Articulation Rate and Surprisal in Swedish Child-Directed Speech

Sammanfattning: Child-directed speech (CDS) differs from adult-directed speech (ADS) in several respects whose possible facilitating effects for language acquisition are still being studied. One such difference concerns articulation rate --- the number of linguistic units by the number of time units, excluding pauses --- which has been shown to be generally lower than in ADS. However, while it is well-established that ADS exhibits an inverse relation between articulation rate and information-theoretic surprisal --- the amount of information encoded in a linguistic unit --- this measure has been conspicuously absent in the study of articulation rate in CDS. Another issue is if the lower articulation rate in CDS is stable across utterances or an effect of local variation, such as final lengthening. The aim of this work is to arrive at a more comprehensive model of articulation rate in CDS by including surprisal and final lengthening. In particular, one-word utterances were studied, also in relation to word-length effects (the phenomenon that longer words generally have a higher articulation rate). To this end, a methodology for large-scale automatic phoneme-alignment was developed, which was applied to two longitudinal corpora of Swedish CDS. It was investigated i) how articulation rate in CDS varied with respect to child age, ii) whether there was a negative relation between articulation rate and surprisal in CDS, and iii) to what extent articulation rate was lower in CDS than in ADS. The results showed i) a weak positive effectof child age on articulation rate, ii) a negative relation between articulation rate and surprisal, and iii) that there was a lower articulation rate in CDS but that the difference could almost exclusively be attributed to one-word utterances and final lengthening. In other words, adults seem to adapt how fast they speak to their children's age, speaking faster to children is correlated with a reduced amount of information, and the difference in articulation rate between CDS and ADS is most prominent in isolated words and final lengthening. More generally, the results suggest that CDS is well-suited for word segmentation, since lower articulation rate in one-word utterances provides an additional cue.