Functional Specialization for Language Processing in Inferior Frontal Regions During Early Childhood: Evidence from functional near-infrared spectroscopy individual functional channels of interest approach

Early language acquisition represents a fundamental achievement in cognitive development, yet the neural mechanisms underlying this process remain debated. Using functional near-infrared spectroscopy (fNIRS) with an innovative functional Channel of Interest (fCOI) approach, we investigated the functional specialization for language processing in bilateral inferior frontal regions during early childhood. In two experiments involving adults (N=20) and toddlers (N=22, ages 2-4 years), participants completed language processing and cognitive control tasks. Results demonstrated early functional specialization in the language-selective region of left inferior frontal gyrus, which showed selective responses to linguistic content while remaining insensitive to cognitive demand manipulations in both age groups. However, language selectivity in the homologous right hemisphere region was present only in adults, suggesting continued development of language organization beyond early childhood. The MD regions showed complementary patterns, with right-hemispheric selectivity for cognitive control emerging early. These findings provide evidence for early neural specialization of language processing in the left hemisphere, while revealing ongoing development in right hemispheric organization. Our results support models of early language-specific neural circuits rather than gradual differentiation from domain-general mechanisms, while highlighting the protracted development of language organization.