Cochlear implant stimulation of a hearing ear generates separate electrophonic and electroneural responses

Electroacoustic stimulation in subjects with residual hearing is becoming more widely used in clinical practice. However, little is known about the properties of electrically induced responses in the hearing cochlea. In the present study, normal-hearing guinea pig cochleae underwent cochlear implantation through a cochleostomy without significant loss of hearing. Using recordings of unit activity in the midbrain, we were able to investigate the excitation patterns throughout the tonotopic field determined by acoustic stimulation. With the cochlear implant and the midbrain multielectrode arrays left in place, the ears were pharmacologically deafened and electrical stimulation was repeated in the deafened condition. The results demonstrate that, in addition to direct neuronal (electroneuronal) stimulation, in the hearing cochlea excitation of the hair cells occurs (“electrophonic responses”) at the cochlear site corresponding to the dominant temporal frequency components of the electrical stimulus, provided these are <12 kHz. The slope of the rate–level functions of the neurons in the deafened condition was steeper and the firing rate was higher than in the hearing condition at those sites that were activated in the two conditions. Finally, in a monopolar stimulation configuration, the differences between hearing status conditions were smaller than in the narrower (bipolar) configurations.