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

SignificanceEarly language acquisition represents a fundamental achievement in cognitive development, yet the neural mechanisms underlying this process remain debated, particularly whether specialized language circuits exist from early life or emerge gradually through development. AimTo investigate functional specialization for language processing in bilateral inferior frontal regions during early childhood, examining whether language-selective regions are distinct from domain-general cognitive networks in toddlers aged 2-4 years. ApproachUsing functional near-infrared spectroscopy (fNIRS) with an innovative functional Channel of Interest (fCOI) approach, we conducted two experiments involving adults (N=20) and toddlers (N=22, ages 2-4 years) who completed language processing and cognitive control tasks. ResultsWe 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. The multiple demand regions showed complementary patterns, with right-hemispheric selectivity for cognitive control emerging early. ConclusionsThese 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.