Left frontal activity measured by fNIRS during a verbal memory task is modulated by predictability in cognitively healthy older adults.

Background: Functional near-infrared spectroscopy (fNIRS) is widely used to measure cognitive load during working memory tasks. However, little is known about how task structure modulates load-dependent neural responses. Objective: We examined how working memory load measured by a verbal memory task is represented in fronto-parietal hemodynamic responses during the encoding phase in cognitively healthy older adults and whether these responses are modulated by the predictability of task difficulty. Methods: Oxygenated hemoglobin (HbO) responses were measured using fNIRS while participants performed digit span trials with load levels presented in either increasing order or randomized sequences. Results: Mixed-effects modeling across all participants and fronto-parietal regions of interest revealed a significant main effect of load levels, with HbO responses during the encoding phase increasing linearly with increasing memory load. However, load sensitivity differed by task structure: the random group showed a significantly steeper load-related increase in HbO responses than the non-random group, driven primarily by the left frontal cortex. Behavioral accuracy remained stable across load levels. Conclusions: These findings indicate that fronto-parietal encoding-phase hemodynamic responses reflect working memory demands in a context-dependent manner, revealing neural sensitivity beyond overt behavioral performance.