Investigating the effect of blurring and focusing current in cochlear implant users with the Panoramic ECAP method

Abstract

Purpose: For some cochlear implants (CIs), it is possible to focus electrical stimulation by partially returning current from the active electrode to adjacent, intra-cochlear electrodes (partial tripolar (pTP) stimulation). Another method achieves the opposite: “blurring” by stimulating multiple electrodes simultaneously. The Panoramic ECAP Method (PECAP) provides a platform to investigate their effects in detail by measuring Electrically-Evoked Compound Action-Potentials (ECAPs) and estimating current spread and neural responsiveness along the length of the CI electrode array. We investigate how sharpening and broadening the electrical current spread is reflected in PECAP estimates.Methods: PECAP measurements were recorded at most comfortable level in 12 ears of Advanced Bionics CI users. Focused thresholds were also determined using steered quadrupolar (sQP) stimulation. For the electrodes with the highest and lowest sQP thresholds, additional PECAP measurements were recorded whilst stimulating in pTP mode and in “blurred” mode with 3 or 5 adjacent electrodes simultaneously stimulated. Current spread and neural responsiveness were then estimated along the electrode array using PECAP.Results: PECAP revealed increased current-spread estimates for blurred stimulation for the targeted electrodes towards the apex of the cochlea. Mixed results for pTP stimulation were found, with increased current spread in two ears, decreased current spread in one ear, and no effect in five. Conclusion: When stimulating multiple electrodes simultaneously, PECAP detected localized increases in current spread toward the apex (but not the base) of the cochlea. pTP stimulation showed mixed effects on PECAP current-spread estimates. These findings are in line with behavioral speech perception studies.

Assessing Array-Type Differences in Cochlear Implant Users Using the Panoramic ECAP Method

“…Although we do not have a complete explanation for why the interaction between electrode number and array type differed between the two measures, it is worth noting a parallel between the effect of electrode position and of another manipulation designed to increase current spread, namely "blurring" the pattern of electrical stimulation by the simultaneous in-phase stimulation of multiple adjacent electrodes (Goehring et al, 2020). It has recently been reported that, compared to single-electrode monopolar stimulation, stimulating either 3 or 5 electrodes simultaneously increased PECAP's estimate of current spread, but only did so when the blurring was applied to more-apical (as opposed to more-basal) electrodes (Garcia et al, 2024). Both the blurring data and the data presented here are consistent with the idea that increasing current spread at the level of the cochlea is reflected in PECAP's estimate of current spread only toward the apex.…”

“…It has been validated by selectively putting neurons near one electrode into a refractory state during PECAP data collection and observing a local reduction in PECAP's estimate of neural responsiveness (Garcia et al, 2021). The current-spread estimate is modelled with symmetric Gaussian curves centred at each electrode, and has been validated using current-manipulation techniques that aim either to increase or decrease the current spread achieved whilst stimulating in standard monopolar (MP) mode (Garcia et al, 2024). PECAP therefore provides a tool to investigate how the synchronous responsiveness of the auditory nerve and the spread of electrical current vary along the length of the electrode array for an individual CI user.…”