Mr May regained his sight in March 2000. He had never been inhibited by his blindness, and was an expert skier. Relying on verbal directions from a guide, he had become adept at navigating bumps and swerving around trees.
But the first time he went on to the slopes after his operation, he had to close his eyes, "as the visual information gave him a sense of imminent collision", the researchers report.
Mr May had learned to understand the world through touch, smell and hearing alone.
He still lives in a world of abstract shapes and colours, and has difficulty interpreting three-dimensional objects.
Researchers tested the firing of his brain cells using magnetic resonance imaging.
Some regions of his brain remained stubbornly inactive. Mr May can see movement and colour, and tell a square from a circle. But when he was shown common objects, he could only name one in four. In tests, he could only identify whether a face was male or female 70% of the time. He has difficulty recognising faces and interpreting facial expressions. He cannot recognise his wife by her face alone.
"The difference between today and two years ago is that I can better guess at what I am seeing," Mr May said. "What is the same is that I am still guessing. A day doesn't go by that I don't appreciate the visual details around me. I have been building my visual catalogue of those details, and although this catalogue is more significantly filled out than it was two years ago, there seems to be an infinite number of visuals to absorb."
The lesson is that seeing is not a simple skill. It involves different kinds of learning, and the cooperation of different areas of the brain. "The old idea that there is one picture of the world on the surface of the visual cortex is far too simple," said Donald MacLeod of the San Diego team. "In fact, we probably have a couple of dozen 'maps', each representing a different mode for sensing and taking in our environment."
