Saberi K, Hafter E R
Department of Psychology, University of Florida, Gainesville 32611, USA.
Nature. 1995 Apr 6;374(6522):537-9. doi: 10.1038/374537a0.
Most naturally occurring sounds are modulated in amplitude or frequency; important examples include animal vocalizations and species-specific communication signals in mammals, insects, reptiles, birds and amphibians. Deciphering the information from amplitude-modulated (AM) sounds is a well-understood process, requiring a phase locking of primary auditory afferents to the modulation envelopes. The mechanism for decoding frequency modulation (FM) is not as clear because the FM envelope is flat (Fig. 1). One biological solution is to monitor amplitude fluctuations in frequency-tuned cochlear filters as the instantaneous frequency of the FM sweeps through the passband of these filters. This view postulates an FM-to-AM transduction whereby a change in frequency is transmitted as a change in amplitude. This is an appealing idea because, if such transduction occurs early in the auditory pathway, it provides a neurally economical solution to how the auditory system encodes these important sounds. Here we illustrate that an FM and AM sound must be transformed into a common neural code in the brain stem. Observers can accurately determine if the phase of an FM presented to one ear is leading or lagging, by only a fraction of a millisecond, the phase of an AM presented to the other ear. A single intracranial image is perceived, the spatial position of which is a function of this phase difference.
大多数自然产生的声音在幅度或频率上是调制的；重要的例子包括哺乳动物、昆虫、爬行动物、鸟类和两栖动物的动物发声及物种特异性通信信号。从调幅（AM）声音中解读信息是一个已被充分理解的过程，需要初级听觉传入神经与调制包络进行锁相。解码调频（FM）的机制尚不清楚，因为调频包络是平坦的（图1）。一种生物学解决方案是，当调频的瞬时频率扫过这些频率调谐的耳蜗滤波器的通带时，监测其幅度波动。这种观点假设了一种调频到调幅的转换，即频率变化以幅度变化的形式传递。这是一个很有吸引力的想法，因为如果这种转换在听觉通路的早期发生，它为听觉系统如何编码这些重要声音提供了一种神经经济的解决方案。在这里，我们证明调频声音和调幅声音在脑干中必须转换为一种共同的神经编码。观察者能够准确地确定呈现给一只耳朵的调频的相位相对于呈现给另一只耳朵的调幅的相位领先还是滞后，时间差仅为几分之一毫秒。人们会感知到一个单一的颅内图像，其空间位置是这个相位差的函数。
