Early Detection of Neural Fatigue in Clinically Healthy Postpartum Mothers Using Functional Near-Infrared Spectroscopy

Background Postpartum mothers often experience depressive symptoms and emotional fluctuations that can affect parenting behavior and increase the risk of child maltreatment. Previous studies have reported reduced prefrontal activation in postpartum women; however, few have clarified the temporal changes in neural activity associated with sustained cognitive load. This study aimed to detect early signs of neural fatigue in clinically healthy mothers by measuring prefrontal activation during task performance using near-infrared spectroscopy (NIRS). Methods Eight mothers at one month postpartum (mean age: 31.5 years) participated in this study. Using a wearable optical topography device (WOT-220, NeU Corporation, Japan), changes in oxygenated hemoglobin (Δoxy-Hb) in the prefrontal cortex were measured during three verbal fluency tasks (initial syllables "to," "se," and "o") and an arithmetic task (serial subtraction of seven from 100). The Wilcoxon signed-rank test was used to compare task and calibration periods, and linear regression analysis was conducted to evaluate temporal changes. Results All participants exhibited task-related changes in Δoxy-Hb. In the left prefrontal cortex, activation was observed during the initial tasks; however, three out of eight mothers showed a gradual decrease in oxygenation responses as the tasks were repeated and sustained. Among these three, one mother had a relatively high Edinburgh Postnatal Depression Scale (EPDS) score, indicating a depressive tendency, whereas the other two were classified as non-depressed by self-report. Task performance (number of words produced and subtraction counts) was largely maintained, suggesting the presence of "latent fatigue," in which neural activity declines despite preserved behavioral performance. Conclusions This study demonstrated that even clinically healthy postpartum mothers may exhibit reduced sustainability of prefrontal activation under continuous cognitive load. The findings visualize early signs of neural fatigue that are undetectable through self-reported scales. Although this study does not propose NIRS as a diagnostic screening tool, it highlights that some mothers deemed clinically healthy may possess latent vulnerability to fatigue. Early recognition and preventive support based on such hidden fatigue could help prevent the onset of depression, improve the quality of life (QOL) for both mothers and infants, and contribute to the primary prevention of child abuse.