Cochlear implants: A remarkable past and a brilliant future

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Abstract

The aims of this paper are to (i) provide a brief history of cochlear implants; (ii) present a status report on the current state of implant engineering and the levels of speech understanding enabled by that engineering; (iii) describe limitations of current signal processing strategies; and (iv) suggest new directions for research. With current technology the “average” implant patient, when listening to predictable conversations in quiet, is able to communicate with relative ease. However, in an environment typical of a workplace the average patient has a great deal of difficulty. Patients who are “above average” in terms of speech understanding, can achieve 100% correct scores on the most difficult tests of speech understanding in quiet but also have significant difficulty when signals are presented in noise. The major factors in these outcomes appear to be (i) a loss of low-frequency, fine structure information possibly due to the envelope extraction algorithms common to cochlear implant signal processing; (ii) a limitation in the number of effective channels of stimulation due to overlap in electric fields from electrodes; and (iii) central processing deficits, especially for patients with poor speech understanding. Two recent developments, bilateral implants and combined electric and acoustic stimulation, have promise to remediate some of the difficulties experienced by patients in noise and to reinstate low-frequency fine structure information. If other possibilities are realized, e.g., electrodes that emit drugs to inhibit cell death following trauma and to induce the growth of neurites toward electrodes, then the future is very bright indeed.

Introduction

In 1802, Ludwig van Beethoven fell into a deep depression following a nearly complete loss of his remaining hearing. His physician recommended rest in Heiligenstadt, a village then and now a part of greater Vienna. There Beethoven wrote in October of that year his famous Heiligenstadt Testament, addressed to his two brothers and meant to be read after his death. In it, he said (translated from the original German into modern English):

“…For me there can be no relaxation in human society; no refined conversations, no mutual confidences. I must live quite alone and may creep into society only as often as sheer necessity demands. …Such experiences almost made me despair, and I was on the point of putting an end to my life – the only thing that held me back was my art… [and] thus I have dragged on this miserable existence.”

Helen Keller wrote in her autobiography, The Story of My Life (first published in 1905):

“…I am just as deaf as I am blind. The problems of deafness are deeper and more complex, if not more important, than those of blindness. Deafness is a much worse misfortune. For it means the loss of the most vital stimulus – the sound of the voice that brings language, sets thoughts astir and keeps us in the intellectual company of man.”

These poignant descriptions convey the feelings of isolation that often accompany deafness. Beethoven stressed loneliness as the major hardship, as opposed to a separation from his music. Helen Keller stressed that “blindness cuts one off from things, but deafness cuts one off from people.”

Just thirty years ago there were no effective treatments for deafness or severe hearing impairments. The advent of cochlear implants (CIs) changed that, and today implants are widely regarded as one of the great achievements of modern medicine.

The purposes of this paper are first to provide a brief history of implants and then to present a status report of where we are and where we are headed with implants. The status report describes current designs and levels of performance. It also presents strengths and weaknesses of present-day implant systems, and offers some possibilities for addressing the weaknesses. These contributions are meant to serve as an introduction and overview for other papers in this special issue on Frontiers of Auditory Prosthesis Research: Implications for Clinical Practice. In addition, the contributions are meant to celebrate the courage and persistence of the pioneers who made this “miracle” of modern medicine possible. The authors of the papers in this issue, along with their many contemporary colleagues, are standing on broad shoulders indeed, and it is our shared challenge and opportunity to move this technology forward and to do so at the same remarkable pace as in the past 30 years.

Section snippets

Historical context

As recently as the early 1980s, many eminent and highly knowledgeable people believed that CIs would provide only an awareness of environmental sounds and possibly speech cadences to their users. Many were skeptical of implants and thought that mimicking or reinstating the function of the exquisite machinery in the normal inner ear was a fool’s dream. Among these critics were world-renowned experts in otology and auditory physiology. Fortunately, pioneers persisted in the face of this intense

Components of implant systems

The essential components in a cochlear prosthesis system are illustrated in Fig. 2 and include (1) a microphone for sensing sound in the environment; (2) a speech processor to transform the microphone output into a set of stimuli for an implanted array of electrodes; (3) a transcutaneous link for the transmission of power and stimulus information across the skin; (4) an implanted receiver/stimulator to (i) decode the information received from the radio-frequency signal produced by an external

Performance with present-day unilateral implants

Each of the major implant systems and the default processing strategies used with them supports recognition of monosyllabic words on the order of 50–60% correct (using hearing alone), across populations of tested subjects (see, e.g., Table 2.4 in Wilson, 2006). Variability in outcomes is high, however, with some subjects achieving scores at or near 100% correct and with other subjects scoring close to zero on this most difficult of standard audiological measures. Standard deviations of the

Efficacy of sparse representations

Present-day CIs support a high level of function for the great majority of patients, as indicated in part by sentence scores of 80% correct or higher for most patients and the ability of most patients to use the telephone. In addition, some patients achieve spectacularly high scores with present-day CIs. Indeed, their scores are in the normal ranges even for the most difficult of standard audiological tests (e.g., the top few patients in Fig. 4 and the patient described in Wilson and Dorman,

Two recent advances

Two recent advances have produced significant improvements in the overall (average) performance of implant systems. The advances are (1) electrical stimulation on both sides with bilateral CIs, and (2) combined EAS of the auditory system for persons with residual hearing at low frequencies. Bilateral electrical stimulation may reinstate at least to some extent the interaural amplitude and timing difference cues that allow people with normal hearing to lateralize sounds in the horizontal plane

Possibilities for further improvements

Tremendous progress has been made in the design and performance of cochlear prostheses. However, much room remains for improvements. Patients with the best results still do not hear as well as listeners with normal hearing, particularly in demanding situations such as speech presented in competition with noise or other talkers. Users of standard unilateral implants do not have much access to music and other sounds that are more complex than speech. Most importantly, a wide range of outcomes

Concluding remarks

We as a field have come a long way in the development of CIs. We and implant patients alike owe a great debt of gratitude to the pioneers who persevered in the face of intense criticism.

With a modern CI, Ludwig van Beethoven’s life, and Helen Keller’s life, would have been changed for the better. Their sense of isolation would have been assuaged if not eliminated. Beethoven probably would have been disappointed in listening to music, however.

The path before us is clear and exciting. We are

Acknowledgments

Some of the findings and thoughts in this paper were first presented in a Special Guest of Honor Address by the first author at the Ninth International Conference on Cochlear Implants and Related Sciences, held in Vienna, Austria, June 14–17, 2006, and with the same title as the present paper. Material also was drawn or adapted from several recent publications (Dorman and Wilson, 2004, Dorman and Spahr, 2006, Wilson, 2006, Wilson and Dorman, 2007, Wilson and Dorman, 2008, Wilson and Dorman, in

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