Auditory brainstem implant: electrophysiologic responses and subject perception.

OBJECTIVES: The primary aim of this study was to compare the perceptual sensation produced by bipolar electrical stimulation of auditory brainstem implant (ABI) electrodes with the morphology of electrically evoked responses elicited by the same bipolar stimulus in the same unanesthetized, postsurgical state. Secondary aims were to (1) examine the relationships between sensations elicited by the bipolar stimulation used for evoked potential recording and the sensations elicited by the monopolar pulse-train stimulation used by the implant processor, and (2) examine the relationships between evoked potential morphology (elicited by bipolar stimulation) to the sensations elicited by monopolar stimulation.
DESIGN: Electrically evoked early-latency and middle-latency responses to bipolar, biphasic low-rate pulses were recorded postoperatively in four adults with ABIs. Before recording, the perceptual sensations elicited by these bipolar stimuli were obtained and categorized as (1) auditory sensations only, (2) mixed sensations (both auditory and nonauditory), (3) side effect (nonauditory sensations), or (4) no sensation. In addition, the sensations elicited by monopolar higher-rate pulse-train stimuli similar to that used in processor programming were measured for all electrodes in the ABI array and classified using the same categories. Comparisons were made between evoked response morphology, bipolar stimulation sensation, and monopolar stimulation sensation.
RESULTS: Sensations were classified for 33 bipolar pairs as follows: 21 pairs were auditory, 6 were mixed, 5 were side effect, and 1 was no sensation. When these sensations were compared with the electrically evoked response morphology for these signals, P3 of the electrically evoked auditory brainstem response (eABR) and the presence of a middle-latency positive wave, usually between 15 and 25 msec (electrical early middle-latency response [eMLR]), were only present when the perceptual sensation had an auditory component (either auditory or mixed pairs). The presence of other waves in the early-latency response such as N1 or P2 or a positive wave after 4 msec did not distinguish between only auditory or only nonauditory sensations. For monopolar stimulation, 42 were classified as auditory, 16 were mixed, and 26 were classified as side effect or no sensation. When bipolar sensations were compared with monopolar sensations for the 21 bipolar pairs categorized as auditory, 7 pairs had monopolar sensations of auditory for both electrodes, 9 pairs had only one electrode with a monopolar sensation of auditory, with the remainder having neither electrode as auditory. Of 6 bipolar pairs categorized as mixed, 3 had monopolar auditory sensations for one of the electrodes. When monopolar stimulation was compared with evoked potential morphology elicited by bipolar stimulation, P3 and the eMLR were more likely to be present when one or both of the electrodes in the bipolar pair elicited an auditory or mixed sensation with monopolar stimulation and were less likely to occur when neither of the electrodes had an auditory monopolar sensation. Again, other eABR waves did not distinguish between auditory and nonauditory sensations.
CONCLUSIONS: ABI electrodes that are associated with auditory sensations elicited by bipolar stimulation are more likely to elicit evoked responses with a P3 wave or a middle-latency wave. P3 of the eABR and M15-25 of the eMLR are less likely to be present if neither electrode of the bipolar pair evoked an auditory sensation with monopolar stimulation.