A 35-year-old man who had been in a vegetative state for 15 years is showing signs of consciousness after receiving a pioneering treatment based on nerve stimulation.

In the month since a vagus nerve stimulator was put into his chest, the man, who was injured in a car accident, has begun responding to simple orders that had been impossible before.

The findings reported in Current Biology may help to show that by stimulating the vagus nerve "it is possible to improve a patient's presence in the world", according to lead researcher Angela Sirigu of Institut des Sciences Cognitives Marc Jeannerod in Lyon, France.

The researchers say it may challenge the view that a vegetative state which lasts for more than 12 months is irreversible.

"Other scientists have hailed it as "a potentially very exciting finding" but have also urged caution.