Cochlear implants: system design, integration, and evaluation

Abstract

As the most successful neural prosthesis, cochlear implants have provided partial hearing to more than 120000 persons worldwide; half of which being pediatric users who are able to develop nearly normal language. Biomedical engineers have played a central role in the design, integration and evaluation of the cochlear implant system, but the overall success is a result of collaborative work with physiologists, psychologists, physicians, educators, and entrepreneurs. This review presents broad yet in-depth academic and industrial perspectives on the underlying research and ongoing development of cochlear implants. The introduction accounts for major events and advances in cochlear implants, including dynamic interplays among engineers, scientists, physicians, and policy makers. The review takes a system approach to address critical issues in cochlear implant research and development. First, the cochlear implant system design and specifications are laid out. Second, the design goals, principles, and methods of the subsystem components are identified from the external speech processor and radio frequency transmission link to the internal receiver, stimulator and electrode arrays. Third, system integration and functional evaluation are presented with respect to safety, reliability, and challenges facing the present and future cochlear implant designers and users. Finally, issues beyond cochlear implants are discussed to address treatment options for the entire spectrum of hearing impairment as well as to use the cochlear implant as a model to design and evaluate other similar neural prostheses such as vestibular and retinal implants.

Keywords: Auditory prosthesis; auditory brainstem; auditory nerve; biocompatibility; biomaterials; current source; electric stimulation; electrode; fine structure; hermetic sealing; loudness; music perception; pitch; radio frequency; safety; signal processing; speech recognition; temporal resolution.

Figure 1

Three phases defining the major events in the development of cochlear implants. The conceptualization phases demonstrated the feasibility of electric stimulation. The research and development phase legitimized the utility and safety of electric stimulation. The commercialization phase saw a wide spread use of electric stimulation in treating sensorineural hearing loss.

Fig. 2

Exponential growth of cochlear implant research and sales. Note the 10 year delay for sales growth. The data of annual publications on cochlear implants (filled green bars with the unit on the left y-axis) were collected using keywords (cochlear AND implant) in PubMed (http://www.pubmed.gov) on June 19, 2008. The sales data (open purple bars with the unit on the right y-axis) were disclosed in Cochlear Annual Report (http://www.cochlear.com.au). Insert: Annual sale number of 3M/House single-electrode (blue line) and Nucleus multi-electrode (purple line) cochlear implants between 1982 and 1989 [25].

Fig. 3

Sentence recognition scores with a quiet background as a function of time for the 3M/House single-electrode device (first column), the Cochlear Nucleus device (filled bars), the Advanced Bionics Clarion device (open bars), and the Med-El device (shaded bars). Previous results before 2004 were summarized in Zeng [40]. The latest results were obtained in the following references: Nucleus Freedom [41], Clarion HiRes system [42], and Med-El Opus device [43]. For a detailed description of the acronyms, see Section III.

Fig. 4

A typical modern cochlear implant system that converts sound to electric impulses delivered to the auditory nerve (www.cochlear.com).

Fig. 5

Architecture and functional block diagram of a modern cochlear implant. The single-electrode systems had an analog external unit and contained no internal active circuits [44]. The Utah six-electrode, four-channel system had a percutaneous plug and contained no internal circuits [22]. The Nucleus 22 device had essentially all components of a modern system, except for the back telemetry circuits [45].

Fig. 6

Classifications of signal processing strategies in cochlear implants.

Fig. 7

A. Block diagram and signal processing in the Continuous-Interleaved-Sampling (CIS) strategy. B. Block diagram of the “n-of-m” strategy.

Fig. 8

Spectrogram (panel A) and electrodograms of CIS (panel B) and SPEAK (panel C) for the sentence: “A large size in stockings is hard to sell”.

Fig. 9

RF transmission coding in the Nucleus Freedom System. Waveform shows the original RF signal, with its amplitude being modulated on and off. The numbers below the waveform show the raw bits recovered from the RF signal. The raw bits are grouped into 6, coding the discrete data token.

Fig. 10

The expanded mode frame coding scheme in the Nucleus system.

Fig. 11

The embedded mode frame coding scheme.

Fig. 12

Block diagram of the cochlear implant internal unit.

Fig. 13

Three electric artifact removal techniques (three rows). The individual recordings and the derived ECAP responses were obtained in an actual Clarion HiRes 90k user (Qing Tang, personal communication).

Fig. 14

Top: Med-El Combi 40+™’ Middle: Advanced Bionics Helix™ ; Bottom: Cochlear Contour™ electrode arrays.

Fig. 15

Panel A: Med-El Combi 40+™; Panel B: Advanced Bionics Helix™ ; and Panel C: helical winding connecting the implanted stimulator and the electrode array in the Advanced Bionics Helix™ and 1J devices.

Fig. 16

To define the initial site and extent of damage to each structure within the cochlea, we dissected and analyzed 13 epoxy embedded temporal bones with traumatic insertions of the Cochlear Contour array. The bars labeled SL, BM, OSL and RM represent the region of injury to the spiral ligament, basilar membrane, osseous spiral lamina and Reissner’s membrane respectively. The location of each of these structures is shown in the inset cross section of the scala tympani. The outlined box between 180° and 270° indicates the region where the electrode perforated the basilar partition. The distance over which the electrode perforated the partition ranged from 7° to 185° with a mean distance of 64.7°. Beyond this perforation the electrode remained in either the scala tympani or scala vestibuli with minimal damage. SpG = Spiral Ganglion, ST = Scala Tympani, SV = Scala Vestibuli.

Fig. 17

Images illustrating the intended insertion path of a pre-molded spiral electrode using the AOS technique. A Cochlear Contour electrode is shown as an example. The electrode array loaded on a straight stylet is shown in image (a) prior to advancing the array. As the stylet is held in position, the electrode is gently pushed off of the stylet and resumes its pre-molded shape (b-d). In this ideal case there is no contact with the lateral wall of the scala tympani. One possible mode of damage with this technique occurs if the electrode and stylet are advanced too far into the ST prior to advancing the array. In this case the straight stylet and electrode will contact the outer wall resulting in trauma similar to that of earlier straight electrodes. Studies at UCSF have shown that the distance from the RW to the first turn of the cochlea varies by more than 50% and that the marker molded in the Contour array may be well outside the cochleostomy when this contact and damage occurs in many subjects.

Fig. 18

A section of Nucleus Freedom Custom Sound fitting program.

Fig. 19

Interaction of human factors that results in either safe and effective use, or unsafe or ineffective use [from 188].

Fig. 20

Basic psychophysical performance in cochlear implants: A. Loudness growth as a function of current level from three studies [67]; B. Pitch estimate as a function of electric stimulation rate (x-axis) and place (symbols) [51]; C. Spatial tuning curves in a cochlear implant user (squares superimposed with the two shallow lines) and a normal-hearing listener (steep lines without symbols) [195]; and D. Temporal modulation detection as a function of modulation frequency in an average cochlear implant user (circles) and an average normal-hearing listener (thick line) [38].

Fig. 21

Challenges facing the present cochlear implant users (open bars); as a control, the comparative normal data are presented as the filled bars. A. Speech recognition in noise, showing signal-to-noise ratio (SNR) needed to achieve 50% correct performance in the presence of either steady-state noise or a competing voice [7]. Music perception, showing melody recognition[202] and timbre recognition [208]. C. Tonal language processing, showing Mandarin tone perception[205] and production[204].

Fig. 22

Treatment of hearing impairment using hearing aids, middle ear implants, and cochlear implants. The hearing aids shown are Exélia micro from Phonak (www.phonak.com). The middle ear implant shown is Soundbridge from Med-El (www.medel.com). The cochlear implant shown is Nucleus-24 from Cochlear (www.cochlear.com). The stimulation site of a brainstem implant may be cochlear nucleus or inferior colliculus. The brainstem implant shown is a penetrating electrode array that stimulates the cochlear nucleus (developed by Huntington Medical Research Institutes: www.hmri.org).