gms | German Medical Science

Neuronal responses to pulse trains with short interpulse intervals (IPIs) smaller than 300 µs are influenced by facilitation effects. These effects within the refractory period can be measured by electrically evoked compound action potentials (ECAPs) of the auditory nerve with cochlear implants (CI). In previous studies (e.g. [1]) intensive ECAP measurements with short IPIs were assessed with CI of the manufacturer Cochlear. The masker pulse level (relative to the probe pulse level) was varied and it was found, that the facilitation effect increased to its maximum for masker levels at or slightly below the ECAP threshold independent of the probe level. A further increase of the masker level revealed a decrease in facilitation.

Similar measurements were repeated in four subjects with CIs of the manufacturer Med-El to investigate if the described effects could be reproduced with a different setup. ECAP threshold were recorded using the manufacturers fitting software Maestro. Afterwards, ECAPs with short IPIs (15 µs) using a custom-made software were recorded with the artifact reduction method of Miller. The probe pulse level was fixed at ECAP threshold, while the masker pulse level varied from 0 CU (absolute value) up to 200 CU (relative to ECAP threshold).

The results of the original study could be confirmed using Med-El implants. As expected, the highest facilitation effect was at masker levels around ECAP threshold, with a steep decrease of ECAP amplitude after the maximum at higher masker levels. Increasing the probe level had no influence on the optimal masker level at the highest facilitation effect.