Homae, F. 1 , Watanabe, H. 2 & Taga, G. 2
1 Department of Language Sciences, Tokyo Metropolitan University
2 Graduate School of Education, University of Tokyo
Infants often pay special attention to speech sounds. They seem to detect key features of speech sounds. In order to investigate the neural foundation related to speech perception of infants, we used near-infrared spectroscopy (NIRS) to measure cortical activation. We presented the following 3 types of auditory stimuli (duration: about 4 s) while 3-month-old infants watched a silent movie: 1) normal speech sounds recorded by a female Japanese speaker, 2) sine wave speech (SWS) sounds consisting of 3 sine waves that tracked the first, second, and third formants of speech sounds, and 3) synthesized sounds composed of 3 pure tones. During the 12-s interstimulus intervals, we alternately presented 2 short pure tones at a lower intensity in order to continuously capture the infants' attention to stimuli. We used 2 multi-channel NIRS instruments (ETG-100, Hitachi Medical Corporation, Tokyo, Japan), one for each hemisphere, with 24 measurement channels over the frontal, temporal, and temporoparietal regions on both sides. Statistical analyses of oxygenated hemoglobin (oxy-Hb) signals revealed that the measurement channels over the left and right auditory areas showed significant activation in all conditions. Direct comparisons of oxy-Hb signal changes between SWS and synthesized sounds showed significant differences in the left frontal and temporal regions. Furthermore, comparisons of oxy-Hb signal changes between speech and SWS sounds exhibited significant differences in the left posterior temporal region. The 3 activated regions in the left temporal cortex adjoined each other, while the stimulus dependencies of activation were not obvious in the measurement channels over the right hemisphere. These results demonstrated that the left temporal cortex of 3-month-old infants was involved in speech processing and that functional differentiation occurred while infants perceived auditory information. Clarification of the functional role of this region will help understand the speech code.