Department of Otolaryngology, University of California, Medical Sciences E, Room E101, Irvine, CA 92697-5310, USA.
J Assoc Res Otolaryngol. 2012 Feb;13(1):67-80. doi: 10.1007/s10162-011-0293-0. Epub 2011 Oct 4.
Cochlear implant listeners receive auditory stimulation through amplitude-modulated electric pulse trains. Auditory nerve studies in animals demonstrate qualitatively different patterns of firing elicited by low versus high pulse rates, suggesting that stimulus pulse rate might influence the transmission of temporal information through the auditory pathway. We tested in awake guinea pigs the temporal acuity of auditory cortical neurons for gaps in cochlear implant pulse trains. Consistent with results using anesthetized conditions, temporal acuity improved with increasing pulse rates. Unlike the anesthetized condition, however, cortical neurons responded in the awake state to multiple distinct features of the gap-containing pulse trains, with the dominant features varying with stimulus pulse rate. Responses to the onset of the trailing pulse train (Trail-ON) provided the most sensitive gap detection at 1,017 and 4,069 pulse-per-second (pps) rates, particularly for short (25 ms) leading pulse trains. In contrast, under conditions of 254 pps rate and long (200 ms) leading pulse trains, a sizeable fraction of units demonstrated greater temporal acuity in the form of robust responses to the offsets of the leading pulse train (Lead-OFF). Finally, TONIC responses exhibited decrements in firing rate during gaps, but were rarely the most sensitive feature. Unlike results from anesthetized conditions, temporal acuity of the most sensitive units was nearly as sharp for brief as for long leading bursts. The differences in stimulus coding across pulse rates likely originate from pulse rate-dependent variations in adaptation in the auditory nerve. Two marked differences from responses to acoustic stimulation were: first, Trail-ON responses to 4,069 pps trains encoded substantially shorter gaps than have been observed with acoustic stimuli; and second, the Lead-OFF gap coding seen for <15 ms gaps in 254 pps stimuli is not seen in responses to sounds. The current results may help to explain why moderate pulse rates around 1,000 pps are favored by many cochlear implant listeners.
人工耳蜗植入者通过调制幅度的电脉冲序列接收听觉刺激。动物的听神经研究表明，在低脉冲率和高脉冲率下，诱发的放电模式有本质的不同，这表明刺激脉冲率可能会影响听觉通路中时间信息的传递。我们在清醒的豚鼠身上测试了听觉皮层神经元对人工耳蜗脉冲序列中缺口的时间分辨能力。与使用麻醉条件的结果一致，时间分辨能力随着脉冲率的增加而提高。然而，与麻醉条件不同的是，在清醒状态下，皮质神经元对包含缺口的脉冲序列的多个不同特征做出反应，其主要特征随刺激脉冲率而变化。对于尾随脉冲序列（Trail-ON）的起始，在 1017 和 4069 脉冲/秒（pps）的速率下，特别是对于短（25ms）的先导脉冲序列，提供了最敏感的缺口检测。相比之下，在 254pps 的速率和长（200ms）的先导脉冲序列条件下，相当一部分单位表现出更强的时间分辨能力，表现为对先导脉冲序列的起始偏移（Lead-OFF）的强烈反应。最后，TONIC 反应在缺口期间表现出放电率的下降，但很少是最敏感的特征。与麻醉条件的结果不同，最敏感单位的时间分辨能力对于短暂的和长的先导爆发几乎一样尖锐。在不同脉冲率下的刺激编码的差异可能源于听觉神经中适应的脉冲率依赖性变化。与对声刺激的反应有两个明显的区别：首先，4069pps 脉冲序列的 Trail-ON 反应编码的缺口比用声学刺激观察到的要短得多；其次，在 254pps 刺激物中，<15ms 缺口的 Lead-OFF 缺口编码在对声音的反应中看不到。目前的结果可能有助于解释为什么许多人工耳蜗植入者喜欢使用 1000pps 左右的中等脉冲率。
