Compression and Cochlear Implants

Abstract

Cochlear compression plays an important role in supporting the exquisite sensitivity, fine frequency tuning, and large operating dynamic range of the ear (Bacon, Chapter 1; Cooper, Chapter 2; Oxenham and Bacon, Chapter 3).With cochlear impairment, hearing threshold is elevated, and frequency selectivity and dynamic range are reduced (Bacon and Oxenham, Chapter 4; Levitt, Chapter 5). In cases of cochlear implants, the cochlear compression and other cochlear functions are bypassed altogether; hearing sensation is evoked by direct electric stimulation of the auditory nerve. The goal of studying compression in cochlear implants is twofold. The first goal is practical. In a cochlear implant, all compression-related functions need to be replaced by a front-end artificial processor. The second goal is theoretical because direct stimulation of the auditory nerve provides a unique opportunity for studying the lack of cochlear compression in auditory perception, thereby complementing the studies in normal-hearing and cochlear-impaired listeners and allowing delineation of peripheral and central contributions to the functions of the overall system. This chapter first briefly reviews how cochlear implants work (see Section 2). The psychophysical, or more precisely, psychoelectrical capabilities in cochlear implant users is reviewed, with an emphasis on the effect of loss of cochlear compression on perception (see Section 3). Section 4 contrasts the psychoacoustical and psychoelectrical data and discusses their inference on the theoretical models of auditory processing. Section 5 discusses practical issues on compression in cochlear implants in terms of restoring normal loudness growth, increasing electric dynamic range, and improving speech performance in implant users. Section 6 summarizes the current data and discusses future research directions.