IntroductionTheta-band transcranial alternating current stimulation (tACS) has been proposed to enhance working memory (WM) by entraining endogenous oscillations, yet the haemodynamic signatures accompanying theta-tACS–related WM gains remain unclear. We investigated whether 6 Hz tACS modulates prefrontal task-evoked haemodynamic responses during WM and whether such changes relate to behavioral improvement.MethodsIn a randomized, single-blind, sham-controlled design, healthy adults (18–30 years) were allocated to 6 Hz tACS or sham (initially n = 25 per group). Stimulation (1mA peak-to-peak, 20min, 30-sramp-up/down) was delivered bilaterally over the dorsolateral prefrontal cortex (DLPFC; F3/F4). Participants completed the Digit Span Test and N-back tasks (1-, 2-, 3-back) before and after stimulation. During N-back performance, fNIRS recorded bilateral DLPFC signals. Task-evoked oxygenated hemoglobin (HbO) β coefficients were estimated using a general linear model. Group differences in pre–post changes were tested using within-/between-group analyses and ANCOVA, with brain–behavior coupling assessed via Spearman correlation.ResultsAfter fNIRS quality control, 44 participants were included in the final analysis; six participants were excluded because more than 30% of channels were identified as low quality. Relative to sham, 6 Hz tACS produced greater improvements in backward Digit Span and total Digit Span. In the N-back task, accuracy improved selectively under the high-load 3-back condition (between-group change: [F(1.40) = 12.29, p_adj = 0.0034, η2p = 0.24)], whereas reaction time showed no significant between-group differences. fNIRS revealed post-stimulation increases in left DLPFC HbO-β during 3-back, with a significant between-group β-change at one channel [(F(1.40) = 10.69, p_adj = 0.035)]. Within the 6 Hz group, 3-back accuracy gains correlated positively with β-change in the left DLPFC (ρ = 0.44, p_adj = 0.039).ConclusionBilateral 6 Hz tACS selectively enhances high-load WM accuracy and is accompanied by increased task-evoked haemodynamic activation in the left DLPFC. The observed brain–behavior coupling suggests that theta-frequency neuromodulation may facilitate executive control under high cognitive demand via strengthened prefrontal neurovascular responses.
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