The multi-channel cochlear implant: past, present and future perspectives

Initial research demonstrated that only low frequencies could be mimicked with rate of electrical stimulation, and thus multi-channel rather than single-channel stimulation was required for the place coding of the mid-high speech frequencies. Place coding of mid-high frequencies was best achieved with electrodes inside the cochlea. Furthermore, correct biomechanical properties of a multiple electrode bundle were required for it to pass around the cochlear spiral to the speech frequency region. Biological studies showed too that intra-cochlear electrodes could be used with minimal trauma, safe electrical stimulus parameters, and methods to prevent inner ear infection and meningitis. The crucial discoveries for coding speech with electrical stimulation have been based on the discovery of: 1) the fact the brain processes frequency information along spatial and temporal channels, and 2) that the first patient experienced vowels when stimulating different electrodes that corresponded to the place of excitation for single formant vowels in people with normal hearing. The inaugural and subsequent speech processing strategies extracted frequencies of special importance for speech intelligibility, and transmitted the information along place coding channels. The voicing frequency and/or amplitude, was coded as temporal information across these spatial channels. As a result a great majority of severely-to-profoundly deaf people with previous hearing can not only communicate when electrical stimulation is combined with lipreading, but with electrical stimulation alone. In addition, the benefits of binaural hearing with bilateral cochlear implants or an implant in one ear and hearing aid in the other ear have been realized. Related psychophysical research has discovered the basic perceptual skills that process the complex patterns of brain excitation that underlie speech recognition both in the one ear as well as bilateral implants.In addition the development of the perceptual skills in the maturing child for speech recognition, have been discovered. In the future high fidelity sound should be achieved by providing the fine temporo-spatial patterns of excitation and preserving the peripheral nerve network. This could require the release of nerve growth factors and the development of electrodes using nanotechnology.