Vagus nerve stimulation involves delivery of electrical pulses to the vagus nerve, which plays an important role in waking, alertness and other essential functions. The treatment is already used to treat epilepsy and depression and, in this latest work, the researchers tested its ability to restore consciousness. To ensure that any improvements couldn't be explained by chance, they deliberately chose a difficult case: a 35-year-old man who had been in a vegetative state for more than a decade with no sign of improvement.

The researchers performed behavioural, electroencephalographic (EEG) and 18F-FDG PET measurements before and after surgical implantation of a vagus nerve stimulator into the patient's chest. After one month of stimulation, the patient's attention, movements and brain activity significantly improved. He began responding to simple orders, such as following an object with his eyes and turning his head upon request.

After stimulation, the patient also exhibited "threat" responses that were previously absent. For instance, when the examiner's head suddenly approached the patient's face, he reacted with surprise by opening his eyes wide. After many years in a vegetative state, he had entered a state of minimal consciousness.

EEG data recorded after vagus nerve stimulation revealed a significant increase in the theta band signal, a brain signal found to reliably distinguish minimally conscious patients from vegetative ones. The treatment was also seen to increase the brain's functional connectivity. PET results corroborated the EEG findings by showing extensive increases in metabolic activity in both cortical and subcortical regions of the brain, as early as three months after implantation of the stimulator.

These findings demonstrate that stimulation of the vagus nerve promoted the spread of cortical signals and caused an increase of metabolic activity leading to behavioural improvement. "Brain plasticity and brain repair are still possible even when hope seems to have vanished," said corresponding author Angela Sirigu, from Institut des Sciences Cognitives Marc Jeannerod.

"Our study demonstrates the therapeutic potential of vagus nerve stimulation to modulate large-scale human brain activity and alleviate disorders of consciousness," the authors write. They are now planning a large collaborative study to confirm and extend the therapeutic potential of vagus nerve stimulation for patients in a vegetative or minimally conscious state.