Morris D J, Pfingst B E
Kresge Hearing Research Institute, Department of Otolaryngology, University of Michigan, Ann Arbor 48109-0506, USA.
J Assoc Res Otolaryngol. 2000 Nov;1(3):211-23. doi: 10.1007/s101620010022.
Recent studies have demonstrated that speech perception with cochlear implants can be significantly affected by electrode configuration. Contrary to expectations, broader configurations (monopolar or broad bipolar) produced equal or better speech recognition compared with narrower configurations (narrow bipolar or common ground). One hypothesis that would account for these results is that broader configurations excite larger populations of neurons providing a more robust representation of information on each channel of the prosthesis. It is known that the number of neurons excited by an electrical stimulus increases considerably as the stimulus level increases. Furthermore, many types of discrimination improve as a function of stimulus level. If the discrimination improvements seen with increasing stimulus level are due to increasing the size of the neural population carrying the signal, and if broadening the electrode configuration also increases the size of the activated neural population, then one would expect level and electrode configuration to affect discrimination in similar ways. To test this hypothesis, we studied several types of discrimination as a function of level and electrode configuration in four nonhuman primates with cochlear implants. We tested electrode configurations that produced current fields ranging from very restricted (tripolar) to broad (parallel monopolar). For each configuration, pulse-rate discrimination, amplitude-modulation-frequency discrimination, and level discrimination were tested at current levels spanning much of the psychophysical dynamic range. Results showed large effects of current level on discrimination in many cases. However, effects of electrode configuration at comparable levels within the dynamic range were smaller or absent. Furthermore, the effect of level on discrimination was independent of electrode configuration in most cases even though the rate of spread of neural activation with level is expected to depend on electrode configuration. Possible interpretations of these results are that (1) the current level adjustments necessary to achieve comparable loudness for the various configurations significantly countered any effects of electrode configuration on the size of the activated neural population, or (2) the effects of level on discrimination do not result from its effects on the spatial extent of neural activation.
最近的研究表明，人工耳蜗的言语感知会受到电极配置的显著影响。与预期相反，较宽的配置（单极或宽双极）与较窄的配置（窄双极或公共接地）相比，产生了相同或更好的言语识别效果。一个可以解释这些结果的假设是，较宽的配置会激发更多数量的神经元，从而在假体的每个通道上提供更强大的信息表示。众所周知，随着刺激水平的增加，电刺激激发的神经元数量会大幅增加。此外，许多类型的辨别能力会随着刺激水平的提高而改善。如果随着刺激水平增加而观察到的辨别能力提高是由于携带信号的神经群体规模增大所致，并且如果拓宽电极配置也会增加激活的神经群体规模，那么人们会预期刺激水平和电极配置会以类似的方式影响辨别能力。为了验证这一假设，我们在四只植入人工耳蜗的非人类灵长类动物中研究了几种类型的辨别能力与刺激水平和电极配置的关系。我们测试了产生从非常受限（三极）到宽泛（平行单极）电流场的电极配置。对于每种配置，在跨越大部分心理物理学动态范围的电流水平下测试了脉冲率辨别、调幅频率辨别和刺激水平辨别。结果表明，在许多情况下，电流水平对辨别能力有很大影响。然而，在动态范围内相当的水平下，电极配置的影响较小或不存在。此外，在大多数情况下，刺激水平对辨别能力的影响与电极配置无关，尽管神经激活随刺激水平的扩散速率预计取决于电极配置。这些结果的可能解释是：（1）为使各种配置达到可比响度所需的电流水平调整显著抵消了电极配置对激活神经群体规模的任何影响；或者（2）刺激水平对辨别能力的影响并非源于其对神经激活空间范围的影响。
