Cortical Tracking of Speech Across Rhythm Classes in English-, French-, and Japanese-Learning 4- and 8-Month-Old Infants.

It has been proposed that the world's languages fall into three rhythmic classes according to the units that determine their rhythmic structure: stress-based (e.g., English), syllable-based (e.g., French), and mora-based (e.g., Japanese). These rhythmic differences modulate language-specific processing in adults and young infants. It has also been proposed that the syllable is the default rhythmic unit across languages, more prominent in acquisition and processing. This study investigated the development of these language-specific versus language-general effects on the neural processing of speech, focusing on cortical tracking at frequency bands potentially corresponding to three rhythmic units (2.5 Hz/foot, 5 Hz/syllable, 10 Hz/mora). We assessed cortical tracking in 119 infants belonging to one of three native-language groups: English, French, and Japanese, and two age groups: 4 and 8 months. Infants heard CVN (consonant-vowel-nasal) syllable strings synthesized using a native-language voice in their respective native language, a non-native voice (Polish), and a non-speech (vocoded) version of the non-native voice stimuli. In all conditions, the stimuli consisted of 100-ms morae, combining into 200-ms syllables, combining into 400-ms bisyllabic feet. In all three language groups and at both ages, results showed (1) efficient cortical tracking at 5 and 10 Hz, but inconsistent tracking at 2.5 Hz; (2) stronger tracking at 5 Hz, and (3) differential tracking of speech and non-speech stimuli. These findings establish that language-general rhythmic processing guides cortical tracking, that the cross-linguistic syllabic level is dominant until at least 8 months, and that speech stimuli are processed differently than non-speech stimuli by at least 4 months.