The fact that the first few generations of hearing aids were only worn monaurally (in one ear) had little to do with the best fitting arrangement and everything to do with their size. These aids were real clunkers, with a separate battery pack and microphone/amplifier, each about twice the size of personal stereo, connected to each other and earphone/receivers by thick wire cords. Even the most determined and motivated person would have difficulty wearing and getting "hooked up" to two such hearing aids. This same attitude carried through when the first "monopack" vacuum tube hearing aids were introduced, about l950. Not only were there few people who were willing to wear two of them, but very few professionals would seriously recommend them. So the first few generations of hearing aid wearers used monaural aids by default. No other possibility was very realistic (An exception was made with children who, in the late l960's, were routinely fit with body-worn binaural hearing aids; this practice begs the question why a desirable practice with children was not even considered for older people!).
If the first electronic hearing aids could have been worn at ear level, then it is likely that the lesson that vision has taught us would have been unconsciously applied. We would have just assumed that two ears were better than one, in the same way we know that two eyes are better than one (monocles are only "in" in bad WW II movies). In other words, two-ear listening would have been the routine expectation, and anything else would have had to be justified. This explains why when hearing aids appeared that could conveniently be worn at ear level, Audiologists found themselves having to rationalize a recommendation for a second hearing aid (a binaural fitting), since the routine fitting at that time was just one hearing aid. They felt they couldn't suggest a binaural fitting unless they could demonstrate that the addition of the second ear provided significant benefits to the hearing-impaired person.
The practice that evolved was for many audiologists to first fit a person with one hearing aid, and then, given some benefit from the hearing aid, convince the person to try a second one. The problem with this practice was the borderline hearing aid candidate or the person whose expectations could not be fullfilled with a single hearing aid. What frequently happened, and still does, is that the monaural fitting does not provide some mild or moderately hearing- impaired people with quite enough assistance in important situations to meet their expectations. For others, fitting a hearing aid to just one ear minimizes or eliminates the possible contribution of the unaided ear. Sometimes, particularly in noise, a person with a bilaterally symmetrical mild-to-moderate hearing loss will function better without hearing aids than with just a monaural fitting. Because of these factors, many such people reject the monaural hearing aid. By beginning with a binaural fitting, however, many, if not most such people, could have realized sufficient subjective benefits from amplification to benefit from and accept hearing aids. Clearly, then, the "traditional" fitting routine should be reversed; it should initially be assumed that a hard of hearing person is a binaural hearing aid candidate - unless there are definite and clear contraindications to this kind of fitting (see below).
In the years since binaural hearing aids became a practical possibility, Audiologists conducted literally hundreds of research studies to reach the obvious conclusion that two ear listening is generally superior to one ear listening; they should have trusted the evidence of their own ears! Now, just about everybody would agree with this judgment. Everybody, that is, except many new and prospective hearing aid users. Some of these people react very negatively when an audiologist first broaches the idea of a binaural fitting. Their response may vary from "my hearing is not that bad!" to dismay about the extra cost involved. It is important for consumers to realize that their Audiologist is not trying to rip them off when he/she recommends a hearing aid for each ear. In most instances, this is the professionally responsible judgment. There are definite advantages in binaural listening as well as possible problems associated with not wearing a hearing aid in each ear.
The usual explanation given by audiologists is that localization of sounds is easier with two ears than with one. And this is generally true, even for hearing aid wearers. The ability to hear better in noise relates to the localization function; localization permits someone to selectively attend to the desired signal and squelch the undesired sounds. This is the kind of information developed by the years of research referred to above. But there are other, less formal advantages, which nevertheless could be very significant for a hearing aid user. With two hearing aids, there is a sense of spaciousness, of acoustic depth, sensations which do not occur with monaural listening. This is an esthetic dimension and does not easily lend itself to formal measurements. Additionally, understanding speech, particularly in acoustically stressful situations, takes less energy when one can listen binaurally. Finally, with two hearing aids, one always has an "ear" ready to go on both sides of the head. (I well remember the times, when I wore an aid only in my left ear, practically breaking my neck when someone tried talking to me from the right side!).
What happens if someone has a bilateral hearing loss and does not wear a hearing aid in each ear? In recent years, a phenomenon termed "adult onset auditory sensory deprivation" has been identified. What this refers to is a decrement in the unaided ear's ability to comprehend speech signals after a number of years in which it had not been stimulated by sound. (not a worsening of the basic hearing thresholds). This is not necessarily a major decrement, nor one that cannot be eliminated or reduced if the deprivation did not last too long. While the phenomenon is real and has been demonstrated in a number of retrospective research studies, the details are still undefined. People vary in their susceptibility, the extent to which they may be affected, and the reversibility of the deterioration in speech perception skills. We do know enough to be cautious and to try to preclude any possibility of adult onset auditory deprivation by Audiologists routinely recommending binaural hearing aids. Incidentally, this recommendation accords with one of the position papers taken by the consumer group, Self-Help for Hard of Hearing People.
Certainly, not every hearing-impaired person can potentially benefit from binaural hearing aids. If someone is completely deaf on one side, then no amount of amplification is going to permit this ear to make a hearing contribution (but, it should be noted, that there are some fittings that actually place a hearing aid in a "dead" ear, with the intent of stimulating the "good" ear via bone conduction vibrations transmitted through the head). Great disparity in hearing thresholds between the two ears is also thought to be a contraindication of binaural success. This may be so, but I would suggest that the initial clinical presumption be that the distinct auditory contribution made by each ear can be integrated and reconciled in the brain. The possible result being higher overall speech perceptions than observed with each ear separately. Unless the addition of a second hearing aid can be demonstrated to significantly reduce overall speech perception abilities, I would suggest trying a second aid for increasing periods of time each day for at least a month or so. Only then, can one be moderately certain that the second aid is inhibiting rather than enhancing overall speech perception.
Trinaural Hearing
No, I don't know any three-eared people, but I do know people (including myself) who make use of three microphones on occasion. What I'm referring to is a fairly recent addition on the hearing aid scene: BTE/FM hearing aids that come with an external FM microphone/transmitter. For adults, this latter device is the key to its possible benefits. They are about an inch in diameter and five to six inches long, smaller than an eyeglass case and easy enough to keep in a pocket or purse. These devices transmit an FM radio signal, on a special wave length, to a BTE hearing aid that also incorporates an FM "radio" receiver (either built into the aid or appearing as an external "shoe" fitting under the base of the hearing aid). The BTE/FM system can function only as hearing aids, picking up only acoustic signals; just as a radio receiver, picking up only the signals emanating from the FM microphone transmitter, or set to detect both radio and acoustic signals simultaneously.
What's neat about this is that the FM microphone/transmitter serves as an external "ear", a third ear if you will. This can be placed next to any sound source and it will transmit the signals directly to the person's BTE/FM receivers. It is the equivalent of someone talking directly into your ear (or hearing aid), or placing your ear (or hearing aid) right next to a loudspeaker. Though these are not exactly practical possibilities, they would improve speech perception. With the "third" ear, this can be done without embarrassment or physical gyrations. The simple act of locating a microphone close to a sound source increases the signal strength relative to the background sounds. What results is an increase in the speech-to-noise ratio, very likely the most important factor underlying speech perception for anybody, hearing-impaired or not.
I use my personal BTE/FM system whenever I go to a reception or any noisy gathering. By casually holding it as close as I conveniently can to a speaker, my ability to understand is significantly improved. In a restaurant with one other person, I ask that the microphone be clipped six or so inches from my companion's mouth. In a noisy car with a number of other people, I pass the microphone around. In a motel, watching TV, I place the FM microphone next to the loudspeaker. Walking the streets of a noisy city, I ask my wife to wear the microphone; that way I don't have to practically hug her in order to get close enough to hear. In meetings, or at a dinner with several other people, I place it next to the most difficult person to understand, using my hearing aid microphones to pick up the easier ones. In all of these situation, and I've just touched the surface, the addition of a movable "ear" tremendously increases my ability to understand.
Until recently, I did not use my personal BTE/FM system routinely because of an inadequate telephone coil. This has since been corrected in the two systems I recently tried. The FM microphones on these two systems also include directional capabilities, which increases their capacity to reject unwanted background sounds. Of all the recent developments in hearing aids, and I include the advent of the impressive digital instruments, this is my favorite. It is the one development, and really the only one, that can directly increase the speech to noise ratio. Having a "remote" ear available, being able to function in a "trinaural" mode, gives a user a great deal of listening flexibility. The down side, for some people, is that one can't hide these instruments in the ear canal; they're BTE instruments and they look like hearing aids. This factor has to be weighed against the auditory advantages they can confer. The problem is that most hard of hearing people don't know that these systems exist, and unless so informed by their Audiologist, they never will. While most people would probably elect not to wear this kind of system, some people will - after being apprised of its potential contribution to their functional auditory capabilities.
Acknowledgement:
This article was supported, in part, by Grant #RH133E30015 from the US Department of Education, NIDRR, to the Lexington Center, Jackson Heights, NY.