Dr. Ross on Hearing Loss
Are Binaural Hearing Aids Better?
by Mark Ross, Ph.D.
Are binaural hearing aids better? An interesting and provocative article in the September 2005 issue of Journal of the American Academy of Audiology (JAAA) by several respected researchers (Therese and Brian Walden) has reopened this question, one that I thought had long been settled in favor of binaural relative to monaural hearing aids. Before reviewing and commenting on this article, it may be helpful to first put the issue into a broader historical perspective.
With the first few generations of electronic hearing aids, there was no choice but to amplify just one ear. These were big and bulky instruments and wearing one was just about all a person could easily tolerate. This same reasoning prevailed during the era of the smaller body-worn hearing aids. The norm at the time was to amplify just one ear, with the determination of which ear to amplify emerging as a major clinical question. (Generally, the decision was to amplify the poorer ear, provided the unaided ear could make a significant hearing contribution on its own, or the better ear when the poorer ear was just too impaired.)
In the middle l960s, as a consequence of a Maternal Rubella epidemic, audiology centers were suddenly inundated with a large number of congenitally hearing-impaired children, all of whom required careful evaluations and appropriate management measures. In addition to the expansion and development of other educational option (day programs, parent-infant programs, mainstreaming, etc.), their presence inspired a rethinking of the way hearing aids were fit. Many parents, in particular, were not satisfied with fitting just one hearing aid to the children. They reasoned that if one aid helped, two could possibly help even more. Besides, they could see that everybody else had two ears and they couldn’t quite see the logic of amplifying just one ear for their children. Soon, it became quite common to see young children wearing frontal harnesses with two pockets, into which body hearing aids were inserted. (Indeed, the fabrication of these harnesses became a quite a cottage industry among some of the parents.) This practice was followed in spite of the fact that there was little objective evidence attesting to the superiority of binaural versus monaural amplification. Because these children usually exhibited severe language delays, it was not feasible to conduct objective studies to evaluate the presumed listening superiority of binaural hearing. Parents and clinicians made their judgments on the basis of careful behavioral observations and the desire – rightly in my estimation – to give the children the benefit of any doubt.
Generally, hard of hearing adults did not follow the lead of these children and begin wearing binaural body hearing aids. For them, it was not until behind-the-ear (BTE) hearing aids became available that the issue of binaural versus monaural hearing aids emerged as a significant clinical topic. For the first time, it was now possible for people to conveniently wear two hearing aids. But before adopting this practice, however, skeptical professionals wanted to see the evidence of binaural superiority. In other words, the proponents of binaural amplification had to prove their case, as monaural amplification was then the standard practice.
This, it hardly needs to be stated, is unlike the normal human state. The usual assumption is that having an ear on each side of our head – binaural hearing – is the normal condition and that having just one functional ear is an abnormality. Indeed, in the Nov/Dec 2005 issue of “Hearing Loss” several unilaterally hearing-impaired people report on the problems they experience with a hearing loss in “only” one ear. The professional literature has many similar reports.
There are a number of documented advantages of binaural hearing. The most obvious one, perhaps, is the ability to perceive sound signals directed to either ear. For example, consider a driver with a dead right ear attempting to hold a conversation with a passenger sitting besides him or her (or, conversely, a passenger with a dead left ear attempting to converse with the driver). Their difficulties are obvious. The ability to localize the source of sound depends upon the brain processing two incoming acoustic signals rather than just one. This capacity not only has implications for understanding speech and responding to one’s environment, but can also be a safety issue as well (like the likelihood of turning to the side of the good ear when a vehicle is coming from the other direction – not a very good survival practice). Then there is the phenomenon known as “binaural squelch,” in which the central auditory structures are able to suppress interfering noises while focusing on the speech of just one person in a noisy environment.
Binaural redundancy, as when the brain receives same information from both ears independently, has also been cited as a binaural advantage. Because of the brain’s ability to synthesize dissimilar information arriving from each ear, the overall (two-ear) perception is usually greater than that occurring from each ear separately. Central auditory tests in vogue years ago used just this synthesis capacity to evaluate possible problems in the central auditory pathways. An example of such a test was when just low frequencies were delivered to one ear and high frequencies to the other. A normal hearing listener would be able to fuse the separate information arriving from the two ears and understand speech better than with either ear alone. It is, indeed, just this capability of the central auditory system that rationalizes the fitting of binaural hearing aids to someone with a somewhat different hearing loss in each ear. Finally, the subjective appreciation of auditory space, the reduction in communicative effort, the confidence and speed of one’s responses, have all been noted by hearing aid users as unique to the binaural listening experience.
Audiologists were very familiar with these theoretical and practical advantages of binaural relative to monaural hearing for normal hearing people. As the issue emerged in professional circles, the question posed was whether these same advantages also applied to people with hearing loss who, by definition, evinced some sort of psychoacoustic abnormality in the auditory system. Furthermore, clinicians were faced with the practical issue of recommending a second hearing aid to adults who not only didn’t even want to wear one aid, but who were also understandably reluctant to pay for a second expensive hearing aid. There was an obvious need for research that could provide the evidence that audiologists needed first to convince themselves, and then to convince their clients, regarding the merit of binaural hearing aids.
The profession soon responded with a large number of studies designed to compare binaural and monaural amplification. By 1980 there were at least 25 such studies that I personally reviewed in a book chapter. Since then there have been many others. Generally, the conclusions made 26 years ago still appear to be valid: “...the overwhelming preponderance of evidence still supports the notion that for most hearing-impaired, two ears are better than one.” But this is a general conclusion. We must also recognize that there have always been credible reports and studies of individuals who did better with one than with two hearing aids. It has always been felt that this applied to only a relatively few individuals. However, the implications of the JAAA study are that this number may be greater than we had believed.
The purpose of the study was to compare speech perception scores obtained with monaural and binaural hearing aids. Rather than using conventional speech discrimination scores, the researchers used a measure termed the signal-to-noise ratio (SNR) loss to make this comparison. Essentially, this is the intensity level of a speech signal relative to the intensity of the noise (the signal to noise ratio) that permits a listener to obtain a 50% score in a sentence comprehension task. For example, if someone with a hearing loss needed speech to be 8 dB louder than the noise in order to understand 50% of the words in a sentence (e.g. 68 dB speech and 60 dB noise), then this would translate into an 8 dB SNR loss. The average normally hearing person would achieve the same 50% at 0 dB S/N. In this test, therefore, the higher the number, the poorer the performance. While 8 dB may not sound like much, we should keep in mind that even a one dB increase or decrease in the SNR can change conventional word intelligibility scores by 6 or 8 percent. Quite clearly, even this much can make quite a difference to the way someone understands speech.
Twenty-eight patients were tested, all of whom had moderate, bilaterally similar losses in both ears. They ranged in age from 50 to 90 with an average age of 75. Sitting in front of a loudspeaker which delivered both the speech and noise signals, the subjects were first tested without hearing aids, then with the hearing aid in the right ear, then in the left ear, and finally in both ears. The task for the subjects was to repeat six five-word sentences played at increasingly smaller signal to noise (S/N) ratios. The SNR loss was computed from the total number of words the subjects correctly repeated.
What they found was rather surprising and may herald a new interest in this old controversy. They showed consistently poorer average performance in the binaural condition than that obtained monaurally. The scores were not only poorer than the ones obtained in the better ear, but often than those obtained in the poorer ear as well. Overall, 23 of the 28 subjects performed better with monaural hearing aids than with binaural amplification (83%). In only three instances were the binaural scores better than the best monaural scores (two others were equal). According to these results, therefore, binaural amplification not only didn’t make things better but actually worsened the overall listening situation; most of these people would have done better with just one hearing aid.
The authors’ explanation for this anomalous result is that in the presence of background noise some people, particularly older folks, are more susceptible to the interfering effects of conflicting information being presented to the brain by the two ears. They invoke the possibility of an age related progressive atrophy (or demyelination) in the neural fibers connecting the two hemispheres of the brain. This would produce a delay or loss of efficiency in the transfer of information from one side of the brain to the other, producing binaural interference rather than the expected binaural enhancement. While it would be a stretch (in my mind) to label this condition pathological, at the least it appears that the connection between the two hemispheres of the brain is malfunctioning to some extent, particularly for some older people.
The notion of binaural interference is given further support in another of their key findings. On average, the scores obtained in the right ear were significantly better (i.e. less SNR loss) than those observed in the left ear. This was true for 22 of the 28 subjects in the study. Since the hearing losses in both ears were symmetrical, this result cannot be explained on the basis of different audiograms in the two ears. We know that there are more and richer neural connections crossing over from the right ear to the left hemisphere than directly to the right hemisphere. The converse is true for signals arriving at the left ear. Since the left hemisphere is usually dominant for speech perception this helps explain the superior scores in the right ear. The poorer scores in the left ear were attributed to problems with the interhemispheric transfer of information from the right to the left side of the brain.
The authors suggest that binaural hearing aid users experiencing difficulty hearing in a noisy situation (and when doesn’t this occur?) be advised to remove one hearing aid (the left aid for most) and then try to determine if their overall listening abilities are improved or worsened. Since the investigators found a relationship between age and the extent of the interference caused by the poorer ear, this may be particularly important suggestion for older individuals. Since no one can predict in advance who would and who would not benefit from this practice, it does seem to be worth a try since we do know that some people apparently do better with one than with two hearing aids. But the total binaural experience has to be considered, and not just the ability to understand speech in one specific type of situation. Additionally, these results only apply when listening to speech in noise and not in quiet. . .
The more zealous binaural hearing aid advocates (and I number myself among them) may find the results of this study a bit disquieting. From their perspective, it may seem that the profession is back to where it was some 50 years ago. But this is a well-executed study conducted by very respected professionals and it must be addressed on its own merits. In interpreting the results, however, we should keep a few points in mind.
Only one physical configuration was used in testing for speech; i.e. the speech and noise were both delivered from the same loudspeaker located directly in front of the listener. As the authors themselves point out, the same results may not occur with different speech and noise configurations. It is necessary to evaluate more common and realistic situations, in which the speech source is frontally located, but where the noise emanates from different, often changing, directions (i.e., spatially separating the speech and noise signals). I would hope that we will soon see some follow-up studies to this effect, or where the predominant noise signal randomly shifts from one azimuth to another. Actually, it was this latter occurrence in the real world that rationalized the development of adaptive directional microphones, now found in many modern hearing aids.
Also to be considered are the other benefits of binaural hearing. No one doubts the value of being able to localize the source of sound, or the capacity to receive sound signals directed to either ear. The binaural summation of loudness is another measurable dimension associated with binaural hearing. When two ears receive an auditory signal, the resulting loudness sensation is greater than that occurring with just one ear. And then there are the difficult to quantify, but still very real, subjective sensations of the binaural compared to the monaural listening experience. How does one describe an almost three dimensional feeling of auditory space? What is noteworthy for many of these dimensions is that they are mediated by the lower auditory centers in the brain and thus are unaffected by possible problems at the higher levels. In other words, there are a number of other advantageous binaural sensations, in addition the possible existence of binaural interference in speech perception in noise. The total experience has to be weighted.
The hypothesized reason for the poorer binaural performance has been attributed to problems in synthesizing the auditory information arriving from the two ears, with the inter-hemispheric transfer of information cited as the likely location for the interference. There is nothing in the study that suggests that the subjects had other than a hearing loss of cochlear origin. Presumably, therefore, their central auditory status would be comparable to their normally hearing age peers. The study found that 83% of the hard of hearing subjects did better with one than two ears. What this implies, if we generalize these results to the older people with normal hearing in our society, is that 83% of them would do better listening with one ear in noise (the right ear for the most part) than they would with two ears. This is hard to accept. In my opinion, amore reasonable explanation for the findings in the JAAA study is that the results either reflect super-threshold asymmetries in the sound sensations in the two ears or, more likely, that the experimental set-up (single frontal source for the noise and the speech) did not sufficiently challenge the binaural system.
Hearing aids are not the only type of hearing device for which one ear and two ear listening has been compared. Since the advent of cochlear implants, a number of studies have compared hearing performance with just a cochlear implant and when the subject also used a hearing aid on the non implanted side (for those with some residual hearing). In the last few years this has been a rather popular research paradigm, with published studies conducted in a number of different countries in addition to the U.S. Generally, the results indicate superior performance in the bimodal condition (implant plus hearing aid) than with the implant alone. The dimensions evaluated include sentence perception in quiet and noise as well as localization ability. In noisy conditions, the bimodal scores were higher in one study when the noise and speech were spatially separated, rather then when the noise and speech emanated from the same loudspeaker (which, as will be recalled, was the test condition in the JAAA study). But it is important to keep in mind that this is a general conclusion and would not apply to every person wearing a bimodal arrangement. In the U.S. study, for example, only two of the 12 subjects showed superior localization ability in the bimodal condition (though binaural advantages were noted in other tests). It should also be noted that while bimodal scores were not always higher in all listening conditions for every subject, neither was performance poorer in the bimodal mode.
There have also been numerous studies that compared bilateral to unilateral cochlear implants. Again, the results generally favor the binaural condition. Comments in some of the studies indicate a learning effect, in which bilateral performance continues to improve over unilateral performance as the subject gain more experience wearing two implants.
Considering all the research and acquired experience over the years, it does seem quite obvious that binaural, and not monaural listening, is the normal situation for human beings. Not only are we endowed with the means to detect sounds from either side of the head but, less visible perhaps but no less pertinent, we also have the central nervous system’s capability to process the slightly different auditory information arriving from the two ears. This is the capability that makes, for example, localization and the “squelch” effect (the suppression of competing sounds in a noisy environment) possible. People with normal hearing can easily get some idea of the benefits of two-ear listening simply by blocking up one ear in a noisy situation.
Our intention, when we use a hearing aid in each ear, is to try to replicate the normal advantages of binaural listening. The question we have to ask, however, is whether there is some complicating factor about a specific hearing loss that prevents this from happening. We are often reminded of the fact that a hearing loss may encompass more than just loss of hearing acuity at different frequencies (as displayed on a simple audiogram). Two ears with identical thresholds may still differ in a number of other auditory dimensions, such as the ability to resolve time differences or separating out the components in a complex sound (like in a piano chord). It is likely that problems at any level in the auditory pathways may cause interference in a binaural signal rather than an enhancement. So what to do?
My suggestion is that we go with what is usual and what is normal and begin by considering everyone with a bilateral hearing loss a binaural hearing aid candidate, whether or not their hearing loss is bilaterally symmetrical. We know that for most people binaural listening is advantageous. But we also have to acknowledge that this would not be true for everybody with a bilateral hearing loss.
I would advise potential hearing aid candidates to start with binaural hearing aids and then, if they think that they’re not helping very much, then try using just one aid (ordinarily in the right ear). I would not advise switching to monaural if people who appear to be functioning equally well with two than with one aid; only for those who function poorer with the two compared with one alone. My reasoning for this is twofold: one is that for some people it may take a few weeks or months for the full benefit of binaural hearing to emerge. The second reason is that even the person who feels that two hearing aids are not helping improve speech perception may still experience enough of the other potential binaural advantages to justify continuing to try two. As far as I’m concerned, therefore, binaural hearing aids should still be the norm, to be varied only when two aids are demonstratively poorer than one. But, given the results of the JAAA study, I do think it is time to reevaluate the entire question, but being sure to employ realistic testing conditions and test dimensions. It is necessary to evaluate all the factors underlying the binaural experience. My bet is that we would find that Mother Nature had it right the first time.