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Multi-session tDCS paired with passive mobilisation increases thalamo-cortical coupling during command following

Background Therapeutic options for patients with prolonged disorder of consciousness (PDOC) are limited. PDOC patients often exhibit a dissociation between their retained level of (covert) cognitive ability and their (overt) behavioural responses (cognitive-motor dissociation; CMD). This is linked to reduced coupling between thalamus and the primary motor cortex. Objective To assess whether pairing tDCS with a concurrent passive mobilisation protocol (designed to be feasible in PDOC) can influence thalamo-M1 dynamics and whether these changes are enhanced after multiple stimulation sessions. Methods We used Dynamic Causal Modelling (DCM) on functional magnetic resonance imaging (fMRI) data from 22 healthy participants to assess tDCS changes on effective connectivity within motor network areas during command-following. Results We found that a single anodal tDCS session (paired with passive mobilisation of the thumb) decreased self-inhibition in the motor cortex, with five sessions further enhancing this effect. In addition, anodal tDCS increased thalamo-M1 excitation as compared to cathodal stimulation, with the effects maintained after 5 sessions. In turn, cathodal tDCS had opposing effects on these connections after one session but became more similar to anodal after 5. Conclusions Together, our results suggest that pairing anodal tDCS with passive mobilisation across multiple sessions may facilitate behavioural command-following in PDOC patients with CMD. More broadly, they offer a mechanistic window into the neural underpinnings of the cumulative effects of multi-session tDCS.