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.