Scheperle Rachel A, Abbas Paul J
1Department of Communication Sciences and Disorders, University of Iowa, Iowa City, Iowa, USA; and 2Otolaryngology-Head and Neck Surgery, University of Iowa, Iowa City, Iowa, USA.
Ear Hear. 2015 Jul-Aug;36(4):430-40. doi: 10.1097/AUD.0000000000000143.
The primary goal of this study was to describe relationships between peripheral and central electrophysiologic measures of auditory processing within individual cochlear implant (CI) users. The distinctiveness of neural excitation patterns resulting from the stimulation of different electrodes, referred to as 'spatial selectivity,' was evaluated. The hypothesis was that if central representations of spatial interactions differed across participants semi-independently of peripheral input, then the within-subject relationships between peripheral and central electrophysiologic measures of spatial selectivity would reflect those differences. Cross-subject differences attributable to processing central to the auditory nerve may help explain why peripheral electrophysiologic measures of spatial selectivity have not been found to correlate with speech perception.
Eleven adults participated in this and a companion study. All were peri- or post-lingually deafened with more than 1 year of CI experience. Peripheral spatial selectivity was evaluated at 13 cochlear locations using 13 electrodes as probes to elicit electrically evoked compound action potentials (ECAPs). Masker electrodes were varied across the array for each probe electrode to derive channel-interaction functions. The same 13 electrodes were used to evaluate spatial selectivity represented at a cortical level. Electrode pairs were stimulated sequentially to elicit the auditory change complex (ACC), an obligatory cortical potential suggestive of discrimination. For each participant, the relationship between ECAP channel-interaction functions (quantified as channel-separation indices) and ACC N1-P2 amplitudes was modeled using the saturating exponential function y = a * (1-e). Both a and b coefficients were varied using a least-squares approach to optimize the fits.
Electrophysiologic measures of spatial selectivity assessed at peripheral (ECAP) and central (ACC) levels varied across participants. The results indicate that differences in ACC amplitudes observed across participants for the same stimulus conditions were not solely the result of differences in peripheral excitation patterns. This finding supports the view that processing at multiple points along the auditory neural pathway from the periphery to the cortex may vary across individuals with different etiologies and auditory experiences.
The distinctiveness of neural excitation resulting from electrical stimulation varies across CI recipients, and this variability was observed in both peripheral and cortical electrophysiologic measures. The ACC amplitude differences observed across participants were partially independent from differences in peripheral neural spatial selectivity. These findings are clinically relevant because they imply that there may be limits (1) to the predictive ability of peripheral measures and (2) in the extent to which improving the selectivity of electrical stimulation via programming options (e.g., current focusing/steering) will result in more specific central neural excitation patterns or will improve speech perception.
本研究的主要目标是描述个体人工耳蜗（CI）使用者听觉处理的外周和中枢电生理测量之间的关系。评估了由不同电极刺激产生的神经兴奋模式的独特性，即“空间选择性”。假设是，如果空间相互作用的中枢表征在参与者之间存在差异，且与外周输入半独立，那么外周和中枢空间选择性电生理测量之间的受试者内关系将反映这些差异。归因于听神经中枢处理的受试者间差异可能有助于解释为何尚未发现空间选择性的外周电生理测量与言语感知相关。
11名成年人参与了本研究及一项配套研究。所有受试者均为语前或语后聋，且有超过1年的CI使用经验。使用13个电极作为探针，在13个耳蜗位置评估外周空间选择性，以引出电诱发复合动作电位（ECAP）。对于每个探测电极，在阵列中改变掩蔽电极以得出通道相互作用函数。使用相同的13个电极评估皮质水平的空间选择性。依次刺激电极对以引出听觉变化复合体（ACC），这是一种提示辨别能力的强制性皮质电位。对于每个参与者，使用饱和指数函数y = a * (1 - e) 对ECAP通道相互作用函数（量化为通道分离指数）与ACC N1 - P2振幅之间的关系进行建模。使用最小二乘法改变a和b系数以优化拟合。
在外周（ECAP）和中枢（ACC）水平评估的空间选择性电生理测量在参与者之间存在差异。结果表明，在相同刺激条件下，参与者之间观察到的ACC振幅差异并非仅由外周兴奋模式的差异所致。这一发现支持了这样一种观点，即从外周到皮质的听觉神经通路多个点的处理在病因和听觉经验不同的个体之间可能存在差异。
电刺激产生的神经兴奋的独特性在CI接受者之间存在差异，并且在外周和皮质电生理测量中均观察到这种变异性。参与者之间观察到的ACC振幅差异部分独立于外周神经空间选择性的差异。这些发现具有临床相关性，因为它们意味着（1）外周测量的预测能力可能存在局限性，以及（2）通过编程选项（例如电流聚焦/转向）提高电刺激的选择性在多大程度上会导致更特定的中枢神经兴奋模式或改善言语感知可能存在局限性。
