Neural attunement processes in infants during the acquisition of a language-specific phonemic contrast

Abstract

To elucidate the developmental neural attunement process in the language-specific phonemic repertoire, cerebral hemodynamic responses to a Japanese durational vowel contrast were measured in Japanese infants using near-infrared spectroscopy. Because only relative durational information distinguishes this particular vowel contrast, both first and second language learners have difficulties in acquiring this phonemically crucial durational difference. Previous cross-linguistic studies conducted on adults showed that phoneme-specific, left-dominant neural responses were observed only for native Japanese listeners. Using the same stimuli, we show that a larger response to the across-category changes than to the within-category changes occurred transiently in the 6- to 7-month-old group before stabilizing in the groups older than 12 months. However, the left dominance of the phoneme-specific response in the auditory area was observed only in the groups of 13 months and above. Thus, the durational phonemic contrast is most likely processed first by a generic auditory circuit at 6-7 months as a result of early auditory experience. The neural processing of the contrast is then switched over to a more linguistic circuit after 12 months, this time with a left dominance similar to native adult listeners.

Figure 1.

Spectrograms and pitch contours of the stimuli /mama/ and /mama:/ differing in final vowel durations.

Figure 2.

Location of the NIRS probes and channels. Five emission and four detection probes arranged in a 3 × 3 square lattice (A) and two emission and two detection probes in a 2 × 2 square lattice (B) were fitted on each lateral side of the head.

Figure 3.

Time course of NIRS responses for eight channels around the temporal areas (left and right) in subjects of different age groups: 3-month-old infant (A), 25-month-old infant (B), and adult data adapted from Minagawa-Kawai et al. (2002) (C). The younger infant showed bilateral and broad activations compared with the adult's pattern with left dominance.

Figure 4.

Averaged values of the total Hb changes in the left auditory area (L) in response to the across- and within-phonemic changes of the stimuli. *p < 0.05 for the difference between the responses in the across- and within-sessions.

Figure 5.

Laterality index in different age groups. Left dominance is observed in the older age groups. *p < 0.05. Boxes, The quartiles; bars in the box, the medians; hinges, the ranges.