Fine-grain recordings of the electrically evoked compound action potential amplitude growth function in cochlear implant recipients

Abstract

Background: In cochlear implants (CI) measuring the electrically evoked compound action potential (ECAP) has become an important tool for verifying the electrode-nerve interface as well as establishing a basis for a map to program the speech processor. In a standard clinical setup recordings are averaged over 25-100 repetitions to allow for the detection of ECAPs within the noise floor. To obtain an amplitude growth function, these measurements are normally performed for 5-10 different stimulation levels. We evaluate a recording paradigm where the stimulation intensity is increased in quasi-continuous steps and instead of averaging repeated recordings with identical stimulation parameters, running averages over small intervals of stimulation levels are computed. The first visible nerve response was manually identified by two experts. Results: Both recording paradigms were evaluated in 39 cochlear implants, showing an on average lower threshold of the first nerve response for the quasi-continuous measurement paradigm (Wilcoxon signed-rank test, p = 6.2e-08) compared to the clinical standard paradigm. The mean maximal loudness over all implants and stimulation electrodes was 13% lower at the 80 pulses/s quasi-continuous paradigm compared to the 44 pulses/s clinical standard paradigm. Conclusions: Beside a more robust determination of the ECAP threshold, the proposed quasi-continuous stimulation paradigm results in a more robust behavioral feedback of the CI user upon the maximal acceptable loudness percept. Furthermore this paradigm can also reveal the fine-structure in the amplitude growth function.