Musical pitch interval comparisons in cochlear implants

Music perception remains challenging for many cochlear implant (CI) recipients, due perhaps in part to the frequency mismatch that occurs between the electrode-neural interface and the frequencies allocated by the programming. Individual differences in ear anatomy, electrode array length, and surgical insertion can lead to great variability in the positions of electrodes within the cochlea, but these differences are not typically accounted for by current CI programming techniques. Flat panel computed tomography (FPCT) can be used to visualize the location of the electrodes and calculate the corresponding spiral ganglion characteristic frequencies. Such FPCT-based CI frequency mapping may improve pitch perception accuracy, and thus music appreciation, as well as speech perception. The present study seeks to develop a behavioral assessment metric for how well place-based pitch is represented across the frequency spectrum. Listeners were asked to match the pitch interval created by two tones, played sequentially, across different frequency ranges to estimate the extent to which pitch is evenly distributed across the CI array. This test was piloted with pure tones in normal hearing listeners, using both unprocessed and vocoder-processed sounds to simulate both matched and mismatched frequency-to-place maps. We hypothesized that the vocoded stimuli would be more difficult to match in terms of pitch intervals than unprocessed stimuli and that a warped map (as may occur with current clinical maps) would produce poorer matches than a veridical and even map (as may be achieved using FPCT-based frequency allocation). Preliminary results suggest that the task can reveal differences between veridical and warped maps in normal-hearing listeners under vocoded conditions. A small cohort of CI recipients performed similarly to a vocoded condition employing the same pitch map. The next steps will be to test this procedure in CI users and compare results with traditional clinical maps and FPCT-based frequency allocation to determine whether the FPCT-based maps result in improved pitch-interval perception.许多人工耳蜗（cochlear implant, CI）使用者仍难以获得良好的音乐感知能力，其部分原因可能在于电极-神经界面与编程设定的分配频率之间存在频率失配问题。耳部解剖结构、电极阵列长度以及手术植入方式的个体差异，会导致耳蜗内电极位置出现极大差异，但当前的人工耳蜗编程技术通常未考虑这类差异。平板探测器计算机断层扫描（Flat panel computed tomography, FPCT）可用于可视化电极位置，并计算对应的螺旋神经节特征频率。这种基于FPCT的人工耳蜗频率映射方案，可提升音调感知准确度，进而改善音乐欣赏能力与言语感知能力。本研究旨在开发一项行为评估指标，用于量化基于位置的音调在整个频谱上的表征效果。实验要求受试者对不同频率范围内依次播放的两个音调所产生的音程进行匹配，以此评估音调在人工耳蜗阵列上的均匀分布程度。本研究先以正常听力受试者为对象，使用纯音开展预实验，通过未处理与声码器处理的声音分别模拟频率-位置匹配与失配的映射方案。研究假设：相较于未处理的刺激，声码器处理的刺激在音程匹配任务中难度更高；而相较于精准且均匀的映射方案（可通过基于FPCT的频率分配实现），扭曲的映射方案（当前临床映射可能出现此类问题）会导致更差的音程匹配结果。预实验结果显示，该任务可在声码器处理条件下，区分正常听力受试者所感知的精准映射与扭曲映射之间的差异。一小部分人工耳蜗使用者的实验表现与采用相同音调映射的声码器处理条件下的表现相近。后续研究将在人工耳蜗使用者中验证该实验流程，并将实验结果与传统临床映射方案及基于FPCT的频率分配方案进行对比，以明确基于FPCT的映射方案是否能提升音程感知能力。