Physiologic measures of electrically evoked auditory system responsiveness: effects of pathology and electrical stimulation.

During the past years we have addressed issues of implant application in children related to candidate selection and time of implantation. The studies have incorporated an assessment of the utility of the electrically evoked middle latency response (EMLR) as a measure of responsiveness of the central auditory pathways elicited by electrical stimulation. We have demonstrated that the EMLR in the guinea pig is similar in waveform characteristics to the acoustically elicited MLR, that it is equal in sensitivity to the electrically evoked auditory brainstem response (EABR) and can be used to reflect the sensitivity of different sites of stimulation. Because of its longer latency, the EMLR shows far less sensitivity to electrical artifact than the EABR. While suprathreshold characteristics of the EMLR are affected by anesthesia (ketamine and xylazine), threshold is not. In the ototoxically deafened animal we have found a decrease in the amplitude input/output function slope with decreasing density of spiral ganglion cell survival and that moderately high intensity of electrical stimulation produces a temporary threshold shift (eTTS) in the EMLR. These data suggest that this measure may be useful in assessing surviving spiral ganglion cell populations for the selecting of candidates for implantation and establishing safe, functional, upper-limits of electrical stimulation for young children. Finally, we have found that early electrical stimulation can reduce the progress of spiral ganglion cell degeneration secondary to ototoxic-induced hair cell loss.