People who gain sight after long-term blindness are often frustrated by difficulties in interpreting what they see. Now scientists have taken advantage of a unique opportunity to study the recovery of vision in a man who had his sight restored after 40 years of blindness. Their findings begin to reveal how the brain copes with the sudden torrent of visual information.
Mike May lost one eye and was blinded in the other at age 3 after playing with an explosive mixture of water and calcium carbide. In 2000 he had a cornea transplant operation that restored vision to his remaining eye. Neuroscientist Ione Fine at the University of California, San Diego, and colleagues met with May 15 times over the next two and a half years. At each meeting, they examined his ability to detect motion, depth, and form. During some tests, researchers used functional magnetic resonance imaging to monitor activity in the visual areas of May's brain.
At the beginning of the study, May could detect color and motion reasonably well but he had extreme difficulty recognizing complex forms such as 3D images, faces, and facial expressions. May's brain activity paralleled his performance on the tests: Brain regions that process visual motion lit up in the imager when May was faced with a motion discrimination task, but object recognition areas were conspicuously quiet when he had to identify objects. Over the course of the study, May's motion perception got even better, and his ability to distinguish shadows from the objects that create them improved. However, his object recognition and other aspects of visual perception did not improve, Fine and colleagues report online 25 August in Nature Neuroscience.
Fine, who is now at the University of Southern California in Los Angeles, says the findings suggest that the ability to discern motion and color is little affected by the loss of sight. The ability to recognize complex objects, on the other hand, seems more dependent on visual experience. The brain regions that process motion and color may be wired up earlier in life than those needed for complex form processing, she speculates.
It's a "beautiful" study that reinforces and expands the findings of scattered cases over the past two centuries, says neurologist Oliver Sacks of New York University. He says he's surprised, however, by the evidence that motion perception is hard wired, because previous animal studies have shown that visual experience is important for activating motion perception.