Six-month-olds' categorization of natural infant-directed utterances.

In this study, the authors demonstrated that 6-month-old infants are able to categorize natural, 650 Hz low-pass filtered infant-directed utterances. In Experiment 1, 24 male and 24 female infants heard 7 different tokens from 1 class of utterance (comforting or approving). Then, some infants heard a novel test stimulus from the familiar class of tokens; others heard a test stimulus from the unfamiliar class. Infants categorized these tokens as evidenced by response recovery to tokens from the unfamiliar class but not to novel tokens from the familiar class. Experiment 2 confirmed that the infants were able to discriminate between closely matched tokens from within each category, supporting the conclusion that the results of Experiment 1 indicated categorization. The authors discuss both a mechanism that might explain the development of this ability and the mutual adaptation seen in parent-infant communication.

A combination of vocal fo dynamic and summary features discriminates between three pragmatic categories of infant-directed speech.

To assess the relative contribution of dynamic and summary features of vocal fundamental frequency (f0) to the statistical discrimination of pragmatic categories in infant-directed speech, 49 mothers were instructed to use their voice to get their 4-month-old baby's attention, show approval, and provide comfort. Vocal f0 from 621 tokens was extracted using a Computerized Speech Laboratory and custom software. Dynamic features were measured with convergent methods (visual judgment and quantitative modeling of f0 contour shape). Summary features were f0 mean, standard deviation, and duration. Dynamic and summary features both individually and in combination statistically discriminated between each of the pragmatic categories. Classification rates were 69% and 62% in initial and cross-validation DFAs, respectively.