 This study will test the effects of two intensities of tDCS stimulation either 0.2mA or 2mA applied on left dorso-lateral prefrontal cortex on both behavior, - assessed by the Coma Recovery Scale-Revised CRS-R scores -, as well as quantified EEG recorded during resting state using algorithms previously designed and published by the investigators and event-related potentials using auditory paradigms we previously published in severely brain damaged patients with disorders of consciousness MCS, VS, and conscious but cognitively disabled patients of various etiologies. 