Effects of four days of lower limb motor skill learning and concurrent tDCS on cortical directed connectivity

Motor skill practice reshapes interactions among cortical regions involved in motor planning, action selection, and sensorimotor control. Transcranial direct current stimulation (tDCS) may modulate these interactions when paired with practice. We investigated whether four days of lower-limb motor skill practice, with or without concurrent tDCS over the contralateral primary motor cortex, alters directed cortical connectivity and whether such changes are related to motor performance. Participants were randomized to skill practice with concurrent tDCS (SKILL-STIM), skill practice with sham stimulation (SKILL-SHAM), and sham stimulation during non-skill control movements (CON). Motor performance was assessed using a sequential visuomotor ankle-joint speed-accuracy task and a transfer accuracy task involving 2-min steady dorsiflexion. Electroencephalography (EEG) during steady dorsiflexion was analyzed using spectral Dynamic Causal Modeling (DCM) and Parametric Empirical Bayes (PEB) to estimate group-level changes in effective connectivity within a fronto-parietal-premotor network. Both skill practice groups improved and outperformed the non-skill control group with no additional learning benefit from active stimulation. At baseline, higher accuracy was associated with stronger forward coupling from the dorsolateral prefrontal cortex to dorsal premotor cortex, and baseline-only PEB analysis indicated group-related differences in this coupling-performance relationship. The full PEB model showed that practice increased premotor-parietal coupling without a proportional accuracy change, whereas accuracy gains covaried with prefrontal-premotor coupling. tDCS paired with practice increased prefrontal-premotor coupling over time and was consistent with a condition-dependent sign difference in the premotor-prefrontal coupling-performance relationship. Overall, four days of practice modified directed connectivity within fronto-parietal-motor pathways, while concurrent tDCS further modulated prefrontal-premotor coupling without enhancing learning.