K. SUZUKI, J. ANDO
ponents (e.g., measurement error). Thus, the lack of significant
differences in hemodynamic responses to speech appears to
provide no clear implications. However, in this study, the indi-
vidual differences in hemodynamic responses provide valid
variance as inferred from the degree of genetic and environ-
mental influences. As mentioned in the introduction section,
twin designs decompose variance into additive genetic, shared
environmental, and unique environmental variance. Measure-
ment error is only involved in unique environmental variance.
In this study, the individual differences in hemodynamic re-
sponses to speech have significant variance within the additive
genetic and shared environmental factors. Therefore, the large
individual differences in hemodynamic responses should be
meaningful components for right oxy-Hb and left deoxy-Hb.
This study highlights the genetic and environmental etiology
of hemodynamic responses to speech stimuli. The present find-
ings revealed that additive genetic factors, as well as shared and
unique environmental factors are significant contributors to
each type of hemoglobin within each bilateral temporal area.
These results provide new knowledge regarding infants’ hemo-
dynamic responses to speech stimuli. Further investigation into
the role of genetic and environmental variation within the infant
neural system, using a larger sample size, may provide more
important information for understanding the mechanisms un-
derlying infant neural maturation.
Acknowledgements
We are grateful to the families who participated in the study.
Data collection for this study was supported by a grant from the
Research Institute of Science and Technology for Society wi-
thin the Japan Science and Technology Agency. This manu-
script was supported by Grant-in-Aid for Young Scientists (B)
(#25730101) by the Japanese Ministry of Education, Culture,
Sports, Science and Technology.
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