Understanding variability in newborns' NIRS 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 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 newborns’ brain activations to linguistic stimuli.Approach: Data from 14 near-infrared spectroscopy (NIRS) studies testing responses to different auditory sound patterns in 192 1-4-day-old newborns were pooled together and analyzed to assess the impact of GA and BW on changes in oxygenated (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. For HbR, increased BW led to more positive, thus less strong, activations in the frontal and more negative, thus stronger, activations in the temporal regions of the left hemisphere, while in the right hemisphere the opposite pattern was observed. Besides, when considering effect sizes reflecting discrimination abilities, these were more strongly associated with BW in the frontal regions, while in the temporal regions they were more strongly associated to GA. Critically, HbR was overall more strongly correlated with prenatal experience and maturation than HbO, challenging the exclusive use of HbO in newborn studies. Conclusions: The findings suggest a differential impact of BW and GA on brain activations, reflecting their roles in biological maturation and auditory discrimination, respectively. Overall, the study suggests that both the length of prenatal experience and maturation play significant roles in shaping newborns’ hemodynamic responses.