Assessing the relationship between neural health measures and speech performance with simultaneous electric stimulation in cochlear implant listeners

Abstract

Objectives: The relationship between electrode-nerve interface (ENI) estimates and inter-subject differences in speech performance with sequential and simultaneous channel stimulation in adult cochlear implant listeners were explored. We investigated the hypothesis that individuals with good ENIs would perform better with simultaneous compared to sequential channel stimulation speech processing strategies than those estimated to have poor ENIs.

Methods: Fourteen postlingually deaf implanted cochlear implant users participated in the study. Speech understanding was assessed with a sentence test at signal-to-noise ratios that resulted in 50% performance for each user with the baseline strategy F120 Sequential. Two simultaneous stimulation strategies with either two (Paired) or three sets of virtual channels (Triplet) were tested at the same signal-to-noise ratio. ENI measures were estimated through: (I) voltage spread with electrical field imaging, (II) behavioral detection thresholds with focused stimulation, and (III) slope (IPG slope effect) and 50%-point differences (dB offset effect) of amplitude growth functions from electrically evoked compound action potentials with two interphase gaps.

Results: A significant effect of strategy on speech understanding performance was found, with Triplets showing a trend towards worse speech understanding performance than sequential stimulation. Focused thresholds correlated positively with the difference required to reach most comfortable level (MCL) between Sequential and Triplet strategies, an indirect measure of channel interaction. A significant offset effect (difference in dB between 50%-point for higher eCAP growth function slopes with two IPGs) was observed. No significant correlation was observed between the slopes for the two IPGs tested. None of the measures used in this study correlated with the differences in speech understanding scores between strategies.

Conclusions: The ENI measure based on behavioral focused thresholds could explain some of the difference in MCLs, but none of the ENI measures could explain the decrease in speech understanding with increasing pairs of simultaneously stimulated electrodes in processing strategies.

Fig 1. Sequence of stimulation for the sound coding strategies used in the speech performance task.

Electrodes over time show the principle of the order of stimulation with one (Sequential), two (Paired) or three (Triplet) channels at the same time.

Fig 2. Quadrupolar stimulation mode: Two electrodes (E₂ and E₃) stimulate with the current divided by α with cathodic-first biphasic pulses.

The other adjacent electrodes E₁ and E₄ act as anodic-first return electrodes in which the current is divided by σ between the two electrodes and the ground electrode. The effective current spread across cochlear place is depicted above as a solid black line, while the individual current spread of each electrode is depicted as dotted lines.

Fig 3. Speech understanding performance differences from all participants between Sequential and Paired as well as Sequential and Triplet.

Fig 4. EFIW as a function of electrode number averaged across participants (top plot) and as a function of participants averaged across electrodes (bottom plot).

The median is in red, edges show the 25th and 75th percentiles and whiskers the most extreme data.

Fig 5. Ideal AGF derived from a sigmoidal fit (upper left plot) and assessed AGFs across all participants fir IPG = 2 μs (upper middle plot) and 30 μs (upper right plot).

Distribution of slopes (bottom left plot) and level 50% to calculate the significant dB offset effect (bottom right plot).