# Effects of stimulus polarity on the local evoked potential in auditory brainstem implant users

## TL;DR

This study examines how different electrical pulse polarities affect hearing in auditory brainstem implant users, finding that cathodic pulses are more effective for measuring local evoked potentials.

## Contribution

The paper introduces a new baseline-dependent definition for analyzing LEP amplitude and suggests cathodic pulses for ABI-specific coding strategies.

## Key findings

- Cathodic leading pulses showed significantly shorter LEP peak latencies compared to anodic pulses.
- LEP thresholds with both cathodic and anodic pulses correlated with perceptual hearing thresholds, though anodic results were affected by outliers.
- No significant differences were found in LEP thresholds or slopes between cathodic and anodic leading pulses.

## Abstract

Auditory brainstem implants (ABI) can enable hearing sensation through electrical stimulation of the cochlear nucleus. The basic stimulation and signal coding strategies of the ABI are based on those of the cochlear implant. This may not always be optimal, and ABI-specific strategies may be preferred. In a cohort of ten ABI users, we examined the feasibility of measuring local evoked potentials (LEP) via fine-grained stimulation with a forward masking paradigm. We introduce a new baseline-dependent definition of LEP amplitude for analyzing the LEP amplitude growth function to obtain threshold stimulation levels and slope values. The processing of biphasic pulses by the cochlear nucleus and the influence of the leading phase polarity were examined. There were no statistically significant differences in LEP thresholds or slopes between cathodic and anodic leading pulses. LEP thresholds measured with cathodic leading pulses (r = 0.77, t31 = 6.81, p < 0.0001) and anodic leading pulses (r = 0.70, t27 = 45.14, p < 0.0001) correlated significantly with perceptual hearing thresholds. The correlation analysis was impacted by outlier values, especially in the case of LEP thresholds measured with anodic leading pulses. Cathodic leading pulses had significantly shorter LEP peak latencies (t104.8 = 2.63, p < 0.01). These results show that the cathodic leading pulses are superior for eliciting LEPs. We suggest that cathodic leading pulses should be the basis for ABI-specific coding strategies.