Effect of Chronic Stimulation and Stimulus Level on Temporal Processing by Cochlear Implant Listeners

Robert P Carlyon,Frances Harris,Alexander J Billig,John M Deeks,François Guérit,Yu Chuen Tam

doi:10.1007/s10162-018-00706-y

Abstract

A series of experiments investigated potential changes in temporal processing during the months following activation of a cochlear implant (CI) and as a function of stimulus level. Experiment 1 tested patients on the day of implant activation and 2 and 6 months later. All stimuli were presented using direct stimulation of a single apical electrode. The dependent variables were rate discrimination ratios (RDRs) for pulse trains with rates centred on 120 pulses per second (pps), obtained using an adaptive procedure, and a measure of the upper limit of temporal pitch, obtained using a pitch-ranking procedure. All stimuli were presented at their most comfortable level (MCL). RDRs decreased from 1.23 to 1.16 and the upper limit increased from 357 to 485 pps from 0 to 2 months post-activation, with no overall change from 2 to 6 months. Because MCLs and hence the testing level increased across sessions, two further experiments investigated whether the performance changes observed across sessions could be due to level differences. Experiment 2 re-tested a subset of subjects at 9 months post-activation, using current levels similar to those used at 0 months. Although the stimuli sounded softer, some subjects showed lower RDRs and/or higher upper limits at this re-test. Experiment 3 measured RDRs and the upper limit for a separate group of subjects at levels equal to 60 %, 80 % and 100 % of the dynamic range. RDRs decreased with increasing level. The upper limit increased with increasing level for most subjects, with two notable exceptions. Implications of the results for temporal plasticity are discussed, along with possible influences of the effects of level and of across-session learning.

Highlights

The processing of electrical stimulation changes in the months following initial activation of a cochlear implant (CI)
The moderate size of this improvement imposes an upper constraint on the amount of plasticity in temporal processing that occurs during the first 2 months after implant activation
Strands of evidence against this latter interpretation are that (a) the effect size was larger for the upper limit than for the low-rate rate discrimination ratios (RDRs), whereas the most comfortable level (MCL) changes were similar across rates, (b) there was no across-subject correlation between level- and upperlimit differences between 0 and 2 months and (c) some subjects showed an increase in MCL but no increase in upper limit between 2 and 6 months

Summary

Introduction

The processing of electrical stimulation changes in the months following initial activation of a cochlear implant (CI). The most important change from the patient’s perspective is an improvement in speech perception (Blamey et al 2013), a finding that has been attributed to the patient learning the relationship between the novel form of stimulation and speech segments (Davis et al 2005). Other changes, not attributable to learning, occur. We extend the study of plasticity in CI patients to temporal processing and investigate whether it occurs in adult-deafened human CI patients following activation of their device. Neurons in the central nucleus of the IC phase-lock to pulse trains only up to a certain Bupper limit^

Methods

Results

Discussion

Conclusion