The multiple-channel cochlear implant: the interface between sound and the central nervous system for hearing, speech, and language in deaf people-a personal perspective

Abstract

The multiple-channel cochlear implant is the first sensori-neural prosthesis to effectively and safely bring electronic technology into a direct physiological relation with the central nervous system and human consciousness, and to give speech perception to severely-profoundly deaf people and spoken language to children. Research showed that the place and temporal coding of sound frequencies could be partly replicated by multiple-channel stimulation of the auditory nerve. This required safety studies on how to prevent the effects to the cochlea of trauma, electrical stimuli, biomaterials and middle ear infection. The mechanical properties of an array and mode of stimulation for the place coding of speech frequencies were determined. A fully implantable receiver-stimulator was developed, as well as the procedures for the clinical assessment of deaf people, and the surgical placement of the device. The perception of electrically coded sounds was determined, and a speech processing strategy discovered that enabled late-deafened adults to comprehend running speech. The brain processing systems for patterns of electrical stimuli reproducing speech were elucidated. The research was developed industrially, and improvements in speech processing made through presenting additional speech frequencies by place coding. Finally, the importance of the multiple-channel cochlear implant for early deafened children was established.

Figure 1

The structure of the ear and a diagram of the multiple-channel cochlear implant. The components are: A, microphone; B, behind the ear speech processor; C, body worn speech processor; D, transmitting aerial; E, receiver–stimulator; F, electrode array (Clark 2000).

Figure 2

The cochlea with 2½–2¾ turns spiralling around the modiolus (M). The fluid-filled canals in the turns are: SV, scala vestibuli; SM, scala media; ST, scala tympani. Inset: the organ of Corti (OC) rests on the basilar membrane (BM). The OC has outer (OHC) and inner (IHC) hair cells connected to auditory nerve fibres (AN). Reprinted with permission from Cochlear Corporation (1987).

Figure 3

An overview of the discoveries leading to the development of the multiple-channel cochlear implant.

Figure 4

(a) The phase locking of action potentials in a group of nerve fibres. (b) An inter-spike interval histogram of the neural responses to an acoustic stimulus of 0.416 kHz.

Figure 5

Field potentials from the superior olivary complex in the auditory brainstem of the cat for 1 and 300 pulses s⁻¹ rates of simulation of the auditory nerve in the cochlea (Clark, 1969b).

Figure 6

Intracellular inter-spike interval histograms from globular bushy cells in the anteroventral cochlear nucleus of the rat for increasingly higher rates of electrical stimulation (200, 800, 1200 and 1800  pulses s⁻¹) (Paolini and Clark, 1997). Reprinted from Clark (2003) with kind permission of Springer Science and Business Media.

Figure 7

(a) A diagram of the voltage field for bipolar stimulation; (b) ‘pseudo-bipolar’ (common ground) stimulation; (c) monopolar stimulation.

Figure 8

A banded, free-fitting, smooth, tapered, electrode array with graded stiffness that has passed into the scala tympani of the basal turn of the human cochlea. M, modiolus; RW, round window or entry point to the basal turn of the cochlea; BT, basal turn of the cochlea. Reprinted from Clark (2003) with permission of Springer Science and Business Media.

Figure 9

A photomicrograph of an implanted cat cochlea. M, infection in the middle ear with proliferation of the mucous membrane; R, thickened round window extending into a well-formed electrode sheath. There is no extension of the infection to the cochlea. Reprinted from Clark et al. (1990) with permission from Elsevier.

Figure 10

The University of Melbourne's prototype receiver–stimulator ready for implantation in the second profoundly deaf patient in 1979.

Figure 11

The first (F1) and second (F2) formant frequencies for the plosives /b/, /d/, /g/, and the burst of noise produced when the sound is released after the vocal tract has been closed. VOT, voice onset time, one of the cues for voicing.

Figure 12

Rate and place discrimination versus duration. The percentage judgments called different for shifts in rate and electrode site when compared with a standard stimulus are shown versus pulse rate and stimulus place trajectories. The trajectories were 25, 50, and 100 ms in duration. (a) The initial pulse rates of the trajectory were varied from 240, 210, 180, to the baseline, 150 pulses s⁻¹. (b) The initial electrodes of the trajectory varied from electrodes 4, 3, and 2 to the baseline 1. Reprinted from Tong et al. (1982) with permission.

Figure 13

A diagram of the processing of frequency information and the perception of pitch through central auditory spatial, coarse temporal and fine temporo-spatial perceptual systems.

Figure 14

Open-set word and sentence scores for the F0/F1/F2, Multipeak, SPEAK and ACE strategies. All frequencies were coded on a place basis.

Figure 15

(a) Spectrogram for the word ‘choice’ with the intensity at each frequency indicated by brightness. (b) The electrode representations (electrodogram) for ‘choice’ using the F1, F2, and high spectral frequency strategy (Multipeak). (c) The electrodogram for the SPEAK strategy.

Figure 16

Electrode ranking according to whether the pitch changed monotonically with the electrode versus word scores for the Bench–Kowal–Bamford open-set sentences for electrical stimulation alone on 16 children using cochlear implants (Busby and Clark, 2000; Clark, 2002). Reprinted from Clark (2002) with permission.