Infant's developmental changes in cerebral lateralization in response to speech stimuli were assessed using a multi-channel near infrared spectroscopic measurement system. Analysis-synthesized Japanese word stimuli were used as phoneme (/itta/ and /itte/) and pitch (/itta/ and /itta?/) contrasts. The baseline blocks contained only/itta/, whereas /itta/ and its phoneme or pitch counterpart were presented randomly with equal probability during the contrast blocks in each session. To analyze cerebral lateralization, we calculated laterality indices using maximal total-Hb changes during the contrast blocks in the left and right auditory areas. Significant differences were found in the laterality indices between phoneme and pitch sessions for the infant groups of 11-12 months and older, as well as for 2- to 5-year-old infants and adults. The results imply that cerebral dominance for processing speech is formed just before one year after birth in normal infants. The method used in this study can be used clinically as an objective test of language development for infants with increased risk in language development.
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