Simmons A M, Reese G, Ferragamo M
Department of Psychology, Brown University, Providence, Rhode Island 02912.
J Acoust Soc Am. 1993 Jun;93(6):3374-89. doi: 10.1121/1.405693.
Discharge patterns of single eighth nerve fibers in the bullfrog, Rana catesbeiana, were analyzed in response to signals consisting of multiple harmonics of a common, low-amplitude fundamental frequency. The signals were chosen to reflect the frequency and amplitude spectrum of the bullfrog's species-specific advertisement call. The phase spectrum of the signals was manipulated to produce envelopes that varied in their shapes from impulselike (sharp) to noiselike (flattened). Peripheral responses to these signals were analyzed by computing the autocorrelation functions of the spike trains and their power spectra, as well as by constructing period histograms over the time intervals of the low-frequency harmonics. In response to a phase aligned signal with an impulsive envelope, most fibers, regardless of their characteristic frequencies or place of origin within the inner ear, synchronize to the fundamental frequency of the signal. The temporal patterns of fiber discharge to these stimuli are not typically captured by that stimulus harmonic closet to the fiber characteristic frequency, as would be expected from a spectral coding mechanism for periodicity extraction, but instead directly reflect the periodicity of the stimulus envelope. Changing the phase relations between the individual harmonics constituting the signal produces changes in temporal discharge patterns of some fibers by shifting predominant synchronization away from the fundamental frequency to the low-frequency spectral peak in the complex stimuli. The proportion of fibers whose firing is captured by the fundamental frequency decreases as the waveform envelope becomes less impulselike. Fiber characteristic frequency is not highly correlated with the harmonic number to which synchronization is strongest. The higher-harmonic spectral fine structure of the signals is not reflected in fiber temporal response, regardless of the shape of the stimulus envelope, even for those harmonics within the range of phase locking to simple sinusoids. Increasing stimulus intensity also shifts the synchronized responses of some fibers away from the fundamental frequency to one of the low-frequency harmonics in the stimuli. These data suggest that the synchronized firing of bullfrog eighth nerve fibers operates to extract the waveform periodicity of complex, multiple-harmonic stimuli, and this periodicity extraction is influenced by the phase spectrum and temporal fine structure of the stimuli. The similarity in response patterns of amphibian papilla and basilar papilla fibers argues that the frog auditory system employs primarily a temporal mechanism for extraction of first harmonic periodicity.
对牛蛙（Rana catesbeiana）单根第八神经纤维的放电模式进行了分析，这些纤维对由共同的低振幅基频的多个谐波组成的信号做出反应。选择这些信号以反映牛蛙物种特异性广告叫声的频率和振幅谱。对信号的相位谱进行处理，以产生形状从脉冲状（尖锐）到噪声状（平坦）变化的包络。通过计算尖峰序列的自相关函数及其功率谱，以及通过在低频谐波的时间间隔上构建周期直方图，来分析对这些信号的外周反应。对于具有脉冲包络的相位对齐信号，大多数纤维，无论其特征频率或在内耳中的起源位置如何，都会与信号的基频同步。纤维对这些刺激的放电时间模式通常不是由最接近纤维特征频率的刺激谐波捕获的，这与用于周期性提取的频谱编码机制所预期的情况不同，而是直接反映了刺激包络的周期性。改变构成信号的各个谐波之间的相位关系，会通过将主要同步从基频转移到复杂刺激中的低频频谱峰值，从而使一些纤维的时间放电模式发生变化。随着波形包络变得不那么像脉冲，被基频捕获放电的纤维比例会降低。纤维特征频率与同步最强的谐波数量没有高度相关性。信号的高谐波频谱精细结构在纤维时间响应中没有得到反映，无论刺激包络的形状如何，即使对于那些在与简单正弦波锁相范围内的谐波也是如此。增加刺激强度也会使一些纤维的同步反应从基频转移到刺激中的一个低频谐波。这些数据表明，牛蛙第八神经纤维的同步放电用于提取复杂的多谐波刺激的波形周期性，并且这种周期性提取受刺激的相位谱和时间精细结构的影响。两栖类乳头体和基底乳头体纤维反应模式的相似性表明，青蛙听觉系统主要采用时间机制来提取一次谐波周期性。
