Theta band oscillations reflect more than entrainment: behavioral and neural evidence demonstrates an active chunking process

Abstract

Parsing continuous acoustic streams into perceptual units is fundamental to auditory perception. Previous studies have uncovered a cortical entrainment mechanism in the delta and theta bands (~1-8 Hz) that correlates with formation of perceptual units in speech, music, and other quasi-rhythmic stimuli. Whether cortical oscillations in the delta-theta bands are passively entrained by regular acoustic patterns or play an active role in parsing the acoustic stream is debated. Here, we investigate cortical oscillations using novel stimuli with 1/f modulation spectra. These 1/f signals have no rhythmic structure but contain information over many timescales because of their broadband modulation characteristics. We chose 1/f modulation spectra with varying exponents of f, which simulate the dynamics of environmental noise, speech, vocalizations, and music. While undergoing magnetoencephalography (MEG) recording, participants listened to 1/f stimuli and detected embedded target tones. Tone detection performance varied across stimuli of different exponents and can be explained by local signal-to-noise ratio computed using a temporal window around 200 ms. Furthermore, theta band oscillations, surprisingly, were observed for all stimuli, but robust phase coherence was preferentially displayed by stimuli with exponents 1 and 1.5. We constructed an auditory processing model to quantify acoustic information on various timescales and correlated the model outputs with the neural results. We show that cortical oscillations reflect a chunking of segments, > 200 ms. These results suggest an active auditory segmentation mechanism, complementary to entrainment, operating on a timescale of ~200 ms to organize acoustic information.

Linguistic structure and meaning organize neural oscillations into a content-specific hierarchy

“…For the syllable-level timescale, we found no phase MI effects between the three forward conditions. We did find enhanced MI for forward compared to backward conditions, which is in line with previous findings, showing that tracking of the syllable envelope is enhanced for intelligible and attended speech (e.g., Peelle & Davis, 2012;Teng, Tian, Doelling, & Poeppel, 2018;Zoefel, ten Oever, & Sack, 2018). This could be due to envelope-tracking in the delta-theta band being facilitated by sharp edges in the acoustic signal, or "acoustic landmarks" (Doelling et al, 2014).…”

Orchestration of brain oscillations: principles and functions

“…The empirical study contributed by Teng et al . () further provides compelling evidence that theta oscillations play a role in the segmentation of auditory stimuli, a critical prerequisite for understanding speech. Heideman et al .…”

Theta and Gamma Bands Encode Acoustic Dynamics over Wide-ranging Timescales

“…Although different stimulus types have acoustic dynamics on different timescales, 467 this chunking process actively groups acoustic information into chunks in the 468 approximate time window of a theta period. The theta band signals reflect this chunking 469 process and therefore can be used to classify all stimulus types (Teng et al, 2018). On the 470 other hand, each gamma cycle integrates fine-grained acoustic information on a local 471 scale (e.g.…”