A man whose sight was restored after 40 years of blindness had to shut his eyes when he began skiing again, according to new research today.
A study in the journal Nature Neuroscience throws new light on one of evolution's darker mysteries - how the visual system develops.
Sight is not just a matter of lens, retina and optic nerve: the brain must learn to make sense of the assault by differing wavelengths of reflected light, at varying intensities.
Michael May, who went blind at three but regained the use of his right eye after a stem cell transplant at the age of 43, provided the chance for psychologists from the University of San Diego, Stanford University and the Salk Institute in California to make the first comprehensive study of a journey from darkness to light.
"What we knew going into this research was that people who regained their sight later in life - after many decades of blindness - seemed to experience more difficulties in adapting to and functioning in the visual world than did those individuals who lost and regained their sight in later life," said Ione Fine of the University of San Diego.
"What we did not know was what was going on in the visual system that was causing this to happen."
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."
