The amplitude envelope of speech is crucial for accurate comprehension. Considered a key stage in speech processing, the phase of neural activity in the theta-delta bands (1-10 Hz) tracks the phase of the speech amplitude envelope during listening. However, the mechanisms underlying this envelope representation have been heavily debated. A dominant model posits that envelope tracking reflects entrainment of endogenous low-frequency oscillations to the speech envelope. Alternatively, envelope tracking reflects a series of evoked responses to acoustic landmarks within the envelope. It has proven challenging to distinguish these two mechanisms. To address this, we recorded MEG while participants (n = 12, 6 female) listened to natural speech, and compared the neural phase patterns to the predictions of two computational models: an oscillatory entrainment model and a model of evoked responses to peaks in the rate of envelope change. Critically, we also presented speech at slowed rates, where the spectro-temporal predictions of the two models diverge. Our analyses revealed transient theta phase-locking in regular speech, as predicted by both models. However, for slow speech, we found transient theta and delta phase-locking, a pattern that was fully compatible with the evoked response model but could not be explained by the oscillatory entrainment model. Furthermore, encoding of acoustic edge magnitudes was invariant to contextual speech rate, demonstrating speech rate normalization of acoustic edge representations. Together, our results suggest that neural phase-locking to the speech envelope is more likely to reflect discrete representation of transient information rather than oscillatory entrainment.

SIGNIFICANCE STATEMENT This study probes a highly debated topic in speech perception: the neural mechanisms underlying the cortical representation of the temporal envelope of speech. It is well established that the slow intensity profile of the speech signal, its envelope, elicits a robust brain response that "tracks" these envelope fluctuations. The oscillatory entrainment model posits that envelope tracking reflects phase alignment of endogenous neural oscillations. Here the authors provide evidence for a distinct mechanism. They show that neural speech envelope tracking arises from transient evoked neural responses to rapid increases in the speech envelope. Explicit computational modeling provides direct and compelling evidence that evoked responses are the primary mechanism underlying cortical speech envelope representations, with no evidence for oscillatory entrainment.

语音的幅度包络对于准确理解至关重要。θ-δ 频带 (1-10 Hz) 中的神经活动相位被认为是语音处理的关键阶段，它跟踪收听期间语音幅度包络的相位。然而，这种信封表示背后的机制一直备受争议。一个主导模型认为包络跟踪反映了语音包络的内源低频振荡的夹带。或者，包络跟踪反映了对包络内的声学地标的一系列诱发反应。事实证明，区分这两种机制具有挑战性。为了解决这个问题，我们在参与者（n= 12, 6 名女性）聆听自然语音，并将神经相位模式与两个计算模型的预测进行比较：振荡夹带模型和对包络变化率峰值的诱发反应模型。至关重要的是，我们还以减慢的速率呈现语音，这两个模型的谱时预测存在分歧。我们的分析揭示了常规语音中的瞬态 θ 锁相，正如两个模型所预测的那样。然而，对于慢速语音，我们发现瞬态 theta 和 delta 锁相，这种模式与诱发反应模型完全兼容，但无法用振荡夹带模型解释。此外，声学边缘幅度的编码对于上下文语速是不变的，这证明了声学边缘表示的语速归一化。一起，

SIGNIFICANCE STATEMENT This study probes a highly debated topic in speech perception: the neural mechanisms underlying the cortical representation of the temporal envelope of speech. It is well established that the slow intensity profile of the speech signal, its envelope, elicits a robust brain response that "tracks" these envelope fluctuations. The oscillatory entrainment model posits that envelope tracking reflects phase alignment of endogenous neural oscillations. Here the authors provide evidence for a distinct mechanism. They show that neural speech envelope tracking arises from transient evoked neural responses to rapid increases in the speech envelope. Explicit computational modeling provides direct and compelling evidence that evoked responses are the primary mechanism underlying cortical speech envelope representations, with no evidence for oscillatory entrainment.