The clinical application of potentials evoked from the peripheral auditory system

The auditory nerve is the obligatory pathway between the cochlea and the central nervous system. As the fibers of the mammalian auditory nerve share with each other many response properties when stimulated electrically (Kiang, 1965), it is reasonable to assume that the summed, or gross, electrical response of a fiber ensemble can provide researchers with meaningful information about nerve function. This chapter describes the gross responses of the auditory nerve that can be evoked by stimuli delivered by one or more electrodes implanted within the cochlea. Two manifestations of this potential--the electrically evoked compound action potential (ECAP) and the electrically evoked auditory brainstem response (EABR)--are introduced and compared. Some implant devices include systems that allow ECAPs to be routinely recorded from electrodes within the cochlea in clinical settings. While such systems have increased the popularity of the ECAP, unique advantages of the EABR are noted. Both potentials have assisted the clinical management of implant recipients and increase our understanding of how the human auditory system responds to electrical stimulation. The goals of this chapter are to review response characteristics of the whole-nerve response, the extent to which they provide information about underlying auditory nerve fiber activity, and limitations to their interpretation. This report will focus on describing and reviewing data collected from various clinical studies and interpreting these results on the basis of theoretical considerations and pertinent results from animal studies. Future research directions, which will likely involve the integration of various dimensions of the electrically evoked response that have been studied in isolation, are also suggested.