Linear phase property of speech envelope tracking response in Heschl's gyrus and superior temporal gyrus

Understanding how the brain tracks speech during listening remains a challenge. The phase resetting hypothesis proposes that the envelope-tracking response is generated by resetting the phase of intrinsic nonlinear neural oscillations, whereas the evoked response hypothesis proposes that the envelope-tracking response is the linear superposition of transient responses evoked by a sequence of acoustic events in speech. Recent studies have demonstrated a linear phase property of the envelope-tracking response, supporting the evoked response hypothesis. However, the cortical regions aligning with the evoked response hypothesis remain unclear. To address this question, we directly recorded from the cortex using stereo-electroencephalography (SEEG) in nineteen epilepsy patients as they listened to natural speech, and we investigated whether the phase lag between the speech envelope and neural activity linearly changes across frequency. We found that the linear phase property of low-frequency (LF) (.5-40 Hz) envelope tracking was widely observed in Heschl's gyrus (HG) and superior temporal gyrus (STG), with additional sparser distribution in insula, postcentral gyrus, and precentral gyrus. Furthermore, the latency of LF envelope-tracking responses derived from phase-frequency curve exhibited an increase gradient along HG and in the posterior-to-anterior direction in STG. Our findings suggest that auditory cortex can track speech envelope in line with the evoked response hypothesis.