A cross-language comparison of /d/-/th/ perception: evidence for a new developmental pattern.
Previous studies have shown that infants perceptually differentiate certain non-native contrasts at 6-8 months but not at 10-12 months of age, whereas differentiation is evident at both ages in infants for whom the test contrasts are native. These findings reveal a language-specific bias to be emerging during the first year of life. A developmental decline is not observed for all non-native contrasts, but it has been consistently reported for every contrast in which language effects are observed in adults, In the present study differentiation of English /d-th/ by English- and French-speaking adults and English- and French-learning infants at two ages (6-8 and 10-12 months) was compared using the conditioned headturn procedure. Two findings emerged. First, perceptual differentiation was unaffected by language experience in the first year of life, despite robust evidence of language effects in adulthood. Second, language experience had a facilitative effect on performance after 12 months, whereas performance remained unchanged in the absence of specific language experience. These data are clearly inconsistent with previous studies as well as predictions based on a conceptual framework proposed by Burnham [Appl. Psycholing. 7, 201-240 (1986)]. Factors contributing to these developmental patterns include the acoustic properties of /d-th/, the phonotactic uniqueness of English /th/, and the influence of lexical knowledge on phonetic processing.