Functional specialization for language processing in inferior frontal regions during early childhood : evidence from functional near-infrared spectroscopy individual functional channels of interest approach

Abstract

Significance: Early language acquisition represents a fundamental achievement in cognitive development, yet the neural mechanisms underlying this process remain debated, particularly whether specialized language regions exist from early life or emerge gradually through development. Aim: We aim to investigate the functional specialization for language processing in early childhood. We first aimed to validate an individual functional channel of interest (fCOI) approach for dissociating language and cognitive control regions in adults and then to apply this method to examine whether these functional profiles are present in toddlers. Approach: Using functional near-infrared spectroscopy with the fCOI approach, we conducted two experiments involving adults (N=20, ages 18 to 26 years) and toddlers (N=22, ages 2 to 4 years) who completed language processing (intact versus degraded speech) and cognitive control tasks (spatial working memory task for adults, go/no-go task for toddlers). Results: For language regions within the left inferior frontal gyrus (LIFG), both adults and toddlers showed a significantly stronger response to intact versus degraded speech, with no significant modulation by cognitive demand manipulation. However, language selectivity in the homologous right hemisphere region was present only in adults. The multiple demand regions showed complementary patterns, with selectivity for cognitive control of regions within the right inferior frontal gyrus (RIFG) emerging early. Conclusions: These findings provide evidence for early neural specialization of language processing in LIFG while revealing ongoing development in RIFG organization. Our results support models of early language-specific neural regions rather than gradual differentiation from domain-general mechanisms while highlighting the protracted development of language organization.