Hughes Michelle L, Laurello Sarah A
Boys Town National Research Hospital, 555 North 30th Street, Omaha, NE, USA.
University of Northern Colorado, Greeley, CO, USA.
Hear Res. 2017 Aug;351:116-129. doi: 10.1016/j.heares.2017.06.004. Epub 2017 Jun 13.
Electrically evoked compound action potentials (ECAPs) have been used to examine temporal response patterns of the auditory nerve in cochlear implant (CI) recipients. ECAP responses to individual pulses in a pulse train vary across stimulation rates for individual CI users. For very slow rates, auditory neurons have ample time to discharge, recover, and respond to each pulse in the train. As the pulse rate increases, an alternating ECAP-amplitude pattern occurs. As the stimulation rate increases further, the alternating pattern eventually ceases and the overall ECAP amplitudes are diminished, yielding a relatively stochastic state that presumably reflects a combination of adaptation, desynchronization, and facilitation across fibers. Because CIs operate over a range of current levels in everyday use, it is important to understand auditory-nerve responses to pulse trains over a range of levels. The effect of stimulus level on ECAP temporal response patterns in human CI users has not been well studied. The first goal of this study was to examine the effect of stimulus level on various aspects of ECAP temporal responses to pulse-train stimuli. Because higher stimulus levels yield more synchronous responses and faster recovery, it was hypothesized that: (1) the maximum alternation would occur at slower rates for lower levels and faster rates at higher levels, (2) the alternation depth at its maximum would be smaller for lower levels, (3) the rate that produces a stochastic state ('stochastic rate') would decrease with level, (4) adaptation would be greater for lower levels as a result of slower recovery, and (5) refractory-recovery time constants would be longer (slower) for lower levels, consistent with earlier studies. The second goal of this study was to examine how refractory-recovery time constants relate specifically to maximum alternation and stochastic rate. Data were collected for 12 ears in 10 CI recipients. ECAPs were recorded in response to each of 13 pulses in an equal-amplitude pulse train ranging in rate from 900-3500 pps for three levels (low, medium, high). The results generally supported hypotheses 1-4; there were no significant effects of level on the refractory-recovery time constants (hypothesis 5). When data were pooled across level, there was a significant negative correlation between alternation depth and refractory recovery time. Understanding the effects of stimulus level on auditory-nerve responses may provide further insight into improving the use of objective measures for potentially optimizing speech-processing strategies.
电诱发复合动作电位（ECAPs）已被用于研究人工耳蜗（CI）植入者听神经的时间响应模式。对于个体CI使用者，脉冲序列中单个脉冲的ECAP反应会因刺激速率的不同而有所变化。在非常低的速率下，听觉神经元有足够的时间放电、恢复并对序列中的每个脉冲做出反应。随着脉冲速率增加，会出现交替的ECAP幅度模式。当刺激速率进一步增加时，交替模式最终停止，整体ECAP幅度减小，产生一种相对随机的状态，这可能反映了纤维间适应、去同步和易化的综合作用。由于CI在日常使用中工作在一定范围的电流水平，了解听神经对不同水平脉冲序列的反应很重要。刺激水平对人类CI使用者ECAP时间响应模式的影响尚未得到充分研究。本研究的第一个目标是研究刺激水平对ECAP对脉冲序列刺激的时间响应各个方面的影响。由于较高的刺激水平会产生更同步的反应和更快的恢复，因此假设：（1）较低水平下最大交替会出现在较慢速率，较高水平下出现在较快速率；（2）较低水平下最大交替深度会更小；（3）产生随机状态的速率（“随机速率”）会随水平降低；（4）由于恢复较慢，较低水平下的适应会更大；（5）与早期研究一致，较低水平下的不应期恢复时间常数会更长（更慢）。本研究的第二个目标是研究不应期恢复时间常数如何具体与最大交替和随机速率相关。收集了10名CI植入者12只耳朵的数据。记录了对13个等幅脉冲序列中每个脉冲的ECAP，脉冲序列速率范围为900 - 3500次/秒，共三个水平（低、中、高）。结果总体上支持假设1 - 4；水平对不应期恢复时间常数没有显著影响（假设5）。当跨水平汇总数据时，交替深度与不应期恢复时间之间存在显著负相关。了解刺激水平对听神经反应的影响可能有助于进一步深入了解如何改进客观测量方法以潜在地优化言语处理策略。
