Is the vowel length contrast in Japanese exaggerated in infant-directed speech?

Vowel length contrasts in Japanese, e.g., chizu “map” vs. chiizu “cheese”, are cued primarily by vowel duration. However, since short and long vowel durations overlap considerably in ordinary speech, learning to perceive vowel length contrasts is complex. Meanwhile, infant-directed speech (IDS) is known to “exaggerate” certain properties of adult-directed speech (ADS). If so, then it is possible that vowel length contrasts might also be exaggerated in IDS. To investigate this, the present study analyzed vowel durations in the RIKEN Japanese Mother-Infant Conversation Corpus, which contains 11 hours of IDS by 22 mothers talking with their 18-to-24-month-old infants, and 3 hours of ADS by the same mothers. Results indicated that vowel length contrasts were generally not exaggerated in IDS, except at the end of prosodic phrases. Furthermore, several factors that systematically affected vowel duration in IDS were identified, including phrase-final lengthening and “non-lexical lengthening”, i.e., the lengthening of vowels for emphatic or other stylistic purposes. These results suggest that vowel duration in Japanese IDS could not only potentially facilitate learning of lexical distinctions, but also signal phrase boundaries, emphasis, or other communicative functions.