The role of syllabic rhythm in speech perception across languages.

The insertion of silences at regular intervals restores the intelligibility of English utterances that have been accelerated beyond comprehension, as long as the duration of the resulting speech-silence chunks falls within the theta rhythm of natural speech, i.e. the temporal modulation associated to the syllabic rate. We test whether such a rhythmic strategy works in languages rhythmically different from English, a stress-timed language. Thus, we assess whether comprehension of time-compressed Semantically Unpredictable Sentences (SUS) is restored in the syllable-timed language French and the mora-timed language Japanese, when silences re-establishing theta rhythm are inserted. Restoring the theta rhythm also improved intelligibility in French, but not in Japanese, in which best performance was instead achieved at faster rhythms, which suggests that modulation at the rate of a language's basic rhythmic unit plays a key role in understanding speech. In a second experiment, French speakers listened to SUS with speech-silence chunks adapted to the range of the temporal modulations of the delta, gamma, and high gamma rhythms, which correspond to the rate of prosodic phrases, phonemes, and subsegmental features, respectively. Unlike the theta rhythm, we found no restorative effects, providing further evidence for the special status of the theta rhythm in speech comprehension.