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Cochlear Implants:
My Perspective

BY WILLIAM F. HOUSE, D.D.S., M.D.
 EDITED BY DAVID HOUSE
Dr. House founded AllHear, Inc.,
a company that manufactures cochlear implants.

 

 

 
AllHear, Inc.
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Footnotes
Please note that all footnote references
in the text below are "active". If you click
on them, you will jump to the footnote text.
The footnote text, as well, is "active",
so that clicking on the text of the footnote
will bring you back to its reference in the text.

 

 

Introduction



"The reasonable man adapts himself to the world;
the unreasonable one persists in trying to adapt the world to himself.
Therefore, all progress depends on the unreasonable man."

George Bernard Shaw

 

This monograph

Not long ago a prominent otologist and medical school faculty member asked me when it was that I became interested in cochlear implants. I explained that my interest in cochlear implants actually started in 1957 (my second year of practice) when a patient brought me an newspaper article about a totally deaf patient in France who could hear because of an electrical device implanted by Dijourno and Eyries. [1] His reply was: "Wow, that was the year I was born!"

The incident made me realize that there is a growing need for a clear presentation of information about cochlear implants, including some information about: their history; some of the current controversies or questions, as well as some of the misconceptions regarding implants; and the metrics of fitting them. As well, I have decided to offer some information about the AllHear implant, since it came into being as a result of this same information.

 

The audience

Beyond the fact that such a presentation does not, currently, exist in vernacular language in one place, it is also true that the number of people who are interested in implants is growing. It is not only the professionals — the otologists, audiologists, speech pathologists and teachers of the deaf — who need information about this medical technology, but as well the patients themselves, and the families of these patients, are increasingly interested in having a full and understandable presentation of some of these facts and issues. They want to be fully informed.

I therefore expect that you are from one of these groups, and thus, primarily for the reasons presented above, this monograph.

 

Truth, bias, and acceptance

Every such document has a viewpoint, and obviously if I am writing the monograph it will be my viewpoint.

There are many who will disagree with my viewpoint, or think of it as being mere bias. That's fine: its their right to have such a viewpoint (or mere bias).

Indeed, alternative points of view are most welcome: one of the most important purposes of this monograph and subsequent is to stimulate critical thinking and discussion about cochlear implants within the professional community, and to help you, if you are a professional, to further refine your approach to advising implant candidates. On the other hand, if you or a member of your family is an implant candidate, then this monograph will offer you some information about the issues being discussed among professionals, and the evidence which exists one way or another. As such, it will help you choose or develop your own position.

I mention this particularly because the saga of cochlear implants has long been dominated by the conflict between clinical observation and theory. As you read this monograph, it will become clear that my viewpoint is strongly shaped by clinical observation. To put it another way, I admit to considerable distrust regarding unsupported statements which disagree with what I can see unfolding in my patient's lives.

However, the last thing that I want is to be dogmatic and not carefully listen to alternative positions. If I expect others to listen and be persuaded, then I also have to be willing to change my ideas when presented with convincing arguments and clear proofs.

The whole process of change, in fact, has much to do with the advance of science and society as a whole, and it figures prominently in any history such as that regarding the development of implants. General acceptance of what is new does not happen by one sudden turning on of the light. It is a gradual process, as it has been with cochlear implants.

After all, much of what is presented as new, or which is outside of the mainstream of thought at one time or another, turns out to be wrong. Skepticism is healthy and necessary, but at the end of every argument the beholder must make a decision for rejection or for the more difficult act: acceptance of the new thought, previously foreign. It is never easy when we have become comfortable with the present status quo — and particularly if we have loudly espoused it-- to accept a new way of thinking. For whatever reason, healthy skepticism can at times regress into mere obstructionism. Perhaps the difference is found in the line between defending the truth and defending one's self.

I write, therefore, in hopes that we would all recognize the difference, and in preservation of our honor and dignity yield, not to each other, but to the truth, to the degree that we can currently understand it.

 

 

 

Chapter I:

A Brief History [2]
of Cochlear Implants

"Every time history repeats itself, the price goes up."

Anonymous

 

IT HAS BEEN MORE THAN A DECADE since the FDA approved for marketing the 3M/House cochlear implant, in November 1984, and amid national fanfare, as the first device ever approved to replace a human sense.

At the time I remember feeling that implants had finally arrived and that the future was now open for their rapid development and widespread use. I was premature: It is only in the last 2 or 3 years that implants seem to have become truly accepted, and progress, as compared with my expectations, has been quite slow.

Many factors prevented this rapid progress, including technological barriers which have only recently been overcome. But the space program, and particularly the moon shot, demonstrate clearly that determination, whenever it is not fettered by unreasonable hindrances or shattered by disunity, is easily able to overcome technologic barriers. With regard to these sorts of hindrances and the further development of cochlear implants, there remains an undercurrent of feeling that new approaches will only be detrimental to already established devices.

But these conditions will, I am confident, prove as temporary as the other barriers which previously slowed or stopped progress in this area.

 

The Sixties

The sixties saw a number of developments which had a significant impact on the development of implants. It was a time of great changes in otology generally.

The operating microscope, for example, opened up vast new possibilities. The tympanoplasty of Wullstein and Zolner, the stapes mobilization of Rosen, and the stapedectomy of Shea were new and exciting. I was busy pursuing new surgical approaches (intact canal wall and facial recess), exploring endolymphatic sac surgery and doing some early acoustic neuroma work. As well I remained interested in the possibility of restoration of hearing by electrical stimulation of the cochlea. During middle ear procedures under local anesthesia I was able to observe the percepts of patients when small electric currents were introduced to the promontory.

But technical barriers proved frustrating. During the early sixties I implanted several devices in totally deaf volunteer patients. Unfortunately these were rejected due to lack of biocompatibility of the insulating material. However, during the short time that these devices worked, it was obvious to me that this was an opening salvo to the conquest of sensorineural deafness.

During the same decade, Robin Michelson (at the University of California at San Francisco), Blair Simmons (at Stanford) and I formed a sort of implant group and we were actively pursing both animal and human research. There was much skepticism and even outright hostility that we, as clinicians, should be invading the cochlear domain of the neurophysiologists. The obvious feeling was "keep your hands out of our cochlea". Simmons later wrote:

 

Jack Urban

One of the most fortunate developments in cochlear implant development was that Jack Urban, a very innovative engineer, became interested in cochlear implants and teamed up with me to ultimately make cochlear implants a clinical reality.

We each brought different skills and differing outlooks to the process. My orientation was the selection of the patients and the surgical approach for implants. Jack applied his genius for electronics to the problems we faced.

At the time research money was completely unavailable, apparently because of the prevailing feeling that implant research "should not be done." I remember being bitterly disappointed when my request for funding was turned down by a prominent otologic research funding foundation. Fortunately for us all, Jack made his shop and electronic expertise available at no charge. I firmly believe that without Jack, cochlear implants would have taken many more years to develop. Many of us owe him an unpayable debt of gratitude: Jack died of cancer in 1985.

 

First steps

I will explain in more detail in the next chapter, but it is important to mention that we believed, at this point in time, and we did all of our early work starting with the assumption, that we would have to stimulate the snail-shell shaped cochlea discretely in order to mimic its natural function. That is, in a naturally functioning ear, the cochlea "sorts" sounds by their frequency along its length (with the highest frequencies nearest the round window) so that, as far as we know, the nerve fibers apparently responsible for a given frequency are all found in one place. It makes sense, then, that we should have to stimulate these groups of dendrites to cause the brain to interpret a given stimulation as having a certain frequency.[4]

This is known as the "tonotopic" (or sometimes as the "traveling wave") theory, and we believed it.

Because we wanted to stimulate only a portion of the cochlea with a signal which we could control completely and monitor continuously, and because we wanted to be able to inject a wide variety of inputs, Jack developed a through-the-skin pedestal for a direct-connection plug. This was connected internally to a five-wire electrode which had been inserted into the cochlea through a facial recess approach I had developed years earlier for chronic ear surgery. These systems had a plug-in button extending through the skin, and therefore offered complete control over separate stimulation of and direct connection of equipment to each electrode. Although there was a common ground which was placed external to the cochlea, the fact that we had direct access to any of the wires meant that we could use any of them as a ground or active electrode.

Because we had been repeatedly told that electrical currents would destroy the remaining neural tissue of the deafened cochlea, we moved with extraordinary caution, and stimulated only during brief sessions in the lab.

Three patients accepted this hard-wired device, and helped us in our trials of many different processing schemes. We worked most extensively with Charles (Chuck) Graser, a high school teacher who had been deafened by streptomycin, and who was an excellent observer of the hearing sensations that were generated through his system. It would often take months to develop a new processor, incorporating a new signal scheme, and sometimes within a couple of hours working together with Chuck, we might discover that the filter system was not right.

Many different systems of stimulation were tried, within the limits of the circuits we had available. Although we started with the idea that the sound signal would have to be separated into frequency bands, each of which would then have to be "presented" to different parts of the cochlea, in practice we could not make it work this way. For example, we tried a "vocoder" circuit, which shifted the higher frequencies lower before presenting them.

But among all the alternatives tried, the modulation scheme which Chuck felt produced the best sound, involved putting precisely the same signal into all electrodes, amplitude modulated on a 16 KHz carrier. Graser kept coming back to this as having the most natural sound, and our other patients agreed.

As time passed, evidence continued to accumulate which told us that, within the limits of the technology we had available, this would be the system of choice. I found that I had to let go of the tonotopic theory: it just did not work. As well, we saw absolutely no evidence of adverse neural effects as a result of the testing process.

Finally Jack and I decided to bite the bullet and give Graser a hard-wired, wearable device. He was delighted. In May of 1972, for the first time, he was able to walk out of the laboratory and perceive the sensation of sound. I called Barbara, his wife, daily for weeks. As each day of use continued to produce sounds that he found very useful, we became ever more convinced that cochlear implants had a great deal to offer.

But one swallow does not make a summer, and each success establishes conditions which enable further efforts. So Jack and I decided to make an entirely implantable device and to implant 8 or 10 patients that I had selected and who had talked to Graser. Because we had discovered that the best sound, as reported to us by our first few patients, was produced when the same signal was injected into all the electrodes, we decided to use only a single, short electrode. By all the evidence we had, nothing more was needed.

This was the next step, and we took it.

 

First frictions

I remember at the time of these studies requesting to present the very preliminary anecdotal findings on Graser a national meeting. I was turned down on the basis that reporters would be at the meeting and their reports of the implant would cause otologists to have to contend with a flood of patients with unrealistic expectations.

Finally, however, in 1973, The American Otological Society Saint Louis meeting held a session on cochlear implants.

Dr. Nelson Kiang, a very prominent neurophysiologist for Harvard, expressed the belief that a single-electrode device, such as all of our patients were then using, would only produce a kind of buzzing, Morse code-like sounds, and from a theoretical standpoint, this was a very reasonable position. Dr. Kiang felt strongly that, if an electric field was generated around the neural tissue in the inner ear, the nerve fibers would all fire, go into a refractory state, and then fire again, repeatedly, for as long as the stimulation lasted. [5] This would result in a buzz sound that turned on and off as the current was turned on and off.

For those who had not actually seen any implant patients, this belief regarding the limitations of cochlear implants was widespread.

The Journal which resulted from the meeting printed Dr. Kiang's thoughts, as well as contrasting comments by noted researchers such as Dr. Merzenich of San Francisco. Dr. Merzenich had seen some of our patients and had begun work with Michelson. He said:

I remember hearing once that, according to standard aerodynamic theory, a bumblebee cannot fly. Apparently no one has successfully convinced the bumblebee of this, however, and these sound theoretical arguments regarding the response of the neural system likewise seemed not to be able to convince our patients that they were not hearing something useful.

As the saying goes, "there are none so blind as those who will not see." Despite repeated requests to observe the patients as Dr. Merzenich had done, there were those whose minds were made up and they continued to insist that no benefit was possible with a cochlear implant.

I remember one remark by a scientist at the 1973 meeting who said: "If I tell you that a lead balloon will not fly, and you go out and build a lead balloon and it does not fly, what have you learned?" I could not help but remark that I had flown to St. Louis in a lead balloon. (Two Wrights had not been proven wrong.)

 

Multiple electrodes

But eventually, of course, this sort of objection faded. It became clear that regardless of what we thought we knew about the neural system, in this instance we were wrong: for obviously much more than Morse code was being heard by these patients.

The conflict between the expectations of theory and the demonstrations of clinical practice has continued to this day, but its next manifestation during this period was found in the growing support for the thought that multiple electrodes were the wave of the future. This thinking was based in the tonotopic theory.

A few months after the American Otologic meeting, a conference entitled "The First International Conference on Electrical Stimulation of the Acoustic Nerve as a Treatment for Profound Sensorineural Deafness in Man" was held at the University of California in San Francisco. [7] In the foreword to the printed proceedings, it states that the purpose of the conference was:

The implication was that then-existing implants would never provide anything more than limited contact with the environment through noise (becoming aware, for example, of a car horn) as contrasted with assistance in understanding speech. The foreword further said:

The thoughts contained in the foreword to the proceedings of the first major meeting on cochlear implants represent some of the attitudes that carried throughout the seventies.

Merzenich and Michelson were working on a multiple-electrode implant, and the common thought seemed to be that "multi-channel" devices [8] were just around the corner, and "single-channel" implants should not be pursued. In other words, solely on the basis of tonotopic theory, it was felt that since single electrode systems could not provide discrete stimulation of limited areas of the cochlea, they therefore could never provide frequency discrimination or access to speech; and since "better" devices would be widely available soon, we should stop using single-electrode devices. These thoughts were not merely implied: a vote was taken at the meeting, and the majority of those present felt I should no longer use single-channel implants.

Indeed, many apparently believed that it had already been demonstrated that multi-electrode implants, as represented at this conference by the as yet unimplanted Clarion device, were the only practical approach. However, it must be borne in mind that at that time there were only a few patients using single-channel implants and there were no patients using any form of multi-channel device: so this unfortunate belief had no possible basis in fact. Beyond this, it bears mention that it took almost 20 years before the Clarion, developed by the San Francisco program of Merzenick and Michelson, became available.

 

Other attitudes

Another position represented at this conference was that animal work was all that was needed to solve the major problems regarding implants. This perspective fueled the controversy over human implantation.

It is difficult to see, however, how work with animals could assist with solving many of the pivotal research problems for a system which is intended to provide access to speech.

Beyond the fact that the alternative was, essentially, non-existent, the criticism seemed to ignore the fact that we exercised utmost caution, worked only with volunteer adults who had demonstrated that they had no measurable auditory function, and who were well-aware and fully advised of the risks.

There were others who felt that all implant work was useless. A prominent ENT department head stated: [9]

This reluctance to break new ground was not an isolated thought. I remember in 1974 at the American Otologic meeting joining Walter Work for breakfast. (At that time, he was the head of the ENT department at the University of Michigan.) After some preliminary discussion, he said to me, "Bill, you know that the cochlear implant is no better than vibro-tactile devices."

I protested that I had read the vibro-tactile literature starting with Gault [10] in 1926. At that time there were no wearable vibro-tactile devices and I pointed out that they were not practical because the speech signal vibrations were overwhelmed by the background noise. The three-year study by Geers and Moog [11] of 13 matched groups of children in the same educational setting, one group continuing on with hearing aids, one with a vibro-tactile device and one with a 22-channel implant, has finally put the matter to rest: vibro-tactile devices cannot provide sufficient speech information, and implants do.

These sorts of attitudes did not disappear overnight, even after considerable success had been clearly demonstrated. The 3M/House single-channel cochlear implants in children had been on-going for several years, and many of these children were demonstrating excellent speech reception. Yet in a 1984 news magazine article, a well-known department head and pediatric otolaryngologist was interviewed on the topic of cochlear implants in children:

(Medical World News, June 11, 1984, p. 34)

 

Meningitis

This doctor may have held these views in part because of a fear of meningitis. At a major meeting the same otologist quoted above stated:

In other words, the concern, in children particularly, was that infections such as otitis media would spread along the electrode to the cochlea, and from there to the spinal fluid, causing meningitis. [12]

I reasoned that the risk was not high because we had not seen otitis media progress to meningitis in the many thousands of stapedectomy patients, who had a wire running from the middle ear into the cochlea's oval window. By this time we also had considerable experience with implants in adults, including the early cases where a plug penetrated the skin barrier directly, and the wires to which it was attached then penetrated the cochlea. While adults are less susceptible to middle-ear infections than children are, an increased risk would surely have demonstrated itself in some manner. There had been no such effect.

Still, this was a concern. In our first 5 or 6 years of implants in children we constantly queried pediatricians and parents about otitis media in the implanted children. The children who developed otitis media were successfully given the same course of treatment as before the implant. There were no reports of meningitis, and there was no increase in the child's susceptibility to otitis media. And so this concern subsided as well.

 

Multiple electrodes, revisited

But the criticism which was then (and to a degree it remains today), most pervasive and apparently persuasive, is the thought that single-electrode implants can never provide patients with the auditory information that multiple-electrode implants can.

As I will demonstrate below, there is no evidence on which to base this conclusion, but it is one of those ideas which seems so right, so logical, that it has gained great strength in spite of its meager diet, which is devoid of facts.

It is difficult to trace the precise genesis of such ideas, but we can point to a few places where it was fostered.

 

The Bilger Report

In 1975 the National Institutes of Health (NIH) asked Dr. Michelson, Jack and I if they could sponsor an independent evaluation of our patients. We readily agreed and our patients, who were by that time enthusiastic about their implants, also agreed to this. A research contract was awarded to Pittsburgh Eye and Ear Hospital for this study.

This extensive report, published in 1977, became known as the Bilger report. [13] This was a study of what were then the world's total population of cochlear implant patients: 13 adults. Eleven of these used an early version of what was later to become known as the 3M/House implant, and 2 used the Robin Michelson's U.C. San Francisco implant, which never became a production unit. [14] Thus, all 13 patients had implants which used a single electrode.

This report for the first time firmly legitimized cochlear implants, but the authors went considerably beyond clinical observation in order to speculate about how and how well implants work, and what should be done in the future.

For example, they assumed that single-electrode implant patients would never be able to understand speech. On page 4 of the report they state:

It is worthy of note that what was then "well-accepted" had no scientific basis for being widely assumed to be true, if indeed it was. Further consider that the implication of the statement seems to be that certain experiments did not need to be done: because certain limits of implants were well-accepted, these limits did not need to be clinically tested. What is truly surprising is that some of these first few patients were reporting and demonstrating speech reception, albeit limited. One may take refuge in the fact that reality has never been subject to popular vote, or the earth would today be flat.

On page 9 of that same report, under Conclusions the authors state:

It is difficult to understand how such sweeping conclusions can be reliably reached on the basis of the information then — or even now-- available, and in fact the statements made offer their own proof that these conclusions were based on the authors' assumptions, theories and biases, and not on clinical observation.

To it's credit, the report had a very positive effect on acceptance of the general idea of implants. The conclusion that cochlear implants work (offering more than 'Morse code' input) could no longer be denied, even if their ultimate utility remained to be discovered. At the same time, however, my opinion is that the report tended to suppress exploration in the field by introducing terms and providing conclusions, based solely in theory, which had no underpinning of solid, data and clinical observation. That is, whether or not it can be traced to this report, it at least pre-figured the very odd and pervasive tendency in this field to develop beliefs which are unsupported by any facts whatsoever.

These beliefs became associated with the use of certain terms, and primary among the set of terms which were promulgated to poor effect were "multi-channel" and "single-channel". The idea behind these phrases was appealing: again, that it is somehow necessary to supply the damaged auditory system with information separated by frequency band, and delivered to the "appropriate spot" within the cochlea.

These terms were, of course, conjured out of the air, and although this would have been a useful act of creation if they had been placed in a context of exploration, they were instead introduced as absolutes: "single-channel" implants were inferior or primitive, [16] and "multi-channel" implants were the necessary future.

In sum, in spite of the fact that these conclusions had no basis whatever, they were repeated by others as if they were factually based, and whether from this or other sources, these or similar ideas have become entrenched as a set of popular truths, which are now "well-accepted".

 

Progress in spite of it all

Even with the pejorative image of single-electrode implants, interest in implants grew, and progress was made. By the mid-seventies numerous teams were established to study and develop implants.

During the late seventies and eighties a number of problems had been solved. One significant problem which had come up involved keeping the external processor centered over the implanted hardware.

All internal devices worked on the principle of induction. That is, a current had to be induced in a coil of wire implanted under the skin. Besides delivering a signal, the external processor, in essence, had to deliver the power for the internal electrodes. If the external power/signal coil was not centered over the internal coil, induction was poor, and there was a significant loss of signal.

Many approaches to keeping the two coils matched up were tried, such as attaching the external devices to an ear mold or to glasses, but all patients noted significant fluctuation of the signal even with slight movements of the jaw or when laughing. Dr. Dorman in Oklahoma and Steve Waldron, an engineer I was working with in California, came up with the idea of including a magnet in the internal coil that could serve as a kind of anchor for a matching magnet in the external coil. Such magnets have now become standard in all implant systems.

With the development of solutions to such problems, and with the increase in knowledge which previous studies provided, we realized that further possibilities had opened: specifically, they made possible the implantation of children.

And so in the early eighties we began implanting children.

At the time this was seen by some as an unwarranted risk, or even an unethical act. I have always thought that concern about unknown dangers was clearly justified as long as questions existed, but I could not understand why the well-known and profound liabilities of deafness itself were never factored in.

Deaf children face a number of wrenching difficulties and extraordinary challenges. The human community is characterized by speech, and that speech is summarized by a mother saying "I love you" to her child, and the child hearing and responding in kind. If we allow opportunities to offer this birthright to pass away, unremarked, if we fail to act to correct a problem when we have the means at hand, we have violated our oath as healers. Implanting children had nothing to do with glory; it had to do with responsibility.

 

Major concerns evaporate

In any case, this step became possible because by the late eighties, virtually all of the major concerns about the long-term success and safety of cochlear implants were largely resolved.

The first concern was that long-term, 18-hour-a-day electrical stimulation of the cochlea would eventually destroy the neural tissue. This concern faded entirely as animal studies were done, [17] [18] [19] and as the person-years of stimulation accumulated and there were no reports of implant failure due to loss of the ability of the VIII nerve to be stimulated. Even though no one has used a cochlear implant for more than 25 years, if we are going to see VIII nerve damage it would be appearing by now. Beyond this, a number of temporal bones have been donated by long-term implant wearers to be studied after their death. No hint of damage traceable to electrical stimulation has appeared in these. [20] Thus, by now it should be clear that cochlear implants are a lifetime solution, even for young children.

Fears concerning meningitis have also faded, for the reasons outlined a few pages previously.

A further concern was ossification: rabbits implanted with cochlear electrodes developed ossification of their cochleas and this seemed to wipe out the VIII nerve. Again, our experience with adults in post-operative polytome X-ray studies had not demonstrated any hint of this problem, but we needed to be wary.

Animal models, however, do not always provide us with a clear picture what will happen in humans. For example, I remember that in the early days of my practice, after stapedectomy had been practiced for several years, experimental stapedectomy in cats showed widespread destruction of their cochleas. At the 1959 meeting of the American Otological Society when the paper about this was presented, many otologists got up and said (in effect): "I have 10, 20, or 50 stapedectomy patients and I have not seen the problems your cats have developed."

In any case, concerns about ossification also subsided.

 

The Nucleus Implant

During this same period, work outside the USA was progressing, most notably in Australia where Clark and colleagues were developing a multi-channel cochlear implant that, in the last half of the eighties, was to become the single-most used implant in the world under the name "Nucleus Multi-channel Cochlear Implant". [21]

The commercial success of the Nucleus device signaled the final acceptance of implants as assistive devices. As implant patients became more numerous, and many clinicians and teachers of the deaf had first-hand observation of these patients, more and more accepted that implants are here to stay, have very few risks, and are very beneficial. For these reasons increasing numbers of cochlear implants are now being recommended.

It's an interesting saga, but I never dreamed it would take so long.

 

 

 

Chapter II:

Controversy
(or)
How Do Implants Work?


"When a subject is highly controversialƅ
one cannot hope to tell the truth.
One can only show how one came to hold
whatever opinion one does hold.
One can only give one's audience the chance of
drawing their own conclusions
as they observe the limitations,
the prejudices, the idiosyncrasies of the speaker. "

Virginia Woolf, A Room of One's Own

 

 

EVEN WITH THE APPARENT broad acceptance of implants, controversies, remain. The most important of these, as I indicated in the introduction, is the apparent conflict between clinical observation and theory.

 

The Tonotopic Theory

The work of von Bekesy [22] in the thirties and forties firmly established the traveling wave theory and the concept of discrete areas of stimulation along the basilar membrane. That is, von Bekesy made it clear that the design of the cochlea "sorted" sounds along its length so that in response to a particular frequency, a specific area of the basilar membrane exhibited the greatest vibration, and correspondingly the hair cells (and their attached dendrites) present at that site were stimulated. This is referred to as tonotopic stimulation, as previously explained.

The key point is that it requires stimulation be site-specific, and limited in its spread. Therefore, the tonotopic theory assumes that the hair cell is the only site of stimulus for the dendrite. In this regard, I would note that because an VIII nerve fiber is a bipolar neuron, it has only one dendrite. (Its spiral ganglion cell body is in Rosenthal's canal in the modiolus.) [23] (See Figure 1)

 

Picture of basilar membrane with dendrites, hair cells missing

 
Figure 1. Original concept of the pathology of sensorineural deafness: hair cells are missing, but stimulable dendrites remain.

 

In any case, on the basis of these assumptions, we reasoned that if discrete clusters of dendrites could be stimulated by placing multiple active and ground electrodes close together along the basilar membrane, the 'natural function' (that is the frequency and intensity discrimination of the cochlea) could be better duplicated. (See Figure 2)

 
Picture of localized stimulation of dendrites on basilar membrane

 
Figure 2. Original concept of localized stimulation of dendrites, based on the tonotopic theory of normal cochlear function.

 

So it was, as I mentioned previously, that in the late sixties, Jack Urban and I began working with five-electrode, hard-wired systems placed in the scala tympani of totally deaf volunteers. I wish to emphasize that our reasoning was entirely molded by the tonotopic theory: it guided our every step, in the beginning. Thus, in early work done with cochlear implants, as mentioned in the previous chapter, our idea was that in total deafness the hair cells were missing, but that the dendrites within the basilar membrane and their associated spiral ganglion cell bodies remained intact, and further that it was through the direct stimulus of the dendrites on the basilar membrane that the electric impulses provided by the implant were transduced into nerve impulses. This was why the first hard-wired implant was "multi-channel".

Many different electrical inputs were tried, but to our amazement putting the same signal into all the electrodes simultaneously seemed — as reported to us by the patients — to give the same or better sound perception as the multiple, limited-area inputs.

This of course was totally unexpected and very confusing. We could not explain it, and it appeared that our reasoning by analogy with what we knew about the cochlear processing of pure tones had somehow provided us with incorrect assumptions. In other words, the theory of tonotopic stimulation which we explored in this early work, as attractive as it was, and regardless that it fit well with what we knew about the natural function of the cochlea, did not, in practice, work.

It was for this reason that when Jack Urban and I developed the original, totally-implanted hardware, a single electrode was used: every test which we had devised demonstrated to us that this was what was needed. The first 10 patients implanted with this simple electrode were among those that went to Pittsburgh for the Bilger study.

It is ironic that between the tonotopic theory and this clinical work, which clearly demonstrated that our theory was wrong, the theory should still have so much apparent potency and wide currency.

 

Anatomic evidence

But there is now much more data that bears on the matter. The studies of Linthicum et al [24] have clearly shown that in 16 temporal bones that had been implanted with cochlear implants, some had very few, and most had no basilar membrane dendrites. (See Figure 3)

 

Picture of stimulation of nerve cell bodies in modiolus

 
Figure 3. Linthicum studies demonstrate that local dendrites are very often missing. As they do not exist, they cannot be locally stimulated.

 

Again, if the tonotopic theory is valid, then stimulable dendrites would have to remain in the basilar membrane: but these dendrites are almost always missing. [25]

This study unequivocally demonstrates that the most probable site of electrical stimulation in the cochlea is the spiral ganglion cell bodies. As such, the clinical data gathered in early work done with implants which puzzled us so completely is now independently supported by these anatomical studies. That is, there were no local dendrites to stimulate, so attempts at local stimulation were useless, and not as well received by the patients as more general stimulation.

(We are of course dealing with pathology here, and thus von Bekesy's concepts of cochlear stimulation remain important, and offer some idea of how a pure tone reacts mechanically within the cochlea. However, we are as yet ignorant regarding the mechanisms by which complex, supra-threshold sounds such as speech are handled by the cochlea. As well, we have virtually no insight into how the derived nerve impulses are processed by the brain and thereby become perceived sounds. Fortunately, cochlear implants are providing some new ideas about mechanisms of cochlear function.)

 

Local stimulation requires small fields

Certainly, the information presented above offers compelling reasons for discarding the tonotopic theory. Beyond this however, further clinical investigations using different electrode placements have offered clear evidence that the tonotopic theory was invalid in the context of implants.

Consider that local stimulation requires closely spaced and paired active and ground electrodes, and as well, it requires that small electrical fields be generated between these electrode pairs. Small fields in turn must be generated by small electrical potentials between the paired electrodes. Note that any enlargement of the field of electrical stimulation, whether caused through the use of a common ground ("de-pairing" the electrodes) or through the use of higher potentials (creating a more intense and larger field) necessarily engulfs more structures, thus widening the area of stimulation from the presumed target: the dendrites on the basilar membrane. If one wishes to be religious in one's reliance on the tonotopic theory, one must likewise be rigorous in using small fields and paired electrodes placed near the proper spot on the basilar membrane: there is no reasonable alternative.

 

Yet larger fields are more efficient

While no one knows enough about the electrical characteristics of the cochlea to predict precisely what the current levels "should be", nor the exact pathways through which artificially induced currents flow within the cochlea, it is clear that, for a given pure tone, local stimulation — if possible — will require less current to attain the threshold of perception than the amount of current required to electrically saturate the cochlea with a single, larger field. In other words, this comparison offers its own benchmark.

What then has the clinical evidence shown? Is the theory borne out in practice? Are measured charges lower with closely paired electrodes, as we would expect, thinking tonotopically?

Our early clinical findings demonstrated that, when stimulating between two closely-spaced electrodes in the scala tympani, more current was required to reach threshold than was required to saturate the cochlea generally. When using a single active and a single ground electrode, placed in such a manner that the current was directed across the modiolus and the spiral ganglion cells, lower currents allowed us to reach threshold.

But of course, this evidence comes from our early results. Is there other evidence which indicates that larger, non-tonotopic fields offer better results?

Of course, there is. In fact this inverse relationship between local stimulation and current requirements appears to be a general finding, and is not restricted to a single electrode system design. In the April 1993 edition of the Cochlear Corporation's Audiologist's Handbook for the Nucleus 22 implant, on page 149, it states:

Typically, the lowest thresholds are obtained in CG [common ground] because of the wider current spread.

"CG" for this device obtains when one electrode is active, and all 21 others are being used as ground electrodes. Note that this "breaks" close pairing between electrodes, and creates a larger field. The chart which is found on the same page of the Handbook (their Figure 41) offers a graph of experimental data which shows that, using common ground in a saline solution (which was likely used due to its presumed electrical similarity to the electrolytic fluid of the scala tympani) the electric field is rather evenly and equally spread from the active to all other electrodes, much as water would drain equally through similarly-sized holes in the bottom of a can.

As the quote makes clear, it "typically" requires less current to achieve threshold when electrode pairing is broken, and the field of stimulation spreads. Indeed, setting the Nucleus system up to use a common ground is a popular practice. [26]

Recently I was told that Cochlear Corporation will in the future use a ground outside the cochlea to further cut their power requirements. Common experience is that batteries are wearing out too fast with closely paired stimuli, because current requirements are higher.

 

Stimulating the spiral ganglion cell bodies

Thus all available evidence points in the same direction: Larger fields are required, and smaller fields will not work.

For example, consider that closely paired electrodes require more current for a given effect. This again demonstrates that closely-paired electrodes must generate more intense fields in order to spread the stimulation: a logical contradiction between design and practice.

With closely paired electrodes, most of the current tends to flow in the most direct path between them. Thus, using such closely-paired electrodes, a more intense current must be passed between them, such that the "spill-over" is sufficient to generate a larger field. This is the sort of effect we might predict if we believed precisely the opposite of the tonotopic theory, to wit, that local stimulation, far from being required, was in fact quite useless, and that, when constrained by designs which direct the current to flow locally, we must "waste" enough current that the excess current will produce the desired result.

But if we are not stimulating local dendrites on the basilar membrane, what then are we stimulating?

Our belief is that the most probable site of stimulation is the spiral ganglion cell bodies within the modiolus. Those familiar with the anatomy of the cochlea will have difficulty choosing another potential site. The conclusion, once again, is that the most probable way — indeed one may say the only possible way — to cause the remaining neural structures of the damaged cochlea to fire is to blanket the spiral ganglion cells of the VIII nerve with an electric field.

The generation of this field within the cochlea is most efficiently accomplished (that is, accomplished with the least electrical voltage to the electrodes), when the ground electrode and the active electrode are placed such that the field generated will pass through the cochlea. (See Figure 4)

 

Closely paired electrodes require more current

 
Figure 4. Localized fields require more current to achieve comparable thresholds, implying that stimulation occurs non-locally via the increased spread of the field. Globalized fields require less current, further supporting the conclusion that stimulation is not local to the basilar membrane.

 

 

 

 

The Morse code rationale

We would expect, based on current neurophysiologic understanding, that any electric field, when strong enough to achieve threshold, would cause indiscriminate firing of all the cochlear neurons: what would prevent that? Further, the expectation would be that the neurons would then go into a refractory state, as previously described. So, conventional wisdom tells us, all the patient should hear something like an 400 Hz buzz whenever any current capable of stimulating the nerves, regardless of its other characteristics, is applied generally to the cochlea.

This theoretical model of neural activity in response to artificial stimulation makes so much sense, in fact, that it is difficult to understand that it is not correct, or why. So the mystery is deepened, and the challenge is widened. Not only do the facts which confront us require us to let go of the tonotopic theory, but as well they bring into question some of our basic neurophysiologic expectations.

It should be very clear that as yet we do not understand enough about how the normal auditory system processes pure tones, much less to figure out how implants work. [27] Rather, we must judge cochlear implants by their results through clinical observation, and not be prevented from trying different approaches because preconceived and unfounded notions say something cannot work. Otherwise, we are back to the days of Galileo, battling with the ghost of Aristotle about which iron ball will fall faster.

 

Finding wiggle room

Still there are those who are unwilling to completely abandon the theory of tonotopic stimulation within the cochlea. Rather than believing that implants operate by stimulating dendrites along the basilar membrane, however, their feeling is that what is required is to selectively stimulate the spiral ganglion cell bodies (within Rosenthal's canal in the modiolus) which apparently correspond to a certain frequency.

It should be remembered, however that the spiral ganglion cell bodies are closely packed, and are surrounded by blood vessels and fluid spaces. The active electrodes are surrounded by the electrolytic fluid of the scala tympani. This fluid seems ideally suited to conduct the electric fields generated by the cochlear implant widely and generally. Under these circumstances it would seem completely illogical to assert that a current is somehow finding its way to, and stimulating only certain discrete numbers of spiral ganglion cells in exclusion to any others. Further, given that the amount of current required to reach threshold in production implants is lower when using a common ground or a remote ground outside the cochlea, it seems by extension illogical to believe that the tonotopic theory can be salvaged by assuming that discrete sets of dendrites along the basilar membrane have somehow been replaced by discrete sets of spiral ganglion cell bodies within the modiolus, as the site of stimulation.

 

The geography of frequency

Independent studies further demonstrate that it is unnecessary to generate electric fields across either the basilar membrane dendrites or the spiral ganglion cell bodies from different sites in the scala tympani.

Consider that, as we currently understand it, the frequency sensitivity of the basilar membrane moves from higher frequencies near its beginning at the round window, to lower frequencies as one approaches its end at the apex. As such, we can — at least theoretically — roughly correlate distance along the basilar membrane with an associated site of stimulation for a given frequency.

So we can say that a 25 mm electrode (e.g. of the length used by the Nucleus 22-channel implant or the Ineraid implant) inserted into scala tympani from the round window will extend from 20,000 Hz at the round window up to about the 1500 Hz area. By contrast, a 6 mm single electrode (e.g. of the length used by the 3M/House and the AllHear implant) extends only up to about the 4000 Hz area. Indeed, because stimulation via multiple electrodes is intended to be local to frequency-specific areas of the basilar membrane, then the electrode must be as long, or nearly as long, as the basilar membrane itself. That is, tonotopically-correct electrodes must necessarily be long electrodes. In fact, given that there are no electrodes in any in-production implant which reach to areas on the basilar membrane of 1500 Hz and below, it can also be said that there is, today, no such thing as a tonotopically-correct electrode. We are all, perforce, practicing this particular sin of omission.

The psychoacoustic studies of Bilger of the first 12 single 6 mm electrode patients, found that these patients had normal frequency difference limens in two cases up to 2000 Hz, and in the remaining cases up to 500 Hz. [28] That is, sounds with frequencies below 2000 Hz are accurately discerned despite the fact that active electrode could not possibly "directly" (tonotopically) stimulate the 2000 to 250 Hz frequency-related area of the basilar membrane. Under these conditions, where electrodes cannot stimulate the "correct" spot on the basilar membrane, it would of course be impossible for either the basilar membrane dendrites, or the corresponding spiral ganglion cell bodies, to be locally, discretely, and exclusively stimulated. One must as a result either recognize that the tonotopic theory in all its potential variations is invalid, or reject the clinical data.

It is now commonly observed that patients with short single 6 mm electrodes, and those with multiple long electrodes in the scala tympani have equivalent pure tone thresholds from 250 to 8000 Hz, [29] thus reinforcing the understanding that there is no need to stimulate from "tonotopically correct" sites in the scala tympani.

In sum, we now know with certainty that long multi-electrode cochlear implant systems do not — indeed, the cited studies show that they most often cannot — do what they were designed to do.

 

A death in the family

In sum:


This demise is really too bad: it was a lovely theory, and offered us the comforting illusion that we knew something. However, dead is dead. If we hold the truth higher than our own previous investments and current opinions, we need to recognize that the facts demonstrate that we were all wrong, and move on. This is called progress: it is rarely comfortable.

 

So what?

Of course, regardless of whether the tonotopic theory is invalid, some may ask why we should not have and use long electrodes anyway. After all, further information may somehow and someday show that there is some modest or subtle benefit to a tonotopic approach which we cannot currently discern, and, after all, the market is (as of this writing) filled primarily with long-electrode implants. Therefore even if the theory is wrong, so what? Why not use long (e.g. tonotopically correct) electrodes anyway?

In fact, this question is easy to answer.

Consider first that the category of patients for whom implants are seen as useful is rapidly broadening. For example, because of the uncertainties regarding the potential damage that the electrical fields which implants generate, implants were at one time not used in very young children. The questions regarding whether these electrical fields will damage a developing auditory system, however, are virtually resolved. We now know that electric field blanketing of the spiral ganglion cells do not cause damage, indeed, they appear to be beneficial in preventing atrophy.

There were uncertainties, as the Bilger report evidences, regarding whether cochlear implants could help patients detect and produce speech. These questions too have been resolved: we are now certain they will so assist.

One result of the gradual and now virtually complete resolution of these particular uncertainties — that is, we know they will not harm and will greatly assist — has been that cochlear implants are being seen by more and more clinicians as assistive devices useful both for younger patients, and for patients who are less than profoundly deaf. As such, many of the patients now being considered for implants, therefore, will have some remaining hair cells, and some residual hearing.

This brings up many possibilities. For example, one can visualize a time when implants might be used in combination with hearing aids to assist patients with frequency-limited losses, such as ski-drop high frequency losses, where these losses are caused by damage to the hair cells.

 

Long electrode damage

The widening of the group of those who are seen as suitable candidates and the fact of long electrodes however, have a tragic collision, as shown by the temporal bone studies of Linthicum, which reveal that long electrodes damage the cochlear structures.

The mechanism of the damage is apparently that as the electrode is inserted into the basal coil, at about the 7 mm depth it contacts and is constrained by the spiral ligament, which in turn is bounded by the bony canal of the cochlea itself. From that point, it is forced to turn inward and upward, following that spiral. (See Figure 5)

 

Insertion of long electrodes harms the cochlea

 

Figure 5: A long electrode array contacts the wall of the cochlea at about 7 mm, and begins to strip off the spiral ligament.

 

What appears to happen is that the electrode, which is forced thus to bend, presses against and strips off the spiral ligament, (See Figure 6) and damages the stria vascularis, the basilar membrane, and the organ of Corti. (See Figure 7)

 

Long electrode begins to press against the spiral ligament

Figure 6. Cross sectional view showing a conceptual model of the point of contact of a long electrode array with the wall of the cochlea. From this point on, as the electrode is pushed further into the spiral of the cochlea, it apparently strips off the spiral ligament.

 

 

Destruction of the spiral ligament results in dendrite degeneration

 
Figure 7. Damage increases as the long electrode array is pushed deeper into the spiral of the cochlea. Sometimes the bony structures of the cochlea are fractured, apparently by the force of insertion. Damage caused mechanically is apparently increased by the disruption of blood supply, or perhaps other factors, and causes a consequent degeneration of the spiral ganglion cells.

 

Other studies, [30] [31] [32] have offered similar findings, and the matter now seems well established: The insertion of a long electrode damages and in some cases completely destroys cochlear structures which are essential to residual hearing. As a direct result, residual hearing is damaged or destroyed.

Schuknecht [33] in a discussion of degeneration of the cochlear neurons states:

In a personal communication, Schuknecht indicated that it was his estimate that 40% of the intact spiral ganglion cells would die if their dendrites were damaged. [34]

What evidence do we have regarding the damage which may or may not be associated with short electrodes? Case 580, case 591, and case 486 in the Linthicum study [35] show that a 6 mm electrode causes little or no damage. As such we conclude that any risk is greatly minimized if the electrode is inserted no deeper than 5 or 6 mm. Indeed, the evidence presented gives us reason to believe that all that is necessary is to simply insert the electrode; whether it is inserted 1 mm or 6 mm will not make any functional difference.

The remaining 10 bones studied by Linthicum, all of which had been implanted with long electrodes, each showed extensive damage to the spiral ligament and basilar membrane, and some even had fractures of the bone of the spiral limbus, testifying to the relative violence suffered by these very delicate structures.

 

Loss of Residual Hearing

As indicated, the anatomic damage from long electrodes is clinically manifested through the loss of pre-implant, unaided hearing. Bogies [36] and numerous clinicians, [37] and the more extensive study of severely hearing-impaired patients with Nucleus 22-channel implants done by the Cochlear Corporation, [38] have shown that residual hearing is likely to be lost as an apparent result of long electrode implantation.

By contrast, previous studies reported by Berliner et al [39] have shown that introduction of a 5 or 6 mm electrode into the scala tympani has very little chance of eliminating any residual hearing. On page 73 of the cited reference, they wrote:

In sum, the use of long electrodes in cochlear implants endangers and can often destroy the very capacity these implants are now meant to replace or which in the future they would supplement. This is the tragic irony: even if we wish to assert that there is, as yet, some subtle tonotopic effect which our tests have not revealed, current designs of long electrodes destroy the very structures they purport to stimulate.

 

The risks of surgery

One of the results of the space program is considerable information regarding the reliability of electronic assemblies. It is thus now well-known that each additional part added to such an assembly imposes an additional risk of failure. One practical consequence that we might expect is that complex internal hardware would be subject to a higher failure rate. This implies that our electrode designs must be as simple as possible, so that there are fewer parts to wear out or break, since whenever internal hardware fails, it must be explanted and a new electrode set implanted, necessitating surgery, with all its consequent risks.

I am not aware of any data which has been published on the failure rate of the Nucleus 22 channel device, but several years ago the failure rate of the 22 channel system was reported at 2%. At the meeting on children's cochlear implants held in New York February 4th and 5th of 1994, the failure rate was stated to have increased to 8%.

The risk of surgery is also imposed with complex internal hardware whenever this complexity is the result of a close coupling between the theory of the processor and the design of the internal hardware. New processing schemes, in that instance, would require new internal hardware. Because of the primitive state of our knowledge, today's best internal hardware designs must be those which would accept almost any conceivable signal over the 50-100 year life of the implant.

 

Inescapable conclusions

If long electrodes do not and cannot serve the purpose intended by the tonotopic theory of stimulation, there is no reason to use them. If, further, they cause damage to the cochlea and its structures, then there are important reasons not to use them. "First do no harm." Under what circumstances is it permissible to permanently destroy something so valuable as the sensation of hearing, when no demonstrated benefit results?

Of course, before this information was available, it could not be said that this destruction was culpable; but now we know, and this knowledge has removed our innocence and made us liable. The future, and indeed our patients, will judge us on that basis.

In fact, in spite of this knowledge, we may have to continue implanting long electrodes in implant patients for the time being because of the lack of immediate alternatives. And for those who are profoundly deaf, it should be clear any implant is the better of two options.

In sum, short electrodes are today's design of choice: they are neither complex nor strongly tied to any particular processing scheme, and they guard cochlear structures and residual hearing. Based on the evidence available to us, we can now assert that long electrode designs are yesterday's solution, and should be discarded for future designs.

 

 

 

Chapter III:

Fitting Implants
and
The Articulation Index

" To overturn orthodoxy
is no easier in science
than in philosophy, religion, economics,
or any of the other disciplines
through which we try to comprehend the world
and the society in which we live."

Ruth Hubbard (b. 1924), U.S. biologist.
"Have Only Men Evolved?" in Women Look at Biology Looking At Women
(ed. by Ruth Hubbard, Mary Sue Henifin and Barbara Fried, 1979).

 

 

THE WHOLE FIELD OF IMPLANTS is in a rapid state of flux, which in itself is one of the characteristics of immaturity: the plant grows fastest, relative to its size, when very young.

Beyond the controversy surrounding the issues of electrode design and its effect on the ability of the patients to perceive sounds and thereby recognize speech, lies another controversy regarding how we can determine whether implants will allow a given patient to understand speech. It seems to me that the whole discussion regarding evaluating implants has focused inappropriately on final results rather than on measurable inputs.

 

Studied speech

In my mind, there is a confusion and as well a bias which feeds this controversy. The confusion is regarding the precise role of hardware in the complex chain of speech. The bias is regarding long electrodes, and we have discussed that in some detail above.

With regard to this bias, I have shown, I believe conclusively, that long electrodes do not and cannot add any significant benefit to implants. However, beyond the fact that this bias first arose in the absence of any facts whatsoever, it is also true that some studies of the speech skills of implant patients have shown that there are some reasons to believe that patients more recently implanted (virtually all of whom use multiple electrode implants) do better on some speech tests.

If my premise is true, how can this be explained?

Many of these studies are certainly valid and well done, when the conclusions are restricted to those areas where their data allows statistically valid conclusions to be made. However, with regard to reaching the conclusion that multiple electrode designs are inherently superior to single electrode designs, these studies cannot help us. The problem with each of these studies in that context is that they have ignored or failed to report many significant factors, such as the sound encoding scheme of external processor or the educational background of the patients. Absent data which allows these factors to be examined, how can anyone authoritatively assert that the differences revealed have to do with electrode design, rather than one or multiple other factors?

Clearly, no one can.

 

A sound by any other name

The confusion regarding the role of hardware arises because we have failed to realize that regardless of how it is that implants actually work, there are only a few vital parameters of sound: intensity, pitch and timing.

It seems almost as if some of us believe that implants are offering the brain some unknown stimulus, as contrasted with giving a known stimulus in an unknown way. Implants provide access to sound, do they not? To say no is to engage in a semantic dispute which begins in words and ends in words, and which has no pragmatic consequence. Come, let us admit the matter until we have some useful reason to deny it: implants provide access to sound. [40]

Boothroyd [41] offers a clear statement about what prosthetic devices do:

Therefore it is inappropriate to evaluate implants on the basis of the patient's ability to recognize speech because it ignores almost everything we have learned in more than 50 years of fitting hearing aids, it is impractical, and because it is completely unnecessary. Consider each of these points:

 

Fitting by speech results ignores history

Many of us are young enough that we have little perspective on how past controversies mirror current ones.

In 1946 Carhart [42] introduced the comparative method of hearing aid fitting, which was to select a hearing aid based on the percentage of correctly identified monosyllabic words. It was reasoned that since the right true end of fitting a hearing aid is to enable the patient to understand speech, the shortest distance between fitting and goal was to present the patient with speech, and evaluate which is the best aid accordinglyƅ as is now done with implants. This thought held sway for many years, but by the mid-sixties it had been concluded, as Jerger et al [43] pointed out in 1966:

These contaminating errors occurred because there are so many factors, far beyond the reach and ken of hearing aids, which combine to allow any patient to correctly report speech, or conversely which cause the patient to fail to do so.

Lets be very clear about this: what the aid does — indeed all the aid does — is provide better access to sound, and that benefit is best measured by standard pure-tone audiometric and similar techniques, aimed at discovering how well the aid assists the patient in perceiving intensity, pitch, and timing.

On the other hand, when we test speech recognition or understanding, we have introduced a whole set of factors far beyond the well-focused question of how well the aid helps the patient hear the sounds: we are then testing the patient, as well as the aid.

Further, as this is true of hearing aids, it is also true of implants. They may not be hearing aids, but they are certainly aids to hearing.

However, we should draw an important distinction. By saying that the measurement of speech is not useful or necessary for the evaluation of hearing aids or implants, we do not intend to say that it is no longer the primary goal as regards their use: certainly it is. As such, speech testing and evaluation has a vital part in working with a given patient as they strive to enhance their speech and language skills. [44]

Thus for the purpose of evaluation, we must test how well the implant provides intensity, pitch, and timing — access to sound — and we must focus on this. We very likely will not otherwise develop a body of data which is directly comparable among implants, and will thus seriously retard progress in this field. Returning to the differences found in studies of implant patients tends to illustrate this, because where complete information on potentially relevant factors is missing, we are left only with a series of unanswered questions. This often renders the whole study unusable.

The history of our field shows that, as entrenched as the comparative method once was, times change, and today most hearing aids are fitted by the prescriptive method as defined by Northern: [45]

In other words, the assumption of the prescriptive method is that if the auditory gain supplied by the hearing aid allows the patient to detect the sounds of speech, intelligibility will follow. This is further emphasized by Preves, [46] who states:

For the same reasons, we must also fit implants by "obtaining functional gain — that is, the amount by which the [implant] improves the patient's hearing threshold levels."

 

Fitting by speech results is impractical

Many of those who get implants can neither speak nor understand speech, either because they are too young, or because the vital stimulus of sound was denied them at a critical period.

This being the case, the only possible way for these patients to be fitted with an implant is to measure their sound reception, while they are wearing an implant, in some standard manner. Beyond that, even those who have either good speech reception or well-developed language skills will require time before they can be comfortable with any new implant and use it to the fullest in speech reception.

If the results gained by testing speech reception "are sufficiently contaminated by error that they do not necessarily reflect meaningful differences between various [implants]," what sense does it make to have to wait for the patient to learn enough that we can do such tests, only to obtain a result which — if we assume we have measured something about the implant alone or primarily — is prone to multiple errors?

The extreme case (patients who have very poor speech and language skills, or who lack any such skills) points out the difficulties experienced in all cases: what we are measuring when we measure speech reception is, to a large degree, the patient's skills, talents, native intelligence as regards speech, her or his previous exposure to speech, educational background, and the like.

All of these factors (e.g. contaminating errors) tend to mask the contribution of the implant, which, precisely like the hearing aid, is simply to provide access to the three main parameters of sound: intensity, pitch, and timing. Why not measure these more directly?

 

Fitting by speech results is unnecessary

Finally, it is clear that we have the tools and the experience to evaluate implants without any reference to speech results. As such, the evaluation of speech results — for the purpose of fitting an implant — is unnecessary.

We have a well-explored and well-understood and "predetermined formula": The articulation index (AI), which I will explain momentarily.

The sum, however, of the points above, again, is that we should not, and indeed cannot evaluate implants by measuring speech results: it ignores history, it is impractical and it is unnecessary.

Better sound detection leads to better sound perception. Whereas the former is more mechanical, the latter has to do with bringing the sounds to consciousness, and more particularly with recognizing patterns within complex sounds, such as the pattern of sound which represents our name. Finally we have speech recognition, which is a further internal step: it involves assigning meaning to the perceived sounds and sound patterns. (Consider the difficulties we face when learning a foreign language later in life!) Surely then the emphasis when fitting should be in measuring sound detection: measuring any of the other steps offers us insight into what the patient is able to make of the detected sounds, as contrasted with what the implant is able to supply as far as the quality and quantity of those sounds.

When we want to measure the patient, let us measure the patient; when we want to measure the implant, let us measure the implant.

 

Approving new implants

At present, of course, we do not have a wide choice among commercially-available implants.

Therefore one of the most important near-term implications of this approach to fitting implants has to do with progress in this field generally. Because we have such poor knowledge, at this point, about how implants work, and because (therefore) we have such a poor understanding of what will work better, we must, as the Chinese saying has it, "let a thousand flowers bloom." That is, we must, within the limits imposed by the safety of our patients, examine how we can allow for a significant number of new implants to be tested and approved.

Of course, the processes involved will vary on a per-country basis, but it is also true that in many respects, the U.S. breaks ground in this area, and my concern is primarily with the process by which implants gain approval in this country.

At present, that approval process seems predicated on a statistical demonstration that a given implant will provide patients the ability to develop speech recognition.

Certainly, as far as it goes, the goal is right and true; our patients must gain access to speech. But to insist that a significant number of patients must wear a given implant for long enough that they can then demonstrate speech capacity, as contrasted with demonstrating that patients who have that implant system can hear all the relevant sounds of speech, makes a world of difference.

In the former case (where it is required to have a number of patients who demonstrate speech reception), the approval process will require a long and expensive series of studies which are, as we have demonstrated above and will further demonstrate below, contaminated by errors, because we are testing the patients much more than the implant. Further, such studies will necessarily take several years, greatly slowing the process. [47]

In the latter case — where what is required is to demonstrate that patients have gained access to the relevant parameters of speech sounds — the approval process will be greatly shortened, less expensive, and further much more accurate as regards providing information about the implant, as opposed to information about the patients implanted and their course of training.

The consequence, clearly, of taking the latter course will be a flowering of the field, and much better and more directly comparable information about implants.

 

A measured response

Of course, in any scientific endeavor it is necessary not only to state the logic, but as well to offer the demonstration and proofs. Do studies which bear on the relationship between audiometric and speech studies bear out the thought that good hearing makes for good speech skills, all else being equal? A related and equally important question is: how do we draw out from the audiometric data some meaningful numbers, so that we can compare one audiogram with another as regards the patient's chances at gaining speech? As I mentioned above, the answer is yes and the number is the articulation index, or AI.

 

The articulation index

The AI offers an easily used method of quantifying the possibility of understanding speech, based on the audiogram. [48] [49] It allows us to make broad comparisons between rather different looking audiograms, and to have a convenient indication of the expectation [50] we may have for a given patient.

The articulation index was developed for and has proven very useful in predicting hearing aid success, and should prove equally useful in predicting cochlear implant success.

As the reader can see (in Figure 8), the patient-specific information used in the calculation of the articulation index is precisely the same as that used in a standard audiogram. The difference is that in plotting this information on what is in essence an enhanced audiogram form, interpretation is facilitated.

 

A standard audiogram form enhanced with numbers to facilitate the calculation of the articulation index

Audiogram with numbers for calculating articulation index

Figure 8. A standard audiogram form enhanced with numbers to facilitate the calculation of the articulation index. After drawing the patient's aided measures, adding all of the numbers on or below the line will yield the articulation index. A patient with a flat 30 dB loss, for example, will have an AI, as estimated by this form, of 65. (1 + 1 + 2 + 3 + 4 + 4 + 5 + 6 + 8 + 8 + 8 + 8 + 4 + 3) The articulation index as calculated mathematically for such a loss is 63, demonstrating a good correspondence between form and formula. Please note that the form is deliberately printed large enough that it can be copied and used.

 

This version of the articulation index employs a series of 100 numbers placed on a conventional audiogram, in a shaded area that represents the average speech spectrum (e.g. the "speech banana"). The value of the numbers is related to the importance of that particular frequency for understanding speech. [51] The numbers are of much higher density in the 1 to 4 KHz region, thus emphasizing the importance of high frequencies — which apparently carry most of the information regarding consonants — in understanding speech. Thus, by plotting a patient's warble-tone thresholds on the articulation index form, one sees at a glance which frequencies the hearing aid or implant user can detect, but more significantly their relative importance is made instantly apparent.

Thus one can either use this form, calculating the AI based on the sum of the highest numbers on or below the decibel rating at each of the critical frequencies, or it can be calculated mathematically. [52]

This is not the whole story, of course. It must be said that neither the standard audiogram nor by extension the AI — all by themselves — can tell us if the patient can discriminate between frequencies, only that the patient can detect them. That is, it may be possible for a patient to hear that a tone is present, without being able to tell if it is high or low. (This would be the auditory equivalent of color blindness, if you will.)

In order to discover if the patient can discriminate between frequencies, some fairly simple tests can be done, but generally are not. However, we have quite good indirect evidence that the majority of implant patients can and do discriminate between frequencies, based on the degree of open-set discrimination which they eventually develop. (It should also be said that because we apparently have few if any "frequency blind" patients, it has been a good working assumption that a patient who can detect a given frequency can also tell if it is high or low.)

We will cover this in greater detail below, but the thought concerning indirect evidence is based in the premise that a good deal of the information encoded in speech is available primarily or only to those who can discriminate between frequencies. Of course, the parameters of sound (intensity, pitch, and timing) and their relationship to speech discrimination turns out to be a surprisingly complex area, and has been difficult to quantify. Among other things, it may depend on the language being spoken, the degree of ambient noise, as well as a whole wilderness of non-auditory parameters. Speech as a code also turns out to have a good deal of redundant information, so that where one might get the best information about a particular feature of speech from a particular frequency range, it is rarely the only range where information about the phoneme in question is found.

However, taken from the point of view of the phonemes involved-- the consonants, vowels, diphthongs and modifiers — work has been done which demonstrates many of the relationships between the components of sound and the components of speech. [53] While the matter is more complex than this, one pertinent fact is that the frequency ranges between 2 and 4 KHz are critical to the understanding of consonant sounds. Further, because many consonants are easily confused when lip-reading, hearing these sounds remains important for greatest ease of communication.

The central fact remains: better access to sound means better access to speech.

It should be re-emphasized, however, that the questions which exist with regard to implants are the same as the questions which exist with regard to hearing aids, and the broadly-based conclusions which we reach as a professional group, I believe, will in the end be very much the same for implants as for hearing aids, if they are not identical.

 

AI literature

Of course, I am not the first to suggest that we use the AI to quantify the chances for a patient to develop speech detection.

Moog and Geers state: [54]

It should be emphasized, as Moog and Geers have indicated, that speech tests are necessary as the prime benchmark of the ultimate success of a given patient, using one or more implants. Again, however, this is a very different thing than testing to properly fit or evaluate an implant initially. It is difficult to see how one implant will differ from another in its ability to offer a given patient a foundation for the development of speech skills unless there is, at some point, a measurable difference between the implants with regard to their ability to deliver information about the intensity, pitch, or timing of sound to that patient. Consider that if our testing cannot reveal a difference (because the patient cannot discern and report a difference), then it can be said, clinically speaking, that no difference exists.

Moog and Geers [55] have devised four speech perception categories (which they describe as a measure of the potential to develop normal or near normal language and academic achievement), and have found rough correlations between these and the patient's AI scores:

Children who attain categories 3 or 4 with the help of their auditory prosthesis are considered capable of attaining to a good level of speech recognition.

I might mention that from this point to the end of the chapter, I will assume a good many things about the background of the reader. The narrative should make as much sense to you, if you are reading primarily for concepts, whether you read through the end of this chapter or not. More casual readers, therefore, may wish to skip to the next chapter.

 

Some numbers

Moog and Geers found a correlation between the AI and the ability of the patient to develop speech. Have other studies found a similar correlation? Of course they have. As well, such correlations can be demonstrated where sufficient information is available (audiograms and consequent speech scores) even from studies where this particular question was not asked.

For example, through the courtesy of Richard Tyler [56] I was given the sound field thresholds and the phoneme composite scores for 48 cochlear implant adults who were up to 5+ year users of their implants. Twenty-five of the patients used the Nucleus 22-electrode F0/F1/F2 pulsatile strategy and 23 used the Ineraid 4-electrode analog strategy. (There were no major differences in the performance of these two groups, so in this report their data are pooled together.)

The phoneme composite score is the percent correct of the sum of three open-set phoneme tests, namely: the Iowa Laser Videodisc Medial Consonant Test, the Iowa Laser Videodisc Vowel Test, and the Iowa NU-6 Test. This composite score is called the NU-6p score. In the study referenced above it was found that the composite scores of the two different hardware groups of patients improved steadily and strongly for the first 9 months of implant use, and more slowly and to a smaller degree for 18 months or longer.

I asked Karen Berliner, Ph.D., to analyze the data from these 48 patients, and her report is as follows:

From the last follow-up threshold data I was able to calculate the articulation indices (AIs) of these 48 patients. I also asked Dr. Berliner to look at the AIs in relation to the NU-6p scores. She reports:

The implication of this data is that sound field thresholds appear essentially stable over time and that the 1 month thresholds and AI can be used as a reasonable measure of the potential for patients to develop good speech reception skills.

The small long-term improvement in the 500 and 1000 Hz thresholds, I would speculate, represents the positive effect of long term electrical stimulation on the spiral ganglion cells and auditory CNS pathways.

Correlations imply that there is a relationship of the factors under study, as there is in this study between the AI and NU-6p scores. Of course, these correlations do not tell us the cause of such an effect, but I believe it is clear that a better AI represents better sound detection, which in turn leads to better sound perception and word identification as demonstrated by the NU-6p scores. Given the many factors which work together to produce the ability to perceive speech, these correlations are, in my opinion, quite significant.

Regarding a similar situation, Boothroyd states: [57]

The importance of the audiogram is also emphasized by Watson: [58]

Interestingly, some reported failures of the correlation between AI and the development of speech recognition may provide further strong evidence for depending on it to classify and fit implants.

For instance, Tye-Murray et al make a report of two cases: [59]

The first example labeled in their study Iowa Symbion 14 (IS14) was a 61 year old male, with progressive hearing loss starting at age 16 and becoming profound at age 39. He was profoundly deaf for 22 years before being implanted. His NU-6p score at 1 month was 0, 26 at 9 months and 28 at 18 months indicating he was fairly low implant performer. The striking thing is that his AI was 74 the highest of the entire study group.

The second example is IS12. She is a 33 year old female with progressive hearing loss starting at age 7 and becoming total 1 year before being implanted. Her NU-6p score at 1 month was 51, at 9 months 71 and at 18 months 75 which was the highest NU-6p score attained by any of the subjects. Her AI was 22 which is in the low range for these patients.

Note that in the first case the patient had been deaf a long time, starting from an early age. We can surmise that this patient had poorly developed speech and language skills, but in any case, he apparently had very little experience in connecting sound with language. In the second case, the patient was recently totally deafened, and, it seems, had spent years with gradually diminishing hearing, perhaps enabling her to hone her skills in discerning speech in circumstances where she was getting progressively fewer sound cues.

However, the value of the AI is implicit in the example, regardless that the result is not what we might hope in the first case. The remarkable thing about the first case is that, with so much information, the patient was able to do so little with it; in the latter case, what is remarkable is precisely the opposite. That is, we accept that the AI is the measure of what is being provided through the implant, and the speech score represents what the patient can do with that information. We can clearly see that the speech results have "contaminating errors," if we take these results as being indicative of what the implant is offering the patient. Further, it becomes even clearer that, with different populations of patients, different training methods and so on, the resulting speech scores between two implant programs cannot be taken as a valid comparison of the two different hardware sets.

Thus at present it appears that the best method of measuring the potential for future success is the one month post-implant AI. It is as accurate as any other tool we have available, it offers great convenience, and it will give us an excellent guide to the sound quality the implant is offering the patient.

If we want to evaluate the implant, we have no better tool than the AI; if we want to evaluate what the patient has done with the implant, speech tests are the means of choice.

 

 

 

Chapter IV:

Auditory Success

"When science, art, literature, and philosophy are simply the manifestation of personality they are on a level where glorious and dazzling achievements are possible, which can make a man's name live for thousands of years. But above this level, far above, separated by an abyss, is the level where the highest things are achieved. These things are essentially anonymous."

Simone Weil, "Human Personality"

 
I CAN REMEMBER WHEN the goal of runners was to achieve the four-minute mile. For a good many years it was widely regarded as impossible, but finally Roger Bannister broke this "speed limit" in May of 1954.

Today one almost never hears reference to this measure of distance and speed, and a number of runners have exceeded this benchmark. This been accomplished first by setting certain goals, and second by studying and instituting all the factors which apparently contribute to attaining the goal: physical and mental training systems, nutrition programs, and so on. In fact Bannister not only broke through this barrier, but he established the pattern for others who followed, because he is said to have accomplished his task through scientific training methods and the study of the mechanics of running. (As an interesting aside, Bannister later became a neurologist.)

At this point in the history of cochlear implants, I think we are now ready to set the ultimate goal for implant patients as:

In essence, we want as many of our implant patients as possible to hear as normally as possible. This would be the four-minute mile or moon-shot of cochlear implants. Because of its nature and broad reach, this will be an anonymous victory, requiring the efforts of many people over a long period of time, and more in that respect like the moon landing than the four-minute mile.

For any given patient, once the implant has provided the best sound possible to a given patient, the next step is to achieve open-set recognition. [60] I refer to this as auditory success. Fortunately, we are seeing an increasing number of implant patients attain auditory success: about 80% of implant patients exhibit some degree of open-set recognition.

Clearly, we must continuously improve our record so that more and more of our patients achieve this aim, gradually winning our way to universal, or nearly universal, auditory success.

In attaining any such goal we must first identify the factors that probably contribute to the goal, and second to learn to enhance those factors. Absent better studies of course, we cannot as yet say definitively or precisely which factors contribute most to auditory success. However, some factors clearly have an effect.

 

The contribution of hardware

Above, I covered in some detail the general question of what the hardware provides, that being access to the critical parameters of sound.

As well, there was considerable discussion regarding electrode design, with the summation being that the design of electrodes according to the tonotopic theory is unreasonable and dangerous to residual hearing. Beyond this, the evidence is that either short or long electrodes can deliver all of the critical parameters of sound: so the subject of electrode design is not further pertinent to a discussion of what the hardware can offer the patient.

However, no mention was made of processing schemes, beyond a discussion of the early work which Jack Urban and I did. This area, concerning the manipulation of the sound signal before delivery to the internal implant, is, in my opinion, where the focus of our efforts regarding implant hardware must be.

There is considerable reason to believe the audiogram can be improved, or the AI scores can be very positively affected if you will, by the external sound processing scheme used in the implant system in question.

Consider an interesting finding in the Cohen et al study: [61]

To reiterate, after two years of using the WSP processor the composite score was 56; after three months of using the MSP processor, this increased to 86. In other words, without any change of internal hardware, the change of the external sound processing scheme had a rather significant effect.

This significant improvement apparently occurred because the implant user was provided with better high frequency thresholds. We are not given the pure tone measurement or AI information which would have demonstrated this beyond question, however. (Again, we must recognize the value of these metrics, and include them in all relevant reports in the field.)

As indicated previously, the frequencies from 2 to 4 KHz are of particular importance for proper discrimination of speech. For example, Skinner et al [62] demonstrated improved consonant recognition by emphasizing the high frequencies when the patients' Nucleus processors were changed from the Mini Speech Processor (MSP) to the Multi-Peak (M-Peak) processor.

Given that many such experiences have demonstrated the central value of the sound processing scheme, it bears mention that the 3M/House processor had, in essence, no processing scheme whatsoever. Sounds were simply amplified, without compression, a 16 KHz carrier was added, and that signal was induced in the internal coil. Further, the 3M/House processor had a frequency limiter in it so that any sounds above 3 KHz were simply thrown away.

The remarkable thing is that any patients using such equipment-- that is, with a primitive processor, albeit, as it turns out, with an advanced electrode — were able to have open-set recognition: but many were.

Again, this demonstrates the degree to which non-hardware factors can influence a patient's ability to become a successful implant user, and, again, it shows that comparisons between this older generation processor and newer processors — with schemes which provide for volume compression, feature extraction and the like — are invalid when applied to the question of electrodes.

In sum however, I am convinced that the most fertile field for the improvement of cochlear implants lies in a thorough exploration of the question of processing schemes. How can we best emphasize the relevant features of speech, using the hearing aid processor technology we now have available, in such a way that what is offered to the user gives optimum access to the sounds of speech?

The other very important implication of current information is that we can, for the foreseeable future, virtually ignore the electrode as regards design issues. A short, safe electrode of very simple design and efficient power utilization clearly offers us everything we need as a platform for almost any implant system. Therefore the area of greatest mystery, challenge, and potential for progress in the improvement of implant hardware will be here, in t