05-02-2007, 07:26 AM
I just found a stash of 'National Geographic' books in my loft, and one of the articles is called 'The Sense of Sight', from volume 182, No.5, November 1992. I found it quite interesting.
[I'll type out the rest later]
Quote:THE SENSE OF SIGHT
In the twinkling of an eye - mere trillionths of a second - the latest surgical lasers cut through tissue to restore sight, the magical sense that translates light in to the colours and shapes of our surroundings.
Precious in function, gemlike when magnified, this complex organ drives a visual system that makes billions of calculations every second
A millimeter can make the difference between sight and blindness during microsurgery inside the eye at Johns Hopkins Hospital.
THE LIGHT coming closer to my eye is piercingly bright, brighter than the white-hot ingots I'd tonged in an Ohio steel mill 40 years ago. The voice behind the light is that of Harry A. Quigley, an ophthalmologist at the Wilmer Eye institute in Baltimore, Maryland. A blend of efficiency and empathy, Quigley maneuvers the light - called a slit lamp in his profession - to illuminate the interior of my right eye, then the left.
I have asked him for this eye examination, explaining that I haven't had one for years and that it would be a good way to begin research for this story on human vision that you are now reading. but I don't tell Quigley the truth. I am concerned because I have spent the day watching him minister to patients with glaucoma - a disease caused by excessive pressure within the eye, like too much pressure inside a tire. For the first time in my life I have been in the company off people whose ability to see is severely impaired.
From behind the light Quigley observes and abnormality on my retina, the thin sheet of neural tissue at the back of the eye. He makes a phone call, and I am on my way upstairs to see Susan B. Bressler, a retina specialist. Again the light. Bressler's diagnosis stuns. I have a disease called macular degeneration.
Bressler characterizes this as a deterioration in a tiny circular area in the retina called the macula. At the bull's eye of this circle - the fovea - lies a tightly packed array of specialized photoreceptor cells that provide us with the fine-tuned central vision essential for everyday life. With foveal vision we are able to recognize the faces of friends or foes, write a letter, drive a car, watch TV, or read books, newspapers, signs, or recipes. Without it, having to rely on our much less acute peripheral vision, we can do none of the above and are legally blind.
This macular degeneration is the leading cause of blindness in people older than 50 in the Western world. In its "dry" form, the kind I have, for which there is no treatment, the disease could lead to a gradual, inexorable loss of central vision over the years. The gradualism is the good news. Bressler cautions that I have a risk of the disease progressing to the "wet" form, in which rogue blood vessels erupt under the macula, degrading vision in a matter of days. Begun promptly, laser treatment can retard vision loss in half the cases, at least temporarily. But when vessels erupt beneath the lesses, loss of central vision is inevitable.
A recent strategy, as yet unproven, might reserve this frightening scenario in the future. Henry J. Kaplan, head of opthalmology in the Washington University School f Medicine in St. Louis, envisions surgery to remove the scar tissue left by the invasive vessels. Then, in a procedure pioneered by neurobiologist Martin S. Silverman of the nearby Central Institute for the Dead, tissue from the rear of the eye would be transplanted to nourish the photoreceptors.
As I leave the Wilmer in the darkening tall afternoon, I focus on the faces of people queued for taxis. I realize that my sight, once good enough for me to join that band of aviators who operate from aircraft carriers, is a temporary gift now up for withdrawal with what seems to be a roll of the dice.
The fear of that withdrawal, the loss of our sight, is a primal fear. According to polls sponsored by Research to Prevent Blindness, an organization in New York City that promotes eye research, Americans dread blindness more than any other disability. To be blind is to lose access to the most significant part of our perceptual world.
As much as a third of the highest level of our brain, the cerebral cortex, is devoted to visual processing. Our eyes funnel two million fibers into the optic nerves, while the auditory nerve, the conduit for hearing, carries a mere 30,000 fibers. Sight mediates and validates the other senses; when we hear, smell, or touch something, we usually turn to see it also. Before we eat food, we inspect it.
In a profound and mysterious alchemy, sight combines with memory to energize the will. After his return from more than six years of captivity in Lebanon, American hostage Thomas Sutherland related that he had tried to commit suicide three times, but that each time, "The vision of my wife and three daughters appeared before," and he could not follow through.
Sight is the sense most intensely studied by brain researches and the best understood.But any of them will tell you they are still on the ground floor of a building that may be tens of stories high. Try this where you're sitting. Look at an object and then turn away so you do not see it. reach out and you will, most likely, be able to touch it. The manufacturers of industrial robots would like to impart this ability to their creations, but neuroscientists do not understand it.
Nor do they fully understand blindsight, the raw ability of certain people blinded by brain damage to act on visual information. In tests some distinguished between colours, without being able to explain how. Apparently their optic fibers bypassed the primary visual cortex to communicate with other areas.
Sight enlivens our language. We celebrate poets for their "insight." The politician we acclaim today for "foresight" may tomorrow, in "hindsight," be abhorred. For the skeptic, "Seeing is believing." For the forgetful, "Out of sight is out of mind."
And blind people, who have lost this precious gift, become the most poignant reminders of our fear.
In pursuing this story, I have learned that most blind people have succumbed that fear, and many are busily building careers. In the course of months of research from San Fransisco to Moscow, I have also glimpsed the many-faceted dimensions of ours visual universe: How nature devises different solutions to the task of seeing, how the brain transforms photons of light in the images we see, how visual illusion deceive us, how the developing world is stricken with eye diseases that we in the industrialized world cannot imagine.
I also learned that important medical breakthroughs are forthcoming. Looking to the year 2000, Carl Kupfer, director of the National Eye Institute, part of the National Institutes of health in Bethesda, Maryland, predicts advances in several areas, including:
-Cataracts, lens opacities that prevent light from reaching the retina. New drugs will slow the development of these opacities that are the leading cause of blindness worldwide.
-Retinitis pigmentosa. researchers are now beginning to unravel genetic clues to this family of diseases that ravage photo receptors. As understanding advances, strategies will develop to retard the disease.
-Optic-nerve regeneration. The stunning ability of proteins to regenerate damaged spinal nerves in experimental animals has also been demonstrated in the optic nerve, typically affected by glaucoma and other diseases.
As founding director of the National eye Institute, which funnels most of our federal tax dollars that support eye research, Kupfer looks back with satisfaction at recent clinical trials that have resulted in a 90 percent reduction in blindness in diabetics and 50 percent in premature infants. Plus the development of the scanning laser opthalmoscope that maps the retina in three-tenths of a second.
What would George Bartisch have done with all this technology? In Renaissance times Bartisch and other itinerant surgeons traveled Germany treating cataracts by jabbing the eye with a pencil-like instrument, a procedure that might sometimes jar the lens from the protective capsule surrounding it and restore some vision but always generated intense pain. Bartisch published a compendium of opthalmology in 1583 that contains a woodcut illustrating the state of anesthetics - a patient's arms and legs are tied to a chair.
In the modern, virtually painless method, a needle inserted behind the eye deadens and immobilizes it while the patient remains conscious. At the Wilmer Institute I watched Harry Quigley anesthetize Esther Morse of York, Pennsylvania, for cataract and glaucoma surgery. Her right eye, magnified 30 times by the surgical microscopes, assumed the dimensions of a small planet. I felt like a visitor from Lilliput as Quigley made tiny incisions to create a trapdoor like flap on the white of her eye near the upper eyelid, perforated the tissue beneath the flap to relieve the pressure of glaucoma, and sutured the trapdoor with wisps of nylon thin as eyelashes.
To remove the cataract-marred lens, Quigley made another tiny cut, "Now with a little pressure I'll pop it right out," he said to me; and the jelly-bean-size lens, brown with age, slid out.
"Take the wrinkles too, doctor," said the attentive Mrs. Morse. Promising to do his best, Quigley inserted a new plastic lens.
THE EYE FUNCTIONS like a camera to focus light rays on the retina. The rays are bent first by the cornea, the thin transparent tissue covering the iris, the diaphragm that gives the eye its distinctive blue, brown, or other colour. The iris expands or contracts to regulate the amount of light transformed through the round aperture at its center; the pupil.
Then the rays are bent further by a protein rich crystalline ens behind the iris, this completes the focusing task.
Unfortunately for a hundred million Americans - who have to wear glasses or contact lenses - nature gets it wrong a significant amount of the time. If the light rays are not bend enough to focus on the retina, you are farsighted. If the rays are bend too much, you are nearsighted, or myopic, a much more common condition. A focal point even a millimeter in front of the retina results in significant myopia, the plight of at least a quarter of the human race, including 70 million Americans.
Though scientists have long know n that myopia is often associated with persons of high income, education, IQ, and status, they do not know its cause. A five-year study, sponsored by the National Eye-Institute, employs sophisticated measuring devices to track eye development among school children in Orinda, California, and may yield clues, says Anthony J. Adams, dean of the School of Optometry at the University of California at Berkeley. "This is the first study to measure every optical component of the eye and track it over a period of time," he explains.
Though conclusions about the cause of myopia are years away, surgical deliverance is at hand, according to Russia's Svyatoslav Fyodorov, perhaps the world's leading exponent of an operation to correct myopia called radial keratotomy.
In this procedure a surgeon makes tiny spokelike incisions deep in the cornea, which flattens a bit and focuses light rays correctly on the retina.
"I guarantee beautiful sight," Fyodorov told me at the Eye Microsurgery Institute, his clinic on the northern edge of Moscow. "Four to 12 incisions. After 48 hours, some tears, then beautiful. For 99 percent of patients, I can guarantee 20/20."
But his crusade, joined by many U.S. practitioners, has produced skeptics as well as believers. In 1980 the National Advisory Eye Council declared radial keratomy an "experimental" procedure and expressed "grave concern" until careful research validates its safety and effectiveness.
The National Eye Institute commissioned a study that monitored 435 patients who had the surgery. Carl Kupler summarizes the results after four years: "Roughly two-thirds of the patients are correct and do not need glasses. The others are either overcorrected or undercorrected. So you have a 67 percent chance of getting rid of your glasses. And then there are the long-term effects of the procedure to consider. We just don't know yet."
[I'll type out the rest later]