Phase Alignment of Low-Frequency Neural Activity to the Amplitude Envelope of Speech Reflects Evoked Responses to Acoustic Edges, Not Oscillatory Entrainment

The amplitude envelope of speech is crucial for accurate comprehension, and several studies have shown that the phase of neural activity in the theta-delta bands (1 - 10 Hz) tracks the phase of the speech amplitude envelope during listening, a process referred to as envelope tracking. However, the mechanisms underlying envelope tracking have been heavily debated. A dominant model posits that envelope tracking reflects continuous entrainment of endogenous low-frequency oscillations to the speech envelope. However, it has proven challenging to distinguish this from the alternative model that envelope tracking reflects a convolution of evoked responses to acoustic landmarks within the envelope. To address this, we recorded magnetoencephalography while participants listened to natural and slowed speech. First, we found that peaks in the rate of envelope change, acoustic edge landmarks, induced a strong evoked response. We then contrasted central diverging predictions of evoked response and oscillatory models regarding the spectral and temporal extent of neural phase locking in the theta-delta range at different speech rates. Our analyses revealed transient theta phase locking at both speech rates and delta phase locking in slow speech. This pattern was predicted by the evoked response model but not by oscillatory entrainment. Furthermore, the encoding of acoustic edge magnitudes was invariant to contextual speech rate, demonstrating speech rate normalization of acoustic edge representations. Taken together, our results show that an evoked response model provides a better account of neural phase locking to the speech envelope than oscillatory entrainment.