An exploratory fNIRS study on directional asymmetry in segmental discrimination: Lateral–rhotic (/la–ra/) and labial–dorsal (/ba–ga/) contrasts tested in 5- and 9-month-old Japanese infants

Directional asymmetry refers to the order-dependent difference in segmental discrimination; however, its underlying cortical mechanisms remain unclear. This exploratory study used near-infrared spectroscopy to measure hemodynamic responses in 5- and 9-month-old Japanese infants (N = 30 [14 girls, 16 boys] and N = 40 [16 girls, 24 boys], respectively) to the lateral-rhotic /la-ra/ and labial-dorsal /ba-ga/ contrasts and to examine whether directional asymmetry was observed. Post hoc analysis revealed significant directional asymmetry for the /la-ra/ contrast in 5-month-old infants, characterized by decreased oxyhemoglobin levels in the right middle-anterior temporal channels, specifically for the /ra/-to-/la/ change. This localized response in the middle portion of the superior temporal gyrus, a region associated with tracking acoustic properties, suggests that infants at this age may rely on acoustic-level processing rather than on fully established phonological categories. This relative deactivation, observed in 5-month-olds but not in 9-month-olds and in the direction that appeared more difficult to discriminate, suggests a potential association between discrimination difficulty and the hemodynamic response. Specifically, the acoustic complexity of the /ra/ standard stimuli could have imposed a higher computational load, potentially leading to incomplete encoding during the baseline periods. This incomplete processing may have elevated hemodynamic activity, thereby resulting in a relative decrease in response to the subsequent change to /la/ stimuli. This phenomenon aligns with the perceptual space learning account, in which hemodynamic activity potentially reflects the computational load involved in the continuous calibration of an infant's developing perceptual space. Future studies should directly assess this hypothesis.