Rapid neural analysis of linguistic stress and meter in continuous speech
Continuous speech evolves around vowels, the centerpieces of individual syllables. Vowels vary in linguistic and acoustic salience: Linguistically, stressed syllables are more salient than unstressed syllables: Stress patterns convey critical lexico-semantic and prosodic information, and their regularity defines the speech meter. Acoustically, English vowel intensity cues lexical stress but also marks salient syllables irrespective of stress status. Recent evidence demonstrates rapid neural analysis of vowel intensity and identity during perception of continuous speech. Here, we probe how these processes integrate lexical stress and metrical regularity. We recorded EEG while participants (n=26) listened to childrens stories with either an irregular, speech-like meter, or a regular poetic meter. Stress and meter modulated cortical encoding of vowels throughout processing: Preparatory activity preceded vowel onsets in an irregular meter only, and early sensory responses were enhanced for unstressed vowels, suggesting additional resource allocation during processing of uncertain and less discriminable speech sounds. In contrast, later processing (300-500ms) was stronger for stressed syllables and in irregular meters, suggesting a combined effect of uncertainty and informational content. Finally, responses were stronger for small intensity rises within metrically predicted stressed vowels than in all other conditions. In the time-frequency domain, the spectral profile of neural phase-locking corresponded to spectral signatures of individual evoked responses, syllable and stress rates in the stimuli. Overall, our findings reveal rapid neural integration of stress and metrical expectations in neural processing of continuous speech. These dynamics may underlie the perceptual benefits of metrically regular speech, such as poetry and song lyrics.