Concurrent temporal channels for auditory processing: Oscillatory neural entrainment reveals segregation of function at different scales

Natural sounds convey perceptually relevant information over multiple timescales, and the necessary extraction of multi-timescale information requires the auditory system to work over distinct ranges. The simplest hypothesis suggests that temporal modulations are encoded in an equivalent manner within a reasonable intermediate range. We show that the human auditory system selectively and preferentially tracks acoustic dynamics concurrently at 2 timescales corresponding to the neurophysiological theta band (4–7 Hz) and gamma band ranges (31–45 Hz) but, contrary to expectation, not at the timescale corresponding to alpha (8–12 Hz), which has also been found to be related to auditory perception. Listeners heard synthetic acoustic stimuli with temporally modulated structures at 3 timescales (approximately 190-, approximately 100-, and approximately 30-ms modulation periods) and identified the stimuli while undergoing magnetoencephalography recording. There was strong intertrial phase coherence in the theta band for stimuli of all modulation rates and in the gamma band for stimuli with corresponding modulation rates. The alpha band did not respond in a similar manner. Classification analyses also revealed that oscillatory phase reliably tracked temporal dynamics but not equivalently across rates. Finally, mutual information analyses quantifying the relation between phase and cochlear-scaled correlations also showed preferential processing in 2 distinct regimes, with the alpha range again yielding different patterns. The results support the hypothesis that the human auditory system employs (at least) a 2-timescale processing mode, in which lower and higher perceptual sampling scales are segregated by an intermediate temporal regime in the alpha band that likely reflects different underlying computations.自然声音可在多个时间尺度上传达感知相关信息，而要提取多时间尺度的必要信息，听觉系统需在不同时间范围内运作。最简假说提出，在合理的中间时间范围内，时间调制（temporal modulations）会以等效方式被编码。本研究表明，人类听觉系统会选择性且优先地同时追踪两个时间尺度下的声学动态，这两个尺度分别对应神经生理学中的θ频段（theta band，4–7 Hz）与γ频段（gamma band，31–45 Hz）；但与预期相悖的是，其并未追踪对应α频段（alpha band，8–12 Hz）的时间尺度——而α频段此前也被发现与听觉感知相关。受试者聆听具备三种时间尺度调制结构的合成声学刺激（调制周期分别约为190 ms、100 ms与30 ms），并在接受脑磁图（magnetoencephalography）记录的同时对刺激进行识别。对于所有调制速率的刺激，θ频段均表现出显著的试间相位相干性（intertrial phase coherence）；而γ频段则在对应调制速率的刺激中呈现出相同特征，α频段并未表现出类似的响应模式。分类分析结果还显示，振荡相位能够可靠地追踪时间动态，但在不同调制速率下的追踪效果并不均等。最后，通过量化相位与耳蜗尺度相关性（cochlear-scaled correlations）之间关系的互信息分析（mutual information analyses），研究同样发现听觉系统会在两种不同的机制下优先处理信息，而α频段再次呈现出不同的变化模式。本研究结果支持这一假说：人类听觉系统至少采用了双时间尺度的处理模式，即较低与较高的感知采样尺度被α频段的中间时间机制分隔开来，这一机制很可能反映了不同的底层计算过程。