Internal rhythms to sentences
Cross-linguistic evidence from English and Mandarin Chinese suggests that low-frequency cortical rhythms (~delta band, 1-4 Hz) can track the constituent structure in a linguistic hierarchy, e.g. phrasal and sentential organization (Ding et al., 2016). Here we extend this paradigm to German and also investigate frequencies below 1 Hz. Stimuli were sentences comprised of four monosyllabic words, five monosyllabic words, and five disyllabic words delivered at a constant rate (4 Hz for monosyllabic words, and 2 Hz for disyllabic words) and concatenated in blocks of ten sentences. Phrase and sentence boundaries were not marked by acoustic cues. Participants detected oddball blocks which contained ungrammatical sentences, interspersed with regular blocks containing only correct sentences. Using spectral analyses on EEG data from each regular block, we find that cortical rhythms reliably integrate sentence-based rhythms also in the low delta band: 0.8 Hz and 0.4 Hz for monosyllabic and disyllabic five-word sequences, respectively. Surprisingly, both these rhythms displayed a clear harmonic structure, supporting the hypothesis of internal rhythmogenesis via harmonic oscillators. We further tested the findings using two attention manipulations: 1) attention to stimuli vs. attention away from stimuli and 2) detection of voice pitch vs. grammaticality oddballs. Our data provide a broader empirical base for understanding the role of brain rhythms in tracking natural spoken language.