Miika Terho, 46, began losing his eyesight as a teenager and was completely blind when he joined a pilot study to test the experimental eye chip at the University of Tubingen in Germany.

Terho was one of three patients who had the chip inserted under part of the retina called the macula, where the highest concentration of light-sensitive cells are found. Terho performed particularly well after the implant.

Terho , who lives in Finland, developed a condition called retinitis pigmentosa, a hereditary disease that causes light-sensitive cells in the eye steadily and irreversibly to die off.

His night vision began to fail when he was 16 and he was severely blind in both eyes by 35. Unaided, he now has just enough peripheral vision to tell night from day.

The chip was connected via thin wire to a battery that the patient wore on a necklace. It contains 1,500 light-sensitive elements that replace the defunct cells in a blind patient's retina. When an image hits the chip, it is converted into electrical pulses that stimulate healthy cells in the retina.

These cells send signals to the brain, where the image is reconstructed. — © Guardian Newspapers Limited, 2010