Department of Linguistics, University of Maryland, College Park, Maryland 20742, USA.
Neuroimage. 2012 May 1;60(4):2118-27. doi: 10.1016/j.neuroimage.2012.02.028. Epub 2012 Feb 21.
Speech elicits a phase-locked response in the auditory cortex that is dominated by theta (3-7 Hz) frequencies when observed via magnetoencephalography (MEG). This phase-locked response is potentially explained as new phase-locked activity superimposed on the ongoing theta oscillation or, alternatively, as phase-resetting of the ongoing oscillation. The conventional method used to distinguish between the two hypotheses is the comparison of post- to prestimulus amplitude for the phase-locked frequency across a set of trials. In theory, increased amplitude indicates the presence of additive activity, while unchanged amplitude points to phase-resetting. However, this interpretation may not be valid if the amplitude of ongoing background activity also changes following the stimulus. In this study, we employ a new approach that circumvents this problem. Specifically, we utilize a fine-grained time-frequency analysis of MEG channel data to examine the co-modulation of amplitude change and phase coherence in the post-stimulus theta-band response. If the phase-locked response is attributable solely to phase-resetting of the ongoing theta oscillation, then amplitude and phase coherence should be uncorrelated. In contrast, additive activity should produce a positive correlation. We find significant positive correlation not only during the onset response but also throughout the response period. In fact, transient increases in phase coherence are accompanied by transient increases in amplitude in accordance with a "signal plus background" model of the evoked response. The results support the hypothesis that the theta-band phase-locked response to attended speech observed using MEG is dominated by additive phase-locked activity.
语音会在听觉皮层中引起与锁相的反应，当通过脑磁图（MEG）观察时，这种与锁相的反应主要由 theta（3-7 Hz）频率主导。这种与锁相的反应可能被解释为叠加在持续 theta 振荡上的新与锁相活动，或者是对持续振荡的相位重置。用于区分这两种假设的传统方法是比较一组试验中与锁相频率的后刺激到前刺激幅度。从理论上讲，增加的幅度表明存在附加活动，而不变的幅度则指向相位重置。然而，如果在刺激后持续背景活动的幅度也发生变化，那么这种解释可能并不有效。在这项研究中，我们采用了一种新的方法来规避这个问题。具体来说，我们利用 MEG 通道数据的细粒度时频分析来检查刺激后 theta 波段反应中幅度变化和相位相干性的共同调制。如果与锁相的反应仅归因于持续 theta 振荡的相位重置，那么幅度和相位相干性应该不相关。相反，附加活动应该产生正相关。我们不仅在起始反应期间，而且在整个反应期间都发现了显著的正相关。事实上，相位相干性的短暂增加伴随着幅度的短暂增加，符合诱发反应的“信号加背景”模型。结果支持这样的假设，即在 MEG 中观察到的注意语音的 theta 波段与锁相反应主要由附加的与锁相活动主导。
