Early influence of language experience in non-native speech perception: Discrimination of three-way Thai stop contrasts by Korean and Japanese infants.

Young infants' remarkable ability to discriminate non-native phoneme contrasts played a critical role in shaping the tenets of the perceptual narrowing hypothesis: early on, infants are sensitive to most phoneme categories, including those not used in their native language, but lose this sensitivity as they attune to their language. However, supporting evidence was derived from limited geographical regions and languages, particularly on early sensitivity, requiring further studies to specify the extent of early sensitivity and reassess the dominant developmental pattern. This study aimed to fill this gap by examining discrimination patterns for three-way Thai stop contrasts by two other Asian language learners (Korean and Japanese) at age 4-6 months. The three stop categories in Thai are distinct along the voice onset time (VOT) dimension, encompassing both negative and positive values. Thai pre-voiced and voiceless (i.e., short lag) stops are similar to stop categories used in languages such as French, Dutch, and Spanish. Thai voiceless and voiceless aspirated (i.e., long lag) stops are similar to those in English, Chinese, and German. Therefore, Thai stop categories provide an ideal test continuum for confirming early universal sensitivities to two supposedly language-general VOT boundaries (-30 ms, +30 ms). We presented two Thai phoneme pairs (pre-voiced vs. voiceless, voiceless vs. voiceless aspirated) to Korean and Japanese infants aged 4-6 months and observed their discrimination patterns using a visual habituation paradigm. The results showed divergent discrimination between the two language learners. Korean infants showed sensitivity to the pre-voiced-voiceless pair, whereas Japanese infants did not. By contrast, only Japanese infants showed some sensitivity to the voiceless-voiceless aspirated pair with some directionality effect, whereas Korean infants did not. These results demonstrate systematic cross-linguistic differences reflecting input influence in early perceptual sensitivity and suggest the ambient language environment may influence consonant perception much earlier than has been considered by the perceptual narrowing theory, calling for further refinement of the extent of initial perceptual state in the theory.