Hancock Kenneth E, Chung Yoojin, McKinney Martin F, Delgutte Bertrand
Eaton-Peabody Laboratories, Massachusetts Eye and Ear, Boston, MA, 02114, USA.
Department of Otolaryngology, Harvard Medical School, Boston, MA, 02115, USA.
J Assoc Res Otolaryngol. 2017 Dec;18(6):771-788. doi: 10.1007/s10162-017-0638-4. Epub 2017 Jul 17.
Modulations in temporal envelopes are a ubiquitous property of natural sounds and are especially important for hearing with cochlear implants (CIs) because these devices typically discard temporal fine structure information. With few exceptions, neural temporal envelope processing has been studied in both normal hearing (NH) and CI animals using only pure sinusoidal amplitude modulation (SAM) which poorly represents the diversity of envelope shapes contained in natural sounds because it confounds repetition rate and the width of each modulation cycle. Here, we used stimuli that allow independent manipulation of the two parameters to characterize envelope processing by inferior colliculus (IC) neurons in barbiturate-anesthetized cats with CIs. Specifically, the stimuli were amplitude modulated, high rate pulse trains, where the envelope waveform interleaved single cycles ("bursts") of a sinusoid with silent intervals. We found that IC neurons vary widely with respect to the envelope parameters that maximize their firing rates. In general, pure SAM was a relatively ineffective stimulus. The majority of neurons (60 %) preferred a combination of short bursts and low repetition rates (long silent intervals). Others preferred low repetition rates with minimal dependence on envelope width (17 %), while the remainder responded most strongly to brief bursts with lesser sensitivity to repetition rate (23 %). A simple phenomenological model suggests that a combination of inhibitory and intrinsic cellular mechanisms suffices to account for the wide variation in optimal envelope shapes. In contrast to the strong dependence of firing rate on envelope shape, neurons tended to phase lock precisely to the envelope regardless of shape. Most neurons tended to fire specifically near the peak of the modulation cycle, with little phase dispersion within or across neurons. Such consistently precise timing degrades envelope coding compared to NH processing of real-world sounds, because it effectively eliminates spike timing as a cue to envelope shape.
时间包络的调制是自然声音中普遍存在的特性,对于使用人工耳蜗(CI)进行听力而言尤为重要,因为这些设备通常会丢弃时间精细结构信息。除了少数例外情况,正常听力(NH)动物和CI动物的神经时间包络处理研究仅使用了纯正弦幅度调制(SAM),而这种调制方式很难代表自然声音中包含的包络形状的多样性,因为它混淆了重复率和每个调制周期的宽度。在此,我们使用能够独立操纵这两个参数的刺激来表征巴比妥麻醉的CI猫下丘(IC)神经元的包络处理。具体而言,刺激是幅度调制的高速率脉冲序列,其中包络波形将正弦波的单个周期(“脉冲串”)与静音间隔交错排列。我们发现,IC神经元在使它们的放电率最大化的包络参数方面差异很大。一般来说,纯SAM是一种相对无效的刺激。大多数神经元(60%)更喜欢短脉冲串和低重复率(长静音间隔)的组合。其他神经元更喜欢对包络宽度依赖性最小的低重复率(17%),而其余神经元对短暂脉冲串反应最强烈,对重复率的敏感性较低(23%)。一个简单的现象学模型表明,抑制性和内在细胞机制的组合足以解释最佳包络形状的广泛变化。与放电率对包络形状的强烈依赖性相反,神经元倾向于精确地锁相到包络,而不管其形状如何。大多数神经元倾向于在调制周期的峰值附近特定放电,神经元内部或神经元之间几乎没有相位离散。与NH对现实世界声音的处理相比,这种始终精确的定时会降低包络编码,因为它有效地消除了作为包络形状线索的尖峰定时。