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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.

Timing cues such as interaural time differences (ITDs) and temporal pitch are pivotal for sound localization and source segregation, but their perception is degraded in cochlear-implant (CI) listeners as compared to normal-hearing listeners. In multi-electrode stimulation, intra-aural channel interactions between electrodes are assumed to be an important factor limiting access to those cues. The monaural asynchrony of stimulation timing across electrodes is assumed to mediate the amount of these interactions. This study investigated the effect of the monaural temporal electrode asynchrony (mTEA) between two electrodes, applied similarly in both ears, on ITD-based left/right discrimination sensitivity in five CI listeners, using pulse trains with 100 pulses per second and per electrode. Forward-masked spatial tuning curves were measured at both ears to find electrode separations evoking controlled degrees of across-electrode masking. For electrode separations smaller than 3 mm, results showed an effect of mTEA. Patterns were u/v-shaped, consistent with an explanation in terms of the effective pulse rate that appears to be subject to the well-known rate limitation in electric hearing. For separations larger than 7 mm, no mTEA effects were observed. A comparison to monaural rate-pitch discrimination in a separate set of listeners and in a matched setup showed no systematic differences between percepts. Overall, an important role of the mTEA in both binaural and monaural dual-electrode stimulation is consistent with a monaural pulse-rate limitation whose effect is mediated by channel interactions. Future CI stimulation strategies aiming at improved timing-cue encoding should minimize the stimulation delay between nearby electrodes that need to be stimulated successively. [ABSTRACT FROM AUTHOR]

A series of experiments investigated the effects of asymmetric current waveforms on the perception of place and temporal pitch cues. The asymmetric waveforms were trains of pseudomonophasic (PS) pulses consisting of a short, high-amplitude phase followed by a longer (and lower amplitude) opposite-polarity phase. When such pulses were presented in a narrow bipolar ("BP+1") mode and with the first phase anodic relative to the most apical electrode (so-called PSA pulses), pitch was lower than when the first phase was anodic re the more basal electrode. For a pulse rate of 12 pulses per second (pps), pitch was also lower than with standard symmetric biphasic pulses in either monopolar or bipolar mode. This suggests that PSA pulses can extend the range of place-pitch percepts available to cochlear implant listeners by focusing the spread of excitation in a more apical region than common stimulation techniques. Temporal pitch was studied by requiring subjects to pitch-rank single-channel pulse trains with rates ranging from 105 to 1,156 pps; this task was repeated at several intra-cochlear stimulation sites and using both symmetric and pseudomonophasic pulses. For PSA pulses presented to apical electrodes, the upper limit of temporal pitch was significantly higher than that for all the other conditions, averaging 713 pps. Measures of discriminability obtained using the method of constant stimuli indicated that this pitch percept was probably weak. However, a multidimensional scaling study showed that the percept associated with a rate change, even at high rates, was orthogonal to that of a place change and therefore reflected a genuine change in the temporal pattern of neural activity. [ABSTRACT FROM AUTHOR]