The Amplitude Modulation Structure of Japanese Infant- and Child-Directed Speech: Longitudinal Data Reveal Universal Acoustic Physical Structures That Accommodate Both Syllabic and Moraic Timing.

Infant-directed speech (IDS) is highly rhythmic, and in European languages, it is dominated by patterns of amplitude modulation (AM) peaking at ∼2 Hz (reflecting prosody) and ∼5 Hz (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 (∼10 Hz) comprising any onset phoneme and vowel phonemes within a syllable, PA-N-DA. As the infant brain must be prepared to acquire any human language, initial speech encoding is likely to utilize language-universal physical acoustic structures in speech. These physical structures are, however, probabilistic and may thereby simultaneously accommodate language-specific structures like morae. Here, a language-blind computational model of linguistic rhythm based on the amplitude envelope (AE) is used to compute the physical acoustic stimulus characteristics for Japanese. Using ∼18,000 samples of natural IDS and child-directed speech (CDS) recorded longitudinally over the ages 0-5 years, the data show that the temporal modulation patterns that characterize the AE of Japanese are 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 (∼2-12 Hz) was elongated in Japanese (2.5-17 Hz), possibly accommodating the faster modulation peaks reflecting morae. Furthermore, the phase synchronization ratios between the two slowest AM bands were as likely to be 1:3 as 1:2, differing from European languages where 1:2 ratios (delivering the perceptual experience of a temporally regular beat) are dominant. Accordingly, the amplitude-driven physical acoustic structures important for cortical speech tracking flexibly accommodate both universality and specificity.