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
The left-hemispheric dominance of speech processing is a prominent characteristic of the human brain. However, the developmental origin of this dominance has never been fully clarified. Here, we used 94-channel near-infrared spectroscopy (NIRS; ETG-7000, Hitachi Medical Corporation, Tokyo, Japan) to measure cortical activation and the temporal correlations between cortical regions in 3-month-old infants (N = 20). We presented Japanese speech sounds (duration: about 4 s) to sleeping infants every 10 or 20 s (63 sentences in total). During the inter-stimulus intervals, no sounds were presented. The averaged time course of oxygenated hemoglobin (oxy-Hb) signals from the onset of stimulus presentation was calculated for each measurement channel in order to determine which cortical regions responsive to speech sounds. Furthermore, we examined the temporal correlations between the oxy-Hb signals with all of the continuous data. We found that the temporal regions of the left and right hemispheres showed significant activation. The frontal, parietal, and occipital regions also showed significant activation in response to speech sounds. Correlation analyses revealed interhemispheric connectivity, especially between homologous regions, as well as intrahemispheric connectivity between adjacent regions and between distant regions in both hemispheres. When we directly compared the correlations of all intrahemispheric pairs between the left and right hemispheres, we found that the correlations between the frontal regions and the temporal regions in the left hemisphere were higher than correlations between the corresponding pairs in the right hemisphere. Our findings demonstrated that both the left and right temporal regions were involved in the processing of speech sounds and that long-range intrahemispheric connectivity was marked in the left hemisphere. The development of frontotemporal connectivity in the left hemisphere might have caused the hemispheric differences in brain function and structure and facilitated language acquisition in infancy.