, ear infections, and middle ear conditions such as otosclerosis. Treatment for conductive hearing loss can include medication, surgery, or assistive devices, including hearing aids and cochlear implants.
If you are diagnosed with hearing loss, the audiologist will give you a treatment recommendation. Hearing aids are most commonly prescribed for mild to moderate losses. Hearing aids work by amplifying sound and sending it into the ear. A cochlear implant bypasses the damaged portion of the ear to stimulate the hearing nerve directly. Like a hearing aid, the cochlear implant system captures sound through one or more microphones. Once the sound is collected, it is processed and transmitted through the headpiece to the implant.
Perhaps one of the most important, but often overlooked words in the hearing loss vocabulary is binaural, which means hearing with both ears. This is one of the most fascinating and powerful abilities of the human brain. Numerous studies have shown that when the brain receives input from both ears, it is able to form a clearer and more complete soundscape. Binaural hearing helps us better localize where sounds come from. It improves the sound quality of music, makes speech clearer in noisy situations, and generally improves the listening experience. That’s why we have two ears!
I had a wonderful speech pathologist who explained to me that I had entered into a hearing marathon. I had to do my exercises and train my brain for this marathon. There would be ups and downs, leaps forward and some setbacks. And she was certainly right. At first, the microwave beep sounded like a clicker.
When people with bilateral hearing loss are only treated in one ear, either with a hearing aid or a cochlear implant, they miss out on binaural hearing. That’s why they often report hearing speech well in quiet situations, but have difficulties in noisy or other challenging listening situations.