θ-Band and β-Band Neural Activity Reflects Independent Syllable Tracking and Comprehension of Time-Compressed Speech

Abstract

Recent psychophysics data suggest that speech perception is not limited by the capacity of the auditory system to encode fast acoustic variations through neural activity, but rather by the time given to the brain to decode them. Whether the decoding process is bounded by the capacity of rhythm to follow syllabic rhythms in speech, or constrained by a more endogenous top-down mechanism, e.g., involving activity, is unknown. We addressed the dynamics of auditory decoding in speech comprehension by challenging syllable tracking and speech decoding using comprehensible and incomprehensible time-compressed auditory sentences. We recorded EEGs in human participants and found that neural activity in both and ranges was sensitive to syllabic rate. Phase patterns of slow neural activity consistently followed the syllabic rate (4 -14 Hz), even when this rate went beyond the classical range (4 -8 Hz). The power of activity increased linearly with syllabic rate but showed no sensitivity to comprehension. Conversely, the power of (14 -21 Hz) activity was insensitive to the syllabic rate, yet reflected comprehension on a single-trial basis. We found different long-range dynamics for and activity, with activity building up in time while more contextual information becomes available. This is consistent with the roles of and activity in stimulus-driven versus endogenous mechanisms. These data show that speech comprehension is constrained by concurrent stimulus-driven and low- activity, and by endogenous activity, but not primarily by the capacity of activity to track the syllabic rhythm.