Sachs M B, Voigt H F, Young E D
J Neurophysiol. 1983 Jul;50(1):27-45. doi: 10.1152/jn.1983.50.1.27.
Responses of auditory nerve fibers to steady-state vowels presented alone and in the presence of background noise were obtained from anesthetized cats. Representation of vowels based on average discharge rate and representation based primarily on phase-locked properties of responses are considered. Profiles of average discharge rate versus characteristic frequency (CF) ("rate-place" representation) can show peaks of discharge rate in the vicinity of formant frequencies when vowels are presented alone. These profiles change drastically in the presence of background noise, however. At moderate vowel and noise levels and signal/noise ratios of +9 dB, there are not peaks of rate near the second and third formant frequencies. In fact, because of two-tone suppression, rate to vowels plus noise is less than rate to noise alone for fibers with CFs above the first formant. Rate profiles measured over 5-ms intervals near stimulus onset show clear formant-related peaks at higher sound levels than do profiles measured over intervals later in the stimulus (i.e., in the steady state). However, in background noise, rate profiles at onset are similar to those in the steady state. Specifically, for fibers with CFs above the first formant, response rates to the noise are suppressed by the addition of the vowel at both vowel onset and steady state. When rate profiles are plotted for low spontaneous rate fibers, formant-related peaks appear at stimulus levels higher than those at which peaks disappear for high spontaneous fibers. In the presence of background noise, however, the low spontaneous fibers do not preserve formant peaks better than do the high spontaneous fibers. In fact, the suppression of noise-evoked rate mentioned above is greater for the low spontaneous fibers than for high. Representations that reflect phase-locked properties as well as discharge rate ("temporal-place" representations) are much less affected by background noise. We have used synchronized discharge rate averaged over fibers with CFs near (+/- 0.25 octave) a stimulus component as a measure of the population temporal response to that component. Plots of this average localized synchronized rate (ALSR) versus frequency show clear first and second formant peaks at all vowel and noise levels used. Except at the highest level (vowel at 85 dB sound pressure level (SPL), signal/noise = +9 dB), there is also a clear third formant peak. At signal-to-noise ratios where there are no second formant peaks in rate profiles, human observers are able to discriminate second formant shifts of less than 112 Hz. ALSR plots show clear second formant peaks at these signal/noise ratios.
从麻醉猫身上获取了听神经纤维对单独呈现以及在存在背景噪声情况下呈现的稳态元音的反应。考虑了基于平均发放率的元音表征以及主要基于反应的锁相特性的表征。当单独呈现元音时，平均发放率与特征频率（CF）的关系曲线（“发放率-位置”表征）在共振峰频率附近可显示出发放率峰值。然而，在存在背景噪声的情况下，这些曲线会发生剧烈变化。在中等元音和噪声水平以及 +9 dB 的信噪比条件下，第二和第三共振峰频率附近不存在发放率峰值。实际上，由于双音抑制，对于 CF 高于第一共振峰的纤维，元音加噪声时的发放率低于单独噪声时的发放率。在刺激开始附近 5 毫秒间隔内测量的发放率曲线，在比刺激后期（即稳态）测量的曲线更高的声级下显示出与共振峰相关的清晰峰值。然而，在背景噪声中，开始时的发放率曲线与稳态时相似。具体而言，对于 CF 高于第一共振峰的纤维，在元音开始和稳态时，元音的添加都会抑制对噪声的反应发放率。当绘制低自发放率纤维的发放率曲线时，与共振峰相关的峰值出现在比高自发放率纤维峰值消失时更高的刺激水平上。然而，在存在背景噪声的情况下，低自发放率纤维并不比高自发放率纤维更好地保留共振峰峰值。实际上，上述噪声诱发发放率的抑制对于低自发放率纤维比高自发放率纤维更大。反映锁相特性以及发放率的表征（“时间-位置”表征）受背景噪声的影响要小得多。我们使用了在 CF 接近（±0.25 倍频程）刺激成分的纤维上平均的同步发放率，作为群体对该成分的时间反应的度量。这种平均局部同步率（ALSR）与频率的关系曲线在所有使用的元音和噪声水平下都显示出清晰的第一和第二共振峰峰值。除了最高水平（元音声压级为 85 dB，信噪比 = +9 dB）外，还有清晰的第三共振峰峰值。在发放率曲线中不存在第二共振峰峰值的信噪比条件下，人类观察者能够辨别小于 112 Hz 的第二共振峰偏移。ALSR 曲线在这些信噪比条件下显示出清晰的第二共振峰峰值。
