Assessing the Electrode-Neuron Interface with the Electrically Evoked Compound Action Potential, Electrode Position, and Behavioral Thresholds

Abstract

Variability in speech perception scores among cochlear implant listeners may largely reflect the variable efficacy of implant electrodes to convey stimulus information to the auditory nerve. In the present study, three metrics were applied to assess the quality of the electrode-neuron interface of individual cochlear implant channels: the electrically evoked compound action potential (ECAP), the estimation of electrode position using computerized tomography (CT), and behavioral thresholds using focused stimulation. The primary motivation of this approach is to evaluate the ECAP as a site-specific measure of the electrode-neuron interface in the context of two peripheral factors that likely contribute to degraded perception: large electrode-to-modiolus distance and reduced neural density. Ten unilaterally implanted adults with Advanced Bionics HiRes90k devices participated. ECAPs were elicited with monopolar stimulation within a forward-masking paradigm to construct channel interaction functions (CIF), behavioral thresholds were obtained with quadrupolar (sQP) stimulation, and data from imaging provided estimates of electrode-to-modiolus distance and scalar location (scala tympani (ST), intermediate, or scala vestibuli (SV)) for each electrode. The width of the ECAP CIF was positively correlated with electrode-to-modiolus distance; both of these measures were also influenced by scalar position. The ECAP peak amplitude was negatively correlated with behavioral thresholds. Moreover, subjects with low behavioral thresholds and large ECAP amplitudes, averaged across electrodes, tended to have higher speech perception scores. These results suggest a potential clinical role for the ECAP in the objective assessment of individual cochlear implant channels, with the potential to improve speech perception outcomes.

Across-site patterns of electrically evoked compound action potential amplitude-growth functions in multichannel cochlear implant recipients and the effects of the interphase gap

“…It should be noted that the change in ECAP threshold as the IPG was increased was not significantly related to neural health characteristics. This last finding is in keeping with a recent report by DeVries and colleagues (DeVries, Scheperle, & Bierer, 2016) who reported that ECAP thresholds, but not amplitudes, measured in human subjects were significantly correlated with electrode position within the cochlea (e.g., distance between the electrodes and the modiolus). Taken together, these findings suggest electrode position may significantly contribute to threshold (or near threshold) measures, but measures obtained at higher current stimulation levels reflect, at least in part, neural health.…”

“…However, it is first important to understand how manipulation of the IPG affects ECAP responses across a multichannel electrode array and if it is feasible to record responses on the majority of electrodes. In deafened human subjects, it is known that both the health of the neural population and the distances from the electrodes to the neurons vary along the length of the cochlea in a subject-specific manner (Nadol, 1997; Long et al, 2014; DeVries et al, 2016). Thus we hypothesized that suprathreshold ECAP measures that reflect cochlear health and or characteristics of the electrode-to-neuron current path would vary along the CI electrode array in a subject specific pattern.…”

Vowel and consonant confusions from spectrally manipulated stimuli designed to simulate poor cochlear implant electrode-neuron interfaces

“…Two CI listeners in particular were selected because they were judged in a previous study (DeVries et al ., 2016) to have poor electrode-neuron interfaces in only the middle frequency region (S43) or the middle and basal frequency regions (S47), somewhat matching those used in this study.…”

The Enigma of Poor Performance by Adults With Cochlear Implants

“…As measured by electrically evoked compound action potential (ECAP), there are data supporting the prediction that a shorter electrode-to-modiolus distance correlates with higher speech recognition score (30). Additionally, there is some evidence that patients with perimodiolar arrays display better speech recognition than those with lateral-wall hugging electrodes (3).…”