Wohlgemuth Sandra, Ronacher Bernhard
Department of Biology, Humboldt University, Berlin, Germany.
J Neurophysiol. 2007 Apr;97(4):3082-92. doi: 10.1152/jn.01235.2006. Epub 2007 Feb 21.
Sound envelope cues play a crucial role for the recognition and discrimination of communication signals in diverse taxa, such as vertebrates and arthropods. Using a classification based on metric similarities of spike trains we investigate how well amplitude modulations (AMs) of sound signals can be distinguished at three levels of the locust's auditory pathway: receptors and local and ascending neurons. The spike train metric has the advantage of providing information about the necessary evaluation time window and about the optimal temporal resolution of processing, thereby yielding clues to possible coding principles. It further allows one to disentangle the respective contributions of spike count and spike timing to the fidelity of discrimination. These results are compared with the traditional paradigm using modulation transfer functions. Spike trains of receptors and two primary-like local interneurons enable an excellent discrimination of different AM frequencies, up to about 150 Hz. In these neurons discriminability depends almost completely on the timing of spikes, which must be evaluated with a temporal resolution of <5 ms. Even short spike-train segments of 150 ms, equivalent to five to eight spikes, suffice for a high (70%) discrimination performance. For the third level of processing, the ascending interneurons, the overall discrimination accuracy is reduced. Spike count differences become more important for the discrimination whereas the exact timing of spikes contributes less. This shift in temporal resolution does not primarily depend on the investigated stimulus space. Rather it appears to reflect a transformation of how amplitude modulations are represented at more central stages of processing.
声音包络线索在多种类群（如脊椎动物和节肢动物）的通信信号识别和辨别中起着至关重要的作用。我们使用基于脉冲序列度量相似性的分类方法，研究了在蝗虫听觉通路的三个层面（感受器、局部神经元和上行神经元）上，声音信号的幅度调制（AM）能够被区分的程度。脉冲序列度量的优势在于能提供有关必要评估时间窗口和处理的最佳时间分辨率的信息，从而为可能的编码原则提供线索。它还能让人区分脉冲计数和脉冲时间对辨别保真度的各自贡献。这些结果与使用调制传递函数的传统范式进行了比较。感受器和两个初级样局部中间神经元的脉冲序列能够很好地区分不同的AM频率，高达约150赫兹。在这些神经元中，可辨别性几乎完全取决于脉冲的时间，必须以小于5毫秒的时间分辨率进行评估。即使是150毫秒的短脉冲序列片段（相当于五到八个脉冲），也足以实现较高（70%）的辨别性能。对于处理的第三个层面，即上行中间神经元，整体辨别准确性有所降低。脉冲计数差异对辨别变得更为重要，而脉冲的确切时间贡献较小。这种时间分辨率的变化并不主要取决于所研究的刺激空间。相反，它似乎反映了在处理的更高级阶段，幅度调制是如何被表征的转变。
