Investigating working memory and brain activation in major depressive disorder with and without insomnia: insights from functional near-infrared spectroscopy (fNIRS)

Background Major depressive disorder (MDD) frequently coexists with insomnia, a comorbidity that exacerbates cognitive deficits, particularly in working memory (WM). While previous research has established links between insomnia and impaired brain function, the barin functional indicators associated with WM deficits in MDD patients with insomnia remain unclear. This study explores cortical activation patterns in MDD patients with and without insomnia using functional near-infrared spectroscopy (fNIRS). Methods A total of 55 MDD patients with insomnia and 67 without insomnia were recruited. Cortical activation during WM tasks was assessed using fNIRS, focusing on oxygenated hemoglobin (Oxy-Hb) concentration changes. Sleep quality was assessed using the Pittsburgh Sleep Quality Index (PSQI), depression severity was assessed using the Hamilton Depression Scale (HAMD), and cognitive function was assessed using the Repeatable Battery for the Assessment of Neuropsychological Status (RBANS). Results MDD patients with insomnia exhibited lower RBANS scores and reduced accuracy in medium-load WM tasks compared to non-insomnia patients. Additionally, fNIRS analysis revealed diminished Oxy-Hb concentrations in the bilateral dorsolateral prefrontal cortex (DLPFC) during medium-load tasks and in the left DLPFC during high-load tasks. Correlation analyses indicated that immediate memory scores negatively correlated with left DLPFC and bilateral medial prefrontal cortex (mPFC) activation during medium-load tasks, while task accuracy negatively correlated with bilateral mPFC activation ( p < 0.05). Conclusion The presence of insomnia in MDD is associated with exacerbated WM deficits and altered prefrontal cortical activation, particularly in tasks with increased cognitive demand. These findings provide potential neurofunctional reference indicators, which may help offer auxiliary assessment means for MDD patients with insomnia. In addition, the sensitivity of fNIRS in detecting these neural brain activation differences suggest it may have potential as an exploratory screening tool for identifying subtypes of depression.