Human acoustic processing becomes attuned to the sounds of our native language within the first year of life. Minagawa-Kawai et al. used near-infrared spectroscopy (NIRS) to track acoustic processing in Japanese infants.
Subjects were gently fitted with headgear containing near-infrared laser emission and detection probe arrays over the lateral auditory areas. Absorption of light by hemoglobin was used to estimate blood flow changes, providing a spatial resolution of 2?3 cm.
NIRS measures only small vessels and thus may be less sensitive to systemic circulatory changes than functional magnetic resonance imaging. The stimuli consisted of two pairs of speech sounds or phonemes, in this case Japanese vowel sounds. The pairs had identical physical structure, but only one contained linguistic information. At 6 months, infants showed phonemic specificity, but this was no longer apparent by 10?11 months. After 12 months, the language specificity was again detectable but now was left lateralized, a pattern similar to adults.