Department of Speech and Hearing Sciences, University of Washington, Seattle, WA, USA.
Ear Hear. 2010 Apr;31(2):247-58. doi: 10.1097/AUD.0b013e3181c7daf4.
The goal of this study was to evaluate the ability of a threshold measure, made with a restricted electrode configuration, to identify channels exhibiting relatively poor spatial selectivity. With a restricted electrode configuration, channel-to-channel variability in threshold may reflect variations in the interface between the electrodes and auditory neurons (i.e., nerve survival, electrode placement, and tissue impedance). These variations in the electrode-neuron interface should also be reflected in psychophysical tuning curve (PTC) measurements. Specifically, it is hypothesized that high single-channel thresholds obtained with the spatially focused partial tripolar (pTP) electrode configuration are predictive of wide or tip-shifted PTCs.
Data were collected from five cochlear implant listeners implanted with the HiRes90k cochlear implant (Advanced Bionics Corp., Sylmar, CA). Single-channel thresholds and most comfortable listening levels were obtained for stimuli that varied in presumed electrical field size by using the pTP configuration for which a fraction of current (sigma) from a center-active electrode returns through two neighboring electrodes and the remainder through a distant indifferent electrode. Forward-masked PTCs were obtained for channels with the highest, lowest, and median tripolar (sigma = 1 or 0.9) thresholds. The probe channel and level were fixed and presented with either the monopolar (sigma = 0) or a more focused pTP (sigma > or = 0.55) configuration. The masker channel and level were varied, whereas the configuration was fixed to sigma = 0.5. A standard, three-interval, two-alternative forced choice procedure was used for thresholds and masked levels.
Single-channel threshold and variability in threshold across channels systematically increased as the compensating current, sigma, increased and the presumed electrical field became more focused. Across subjects, channels with the highest single-channel thresholds, when measured with a narrow, pTP stimulus, had significantly broader PTCs than the lowest threshold channels. In two subjects, the tips of the tuning curves were shifted away from the probe channel. Tuning curves were also wider for the monopolar probes than with pTP probes for both the highest and lowest threshold channels.
These results suggest that single-channel thresholds measured with a restricted stimulus can be used to identify cochlear implant channels with poor spatial selectivity. Channels having wide or tip-shifted tuning characteristics would likely not deliver the appropriate spectral information to the intended auditory neurons, leading to suboptimal perception. As a clinical tool, quick identification of impaired channels could lead to patient-specific mapping strategies and result in improved speech and music perception.
本研究旨在评估一种阈限测量方法的能力，该方法采用受限电极配置，以识别空间选择性相对较差的通道。在受限的电极配置中，通道间的阈值变化可能反映了电极与听觉神经元之间界面的变化（即神经存活、电极放置和组织阻抗）。这些电极-神经元界面的变化也应反映在心理物理调谐曲线（PTC）测量中。具体而言，假设使用空间聚焦部分三极（pTP）电极配置获得的高单通道阈值与宽或尖端移位的 PTC 相关。
本研究的数据来自五名植入 HiRes90k 人工耳蜗（Advanced Bionics Corp.， Sylmar，CA）的人工耳蜗植入患者。使用 pTP 配置获得了在假定电场大小上变化的刺激的单通道阈值和最舒适的听力水平，其中中心活性电极的一部分（sigma）通过两个相邻电极返回，其余部分通过远处的无关电极返回。对于具有最高、最低和中位数三极（sigma = 1 或 0.9）阈值的通道，获得了正向掩蔽 PTC。探针通道和水平固定，并以单极（sigma = 0）或更聚焦的 pTP（sigma> = 0.55）配置呈现。掩蔽器通道和水平变化，而配置固定为 sigma = 0.5。使用标准的三间隔、二选一强制选择程序测量阈值和掩蔽水平。
随着补偿电流 sigma 的增加和假定电场变得更加集中，单通道阈值和通道间阈值的变异性系统地增加。在所有受试者中，当使用窄的 pTP 刺激测量时，具有最高单通道阈值的通道具有明显较宽的 PTC，而最低阈值的通道则较窄。在两名受试者中，调谐曲线的尖端从探针通道偏移。对于最高和最低阈值的通道，与 pTP 探头相比，单极探头的调谐曲线也更宽。
这些结果表明，使用受限刺激测量的单通道阈值可用于识别空间选择性较差的人工耳蜗植入通道。具有宽或尖端移位的调谐特征的通道可能无法将适当的频谱信息传递给预期的听觉神经元，从而导致感知不佳。作为一种临床工具，快速识别受损通道可以导致针对特定患者的映射策略，并提高言语和音乐感知。
