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.

对于许多人工耳蜗（cochlear implant, CI）使用者而言，音乐感知仍是一项颇具挑战性的任务，究其原因或许部分在于电极-神经界面与临床编程设定的分配频率之间存在频率失配。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，会导致耳蜗内电极的位置存在显著差异，但当前的人工耳蜗编程技术通常并未将这些差异纳入考量。平板计算机断层扫描（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。此类基于FPCT的人工耳蜗频率映射方案，有望提升音调感知精度，进而改善音乐欣赏能力与言语感知效果。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率谱上的表征效果。研究者要求受试者对两组依次播放的不同频率范围音调所产生的音程进行匹配，以此估算音调在人工耳蜗阵列上的均匀分布程度。本测试先以正常听力受试者为对象开展预实验，分别采用未处理声音与声码器处理后的声音，模拟匹配与失配的频率-位置映射方案。我们提出如下研究假设：相较于未处理的刺激音，声码器处理后的刺激音在音程匹配任务中的难度更高；且扭曲映射（类似当前临床常用映射可能出现的情况）会比真实且均匀的映射（可通过基于FPCT的频率分配方案实现）带来更差的音程匹配表现。初步结果显示，该任务能够在声码器处理条件下的正常听力受试者中，区分真实映射与扭曲映射之间的差异。一小队列人工耳蜗使用者的表现，与采用相同音调映射的声码器处理条件下的受试者表现相近。后续研究将在人工耳蜗使用者中测试该实验流程，并将结果与传统临床映射及基于FPCT的频率分配方案进行对比，以验证基于FPCT的映射是否能够改善音程感知能力。