Starkey Research Series: Binaural Hearing and the Importance of Bilateral Hearing Aid Fitting

Editor's note: This is an edited transcript of a Starkey live webinar on AudiologyOnline.  Download supplemental course materials. 

Sridhar Kalluri:  By the end of the presentation I hope you will have a modern understanding of the importance of binaural hearing and how bilateral hearing aid fittings are particularly important.  I am a scientist, and I do not have as much of a clinical background as the rest of you, but I will touch on a few clinical implications throughout this presentation. 

First, I would like to make a distinction between binaural hearing and bilateral hearing aid fits.  Binaural hearing relates to the brain making use of information from both of the ears.  It is the processing that the brain applies to the information from the two ears.  Bilateral hearing aid fitting, sometimes called binaural hearing aid fitting, is simply about two hearing aids or assistive devices on two ears.  The bilateral fit is about the technology that we use to assist binaural hearing or binaural processing by the brain. 

Agenda

First on the agenda is the benefits of binaural hearing and bilateral fittings.  I will go over some of the new research about the benefits of binaural hearing and in what ways bilateral hearing aid fittings are beneficial to listeners.  I will touch on the limitations of bilateral fittings and make some concluding comments about the clinical implications of these most recent research observations in the literature. 

Benefits of Binaural Hearing and Bilateral Fittings

Let’s start by looking at the benefits of binaural hearing and bilateral hearing aid fits.  When you hear the same information from both ears, like two sides of the same coin, you are getting redundant information.  As a benefit, binaural hearing gives you two different perspectives in a given auditory environment.  You get two sides of the coin.  This is not a new knowledge. 

Localization

One of the earliest pioneers of this was Lord Rayleigh, who was an accomplished scientist from the 19^th and early 20^th centuries.  He made contributions to a wide range of fields having to do with physics, optics, acoustics, and perception of sound.  He developed a theory called the Duplex Theory of Sound Localization that discussed how differences in the information at the two ears can be useful for localizing sound.  This was what fundamentally drove our understanding of binaural hearing benefits. 

He realized that if there is a sound source coming from one side of the head, it causes a shadow on the opposite side, thus decreasing the intensity of the sound on the contralateral side of the head.  Additionally, it takes sound longer to travel from one side of the head to the other side of the head.  The arrival time and intensities of the sound source at each ear are important differences that give rise to the benefits of binaural hearing. 

For example, if you move the sound source from completely on the side of the head to the midline, the time travel between the ears gets smaller and smaller.  As the angle from the midline of the head gets larger (moving to one side or the other), the interaural time difference increases.  Sound is localized based on an interaural time difference that is perceived by the brain.  Similarly, as you move the sound source to the side of the head, a level occurs between the ears, called the interaural level difference.  That is another cue for the location of a sound relative to your head. 

Speech Recognition

Another binaural benefit is speech recognition.  This head-shadow effect can be useful for understanding speech when it is in the presence of an interfering noise (Figure 1).  If an interfering noise is on the opposite side of the head from the speech, the interfering noise is reduced at the near ear by the head-shadow effect.  The signal-to-noise ratio (SNR) at this near ear is reduced compared to if there was no head shadow effect.  You would not have this without two ears.

Figure 1.  Head-shadow effect is beneficial for speech recognition in noise.

In this situation, the brain also makes use of the interaural time differences that result from the different locations of the target and the interfering sounds.  The brain is able to exploit the interaural time differences to improve the intelligibility of this target speech. 

Spatial Attention

Another way to understand how location and binaural cues can be useful for understanding speech is to look at how we attend to important sounds.  If you can localize where different sounds originate in your environment, you can focus your spatial attention, which is a cognitive capability, on a particular location.  This enables you to suppress interfering sounds around you.

Binaural Processing Starts in the Brain

All of this processing starts in the midbrain.  The cochlea detects sound and transmits that information to the rest of the brain.  Binaural processing, almost by definition, has to happen somewhere centrally in the brain.  The information from the two ears has to come together in some way.  That happens in the lateral superior olive, which is in the midbrain.  The details of it are not critically important; the important thing is to recognize that binaural processing is a benefit we get from the central nervous system. 

Bilateral Hearing Aid Fitting

People have investigated the benefit of using two hearing aids for years, and we recognize that bilateral hearing aid fittings are beneficial for speech perception in noise by way of the head-shadow effect and binaural squelch.  People do better for sound localization with two hearing aids versus one hearing aid.  Sound quality is generally better with two hearing aids than one.  There have been arguments over the years that using only one hearing aid may lead to some deprivation effects, where the ear that did not have the hearing aid becomes less effective at using sound.  Other research suggests that perhaps bilateral hearing aids minimize this deprivation effect. 

I have a timeline of studies that show various benefits of binaural hearing and of two hearing aids compared to one (Figure 2).  Why is this still a topic of interest for people?  It is sometimes puzzling to think why this is an issue, because the percentage of binaural hearing aid fits are up to about 90% now (Kochkin, 2010).  This has picked up rapidly in the last 30 years.  Clinically, practitioners are recognizing that bilateral fittings are beneficial for patients. 

Figure 2. Timeline of select findings regarding bilateral hearing advantages.

Why Two vs. One?

For populations with bilateral hearing loss, why is the true value of binaural hearing still an issue for us?  One reason is that in much of the world the fit rate for bilateral hearing aids is quite low (Arlinger, Brorsson, Lagerbring, Leijon, Rosenhall, et al., 2004).  The bilateral fit rate is fairly high in the United States and Australia.  However, in much of the world including Europe, which is fairly wealthy, bilateral fit rates can be quite low.  It seems most of the world still needs to be convinced of the benefits or binaural fittings.

While the research literature extensively shows that bilateral fittings are beneficial compared to unilateral fittings for most patients, there have been some cautionary findings, some of which have been quite influential.  A study from Walden and Walden (2005) tested the SNR loss in bilateral fittings.  The SNR loss tells the SNR that listeners require to understand at least 50% of the spoken material compared to normal hearing listeners; the lower the SNR loss, the better.  The hearing impaired listeners do not need an SNR that is that much lower than normal hearing listeners to achieve the same performance.  With bilateral fits, the SNR loss can be quite considerable for some of these patients.  In Walden and Walden’s study (2005), the SNR loss for understanding speech in noise was often worse with bilateral hearing aids than with a single hearing aid.  I will explain this later in the presentation.  However, this kind of data gives us pause when recommending two hearing aids versus one hearing aid for patients.

Binaural Benefits are Underappreciated

The benefits of spatial hearing and bilateral hearing aid fits are underappreciated in the hearing aid research literature, as well as in the clinical world.  If the benefits of bilateral fits and binaural hearing are underappreciated, then the cost-benefit trade-off of having two hearing aids versus one hearing aid becomes less apparent to patients, and that can drive some of the conversation that a clinician might have with their patients about the value of the second hearing aid.  We have been quite concerned with how to show the benefits of binaural and spatial hearing in our research. 

Why is this benefit underappreciated?  This goes back to the history of hearing aid development.  The design and engineering of all hearing aid technology has been focused on methods to restore audibility, which has been driven by a desire to improve speech, particularly in noise.  That is a sensible and reasonable way to proceed.  These days, hearing aids do a fairly competent job at restoring audibility, and unlike a few decades ago, we rarely ever have people experience issues understanding speech in one-on-one conversations and situations with steady noise.  The problem is that hearing disability is characterized not just by poor speech communication in situations with steady noise or in one-on-one conversations.  Stuart Gatehouse, a very prominent researcher, pointed this out.  He characterized hearing disability as having poorer speech communication, access to environmental sounds, spatial hearing, selecting/dividing/switching attention between talkers, and ease of listening, most of which require higher-order brain functioning.  These are all domains which hearing technology really has not addressed, due to the nature of the brain function. 

The outcome measures for assessing hearing technologies do not tap into any of these higher-order brain functions, including binaural hearing.  As a result, we are not assessing the benefit of bilateral fits compared to unilateral fits in settings where binaural function is especially beneficial.  That is part of the reason why the benefits of binaural hearing are underappreciated.   There is a need for generating outcome measures that engage these higher-order brain functions.

Auditory Scene Analysis

I want to talk about a way of thinking for how binaural hearing can be beneficial for spatial hearing, selecting and dividing attention, and even for ease of listening.  Binaural hearing incorporates an auditory scene analysis, which is knowing your environment and identifying sound sources in your environment only through sound.  This is only accomplished through your auditory sense. 

Auditory scene analysis works by using various cues, and one of the most prominent cues is spatial location to sort your environment into its constituent objects.  This happens pre-attentively using these objects that have been sorted to focus your selective attention to the sound sources in which you are interested.  By giving this information to the cognitive system for selecting, dividing, and switching attention, it facilitates some of these higher-order functions.  Auditory scene analysis and binaural hearing are engaged in dynamic environments that listeners frequently encounter in their daily living.  These are the kinds of situations in which bilateral hearing aids would show a great advantage over unilateral hearing aids. 

Noble and Gatehouse (2006) did a study asking bilateral and unilateral hearing aid users to assess how they were doing in some of the following situations: multi-stream processing, attention switching, fluctuating and complex backgrounds, perception of reverberation, speech reception, and listening effort.  They found a clear advantage for binaural hearing aids in complex environments where multiple streams of information are processed and flexibility in attention is paramount, in reverberation, and in conditions where you have to exert a lot of listening effort.

In summary, one of the reasons binaural hearing and bilateral hearing aid fits are underappreciated is, to some extent, a failure to incorporate the elements of real-world scenarios in the outcome measures that we use to assess our hearing technology.

New Research

We are now doing a lot of research now on the benefits of binaural hearing and bilateral fittings that takes into account the insight that we need to incorporate more realism.  I am going to go over the research that we are conducting at the Starkey Hearing Research Center along with university colleagues to develop new outcome measures that capture more realism.  We are doing this to focus on the role of binaural and spatial hearing in complex dynamic scenarios.  The benefit for us is that it helps us understand the full scope of the benefits of binaural hearing, and it gives us new outcome measures that can be predictive of the benefit that people will experience in the real world.  Finally, it helps us assess signal processing alternatives in new ways so that researchers can continue to improve hearing aid technology. 

Binaural Hearing is Important in Multiple-Talker Environments

In collaboration with University of California – Berkeley, we showed that binaural hearing is important for comprehending speech, not just recognizing speech, in realistic multi-talker environments, such as a restaurant or cocktail party.  These experiments aimed to capture, in a laboratory setting, aspects of real-world listening environments that are not captured on traditional speech tests like the Hearing in Noise Test (HINT) or the QuickSIN.  The goal of any speech communication situation is to understand and comprehend information, not just recognize that it is audible or not. 

The second thing we wanted to capture was that information flows continuously.  Lastly, we wanted to capture that we do process information from multiple sources, especially in noisy environments.  Furthermore, we try to eliminate interfering information from sources that we do not care about. 

In the laboratory, we had a listener surrounded by a ring of loud speakers.  We presented speech from a handful of these loud speakers in this ring.  Usually we assigned one of the loud speakers as the primary loud speaker to which the listener should have been paying attention.  A listener gets speech information from each of these different talkers as they would in a noisy environment.  Periodically, we asked questions about the information that was coming from these speakers, with a focus mainly on information from the primary talker.  The question we asked was flashed on a screen and the listener was given two answers from which to choose.   These questions came fairly often.

The primary talker may have been saying, “Ever since a young child, she had been instinctively nervous, jumping at shadows all too often.  At one time it had made her parents laugh.”  This is a passage from a story that is continuously playing.  The question we would ask, which would focus on understanding rather than recognition of key words, was, “The woman was: a) easily startled; or b) usually calm.”  If you understood this passage, the answer would be easily startled. 

To give you an example of how this might have gone if you had a traditional speech test like the HINT, the alternatives for the answer would have been instinctively nervous or usually calm.  The correct answer would have been instinctively nervous, which you could have gotten correct, even if you did not actually comprehend the information that you were getting and all you did was recognize the speech sounds that you heard. 

When we focus on comprehension and engage the higher-level cognitive process, we saw that increasing the separation between the target talker and interfering talkers improved speech understanding.  You would need binaural hearing in order to perceive this increasing spatial separation because you use binaural hearing for localization of sound.  In this way, binaural hearing is critically important for understanding speech, not just recognizing speech,

Binaural Hearing Helps Reduce the Cognitive Load of Speech-in-Noise

Binaural hearing not only improves your ability to recognize and understand speech, but it also make it less effortful for you cognitively.  Our next experiment was fairly straightforward.  We compared two different situations, where one situation had three talkers consisting of two males and one female.  The participants in the experiment were instructed to pay attention to the female talker.  In one condition, there was a gender difference to help you pick out the person to whom you wanted to listen.  In the second condition, there was both a gender difference and a difference in spatial location.  Binaural hearing would be very useful to get the spatial location, and our hypothesis was that it would be less effortful in the latter situation. 

We can ask people how much of the talking they understood to determine speech understanding, but how do we get at what their cognitive load is?  We used a dual-task paradigm, where the primary task is to repeat key words of the target speech.  It is to recognize the speech.  I will play for you the kind of speech that we used in this experiment.  There will be a mixture of male and female talkers, and you have to recognize the key words of the female talker. 

This is pretty hard, but if you do it a couple of times, people can do the task.  The female talker said something like, “Ready, Baron, go to green five now.” 

How do we determine the cognitive load of speech understanding?  We do a second task at the same time as the speech task.  The notion here is that we have a limited amount of cognitive effort we can devote to anything.  The primary task takes a certain amount of cognitive load and whatever is remaining would be deployed toward the secondary task.  If the primary task gets easier, if it takes less cognitive load, it frees up more cognitive load to devote to the secondary task.  It is a way to use performance on the secondary task to index or indicate how much cognitive load the primary task has taken.  The better you do at the secondary task, the less cognitive load the primary task has taken. 

Our secondary task used a computer screen and a moving white dot.  The subjects had to track the white dot, which moved at random across the screen, using a red ring that they controlled with the computer mouse.  Keep in mind they were doing this while listening to the speech as well.  The performance on this secondary task indicates how much cognitive load the primary task is taking. 

We evaluated both the cognitive load and the speech recognition for both gender and combined-cue conditions.  When we added the location cue in addition to the gender cue, there was a reduction in the cognitive load compared to the gender-only condition.  That is true even when there was no improvement in speech recognition.  This is a very important result because it shows that the presence of this location cue by way of binaural processing helps with cognitive load; it makes listening easier. 

Bilateral Fittings Increase Ability to Recognize Speech in the Presence of Other Speech

The traditional speech tests like the HINT use speech-shaped noise as the interference.  That is quite different than the speech interference that we encounter in real life.  This research was a collaboration with the University of Manchester.  We simply measured speech recognition performance in a situation where subjects had to listen to female speech in the presence of other female talkers for a unilateral hearing aid fitting compared to bilateral hearing aid fitting.  This was an extremely well-controlled experiment with controls to rule out better-ear effects and those kinds of things.  There was a tremendous amount of attention to detail, and it was a very clear demonstration that there was a significant benefit of (1.5 dB) for bilateral fittings over unilateral fittings.  This may not seem like a lot in dB, but it equates to 10 to 15 percentage points on the percent-correct score for speech understanding. 

Other Experiments

We have several other experiments under way that are investigating the additional benefits of bilateral fittings over unilateral fittings for other deficits that people with hearing impairment face.  One is looking at people’s spatial awareness of their acoustic environment when under high cognitive load.  We think the difference between unilateral and bilateral fittings will be fairly significant in this situation.  Secondly, we are looking at people’s ability to switch attention rapidly between talkers.  This is a situation that people encounter quite often.

What Have we Learned?

What have we learned from this kind of modern research?  I think a more complete picture of the role binaural hearing and bilateral fitting plays is emerging from these outcome measures that incorporate greater realism. 

Factors Limiting Benefits of Bilateral Fittings

Recall one of the cautionary things I had mentioned earlier.  Walden and Walden (2005) showed that there were a significant number of listeners who were not doing better with two hearing aids compared to one. I am going to talk a little bit about what limits the benefits of bilateral hearing aid fittings. 

Mild Hearing Loss

The first thing that can limit the demonstration of bilateral fit over unilateral fit, perhaps even in real world settings, is if the listeners have mild hearing losses in both ears.  If you have a mild hearing loss, the ear that is not aided still provides sufficient amount of hearing to enable listeners to pick up a sufficient number of binaural cues to get some amount of binaural function.  I would contend that it is when you get into really challenging environments where there is a tremendous amount of cognitive load on listeners where the relatively small degradation of binaural cues caused by mild hearing losses might still show up as a deficit.  However, that remains to be seen with future experiments. 

High-Frequency Hearing Loss

A second factor that might limit the demonstration of binaural benefit is high-frequency hearing loss in the presence of normal or near-normal low-frequency hearing.  If you have good low-frequency hearing, you still have decent access to interaural time difference cues, which can provide at least some of the basic binaural hearing benefits, like sound localization. 

Hearing Aid Signal Processing

A third factor is that binaural cues may get disrupted by hearing aid signal processing, especially if the hearing aid is not fit appropriately.  Kalluri and Edwards (2007) demonstrated that very fast-acting compression can distort binaural cues, particularly interaural level difference cues that can reduce the benefit of spatial separation between target talkers and interfering talkers.  We have shown that this benefit of spatial separation in dB could be reduced when you have compression acting independently at both ears. 

Limited Binaural Roles

Another factor would be examining scenarios where binaural hearing has a limited role.  This is something I have mentioned in depth throughout this presentation.  There might be a certain role for binaural hearing in understanding speech in noise, but if you have speech in noise coming from the same location, there is no benefit of binaural hearing as there are no spatial cues to be perceived.  This is exactly what happened in that influential study from Walden and Walden (2005), where they noted that people were doing worse with two hearing aids compared to one hearing aid.  However, they were testing listeners in a situation where target and interfering sounds were coming from the same location, where binaural hearing would not be beneficial anyway.

Binaural Interference

The final factor that drives limitation in the observations of binaural benefits is that some listeners experience something we call binaural interference.  Typically we think that the brain benefits when information is combined from the two ears.  But for approximately 10% of elderly patients, the information from a second ear interferes with the information from the first ear (Jerger, Silman, Lew, & Chmiel, 1993).  For reasons we still do not understand, the information from the second ear causes a negative binaural interaction.  What I would conclude is that except in about 10% of elderly patients who experience binaural interference, bilateral hearing aids should be beneficial, especially in complex dynamic noisy scenarios where you have to be flexible with deploying your cognitive resources. 

What Does this Mean for Clinical Practice?

I would say that bilateral hearing aid fits should be the default in our clinical practice.  Unilateral fittings should be the exception.  For those concerned about binaural interference, it is relatively easy to figure out when that scenario is taking place.  The second thing is that you can counsel that bilateral hearing aid fittings are beneficial in complex dynamic scenarios with this emerging information.  There is a benefit for ease of listening, as well as speech understanding.  These are the benefits that may not be immediately apparent to patients, but when they persist, these things become evident. 

Finally, I think this research means that you will be working with future outcome measures that will incorporate more realism, not just in the kinds of environments that listeners will be experiencing, but also in the kinds of tasks in which we ask them to engage. 

Questions and Answers

Older adults sometimes do better with monaural fittings.  Did your studies address the cognitive status and auditory process?

I will say that the particular studies that I presented do not pull out those 10% of patients who would be hurt by bilateral fittings compared to unilateral fittings, but that is something we will be looking into in the future. 

Are there any measures that we can use to predict success with binaural fittings?

My current recommendation is that your default should be to fit bilateral by default and to fit unilateral as the alternative or exception, if it is warranted. What you can use to predict that could be as simple as doing a standard speech test with one ear and both ears. Worse performance with both ears than one ear can be an adequate predictive measure. There are also more sophisticated measures of binaural interference out there that could be used. However, I am not 100% sure how easy they are to use clinically.

References

Arlinger, S., Brorsson, B., Lagerbring, C., Leijon, A., Rosenhall, U., & Scherstén, T. (2003). Benefit of two hearing aids compared to one. In Hearing aids for adults: Benefits and costs.  Retrieved from https://www.sbu.se/upload/Publikationer/Content1/1/hearingslut.pdf

Jerger, J., Silman, S., Lew, J. & Chmiel, R. (1993). Case studies in binaural interference: converging evidence from behavioral and electrophysiologic measures. Journal of the American Academy of Audiology, 4(2), 122-131.

Kalluri, S., & Edwards, B. (2007, September).  Impact of hearing impairment and hearing aids on benefits due to binaural hearing.  Presentation at the 19th International Congress on Acoustics, Madrid, Spain.

Noble, W., & Gatehouse, S. (2006). Effects of bilateral versus unilateral hearing aid fitting on abilities measured by the Speech, Spatial, and Qualities of Hearing Scale (SSQ). International Journal of Audiology, 45(3), 172-181.

Walden, T. C. & Walden, B. E. (2005). Monaural versus binaural amplification for adults with impaired hearing. Journal of the American Academy of Audiology, 16(8), 574-584.