Effect of sleep stages on patterns of fNIRS hemodynamic response to auditory paradigms in one-month-old Gambian and UK infants

Significance: Functional near-infrared spectroscopy (fNIRS) has advanced our understanding of early brain development, especially infant responses to social and auditory stimuli. Unlike older children and adults, very young infants are often assessed during natural sleep to reduce movement and ensure sufficient data quality. Yet the impact of sleep stage on fNIRS signals and how it might affect interpretations of early brain activation patterns remains unclear. Aim: This study investigates the effect of sleep stages on fNIRS-measured hemodynamic responses to two auditory paradigms across different global populations of one-month-old infants. Approach: In total, 46 Gambian and 40 UK infants in quiet or active sleep were tested using (1) social selectivity and (2) a habituation and novelty detection paradigms. Results: In the UK cohort, active sleep was associated with a stronger initial response and greater habituation compared to quiet sleep. In contrast, Gambian infants in quiet sleep showed more widespread activation and evidence of habituation, while infants in active sleep showed no habituation. No sleep stage effects were observed for response in the social selectivity paradigm in either group. Conclusions: Different effects of sleep stages were observed across the two cohorts and paradigms and should be carefully considered in neuroimaging studies.