Acoustic masking of electric stimulation at basal and extra-cochlear sites

Cochlear implant (CI) candidates with residual low-frequency hearing increasingly receive hearing-preserving electrode arrays to enable electric-acoustic stimulation (EAS). However, assessing very low-frequency hearing in newborns and young children remains difficult. Previous work has shown that acoustic stimulation can mask electrically evoked auditory percepts when stimulation sites are spatially separated. Whether such masking also occurs when electric stimulation is delivered near the round window (RW), prior to cochlear insertion, remains unclear. This study investigated ipsilateral acoustic masking of electric stimulation delivered at apical, basal, and near-RW sites in 12 EAS users with residual low-frequency hearing implanted with short or partially inserted electrode arrays. Using a psychophysical paradigm, threshold elevations of electric pulse trains were measured during simultaneous acoustic stimulation. Electric stimulation was applied at extra-cochlear RW locations and intra-cochlear basal and apical electrodes, while stimulation parameters were systematically varied. With Individually optimized parameters, electric stimulation near the RW reliably evoked auditory percepts without side effects in 8 of 12 participants, though optimal stimulation parameters and masking strength varied across participants. Strong acoustic masking was consistently observed for apical stimulation, whereas weaker but measurable masking occurred for basal and RW stimulation. A cochlear lumped-parameter model indicated that approximately 5% of current delivered at the RW can reach apical regions, explaining the observed masking by low-frequency acoustic tones. Masking strength correlated with residual hearing for apical stimulation only. These findings indicate that acoustic masking of apical electric stimulation, which correlated with residual hearing, holds potential as a diagnostic tool for assessing low-frequency hearing post-operatively. The detection of masking during both RW and basal stimulation represents a promising result, indicating that these stimulation paradigms can produce quantifiable effects. Given the considerable variability observed among participants, further investigation is required to establish the extent to which masking measures reflect residual hearing or hearing loss. Nonetheless, these results provide novel insight into auditory perception and side effects associated with basal and extra-cochlear electric stimulation, informing future efforts to optimize this approach for low-frequency hearing assessment.