[Measuring late electrically evoked potentials of the auditory system in cochlear implant patients]

The current adjustment of cochlear implant (CI) speech processors is based on a knowledge of the lower and upper limits (T- and C-levels) for electrical stimulus currents. These data are usually acquired from subjective classifications of individual patients. In cases with non-reliable patient responses, objective methods are necessary. Especially for the estimation of correct T-levels, auditory evoked potentials (AEP) can be applied, since they allow the determination of response thresholds in a frequency-specific manner. By determining the AEP of different latencies, late cortical responses can be registered almost without artifact contamination. These patients have been examined in 20 patients provided with 22- or 8-channel CI-systems (Nucleus or Med-EI implants). In all cases, clear responses and clearly discernible threshold transitions could be detected. By making use of acoustical stimulation in a free sound field, subjective hearing threshold and the T-levels of electrical stimulation could be verified. Since late responses are generated in the primary auditory cortex, their assessment permits a nearly integral functional control of the aided hearing system. To date, no problems have occurred from maturation or attentional effects in either pediatric or adult patients. The applicability in very young children remains to be explored.