A behavioral method to estimate charge integration efficiency in cochlear implant users

Abstract

BackgroundIn cochlear implants, pulse amplitude (PA) or pulse phase duration (PPD) can be used to increase loudness. Loudness grows more slowly with increasing PPD, resulting in a larger dynamic range (DR), possibly reflecting “leaky” charge integration associated with neural degeneration due to hearing loss. Here, we propose a method to estimate charge integration efficiency for CI users. New methodThe DR was measured with increasing PA or PPD, relative to a common threshold anchor with a short PPD (25μs/ph); DRs were converted to the common unit of charge (nC). Charge integration efficiency was calculated as the dB difference in DR with increasing PPD or PA. Loudness growth functions were also compared as PA or PPD was increased relative to the common threshold. ResultsTen CI ears were tested; all participants were adult users of Cochlear© devices. DR was significantly larger when PPD was increased, requiring (on average) 70 % more charge than when PA was increased. A significant correlation (p = 0.007) was observed between duration of deafness and charge integration efficiency, largely driven by a participant with long auditory deprivation in both ears. Loudness growth was slower when PPD was increased, consistent with previous studies.Comparison to Existing Methods. The present method offers a quick behavioral test with which to measure charge integration efficiency, which may be a useful measure of neural health. DiscussionCharge integration efficiency may be used to probe neural health independent of absolute detection thresholds, which mostly reflect the proximity of electrodes to neural populations.