Pitch adaptation patterns in bimodal cochlear implant users: over time and after experience.

OBJECTIVES: Pitch plasticity has been observed in Hybrid cochlear implant (CI) users. Does pitch plasticity also occur in bimodal CI users with traditional long-electrode CIs, and is pitch adaptation pattern associated with electrode discrimination or speech recognition performance? The goals of this study were to characterize pitch adaptation patterns in long-electrode CI users, to correlate these patterns with electrode discrimination and speech perception outcomes, and to analyze which subject factors are associated with the different patterns.
DESIGN: Electric-to-acoustic pitch matches were obtained in 19 subjects over time from CI activation to at least 12 months after activation, and in a separate group of 18 subjects in a single visit after at least 24 months of CI experience. Audiometric thresholds, electrode discrimination performance, and speech perception scores were also measured.
RESULTS: Subjects measured over time had pitch adaptation patterns that fit one of the following categories: (1) "Pitch-adapting," that is, the mismatch between perceived electrode pitch and the corresponding frequency-to-electrode allocations decreased; (2) "Pitch-dropping," that is, the pitches of multiple electrodes dropped and converged to a similar low-pitch; and (3) "Pitch-unchanging," that is, the electrode pitches did not change. Subjects measured after CI experience had a parallel set of adaptation patterns: (1) "Matched-pitch," that is, the electrode pitch was matched to the frequency allocation; (2) "Low-pitch," that is, the pitches of multiple electrodes were all around the lowest frequency allocation; and (3) "Nonmatched-pitch," that is, the pitch patterns were compressed relative to the frequency allocations and did not fit either the matched-pitch or low-pitch categories. Unlike Hybrid CI users which were mostly in the pitch-adapting or matched-pitch category, the majority of bimodal CI users were in the latter two categories, pitch-dropping/low-pitch or pitch-unchanging/nonmatched-pitch. Subjects with pitch-adapting or matched-pitch patterns tended to have better low-frequency thresholds than subjects in the latter categories. Changes in electrode discrimination over time were not associated with changes in pitch differences between electrodes. Reductions in speech perception scores over time showed a weak but nonsignificant association with dropping-pitch patterns.
CONCLUSIONS: Bimodal CI users with more residual hearing may have somewhat greater similarity to Hybrid CI users and be more likely to adapt pitch perception to reduce mismatch with the frequencies allocated to the electrodes and the acoustic hearing. In contrast, bimodal CI users with less residual hearing exhibit either no adaptation, or surprisingly, a third pattern in which the pitches of the basal electrodes drop to match the frequency range allocated to the most apical electrode. The lack of association of electrode discrimination changes with pitch changes suggests that electrode discrimination does not depend on perceived pitch differences between electrodes, but rather on some other characteristics such as timbre. In contrast, speech perception may depend more on pitch perception and the ability to distinguish pitch between electrodes, especially since during multielectrode stimulation, cues such as timbre may be less useful for discrimination.