Neural reorganization of dialect perception within a critical two-month window in infancy: A longitudinal fNIRS study

Infants are sensitive to linguistic variation from early in life, yet little is known about whether closely related language varieties elicit differential neural responses already at 4 months and, more importantly, how responses to familiar and unfamiliar varieties change over development. Here we present the first longitudinal fNIRS study examining age-related changes in infants' neural processing of dialectal variation. Thirty-seven monolingual Czech-learning infants were tested at 4 and 6 months, a key period of early speech perceptual reorganization, while listening to sentences spoken in Standard Czech (familiar) and in the Silesian dialect (unfamiliar). These varieties differ in intonation and vowel-length realization. At both ages, both varieties elicited robust bilateral temporal activation, indicating stable auditory speech processing. At 4 months, infants showed stronger right prefrontal cortex (PFC) responses to Standard Czech, consistent with a neural familiarity preference. In the same region, longitudinal analyses revealed age-related changes. By 6 months, PFC oxygenation to the unfamiliar Silesian dialect increased to the level of the familiar variety, reflecting a progressive reorientation of attention. We interpret this pattern as indicating a transitional state in which the familiarity preference evident at 4 months has disappeared, while the novelty preference documented behaviorally at later ages has not yet fully emerged. The right hemispheric, frontal locus of this developmental change may indicate increased sensitivity to slowly varying intonational contours of the unfamiliar dialect, though this interpretation requires confirmation through a more controlled experimental design.