By Blake S. Wilson, PhD, DSc, DEng, Dr.med.hc (mult.)
I had the great privilege of presenting the opening keynote address for the 10th Asia-Pacific Symposium on Cochlear Implants and Related Sciences (APSCI 2015), which was held in Beijing in May 2015. The Symposium and its grand spirit are beautifully described in the July 2015 issue of this journal. The present article is a summary of my talk; the complete set of slides with annotations is available at http://bit.ly/29u97Xz.
In the talk, I first recalled with the greatest fondness a trip Fan-Gang Zeng, Steve Rebscher, Bob Shannon, Jerry Loeb, and I took in 1993 to participate in the Zhengzhou International Symposium on Electrical Hearing and Linguistics, which I believe was the first conference of its type in China. Approximately 130 persons participated. Fan-Gang, Steve, Bob, and I are shown in the photo, one that brings back happy memories indeed, including memories of all the wonderful people we met at the conference and our marvelous tour of China afterward.
Fan-Gang Zeng, Steve Rebscher, Bob Shannon,and Blake Wilson in China in 1993.
Everyone in the photo was at the APSCI 2015, which was a lovely reunion for us. We noticed that we are a bit younger in the photo!
1993 was about the time when highly effective processing strategies were introduced into clinical practice and after implants with multiple sites of stimulation in the cochlea had been developed. And 1993 was near the clear onset of what later would prove to be an exponential growth in the number of implant recipients worldwide, a growth that continues to this day (Fig. 1).
LARGE STEPS FORWARD
Figure 1 also shows large steps forward in the development and applications of cochlear implants (CIs) and related treatments. The first step was to develop safe devices that could be used outside of the laboratory by patients in their daily lives for many years. The person most responsible for that huge achievement was Dr. William F. House, M.D., D.D.S. Later steps included: (1) development of implants that could provide multiple and perceptually separable sites of stimulation in the cochlea; (2) development of the processing strategies mentioned previously; and (3) stimulation in addition to that provided by a unilateral CI, either with electrical stimulation on both sides or with electric plus acoustic stimulation (combined EAS) for persons with residual hearing.
Figure 1. Cumulative number of implant recipients over the years. The dots represent published numbers. Industry records indicate that the number approximated 460,000 in June 2015. Major events in the development of the cochlear implant are also indicated in the figure. Abbreviations are CIs for cochlear implants and EAS for combined electric and acoustic stimulation. (The figure is adapted and updated from IEEE Sensors J 2008;8:131-142 and is presented here with permission.)
Although the present-day CIs and related treatments are great, they are not perfect. Some of the remaining gaps between prosthetic and normal hearing are that:
- Some users of CIs still do not have high levels of speech reception
- Speech reception in adverse acoustic environments such as noisy restaurants or workplaces is worse for even the best CI users compared to listeners with normal hearing
- The averages of scores for difficult tests in quiet, such as recognition of monosyllabic words, are still far lower for CI users than for listeners with normal hearing, although some CI users score at or near 100 percent correct in these tests
- Sound localization is absent or nearly so for users of unilateral CIs
- Reception of sounds more complex than speech, e.g., most music, is impaired for CI users
In addition, some experts have suggested that reception of tone languages may pose further difficulties for CI users, in that perception of fundamental frequencies (and therefore the tone contours in tone languages) may be at least somewhat impaired with the present-day devices. However, results from the recent clinical trial in China of the Nurotron device indicate that recognition of sentences in Mandarin is as good as recognition of sentences in languages that do not use tone contours to convey phonetic information (Hear Res 2015;322:188-199). Possibly, the perception of tone contours is sufficient for robust reception of tone languages with the present-day CIs, or co-varying cues convey the necessary phonetic information in any case.
A VEXING LIMITATION
A likely roadblock to better performance for CIs is a limitation in the number of effective channels, even with a higher number of intracochlear electrodes. The limitation is illustrated in Fig. 2, which shows speech reception scores as a function of the number of processing channels and corresponding electrodes for users of unilateral CIs. The top panel presents data from one of my earlier laboratories and the bottom panel presents data from Garnham et al. (Ear Hear 2002;23:540-552). The presented data are representative of findings from other studies. The subject in the top panel used a Cochlear Ltd implant with its 22 intracochlear electrodes, and the 11 subjects in the bottom panel used MED EL GmbH implants with their 12 intracochlear electrodes. (Today's CIs use 12-24 intracochlear electrodes.) Tests of consonant identification in quiet and in noise were administered for the subject in the top panel, and various tests of speech recognition in quiet and in noise were administered for the subjects in the bottom panel. Means and standard errors of the means of the scores are shown.
Figure 2. Means and standard errors of the means for tests of speech reception conducted in one of the author's earlier laboratories, at the Research Triangle Institute (RTI) in North Carolina, USA, and by Garnham et al. (Ear Hear 2002;23:540-552). The subjects were users of unilateral cochlear implants and the tests included identification of consonants in quiet and in noise; recognition of the Bamford-Kowal-Bench (BKB) sentences in noise; identification of vowels in noise; and recognition of the Arthur Boothroyd (AB) monosyllabic words in quiet and in noise. The speech-to-noise ratios (S/Ns) for the tests in noise are indicated in the legends. Scores for sound processors using different numbers of channels – and the electrodes associated with those channels – are shown along the abscissa in each panel. The tests of consonant identification at the RTI included 24 consonants. Additional information about the RTI tests is presented in Br J Audiol 1997;31:205-225. (The figure is from Hear Res 2008;242:3-21 and is reproduced here with permission.)
Both panels indicate a plateau in scores once the number of channels rises above 3-6 depending on the test. Indeed, no implant subject tested to date has reached more than eight channels in any test before encountering asymptotic performance. That means that the number of effective channels with the present-day unilateral CIs is below, and often far below, the number of intracochlear electrodes and the maximum possible number of channels.
The reason(s) for the limitations in the numbers of effective channels remain to be identified. A lack of discrimination among electrodes is not a candidate reason, as many subjects can discriminate most or all of their intracochlear electrodes when the electrodes are stimulated in isolation, one after the other. For example, the subject in the top panel could reliably discriminate any pairing of electrodes from among the available 22 and yet the number of effective channels was three for consonant identification in quiet and four for consonant identification in noise. Thus, an apparent disconnect exists between the number of discriminable electrodes and the number of effective channels. Possibly, temporal interactions that are produced in the speech processor context, with rapid sequential presentations of overlapping electric fields, and that are not produced in the electrode discrimination context, may provide a partial or complete explanation for the disconnect. However, that is speculation at this point and more research is needed to learn why the numbers of effective channels are so low.
BENEFITS OF ADJUNCTIVE STIMULATION
The speech reception performance of unilateral CIs has been relatively constant since the early 1990s despite great efforts by many outstanding teams to produce improvements (Hear Res 2015;322:24-38). Fortunately, however, another way was found to improve performance and that was to provide stimulation in addition to that presented with a unilateral CI.
In broad terms, bilateral CIs and combined EAS can each produce highly significant increases in speech reception, especially for difficult speech items and for speech presented in competition with noise or other talkers. In addition, bilateral CIs can reinstate to some extent sound localization abilities through a representation of the interaural level difference (ILD) cues that can indicate the lateral positions of sound sources. Combined EAS also greatly enhances music reception, perhaps through a representation with the acoustic stimulus of the first several harmonics for periodic sounds, which are critical for robust perception of the fundamental frequencies for those sounds.
THE CONTINUED IMPORTANCE OF UNILATERAL IMPLANTS
Despite these wonderful gains with adjunctive stimulation, unilateral CIs are still vitally important. In particular, not all patients have useful residual hearing (although many do); not all patients or prospective patients have access to bilateral CIs due to national health policies or restrictions in insurance coverage; and the unilateral CI and its performance is the "bedrock" of the adjunctive stimulation treatments. With respect to the last point, improvements in the performance of unilateral CIs would be expected to boost the performance of the adjunctive stimulation treatments as well.
POSSIBILITIES FOR NARROWING THE GAPS
Each of the gaps listed previously can be narrowed but not eliminated with adjunctive stimulation. Additional possibilities for narrowing the gaps include: (1) identifying the mechanism(s) underlying the difference between the number of discriminable electrodes and the lower number of effective channels; (2) an increase in the latter number, using that knowledge and perhaps with a greater spatial specificity of stimulation; (3) prudent pruning of interfering or otherwise detrimental electrodes; (4) a further relaxation in the criteria for implant candidacy; and (5) "brain centric" approaches to designs and applications of CIs. Many other possibilities could be suggested and indeed are being pursued. However, the listed possibilities are the most promising in my opinion.
Understanding the apparent disconnect between the number of discriminable electrodes versus the number of effective channels is vital for guidance in increasing the latter number. Modeling studies are underway in my present laboratory to evaluate various possible mechanisms for the observed effects. (Josh Stohl, Ph.D., is our highly able leader in this research.) Results from the studies may inform the design of electrodes or stimuli or both that will produce increases in the number of effective channels.
Based on what we know now, increases also might be produced with greater spatial specificities of stimulation at different sites in the cochlea or auditory nerve. Three promising possibilities along these lines are optical rather than electric stimulation in the cochlea (Hear Res 2015;322:224-234); delivery of electrical stimuli within the auditory nerve rather than in the scala tympani (ST) of the cochlea (J Assoc Res Otolaryngol 2007;8:258-279); and promoting the growth of neural processes from the spiral ganglion cells toward electrodes in the ST (Sci Transl Med 2014;6:233ra54). Each of these approaches may sharpen the neural excitation fields.
Pruning of interfering or otherwise detrimental electrodes makes great sense for the present-day CIs, in that those devices can support only a maximum of eight effective channels. Choosing the best eight electrodes, and deactivating the others, may produce improvements in performance compared with using most or all of the available electrodes. Certainly, deleterious interactions among electrodes would be reduced with the pruning.
A further relaxation in the criteria for implant candidacy would make CIs available to many more persons who could benefit from these marvelous devices. In addition, the newly included persons would have even more residual hearing than the present CI users and candidates for CIs. The average scores would increase, through the demonstrated benefits of combined EAS, which also include better reception of sounds more complex than speech. Even a slight relaxation in the criteria would increase the number of candidates substantially. Possibly, for example, persons who now suffer from the debilitating effects of presbycusis may become candidates for a CI and the benefits of combined EAS.
Gentle explorations of the boundaries for implant candidacy seem warranted for these reasons and the fact that persons presently at the boundaries still receive benefits from CIs that are at least as great as the benefits received by persons with much less residual hearing (J Hear Sci 2012;2:19-32 and Ear Hear 2010;31:186-194). The combination of electric plus acoustic stimulation is powerful and a point of diminishing returns in providing a CI for persons with progressively more residual hearing has yet to be identified.
And finally, the brain centric approaches to designs and applications of CIs may be especially helpful to patients presently at the low end of the performance spectrum (Prog Brain Res 2011;194:117-129). Indeed, accumulating evidence is indicating that a large portion of the remaining variability in outcomes with CIs may be due to differences in the function of the "hearing brain" among the recipients. If so, then a better match between what the prosthesis provides and what the brain can process may improve performance for the patients who are still struggling.
The modern CI is a triumph of otology, engineering, and neuroscience, among other disciplines. Its success has surprised many renowned experts in the relevant fields and indeed it has produced the first substantial restoration of a human sense using a medical intervention.
With that said, room remains for improvements. Most fortunately there are many possibilities for that and the quest for narrowing the remaining gaps between prosthetic and normal hearing continues.