A blind man had his vision restored by an experimental stem cell treatment -- going from 20/400 to 20/40.

Intermediate age-related macular degeneration in a human eye. (Credit National Eye Institute/National Institutes of Health)

People undergoing an experimental stem-cell treatment have experienced modest improvements in their vision since the trial began in 2011, but one man had his vision restored from 20/400 -- basically blind -- to 20/40.

"There's a guy walking around who was blind, but now can see," said Gary Rabin, CEO of Advanced Cell Technology, the Massachusetts company that devised the treatment. "With that sort of vision, you can have a driver's license."

The man was taking part in a trial examining the safety of using human embryonic stem cells (hESCs) to reverse two common causes of blindness, reports New Scientist.

In all, ACT has treated 22 patients who either have dry age-related macular degeneration, a common condition that leaves people with a black hole in the centre of their vision, or Stargardt's macular dystrophy, an inherited disease that leads to premature blindness.

In both diseases, people gradually lose retinal pigment epithelial (RPE) cells. These cells recycle protein and lipid debris that accumulates on the retina, and supply nutrients and energy to photoreceptors -- the cells that sense light and transmit signals to the brain.

The company is testing treatments for both conditions by turning hESCs into new RPE cells, then giving each trial participant a transplant of the cells beneath the retina in one eye.

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Blind man's sight restored by stem cell treatment