A new study completed at the University of California, San Diego describes the effects of long-term blindness on the human visual system.
"The Effects of Long-Term Deprivation on Visual Perception and Visual Cortex," will appear in the Aug. 25 online issue of Nature Neuroscience and in the September printed edition of the journal. The collaborative study was led by Ione Fine and Donald MacLeod in the UCSD Psychology Department, and combined psychophysical and neuroimaging techniques to measure the effects of long-term blindness on the visual cortex of the brain. Other researchers involved in the study include Alex Wade, Alyssa Brewer and Brian Wandell, Stanford University and Geoffrey Boynton, Salk Institute.
The two-year study focused on the experiences of Michael May, who, after having been completely blind since the age of 3 1/2, successfully underwent an experimental limbal stem cell transplant in his right eye at the age of 43. After regaining his sight in March of 2000, May, like many others who have regained their sight after decades of blindness, could see the world, but could not interpret what he was seeing. Two years after surgery, he still lives in a world of abstract shapes and colors instead of the environment of recognizable objects with three-dimensional shapes that normally sighted people take for granted.
While several neurologists, among them Oliver Sacks, have documented the frustration experienced by those undergoing sight restoration after long-term blindness, cases of restored sight are so rare that this research represents the first comprehensive study of this phenomenon.
"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 later in life," said Fine. "What we didn't know was what was going on in the visual system that was causing this to happen."
The study found that May was able to detect motion and color, but had a difficult time identifying objects, and was especially challenged by faces and facial expressions. Fine, MacLeod and their co-authors suggest that some visual mechanisms such as motion processing are more hard-wired than others and may develop early in infancy. Complex form processing, including object and face recognition, may develop later in life, and consequently remain virtually undeveloped in individuals like May who lose their sight at an early age. Complex form processing may take a long time to develop, the study postulates, because individuals need to recognize novel objects and faces throughout their lives.
"The old idea that there is one picture of the world on the surface of the visual cortex is far too simple," said MacLeod, who specializes in visual perception. "In fact, we probably have a couple dozen maps, each representing a different mode for sensing and taking in our environment."
Brain (fMRI) scans--which track activity in an individual's brain while a function is being performed--revealed that when May viewed faces and three-dimensional objects the part of his brain which is normally responsible for processing faces and objects was inactive. But, when he was shown an object in motion, the motion-detection part of his brain showed a high level of activity.
Two years after the operation, May continues to grapple with the same visual challenges as he did immediately after his sight was restored. However, he has made noticeable progress in interpreting motion and shading cues. For example, May-- who was never inhibited by his blindness--had been an expert skier. Using verbal directions from a guide, he had become amazingly adept at navigating bumps and trees.
Immediately after his operation, however, he closed his eyes when skiing because the overwhelming sensation of motion gave him a frightening sense of imminent collision. After three years of visual experience he can now ski with his eyes open, and has begun to use the patterns of light and shade to estimate the shape of the ski slope.
However, even after three years of vision, May still has great difficulty in identifying faces and facial expressions as well as more complex, multidimensional objects. He still, for example, cannot identify his wife by her face alone. To compensate for these limitations, he relies on other less complex visual clues such as the length of a person's hair, their gait, or the shape of their eyebrows. "The difference between today and two years ago," said May, "is that I can better guess at what I am seeing. 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 catalog of these details and although this catalogue is significantly more filled out than it was two years ago, there seems to be an infinite number of visuals to absorb."
The case study of Michael May, said MacLeod, reveals the many layers of complexity involved in perceiving and interpreting the world around us. A brain that has been programmed to navigate the world through touch, taste, smell, and sound will be challenged by the introduction of a new and very foreign sense – vision.
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