The Amplitude Modulation Structure of Japanese Infant- and Child-Directed Speech: Longitudinal Data Reveal Universal Acoustic Physical Structures Underpinning Moraic Timing

Infant-directed speech (IDS) is highly rhythmic, and in European languages IDS is dominated by patterns of amplitude modulation (AM) at ~2Hz (reflecting prosody) and ~5Hz (reflecting individual syllables). The rhythm structure of spoken Japanese is thought to differ from European stress-timed and syllable-timed languages, depending on moraic units. Morae comprise any onset phoneme and vowel phonemes within a syllable, PA-N-DA. Arguably, initial speech encoding via cortical tracking by infants is likely to utilize language-universal physical acoustic structures in speech rather than language-specific structures like morae, since the infant brain must be prepared to acquire any human language. Here a language-blind computational model of linguistic rhythm based on features of the amplitude envelope is used to compute these physical acoustic stimulus characteristics for Japanese. Using ~18,000 samples of natural IDS and child-directed speech (CDS) recorded longitudinally from 6 parents while speaking to their children over the ages 0-5 years, we find that the temporal modulation patterns that characterise the amplitude envelope of Japanese are highly similar to those found for stress-timed and syllable-timed European languages. However, the AM band corresponding to the syllabic level in CDS/IDS in European languages (~2-12Hz, theta-rate cortical tracking) was elongated in Japanese (2.5-17Hz). Further, the phase synchronization ratios between the two slowest AM bands were as likely to be 1:3 as 1:2, which differs from European languages where 1:2 ratios are dominant. Accordingly, the language-universal amplitude-driven physical acoustic structures important for cortical speech tracking flexibly accommodate language-specific differences in core rhythmic units.