Two Stages of Speech Envelope Tracking in Human Auditory Cortex Modulated by Speech Intelligibility

Abstract

The envelope is essential for speech perception. Recent studies have shown that cortical activity can track the acoustic envelope. However, whether the tracking strength reflects the extent of speech intelligibility processing remains controversial. Here, using stereo-electroencephalogram (sEEG) technology, we directly recorded the activity in human auditory cortex while subjects listened to either natural or noise-vocoded speech. These two stimuli have approximately identical envelopes, but the noise-vocoded speech does not have speech intelligibility. We found two stages of envelope tracking in auditory cortex: an early high-{gamma} (60-140 Hz) power stage (delay {approx} 49 ms) that preferred the noise-vocoded speech, and a late {theta} (4-8 Hz) phase stage (delay {approx} 178 ms) that preferred the natural speech. Furthermore, the decoding performance of high-{gamma} power was better in primary auditory cortex than in non-primary auditory cortex, consistent with its short tracking delay. We also found distinct lateralization effects: high-{gamma} power envelope tracking dominated left auditory cortex, while {theta} phase showed better decoding performance in right auditory cortex. In sum, we suggested a functional dissociation between high-{gamma} power and {theta} phase: the former reflects fast and automatic processing of brief acoustic features, while the latter correlates to slow build-up processing facilitated by speech intelligibility.