Michael Schumacher might be able to "cry at stories" and "move his thumbs" after his stem-cell operation, a top brain expert has claimed.

The seven-time Formula One world champion turned 50 in January but hasn't been seen in public since suffering a horrific skiing accident in December 2013.

On Monday, he was taken to the Georges Pompidou Hospital in Paris, where he will be given cutting-edge stem-cell treatment.

He is set to be treated at the state-of-the-art facility by Professor Philippe Menasche, a top surgeon specialising in stem-cell breakthroughs.

Schumacher has been in a coma since his accident in 2013 when he suffered catastrophic head injuries.

Michael Schumacher may be able to cry and move his thumbs after his latest stem-cell therapy
Michael Schumacher may be able to cry and move his thumbs after his latest stem-cell therapy
Schumacher has remained unconscious since suffering a horrific skiing accident in 2013
Schumacher has remained unconscious since suffering a horrific skiing accident in 2013

Skiing with his son Mick in the French Alps, he fell and cracked his head on a boulder.

He hit the right side of his head on the rock, splitting open his helmet, triggering blood clots in his brain.

Speaking to Msn, neurological expert Matilde Leonardi said Schumacher's condition may improve to the level where he can move his thumbs and even cry at sad stories.

She urged caution, however, admitting that stem-cell research is still in its early days when it comes to treating conditions of the brain.

The Georges-Pompidou Hospital in Paris, where Schumacher went for stem-cell therapy this week
The Georges-Pompidou Hospital in Paris, where Schumacher went for stem-cell therapy this week
Schumacher won seven Formula One World Championships
Schumacher won seven Formula One World Championships

Leonardi is head of neurology in the scientific department of the Carlo Besta Neurological Institute in Milan.

She said that although "very rare," there have been cases where patients' conditions have improved after years in a state of "minimum consciousness".

The professor went on: "Patients in Schumacher's condition open their eyes, they can turn their head when you call them, they can move their thumbs in response to a question, and they can cry at the end of a story."

But she stressed that there is often confusion between a vegetative state and the state of minimum consciousness.

Schumacher returned home after the treatment with his wife Corinna, left
Schumacher returned home after the treatment with his wife Corinna, left
Italian neurological expert Matilde Leonardi
Italian neurological expert Matilde Leonardi

Le Parisien said the F1 legend had been injected with secretome, called "stem-cell juice" by Professor Menashe, which is more commonly used to repair the heart.

The 69-year-old star surgeon is famous for performing the world's first embryonic cell transplant on a patient with heart failure in 2014.

Schumacher is now believed to have returned to his home in Gland, Switzerland, with his wife, Corinna.

His relatives do not discuss the racing star's condition in public, but this week, Schumacher's former boss urged the family to "tell the world the truth" .