Prefrontal tDCS modulates behavioural and neural signatures associated with consciousness

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INTRODUCTION: Transcranial direct current stimulation (tDCS) has shown encouraging results in patients with Disorders of Consciousness (DoC), yet its efficacy and mechanisms remain unclear. This study aimed to evaluate the effects of left dorsolateral prefrontal cortex (DLPFC) tDCS on Coma Recovery Scale-Revised (CRS-R) scores and EEG activity in severely brain-injured DoC patients. METHODS: In a double-blind, sham-controlled crossover design, 2 mA anodal and sham tDCS were randomly administered over the left DLPFC (right orbito-frontal cathode) for 20 min. CRS-R scores, event-related potentials using the local-global paradigm and resting EEG were recorded before and after each stimulation. Follow-up assessments occurred at 6 weeks and 6 months. RESULTS: Between November 2018 and April 2022, 20 patients were included (12 in a Minimally Conscious State (MCS), 7 in a Vegetative State/Unresponsive Wakefulness Syndrome (VS/UWS), and one patient who emerged from MCS (emergence from MCS (eMCS)). Group-level analysis across all patients revealed a significant treatment effect on CRS-R scores (p = 0.02). Nine patients (45%; 6 MCS, 3 VS/UWS) responded to tDCS, with five (25%; 3 MCS, 2 VS/UWS) exhibiting new behavioural items absent in pre- and post-sham evaluations. However, no patient demonstrated a change in consciousness state (e.g., transition from VS/UWS to MCS), nor was any improvement observed at 6-week or 6-month. EEG analysis showed increased complexity, gamma-band power, and delta-band cortico-cortical functional connectivity post-tDCS. Multivariate EEG analyses did not show a significant effect of tDCS on the prediction of conscious state, and ERP assessments using the local-global paradigm revealed no modulation of novelty detection responses following stimulation. Simulation analyses on individual brain anatomical structures revealed that responders displayed stronger tDCS-induced electric fields within a Fronto-parietal network than non-responders. CONCLUSION: Despite the limited sample size, our results provide additional evidence in favour of the behavioural and electrophysiological effects of tDCS in DoC patients.

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