Cochlear Limited, Sydney, New South Wales 2066, Australia.
Ear Hear. 2010 Jun;31(3):380-91. doi: 10.1097/AUD.0b013e3181cb41aa.
The Neural Response Telemetry (NRT) recovery function measures the electrically evoked compound action potential (ECAP) in response to a second biphasic pulse (the probe) after masking by a first pulse (the masker). The masker-probe interval is varied and the ECAP amplitude is measured at each masker-probe interval, giving an inverse exponential recovery. The prevailing understanding of the recovery function has been that faster recovery indicates a more efficient response to the individual pulses within a pulse sequence. Psychophysical data in the past have not supported this view, and in fact, the opposite result has been observed. This study explores this phenomenon from theoretical and experimental viewpoints. Fundamentally, a distinction is made between the refractoriness of a single fiber and the refractoriness of the whole nerve. The hypothesis is that the size of the neural population heavily influences whole nerve refractoriness: large neural populations operate near threshold and are more susceptible to masking, leading to slower ECAP recovery; however, they maintain temporal responsiveness through greater numbers of nonrefractory neurons.
In phase I, the hearing loss durations (indicators of neural survival) of 21 adult Nucleus Freedom implantees were compared with the corresponding median recovery function time-constants (calculated per implant array). The data were separated by implant (nine Contour, 12 Straight) and the means of these two groups were compared. The Straight array, delivering broader excitation, is expected to engage a larger neural population. In phase II, a computational model of the ECAP recovery function was constructed based on data from the cat auditory nerve. The model allows the neural population size to be manipulated; accordingly, recovery functions from different neural populations were compared. In phase III, ECAP thresholds (via AutoNRT), ECAP recovery functions, and T- and C-levels were obtained from a subset of 12 subjects. Psychophysical levels were measured using pulse train stimuli at six different stimulation rates, spanning 250 to 3500 Hz. At each electrode, the recovery function time-constant tau was compared with two measures of temporal responsiveness: (i) the gradient of the linear trend in psychophysical levels with stimulation rate; and (ii) the difference between ECAP threshold (a single pulse measure) and 900 Hz T-level (a pulse train measure).
In phase I, a trend toward shorter recovery function time-constants with increasing hearing loss durations was observed. The mean recovery function time-constant of the Contour implant group (0.51 msec) was significantly shorter than that of the Straight implant group (0.90 msec). When, in phase II, the recovery functions from the computational model were compared at equal ECAP amplitude, the larger neural population was associated with slower ECAP recovery. In phase III, the recovery function time-constant was significantly correlated with both temporal responsiveness measures, with slower ECAP, recovery associated with greater temporal responsiveness, thus confirming the results of previous studies.
: Slower ECAP recovery, at equal loudness, is associated with larger neural populations. The collective results suggest that this neural population view of the recovery function explains the observed association between slower ECAP recovery and greater temporal responsiveness.
神经反应遥测 (NRT) 恢复功能测量响应第二个双相脉冲（探针）的电诱发复合动作电位 (ECAP)，该脉冲在第一个脉冲（掩蔽器）掩蔽后。掩蔽器-探针间隔变化，在每个掩蔽器-探针间隔测量 ECAP 幅度，得到反指数恢复。对恢复功能的普遍理解是，更快的恢复表明对脉冲序列中各个脉冲的反应更有效。过去的心理物理数据并不支持这一观点，事实上，观察到了相反的结果。本研究从理论和实验的角度探讨了这一现象。从根本上讲，对单个纤维的不应期和整个神经的不应期进行了区分。假设是神经元群体的大小对整个神经的不应期有很大的影响：大的神经元群体在接近阈值的情况下工作，更容易受到掩蔽的影响，导致 ECAP 恢复较慢；然而，它们通过更多的无反应神经元保持时间响应性。
在第一阶段，将 21 名 Nucleus Freedom 植入物的成人听力损失持续时间（神经存活的指标）与相应的中位数恢复功能时间常数（按每个植入物阵列计算）进行比较。数据按植入物（九个轮廓，十二个直）分开，比较这两组的平均值。直阵列提供更宽的激发，预计会吸引更大的神经元群体。在第二阶段，根据猫听觉神经的数据，构建了 ECAP 恢复功能的计算模型。该模型允许操纵神经元群体的大小；因此，比较了来自不同神经元群体的恢复功能。在第三阶段，从 12 名受试者的一个子集获得 ECAP 阈值（通过 AutoNRT）、ECAP 恢复功能和 T 级和 C 级。心理物理水平通过在六个不同刺激率下使用脉冲串刺激进行测量，范围为 250 至 3500 Hz。在每个电极上，将恢复功能时间常数 tau 与两个时间响应性测量值进行比较：（i）心理物理水平随刺激率的线性趋势的梯度；和 (ii) ECAP 阈值（单个脉冲测量）与 900 Hz T 级（脉冲串测量）之间的差异。
在第一阶段，观察到恢复功能时间常数随着听力损失持续时间的增加而呈缩短趋势。轮廓植入物组的平均恢复功能时间常数（0.51 毫秒）明显短于直植入物组（0.90 毫秒）。当在第二阶段比较计算模型的恢复功能时，在相等的 ECAP 幅度下，较大的神经元群体与较慢的 ECAP 恢复有关。在第三阶段，恢复功能时间常数与两个时间响应性测量值显著相关，较慢的 ECAP 恢复与更大的时间响应性相关，从而证实了先前研究的结果。
在相等的响度下，较慢的 ECAP 恢复与较大的神经元群体有关。综合结果表明，这种恢复功能的神经元群体观点解释了观察到的较慢的 ECAP 恢复与更大的时间响应性之间的关联。
