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

Natural sounds have broadband modulation spectra and contain acoustic dynamics ranging from tens to hundreds of milliseconds. How does the human auditory system encode acoustic information over wide-ranging timescales to achieve sound recognition? Previous work (Teng et al., 2017) demonstrated a temporal coding preference in the auditory system for the theta (4 - 7 Hz) and gamma (30 - 45 Hz) ranges, but it remains unclear how acoustic dynamics between these two ranges is encoded. Here we generated artificial sounds with temporal structures over timescales from ~200 ms to ~30 ms and investigated temporal coding on different timescales in the human auditory cortex. Participants discriminated sounds with temporal structures at different timescales while undergoing magnetoencephalography (MEG) recording. The data show robust neural entrainment in the theta and the gamma bands, but not in the alpha and beta bands. Classification analyses as well as stimulus reconstruction reveal that the acoustic information of all timescales can be differentiated through the theta and gamma bands, but the acoustic dynamics in the theta and gamma ranges are preferentially encoded. We replicate earlier findings of multi-time scale processing and further demonstrate that the theta and gamma bands show generality of temporal coding across all timescales with comparable capacity. The results support the hypothesis that the human auditory cortex primarily encodes auditory information employing neural processes within two discrete temporal regimes. SignificanceNatural sounds contain rich acoustic dynamics over wide-ranging timescales, but perceptually relevant regularities often occupy specific temporal ranges. For instance, speech carries phonemic information on a shorter timescale than syllabic information at ~ 200 ms. How does the brain efficiently sample continuous acoustic input to perceive temporally structured sounds? We presented sounds with temporal structures at different timescales and measured cortical entrainment using magnetoencephalography. We found, unexpectedly, that the human auditory system preserves high temporal coding precision on two non-overlapping timescales, the slower (theta) and faster (gamma) bands, to track acoustic dynamics over all timescales. The results suggest that the acoustic environment which we experience as seamless and continuous is segregated by discontinuous neural processing, or sampled.