Cochlear implants: a remarkable past and a brilliant future

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.

Fig. 1

Early history of cochlear implants. Developers and places of origin are shown, along with a timeline for the various efforts. Initial stages of development are depicted with the light lines, and clinical applications of devices are depicted with the heavy lines. Most of these devices are no longer in use, and many of the development efforts have been discontinued. Present devices and efforts are described in the text. (This figure is adapted from a historical model conceptualized by Donald K. Eddington, Ph.D., of the Massachusetts Eye & Ear Infirmary, and is used here with his permission. The figure also appeared in Niparko and Wilson, 2000, and is reprised here with the permission of Lippincott Williams & Wilkins.)

Fig. 2

Components of a cochlear implant system. The TEMPO+ system is illustrated, but all present-day implant systems share the same basic components. (Diagram courtesy of MED-EL Medical Electronics GmbH, of Innsbruck, Austria.)

Fig. 3

Block diagram of the continuous interleaved sampling (CIS) strategy. The input is indicated by the filled circle in the left-most part of the diagram. This input can be provided by a microphone or alternative sources. Following the input, a pre-emphasis filter (Pre-emp.) is used to attenuate strong components in speech below 1.2 kHz. This filter is followed by multiple channels of processing. Each channel includes stages of bandpass filtering (BPF), envelope detection, compression, and modulation. The envelope detectors generally use a full-wave or half-wave rectifier (Rect.) followed by a lowpass filter (LPF). A Hilbert Transform or a half-wave rectifier without the LPF also may be used. Carrier waveforms for two of the modulators are shown immediately below the two corresponding multiplier blocks (circles with a “x” mark within them). The outputs of the multipliers are directed to intracochlear electrodes (EL-1 to EL-n), via a transcutaneous link or a percutaneous connector. The inset shows an x-ray micrograph of the implanted cochlea, to which the outputs of the speech processor are directed. (Block diagram is adapted from Wilson et al., 1991, and is used here with the permission of the Nature Publishing Group. Inset is from Hüttenbrink et al., 2002, and is used here with the permission of Lippincott Williams & Wilkins.)

Fig. 4

Percent correct scores for 55 users of the COMBI 40 implant and the CIS processing strategy. Scores for recognition of the Hochmair-Schultz-Moser sentences are presented in the top panel, and scores for recognition of the Freiburger monosyllabic words are presented in the bottom panel. Results for each of five test intervals following the initial fitting of the speech processor for each subject are shown. The horizontal line in each panel indicates the mean of the scores for that interval and test. (The great majority of the data are from Helms et al., 1997, with an update reported by Wilson, 2006. Figure is adapted from Dorman and Spahr, 2006, and is used here with the permission Thieme Medical Publishers.)

Fig. 5

Speech reception scores as a function of the number of stimulated electrodes (and associated channels) using the CIS processing strategy. Means and standard errors of the mean are shown. Results from studies conducted in the first author’s laboratory are presented in the top panel, and results from Garnham et al. (2002) are presented in the bottom panel. The top panel shows scores for identification of 24 consonants in an /a/- consonant-/a/ context, by one subject using a Nucleus cochlear implant system with its 22 intracochlear electrodes. The bottom panel shows scores for recognition of the Bench, Kowal, and Bamford (BKB) sentences, identification of 16 consonants also in /a/- consonant-/a/ context, identification of 8 vowels in a /b/-vowel-/d/ context, and recognition of the Arthur Boothroyd (AB) monosyllabic words, by a maximum of 11 subjects (Ss) using the COMBI 40+ cochlear implant system with its 12 electrode sites. The test items either were presented in quiet or in competition with noise, as indicated in the legends for the two panels. For the presentations in competition with noise, the signalto- noise ratios (S/Ns) are indicated. The experimental conditions used for the study depicted in the top panel are the same as those described in Wilson (1997).