Categorization of infant-directed speech: development from 4 to 6 months.

To extend a previous finding that 6-month-old infants categorized low-pass filtered infant-directed (ID) utterances, we examined a) 6-month-old infants' categorization of more naturalistic, unfiltered ID utterances and b) the developmental progression of ID-speech categorization. In Experiment 1, 6-month-olds heard seven different unfiltered tokens from one class of ID utterance (approving or comforting), followed by a novel token from either the same or an unfamiliar category. Infants recovered, responding only to the unfamiliar category token, suggesting that they categorized naturalistic ID utterances. Four-month-olds' categorization of filtered and unfiltered versions of the ID utterances was assessed in Experiments 2 and 4. Four-month-olds did not recover, responding to a test token from an unfamiliar class, suggesting that they did not categorize either filtered or unfiltered ID utterances. Experiment 3 demonstrated that 4-month-old infants' failure to categorize did not result from their inability to complete the procedure. These results suggest that infants' processing of ID speech changes from 4 to 6 months of age.