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   Rehabilitation Engineering Research Center
   on Hearing Enhancement

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Dr. Ross on Hearing Loss

Cochlear Dead Regions

by Mark Ross, Ph.D.
This article first appeared in
Hearing Loss (Nov/Dec 2002)

Most hearing-impaired people have poorer hearing at the higher frequencies than the lower ones. In responding to this fact, one of the common clinical procedures in fitting hearing aids to such people is to provide additional amplification in the higher frequencies. This practice is important, necessary, and recommended - but often problematical.
It is necessary because high frequency speech sounds, like the consonants in general, convey more of the information required to understand speech than the lower frequencies do. As an important example, consider the sound (phoneme) /s/. Its sibilant acoustical energy extends across a wide frequency range, from about 3000 Hz to over 8000 Hz, thus making it difficult, or sometimes impossible, for people with high frequency hearing loss to perceive. There is good evidence to show that hard of hearing people make more perceptual errors with the /s/ phoneme than with any other sound in the English language. This is doubly unfortunate, since the /s/ sound is not only one of the most frequently occurring sounds in the English language, but it is also one that provides more grammatical information than any other sound. (Think how pluralization, possession, contractions, etc. are formed.) It is, therefore, a sound that, if at all possible, we would like people to detect (or, at least, some of the lowest frequency components of the sound). What is true for /s/ also applies to many other consonants, if to a lesser degree.

But, the decision to provide more high frequency amplification is problematical because of the varied reactions of people initially fit with hearing aids. Some people just love, almost from the very beginning, their re-introduction to the world of high frequency sounds. For them, the clatter of dishes, the noise of food sizzling in a frying pan and the shuffling of paper are greeted like old and welcome friends. They just can't wait to hear more of them.

Then there are some other people, probably the majority, who need time to reintroduce themselves to these sounds that they haven't heard for years. Actually, it's not just their ears that require this "re-introduction," but the entire auditory system, from cochlea to cortex. These are the people for whom a gradual period of adjustment is recommended, perhaps by increasing the amplification of the higher frequencies over some weeks or even months. The rewards of going through this process are worth the trouble, since it can mean increased use and benefit from the important high frequency components in speech signals. Speech, in other words, would be easier to understand.

And then there are some people, relatively few I think, who find that amplification of the higher frequencies is either of no benefit or is positively detrimental. These are the people who may exhibit what has been termed "cochlear dead regions." What this means is a loss of function of inner hair cells within some region in the cochlea. The inner hair cells are the sensory receptors that are responsible for converting acoustic energy into electrical energy by directly stimulating the fibers of the auditory nerve. (There are also outer hair cells within the cochlea, whose function it is to "amplify" and analyze very soft sounds. The outer hair cells tend to be much less robust than the inner hair cells and usually exhibit the pathological changes producing hearing loss long before the inner hair cells are involved.)

Damage or absence to the inner hair cells means that the corresponding auditory nerve fibers will not be stimulated; i.e. there is a dead spot. When a person's hearing loss exceeds about 60 or 70 dB, particularly at the higher frequencies, it is a good indication that there is starting to be some involvement in the inner hair cells. Since there is a great overlapping of inner hair cells and their corresponding auditory nerve fibers, the appropriate fibers may still be stimulated in spite of the loss or damage of a significant number of inner hair cells. As the hearing loss increases, however, so does the likelihood that additional inner hair cells are damaged. In this case, there may be a true "cochlear dead region" affecting an entire region of the cochlea.

What is often deceptive is the apparent measurement of high frequency thresholds in spite of the presence of a cochlea dead region. When presented with very loud high frequency sounds (perhaps at 3000 Hz or 4000 Hz) many people report a change in the quality of the test signal. They report hearing "something", perhaps like clicks, a buzz, or hiss. The audiologist may then dutifully note a measured threshold at that point, thus giving the impression that valid hearing thresholds were present. In actuality, however, there may be no residual hearing at these frequencies (3000 Hz and 4000 Hz in this example). These are the people for whom high frequency amplification may actually interfere with their ability to fully benefit from a hearing aid.

What is happening can be explained by the action of normal cochlea dynamics. Each part of the cochlear has a characteristic frequency to which it is "tuned." When input sounds are very soft, only a tiny, specific region of the outer hair cells in the cochlea is activated (a narrow excitation area). The outer hair cells then energize the corresponding inner hair cells that, in turn stimulate a specific narrow area of the corresponding nerve fibers. Now what happens when the ear is exposed to high intensity sounds, such as those produced by hearing aids?

When exposed to high intensity sounds, the cochlea (actually the basilar membrane within the cochlea) displays a broad excitation pattern and quite a large number of adjacent hair cells may be stimulated. In its normal operation, the auditory system suppresses signals arriving from locations other than the portion for which it is specifically "tuned." But, when the greatest movement of the basilar membrane occurs in areas where there are cochlea dead regions, then these adjacent areas, with intact inner hair cells, may stimulate auditory nerve fibers and produce an auditory sensation. Subjectively, however, the audible result of these "off-center" locations may be perceived as noise and distortion and interfere with speech comprehension. What they are doing is sending misinformation to the brain regarding the acoustic composition of an incoming speech sound.

The fact that some or all of the sensory cells within the cochlea are non-functional will come as no surprise to any hearing professional. This, indeed, is the essence of a "sensory" hearing loss (often more broadly labeled as "sensorineural" or "nerve" hearing loss). What may come as a surprise to some, however, is the fact that apparent thresholds in the high frequencies may actually be cochlear artifacts and not really valid indicators of hearing status. While it has long been possible to measure this phenomenon by administering time-consuming psychoacoustic tests, only recently has a test been described that appears to be relatively easy and practical to administer in the average clinical setting - the "threshold equalizing noise" test, or TEN for short. Perhaps further research with the TEN test can help resolve some of the clinical questions that have recently been raised.

In the past few years, a number of articles in professional journals have raised questions about the utility of high frequency amplification for people with severe and profound hearing losses in the high frequencies. These are very pertinent questions for those of us who fall into this category. On the one hand, no professional denies the potential contribution of high frequency speech acoustic information. But, on the other hand, there is a concern that providing audibility at these high frequencies with powerful hearing aids may be counter-productive, that they will detract from, rather than contribute to, speech comprehension. So what is a reasonable clinical approach?

To begin with, I believe that all hearing aid users be given the benefit of any doubt. The value of the high frequencies for speech comprehension is too important for failure to be simply predicted without making a sincere attempt to utilize them. Unless and until such tests as the TEN are routinely administered, there is no way I know of to be reasonably sure that a hearing-impaired person cannot benefit from amplified high frequency sounds. In my judgment, it does not matter how severe the hearing loss in the high frequencies is; if some degree of audibility can be obtained with powerful hearing aids, then this should be attempted. In the past, when it was difficult to provide a great deal of high frequency amplification without producing acoustic feedback, the issue was frequently overlooked. Now, however, with the advent of advanced feedback control circuits in a number of digital hearing aids, it is possible to provide more amplified high frequencies without the complications of feedback. The issue should now, I believe, be moved to the front burner.

Initially, within the constraints of current selection "prescription" procedures, I would suggest that hearing aid dispensers attempt to ensure audibility for all their clients at whatever frequency residual hearing is present. While this may be in small increasing dosages, it should be defined as a definite goal perhaps confirmed at each step by real-ear measures. Only when there is a clear indication that the inclusion of the higher frequencies actually interferes with speech comprehension should the attempt be terminated. While "cochlear dead regions" is more or less a reality for some people with hearing loss, so is the importance of the higher frequencies for speech perception for everybody.

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