Time-frequency EEG markers of word boundaries in speech production.

Speech production requires orchestration of multiple brain systems that support the unfolding of the spoken message across hierarchical linguistic levels, such as phonemes, syllables, words or phrases. Transitions between linguistic levels are critical for connected speech production, yet the neural dynamics of, for example, syllable-level and word-level transitions remain unknown. In this electroencephalography (EEG) study, we use time-frequency analysis and source localization to determine differences between word-boundary and within-word syllable transitions. To this end, pseudoword pairs comprising a total of six consonant-vowel (CV) syllables with different word-boundary positions were used. Adults produced the pseudoword pairs at the pace of a visual metronome, such that syllables were uttered at matching times independently of their relative positions. Accordingly, target syllables could reference either within-word or between-word syllabic transitions, while other linguistic properties, including articulation, stress patterns, co-articulation or prosody were matched. EEG time-frequency analyses of neural sources revealed sensitivity to hierarchical structure. Sources in left and right inferior frontal lobes, as well as left superior temporal lobe were differentially recruited when producing the same syllables, in the same utterance position, but under different pseudoword boundary contexts. The right inferior frontal source showed extensive time-frequency modulation in word transitions that included elevated event-related synchronization across theta and beta oscillations. Alpha suppression was found in the left superior temporal lobe, possibly related to increased sensory prediction at word onset. Despite efforts to control speech pace and syllable acoustics across conditions, small residual differences in timing and acoustic properties were observed.