|Seeing with New Sight
Vision abilities and deficits after forty years of blindness
By Melissa Lee Phillips |
Neuroscience for Kids Consultant
February 23, 2004
In his book An Anthropologist on Mars, neurologist Oliver Sacks
tells the story of his patient Virgil, a man with blinding cataracts. In the beginning of the story, Virgil is
in his early fifties, and he has just had his cataracts removed. His
retinas are absorbing their first images in nearly half a century. When
Sacks visits Virgil, he finds someone who is trying to learn to see.
Virgil could see color and bold motion immediately after his surgery;
more subtle aspects of vision, however, continued to elude him.
"Virgil's cat and dog bounded in to greet and check us--and Virgil, we noted, had some difficulty telling which was which. This comic and embarrassing problem had persisted since he returned home from surgery: both animals, as it happened, were black and white, and he kept confusing them--to their annoyance--until he could touch them, too...Further problems became apparent as we spent the day with Virgil. He would pick up details incessantly--an angle, an edge, a color, a movement--but would not be able to synthesize them, to form a complex perception at a glance. This was one reason the cat, visually, was so puzzling: he would see a paw, the nose, the tail, an ear, but could not see all of them together, see the cat as a whole."
When this book was published in 1995, the story was extremely rare; very few cases like Virgil's had ever been documented. Vision restoration in an adult has not become much more common in the past nine years. Certain emerging technologies, however, are hinting that some visual repair is possible. Scientists and surgeons are slowly learning how to remove constraints on the eye's ability to see; unleashing the brain's ability to see is another story. In the September 2003 issue of Nature Neuroscience, neuroscientist Ione Fine, at the University of California in San Diego, and her colleagues reported a case study similar to Virgil's. Their subject, whom they call "MM," was blinded in his right eye and lost his left eye completely when he was only three and a half years old. The cause of right eye blindness was chemical and thermal damage to the cornea, the clear tissue that covers the central part of the eye, including the pupil and the colored iris. When MM was still a child, doctors attempted to transplant a healthy cornea into his damaged right eye, but the transplant was unsuccessful. There was too much damage to his limbus, an area of the eye next to the cornea that is essential for proper corneal functioning.
Growing up, MM could perceive a small amount of light in his environment (as could Sacks's patient Virgil), but he had no form or contrast perception whatsoever. He would notice if the room he was in suddenly changed from dark to bright, but he could not discern the shapes of objects in his surroundings, nor could he tell where one object ended and another began.
Forty years after he was blinded, surgeons performed a stem cell transplant in MM's right eye. Stem cells are the least developed type of cell; they have not yet chosen what type of adult cell to become. They can be coaxed by chemicals in their cellular environments to become whatever type of cell the body area needs, and they can then integrate themselves into existing tissue. MM's physicians implanted stem cells from another person's healthy cornea and limbus. Soon, these new stem cells began to grow and move into MM's cornea, where they developed into mature corneal cells, replacing his old, damaged cells.
Immediately after this surgery, MM reported that he could see simple shapes. Today, he seems to have essentially no deficits in recognizing simple forms. He can easily report the orientation of a bar: vertical, horizontal, or any diagonal in between. And he can consistently identify orientation changes in a bar, even if the variations between bars are very slight. Immediately post-surgery, he also began to see and recognize colors very easily. These developments alone are amazing in a person who had seen nothing but vague impressions of light and dark for 40 years. As Fine as her co-workers continued to test MM's sight, however, certain deficiencies became evident. Although his ability to detect contrast at low spatial frequencies -- if the contrasting pieces were each very large -- was nearly as good as the ability in people with normal eyesight, his ability to detect high spatial frequencies -- contrasting colors in very thin stripes -- was substantially impaired. If presented with a panel like the square farthest to the left:
MM would most likely detect both blue and black bars. But, if presented with the square farthest to the right, he would probably be unable to distinguish purple bars from black ones. MM's eyesight reveals shortfalls in vision tasks that others find easy, such as picking coherent patterns out of random noise, or seeing that line segments could form one straight line if they were connected:
MM also does not easily identify implied shapes. For example, most of us would probably say that both images below contain a white square in the middle. MM only sees the square in the picture on the right. If the square's outline is missing, he is unable to interpret its implied presence:
He has incredible difficulty identifying complex shapes (like most objects that we encounter in day-to-day life) and faces. By far the most difficult tasks for MM involve three-dimensional interpretation of his environment. When an image is projected onto the retina, it is two dimensional, because the retina is essentially flat. When we are very young, our brains learn to use depth cues, such as shadows and line perspective, to see the three-dimensional world. Eventually, incorporating these cues into a coherent picture of the world becomes involuntary.
Our ability to judge size correctly is one example of the brain's reinterpretation of two-dimensonal images. When a person walks away from us, the image of her becomes smaller and smaller on our retina. We know that people do not actually shrink as they move away, however. The brain combines the shrinking retinal image with perspective and depth cues from the surroundings, and we "decide" that the person is moving away.
When MM lost his sight when he was three years old, his brain probably had
not yet constructed the connections that incorporate separate perceptions
into one combined perception. When a person walks away from MM, he has to
remind himself that the person is not actually shrinking in size! MM's
difficulties with three-dimensional interpretations are also obvious from
his explanations of drawings.
Additionally, he does not automatically integrate shading cues into his perceptions of objects. People who have had normal eyesight since birth usually say that, in the below drawing, the middle shape in the bottom row is caving inward (concave), and the rest of the shapes are protruding outward (convex).
We interpret these drawings in this way because our brains assume that a light source (maybe the sun) is coming from above. So if the shape is brighter on the top, that part must be protruding outward, into the sun's rays. If the shape is brighter on the bottom, then the top is in shadow and therefore caving inward, out of the sun's light. The researchers report that MM can sometimes answer a problem like this correctly, but that he seems to be reasoning explicitly about the light source and the resulting shadows, rather than seeing the objects' shapes instantly and automatically.
Post-surgery, physicians examined MM's right retina for any abnormalities. They found no retinal degeneration, and his retina's electrical responses were normal. His retina did not seem to be the cause of his vision deficiencies. In fact, his problems didn't seem to be vision deficiencies so much as visual interpretation deficiencies. And deficiencies of this sort lie not with the retina's ability to perceive light and color, but with the brain's ability to process the retina's signals correctly. We usually do not think of the above problems as involving interpretation, because we have performed these interpretations so many times, and from such a young age. But since MM lost his sight at an early stage of development, since he had no visual input into his brain after age three, the researchers suspect that the visual centers in his brain did not develop normally -- and now, they likely never will.
Motion processing develops very early in infancy compared with form processing. By the time MM lost his eyesight in the accident, the motion centers in his brain were probably nearly complete. So when he regained some eyesight in his forties, those connections in the brain were ready to go. The parts of the brain that process complex shapes, however, do not develop until later in childhood, so MM's brain likely missed its chance to establish those particular brain connections. The authors also propose that our brains may retain the ability to modify and refine complex form identifications throughout life, not just throughout childhood. New objects and faces are continually encountered throughout life, and our visual processing centers must be able to adapt and learn to see new shapes and forms. MM's brain never had the chance to learn.
MM's visual capacities continue to improve, but he also remains somewhat uncomfortable with his new sense. As a blind person, MM became extremely proficient at skiing, with the help of a guide to give him oral directions. After his eyesight was restored, skiing frightened him. The trees, snow, slopes, people -- everything whizzed by him, chaotic and uninterpretable. After much practice, he is now a moderate sighted skiier -- but when he really wants to go fast and feel confident, he closes his eyes.
Even long after his cataract surgery, Virgil seemed to prefer to identify and characterize objects by their feel. Sacks watches him one day has he runs his hands over a statue:
"Exploring it swiftly and minutely with his hands, he had an air of assurance that he had neer shown when examining anything by sight. It came to me -- perhaps it came to all of us at this moment -- how skillful and self-sufficient he had been as a blind man, how naturally and easily he had experienced his world with his hands, and how much we were now, so to speak, pushing him again the grain: demanding that he renounce all that came easily to him, that he sense the world in a way incredibly difficult for him, and alien."
Although Virgil, MM and others like them certainly possess a rudimentary form of vision, decades of visual deprivation may never be completely redeemable. The human brain has an amazing capacity for plasticity, but there are some things that it cannot do. MM will likely never see the way that we see.
For references and more information, see:
|GO TO:||Neuroscience In The News||Explore the Nervous System||Table of Contents|