Learning to See Requires More Than Just Eyesight
Although most people take it for granted, learning how to see is a very difficult task. An intriguing case study published in the September issue of Nature Neuroscience describes a man's recovery from 40 years of blindness and should help scientists better understand how the human visual system functions.
Ione Fine of the University of California at San Diego and her colleagues followed Michael May, a 43-year-old man who had been blind since the age of three and a half, as he recovered from experimental stem-cell surgery. The procedure restored sight to his right eye in March of 2000. Ever since, he has been struggling to adapt to a viewable world, a common problem for people who have regained their sense of vision after years of blindness. May finds it particularly difficult to interpret faces and facial expressions--during testing, he could correctly identify a face as male or female only 70 percent of the time, and expressions as happy, neutral or sad 61 percent of the time. In addition, seeing only the face of his own wife is still not enough for him to identify her, and he relies on clues such as hair length or gait to help him recognize people.
To determine what causes these difficulties, the scientists used functional magnetic resonance imaging (fMRi) to track activity in May's brain as he processed the world around him. Although May's ability to perceive simple forms, colors and motion is essentially normal, the investigators found that when he looks at faces or three-dimensional objects, the brain region active in sighted people during identification is not utilized. This suggests that different parts of the visual system develop at different times, the authors note, with motion processing being more hard-wired and forming very early in life. "The old idea that there is one picture of the world on the surface of the visual cortex is far too simple," remarks study co-author Donald I. A. MacLeod of the University of California at San Diego. "In fact, we probably have a couple dozen maps, each representing a different mode for sensing and taking in our environment." As for May, he is slowly coming to terms with his sight. "The difference between today and two years ago is that I can better guess at what I am seeing," he says. "What is the same is that I am still guessing." --Sarah Graham [Scientific American August 25, 2003 ]