The first useful artificial eye is now helping a blind man walk safely around and read large letters, researchers said on Monday.

The 62-year-old man, identified only as Jerry, demonstrated that he was able to collect a black hat off a white wall, then locate a mannequin and place the hat on its head.

The specks inside the box are what Jerry sees

Jerry, blinded by an accident at the age of 36, does not see an image but up to 100 specks of light that appear and disappear as his field of vision shifts. The specks show up the edges of objects. The constellation of dots, called phosphenes, has been likened to stars that glitter and fade behind passing clouds.

"He can do remarkably well," said Dr William Dobelle, chairman of the Dobelle Institute, the US medical device company that has developed the system.

Richard Normann, an artificial vision expert at the University of Utah, stressed that the device was "a very limited navigational aid, and a far cry from the visual experience that normal people enjoy".

However, he said the work suggested that even limited signals to the brain would let blind people do relatively complicated visual tasks.

'Major step forward'

Dr Bill Heetderks, director of a US National Institutes of Health programme to develop electronic implants that work with the brain, believes an implant that helps blind people navigate would be a major step forward.

"When Dr Dobelle provides additional details on his methodology that establishes this result, we may be there," Dr Heetderks said. The device and its performance are described in the January/February issue of the American Society of Artificial Internal Organs Journal.

The artificial vision system works by taking an image from a miniature television camera and distance information from an ultrasound sensor, each of which is mounted on one lens of a pair of sunglasses.

These signals are processed by a five-kilogram (10 pounds) portable computer and then a new signal is sent to 68 platinum electrodes implanted in the person's brain. The electrodes are on the surface of the brain's visual cortex and stimulate the person to 'see' the phosphenes.

Jerry has recognised five-centimetre-tall (two inches) letters from 1.5 metres (five feet) away, said Dr Dobelle. The device provides "tunnel vision", with the field of view being equivalent to a card 20 cm tall by five centimetres wide (eight by two inches) held at arm's length.

Despite this, Dr Dobelle said that Jerry had been able to navigate in unfamiliar environments, including the New York City subway system. He added that , with a special electronic interface taking the place of the camera, Jerry was also learning to use a computer.

The computer software detects the edges of objects

Jerry, along with another volunteer, had the electrodes implanted in his brain in 1978. Since then the computer used has shrunk in size and the software improved.

One aspect of future development will focus on how best to provide depth perception - during the demonstration Jerry had to walk cautiously as he approached the mannequin and the wall, holding out an arm to prevent collisions.

Dr Dobelle said a version of the device should go on sale outside the US later this year.