Delta, theta, beta, and gamma brain oscillations index levels of auditory sentence processing

A growing number of studies indicate that multiple ranges of brain oscillations, especially the delta (delta, <4 Hz), theta (theta, 4-8 Hz), beta (beta, 13-30 Hz), and gamma (gamma, 30-50 Hz) bands, are engaged in speech and language processing. It is not clear, however, how these oscillations relate to functional processing at different linguistic hierarchical levels. Using scalp electroencephalography (EEG), the current study tested the hypothesis that phonological and the higher-level linguistic (semantic/syntactic) organizations during auditory sentence processing are indexed by distinct EEG signatures derived from the delta, theta, beta, and gamma oscillations. We analyzed specific EEG signatureswhile subjects listened to Mandarin speech stimuli in three different conditions in order to dissociate phonological and semantic/syntactic processing: (1) sentences comprising valid disyllabic words assembled in a valid syntactic structure (real-word condition); (2) utterances with morphologically valid syllables, but not constituting valid disyllabic words (pseudo-word condition); and (3) backward versions of the real-word and pseudo-word conditions. We tested four signatures: band power, EEG-acoustic entrainment (EAE), cross-frequency coupling (CFC), and inter-electrode renormalized partial directed coherence (rPDC). The results show significant effects of band power and EAE of delta and theta oscillations for phonological, rather than semantic/syntactic processing, indicating the importance of tracking delta- and theta-rate phonetic patterns during phonological analysis. We also found significant beta-related effects, suggesting tracking of EEG to the acoustic stimulus (high-beta EAE), memory processing (theta-low-beta CFC), and auditory-motor interactions (20-Hz rPDC) during phonological analysis. For semantic/syntactic processing, we obtained a significant effect of gamma power, suggesting lexical memory retrieval or processing grammatical word categories. Based on these findings, we confirm that scalp EEG signatures relevant to delta, theta, beta, and gamma oscillations can index phonological and semantic/syntactic organizations separately in auditory sentence processing, compatible with the view that phonological and higher-level linguistic processing engage distinct neural networks. (C) 2016 Elsevier Inc. All rights reserved.