Understanding variability in full-term newborns’ fNIRS data: the impact of birth weight and gestational age on infants’ speech perception abilities

Significance: Language acquisition is a complex process already influenced by prenatal neural development and auditory experiences. From the onset of the third trimester, fetuses perceive sounds already influencing the fetal brain. Aim: The study investigates how the length of intrauterine language exposure, indexed by gestational age (GA), and overall maturation, indexed by birth weight (BW), affect full-term newborns' brain responses to linguistic stimuli. Approach: Data from 14 near-infrared spectroscopy studies testing responses to different auditory sound patterns in 192 0- to 7-day-old newborns were pooled together and analyzed to assess the impact of GA and BW on changes in oxygenated hemoglobin (HbO) and deoxygenated hemoglobin (HbR). Results: Results showed that, for HbO, activations in both considered conditions were larger in the temporal than in the frontal areas, irrespective of BW or GA. A similar spatial pattern was observed for HbR, with stronger responses in the temporal compared with frontal regions across conditions. In contrast, when considering effect sizes and reflecting discrimination abilities, these were more strongly associated with BW in the bilateral frontal regions, whereas in the bilateral temporal regions, they were more strongly associated with GA. Conclusions: The findings suggest a differential impact of BW and GA on neural measures of linguistic sensitivity in newborns, reflecting their roles in biological maturation and auditory experience, respectively. Overall, the study suggests that both the length of prenatal experience and maturation play significant roles in shaping newborns' hemodynamic responses.