Minimally Conscious (MCS) or Vegetative State (VS) are disorders of consciousness which often occur following traumatic brain injury or ischemia. These alterations result most of the time in patients' loss of autonomy and require long years of special care. No efficient therapy to improve patients' consciousness has been found so far. Investigators propose to use vagal nerve stimulation (VNS) to restore cortical activity and patients' embodied self. The investigators' main hypothesis is that VNS will reestablish the thalamo-cortical connectivity leading to an improvement of the consciousness state. To test this hypothesis, investigators will use behavioral measures as well as fMRI, PET scan and EEG to assess brain activity. Patients will be evaluated before and during eight months following implantation of the stimulation device.