The developmental change in auditory preferences for speech stimuli was investigated for Japanese infants aged 4-14 months old. We conducted three experiments using two speech pairs in the head-turn preference procedure. Infant-directed (ID) speech and adult-directed (AD) speech stimuli were used in a longitudinal study (Experiment 1) and a cross-sectional study (Experiment 2). Native (Japanese) and non-native (English) speech stimuli were used in a cross-sectional study (Experiment 3). In all experiments, infants demonstrated a developmental change in their listening preference. For the ID/AD speech pair used in Experiments 1 and 2, infants show a U-shaped developmental shift with three developmental stages: Stage 1, in which very young infants tend to prefer ID speech over AD speech; Stage 2, in which the preference for ID speech decreases temporarily; and Stage 3, in which older infants again show a consistent preference for ID speech. For the native/non-native speech pair, there is a tendency for an increased preference for native speech over non-native speech, although infants did not demonstrate a U-shaped pattern. The difference in developmental pattern between the two types of speech pairs was discussed.

Effects of L1 phonotactic constraints on L2 speech perception and production were analyzed during the course of audio-visual perceptual training for Japanese adult learners of English by observing articulation of words containing /l/, /r/, and /w/. The speech identification score drastically improved during the training. The improvement in non-native word-initial rl distinction was clearly associated with the changes in the perceptual, articulatory, and neuronal spaces, which represent dissimilarities between the non-native and native phonemes in each domain assessed through perceptual, palatographic, and neuromagnetic measurements. Significant difficulty in identification of consonant-rl clusters, however, remained for some trainees even after one year of training. Analyses of palato-lingual contact patterns during word articulation suggested that more articulatory errors tend to occur in consonant-rl clusters than in other phonotactic contexts in such a way that both /l/ and /r/ are substituted with Japanese /r/ sometimes associated with a vowel inserted. Results suggested that the L2 phonological system can be acquired through a multimodal training, although it is strongly interfered with by L1 phonotactic constraints.

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