Stimulation for the Return of Hearing

This chapter focuses on the use of electrical stimulation at the auditory nerve for returning a person from total deafness or a severe hearing loss to useful hearing. The principal cause of hearing loss is damage to or complete destruction of the sensory hair cells. Therefore, the function of a cochlear prosthesis is to bypass the missing hair cells by stimulating directly the surviving neurons in the auditory nerve. The essential components in a cochlear prosthesis system include a microphone for sensing sound in the environment, a speech processor to transform the microphone input into a set of stimuli for the implanted array of electrodes, and a transcutaneous link for the transmission of power and stimulus information across the skin. The direct stimulation of the nerve is produced by currents delivered through electrodes placed in the scala tympani (ST), which is one of three fluid-filled chambers along the length of the cochlea. Different electrodes in the implanted array stimulate different subpopulations of neurons. The spatial specificity of stimulation with an ST electrode depends on a variety of factors, including the orientation and geometric arrangement of the electrodes, the proximity of the electrodes to the target neural structures, and the condition of the implanted cochlea in terms of nerve survival and ossification. Variability in outcomes is high, with some subjects achieving scores at or near 100% correct and with other subjects scoring close to zero.