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.

A computational model to simulate spectral modulation and speech perception experiments of cochlear implant users

“…The activation function decreases and widens with worse neural health conditions having an impact in the MCL levels, as shown in Figure 6 . Worst neural health conditions resulted in higher MCL levels, which is consistent with the findings of Langner et al ( 2021 ).…”

“…The results of the SRT experiment show that worse neural health conditions result in poorer speech reception performance. Although this degeneration affected more the performance with simultaneous stimulation sound coding strategies, especially with F120-T, the detriment compared to sequential stimulation (F120-S) was rather small and not as relevant, as shown by Langner et al ( 2017 , 2021 ). A more detailed discussion regarding SMT and SRT experiments is presented in the following subsections.…”

“…It has been shown that peripheral aspects such as neural health condition (Nadol, 1997 ), insertion depth and position of the electrode array (Dorman et al, 1997 ), along with more central aspects such as neural plasticity (Han et al, 2019 ) may account for an important part of the inter-subject variability observed in CI users. However, it is not possible to estimate the degree of ANF degeneration without invasive methods unless the individual is already implanted with a CI (Prado-Gutierrez et al, 2006 ; Ramekers et al, 2014 ; Imsiecke et al, 2021 ; Langner et al, 2021 ). Langner et al ( 2021 ) investigated the hypothesis that individuals with good neural health and electrode positioning will show a lower difference in performance when using simultaneous stimulation strategies (F120-P and F120-T) compared to sequential stimulation (F120-S).…”

“…However, it is not possible to estimate the degree of ANF degeneration without invasive methods unless the individual is already implanted with a CI (Prado-Gutierrez et al, 2006 ; Ramekers et al, 2014 ; Imsiecke et al, 2021 ; Langner et al, 2021 ). Langner et al ( 2021 ) investigated the hypothesis that individuals with good neural health and electrode positioning will show a lower difference in performance when using simultaneous stimulation strategies (F120-P and F120-T) compared to sequential stimulation (F120-S). Healthy conditions lead to lower focused thresholds and less channel interaction between virtual channels; therefore, healthy neural conditions could lead to less detriment in performance when comparing sequential and simultaneous stimulation.…”

“…The goal is to show the effects of parallel stimulation and neural degeneration in CI outcome performance. This model was tested with the three sound coding strategies used by Langner et al ( 2021 ) (F120-S, F120-P and F120-T), and the hypothesis that channel interaction affects individuals with poorer neural health conditions to a larger extent is assessed. In the next section, the different parts composing this computational model and how the SRT and SMT experiments were implemented with FADE are described.…”