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.

A comparison of the speech recognition and pitch ranking abilities of children using a unilateral cochlear implant, bimodal stimulation or bilateral hearing aids

“…Pitch perception is further limited by overlaps in the electrical currents generated at adjacent, and more distant, electrodes [43]. Overlapping electrical currents occur because intracochlear electrode arrays are surrounded by highly conductive fluid that fills scala tympani [2]. Current evidence suggests only 4 to 8 independent sites of stimulation are available, even for arrays with 22 electrodes [44,45].…”

“…These low-order harmonics, along with the fundamental frequency (F0), are also important for determining the pitch of a complex sound. However speech processing strategies used in the current generation of cochlear implants (CIs) provide little representation of individual harmonics [2,3] and do not adequately convey F0 information, adversely affecting pitch perception.…”

Toward better representations of sound with cochlear implants

“…These latter persons included, but were not limited to, Graeme Clark, Donald Eddington, and Michael Merzenich, as well as their respective teams (Wilson and Dorman, 2008a). …”

Strategies to Improve Music Perception in Cochlear Implantees

“…There are many factors that can influence functional outcomes post-cochlear implantation including surgical techniques, variability of array placement, device coding strategies, intensity of rehabilitation and pathology of hearing loss (Wilson & Dorman, 2008a, 2008b. In addition to the variability of functional outcomes, music appreciation in cochlear implant recipients is also variable, presumably for similar reasons.…”