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. ### Competing Interest Statement The authors have declared no competing interest.