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 (NIRS) 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.
Japanese has a vowel duration contrast as one component of its language-specific phonemic repertory to distinguish word meanings. It is not clear, however, how a sensitivity to vowel duration can develop in a linguistic context. The present study evaluated infants’ abilities to discriminate Japanese long and short vowels embedded in two-syllable words (/mana/ vs. /ma:na/), using the visual habituation-dishabituation method. The results revealed that 4- (n = 32) and 7.5-month old Japanese infants (n = 33) failed to discriminate the contrast (p = .676, .275, respectively), whereas 9.5-month old group (n = 33) showed the discrimination ability (p = .014). By contrast, the 4-month old group (n = 24) showed sensitivity to a vowel quality change (/mana/ vs. /mina/) (p = .034). These results indicate that Japanese infants acquire sensitivity to long/short vowel contrasts between 7.5- and 9.5-months of age, and that the developmental course of the phonemic category by the durational changes is different from that by the quality change.