Cambridge Hearing Group, MRC Cognition and Brain Sciences Unit, University of Cambridge, 15 Chaucer Road, Cambridge, CB2 7EF, UK.
Bionics Institute, 384-388 Albert Street, East Melbourne, VIC, 3002, Australia.
J Assoc Res Otolaryngol. 2021 Feb;22(1):81-94. doi: 10.1007/s10162-020-00774-z. Epub 2020 Oct 27.
Variations in the condition of the neural population along the length of the cochlea can degrade the spectral and temporal representation of sounds conveyed by CIs, thereby limiting speech perception. One measurement that has been proposed as an estimate of neural survival (the number of remaining functional neurons) or neural health (the health of those remaining neurons) is the effect of stimulation parameters, such as the interphase gap (IPG), on the amplitude growth function (AGF) of the electrically evoked compound action potential (ECAP). The extent to which such measures reflect neural factors, rather than non-neural factors (e.g. electrode orientation, electrode-modiolus distance, and impedance), depends crucially upon how the AGF data are analysed. However, there is currently no consensus in the literature for the correct method to interpret changes in the ECAP AGF due to changes in stimulation parameters. We present a simple theoretical model for the effect of IPG on ECAP AGFs, along with a re-analysis of both animal and human data that measured the IPG effect. Both the theoretical model and the re-analysis of the animal data suggest that the IPG effect on ECAP AGF slope (IPG slope effect), measured using either a linear or logarithmic input-output scale, does not successfully control for the effects of non-neural factors. Both the model and the data suggest that the appropriate method to estimate neural health is by measuring the IPG offset effect, defined as the dB offset between the linear portions of ECAP AGFs for two stimuli differing only in IPG.
耳蜗长度上的神经群体状态的变化可能会降低人工耳蜗传递声音的频谱和时程表示，从而限制言语感知。已经提出了一种作为神经存活（剩余功能神经元的数量）或神经健康（剩余神经元的健康状况）的估计的测量方法，即刺激参数（例如相间间隙 (IPG)）对电诱发复合动作电位 (ECAP) 的幅度增长函数 (AGF) 的影响。这些措施在多大程度上反映了神经因素，而不是非神经因素（例如电极方向、电极-蜗轴距离和阻抗），这取决于如何分析 AGF 数据。然而，目前文献中对于由于刺激参数变化而导致 ECAP AGF 变化的正确解释方法尚无共识。我们提出了一个简单的理论模型，用于解释 IPG 对 ECAP AGF 的影响，以及对测量 IPG 效应的动物和人类数据的重新分析。理论模型和动物数据的重新分析都表明，使用线性或对数输入-输出刻度测量的 IPG 对 ECAP AGF 斜率（IPG 斜率效应）并不能成功控制非神经因素的影响。模型和数据都表明，通过测量 ECAP AGF 的 IPG 偏移效应（仅在 IPG 上有差异的两个刺激之间的 ECAP AGF 的线性部分之间的 dB 偏移）来估计神经健康的适当方法是。
