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

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. Parsing continuous natural sounds into perceptual units is fundamental to auditory perception. We used magnetoencephalography to investigate how the human auditory system groups sounds with 1/f modulation spectra, which mimic modulation profiles of natural sounds and indicate irregular temporal structure. The results revealed an active chunking mechanism reflected by theta band oscillations.