Sound Coding in Cochlear Implants: From electric pulses to hearing

Cochlear implantation is a life-changing intervention for people with a severe hearing impairment [1]. For most cochlear implant (CI) users, speech intelligibility is satisfactory in quiet environments. Although modern CIs provide up to 22 stimulation channels, information transfer is still limited for the perception of fine spectrotemporal details in many types of sound. These details contribute to the perception of music and speech in common listening situations, such as where background noise is present. Over the past several decades, many different sound processing strategies have been developed to provide more details about acoustic signals to CI users. In this article, progress in sound coding for CIs is reviewed. Starting from a basic strategy, the current commercially most-used signal processing schemes are discussed, as well as recent developments in coding strategies that aim to improve auditory perception. This article focuses particularly on the stimulation strategies, which convert sound signals into patterns of nerve stimulation. The neurophysiological rationale behind some of these strategies is discussed and aspects of CI performance that require further improvement are identified.