Infants' speech perception abilities change through the first year of life, from broad sensitivity to a wide range of speech contrasts to becoming more finely attuned to their native language. What remains unclear, however, is how this perceptual change relates to brain responses to native language contrasts in terms of the functional specialization of the left and right hemispheres. Here, to elucidate the developmental changes in functional lateralization accompanying this perceptual change, we conducted two experiments on Japanese infants using Japanese lexical pitch–accent, which changes word meanings with the pitch pattern within words. In the first behavioral experiment, using visual habituation, we confirmed that infants at both 4 and 10 months have sensitivities to the lexical pitch–accent pattern change embedded in disyllabic words. In the second experiment, near-infrared spectroscopy was used to measure cortical hemodynamic responses in the left and right hemispheres to the same lexical pitch–accent pattern changes and their pure tone counterparts. We found that brain responses to the pitch change within words differed between 4- and 10-month-old infants in terms of functional lateralization: Left hemisphere dominance for the perception of the pitch change embedded in words was seen only in the 10-month-olds. These results suggest that the perceptual change in Japanese lexical pitch–accent may be related to a shift in functional lateralization from bilateral to left hemisphere dominance.

The present study using near-infrared spectroscopy examined the neuronal correlates of Japanese long/short vowel contrast discrimination and its relationship with behavioral performance by comparing native Japanese (L1) subjects and Korean subjects learning Japanese as a second language (L2). Phoneme-specific responses were predominantly observed in the left auditory area only in the L1 subjects, although the behavioral scores of the L2 subjects indicated categorical perception (CP) that was indistinguishable from that of the L1 subjects. These inconsistent relationships were more evident in the correlation coefficients between the brain recording and behavior. However, slower reaction times and non-specific brain responses in the L2 listeners suggest differences in their cortical processes from those of the L1 subjects. These findings suggest that the CP of L2 phonemes as determined by behavioral scores alone does not always predict a language-specific neural processing as employed by the L1 listeners.