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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 two and six 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.

Two-electrode stimuli presented on adjacent mid-array contacts in cochlear-implant users elicit pitch percepts that are not consistent with a summation of the two temporal patterns. This indicates that low-rate temporal rate codes can be applied with considerable independence on adjacent mid-array electrodes. At issue in this study was whether a similar independence of temporal pitch cues can also be observed for more apical sites of stimulation, where temporal cues have been shown to be more reliable than place cues, in contrast to middle and basal sites. In cochlear-implant recipients with single-sided deafness implanted with long lateral-wall electrode arrays, pitch percepts were assessed by matching the pitch of dual-electrode stimuli with pure tones presented to the contralateral normal-hearing ear. The results were supported with an additional pitch-ranking experiment, in a different subject population with bilateral deafness. Unmodulated pulse trains with 100, 200, and 400 pulses per second were presented on three pairs of adjacent electrodes. Pulses were separated by the minimal interchannel delay (1.7 µs) in a short-delay configuration and by half the pulse period in a long-delay configuration. The hypothesis was that subjects would perceive a pitch corresponding to the doubled temporal pattern for the long-delay stimuli due to the summation of excitation patterns from adjacent apical electrodes, if those electrodes were to activate largely overlapping neural populations. However, we found that the mean matched acoustic pitch of the long-delay pulses was not significantly different from that of the short-delay pulses. These findings suggest that also in the apical region in long-array cochlear-implant recipients, temporal cues can be transmitted largely independently on adjacent electrodes.

Hypothesis: We hypothesized that cochlear implant (CI) users are able to discriminate tones consisting of 1 and 2 modulation frequencies when the stimuli are applied through direct electrical stimulation. Background: Music perception is a very challenging task for CI users. In music, multiple tones often occur simultaneously, an essential feature of harmony. Proper encoding of simultaneous tones is crucial to musical perception and appreciation. With current implant processing strategies, CI users are severely impaired in the perception of pitch and polyphony. Methods: The ability of CI users to identify the number of simultaneous tones was assessed. Stimuli were applied with direct electrical stimulation. Stimuli with 1 modulation frequency were applied on a basal, a middle, and an apical electrode to determine if there was an effect of cochlear region. Stimuli with 2 modulation frequencies were applied on combinations of an apical electrode together with a basal or a middle electrode. Additionally, 2 modulations frequencies were presented at the same time on an apical electrode only. Results: Results demonstrate that subjects were generally able to identify the number of modulation frequencies in the presented stimuli. Performance for 1 modulation frequency stimuli was significantly above chance level on all 3 electrodes tested. Performance was best on the apical and the middle electrode, followed by the basal electrode. Subjects were also able to identify 2 modulation frequencies significantly above chance level on all 3 combinations tested. Performance was best on combination apical-basal followed by apical-middle. Performance was worst when 2 modulation frequencies were applied on an apical electrode only, but it was still significantly above chance level. Conclusion: If sound processing strategies were to use concurrent modulation frequencies on multiple or single electrodes, then possibly polyphonic tones would be better perceived by CI users yielding better music and language perception.