Speech Processing Strategies for Cochlear Prostheses-the Past, Present and Future: a Tutorial Review

Cochlear Implants are widely accepted prosthetic devices that improve the hearing ability of people with profound hearing loss. The cochlear implant speech processor is responsible for decomposing the input sound into different frequency bands and delivering the most appropriate pattern to the electrodes. The performance of the cochlear prostheses depends on various parameters such as number of channels, number of electrodes, type of stimulation, rate of stimulation and compression function. The objective of this paper is to review how the sound signals are coded in cochlear implants, using different speech processing strategies focusing on waveform, feature-extraction and hybrid. The review describes the coding of sound for single and multi-channel implants based on the type (analog and pulsatile), rate of stimulation and compression function. Also, speech processing strategies used in currently available commercial cochlear speech processors are presented. Results of several investigations show that the strategies based on spectral signal analysis allow for better speech understanding than speech feature extraction. Though the performance of different strategies is variable from patient to patient, certain strategies have variable features which improve the speech perception.