Supernormal Vision: A Focus on Adaptive Optics Improves Images of the Eye and Boosts Vision

Article excerpt

Anyone who grew up reading comic books may recall the advertisement that always graced the back pages, touting weird and wonderful gizmos designed to appeal to fun-loving youngsters. In one classic ad, a crew-cut boy sporting horn-rimmed "X-ray specs" stares gleefully at the bones in his hand. With the specs, the ad promised, mere mortals could acquire the super-vision of Superman.

Most people with failing eyesight are satisfied just to have glasses or contact lenses that can keep their vision within the 20/20 range. One group of researchers, however, has developed a technology that can endow people with a new kind of supernormal vision. Following the tradition of binoculars' helping people to observe distant objects and night vision goggles' allowing them to see in the dark, the most recent technology enables the human eye to detect finer detail than is ordinarily possible.

At a seminar sponsored by Research to Prevent Blindness and held in Los Angeles in September, vision scientist David R. Williams of the University of Rochester in New York explained the approach that he and his group have taken. They have embraced the technique of adaptive optics, originally designed to sharpen images from military surveillance devices and astronomical telescopes. As applied to the human eye, the system allows people to see at high resolution, but it also works in reverse, allowing researchers to capture extraordinarily detailed images of the eye's retina.

That, in fact, is the group's main goal. Clear, detailed images of the retina could improve the diagnosis, understanding, and treatment of diseases such as glaucoma and age-related macular degeneration, both of which can cause blindness. As part of their adaptive optics system, Williams and his group also have adopted a new way of measuring the eye's imperfections that could help to improve the performance of contact lenses and allow more accurate assessment of corrective eye surgery.

Williams and his colleagues recently obtained the clearest pictures yet of individual cells in the retina, an achievement that Howard C. Howland of Cornell University calls "spectacular." The retina acts as the eye's movie screen, receiving a projected image of the ever-changing world. Light entering the eye hits not a white, reflective surface, but a carpet of rods and cones--two kinds of photoreceptor cells that serve different purposes. Rods detect dim light and are needed for peripheral vision, while cones function in bright light and are responsible for color vision. Cone cells come in red, blue, and green types, the names indicating their color sensitivities.

"Spatial arrangement and relative numbers [of rods and cones] are not well characterized in the human retina," says Williams. So far, most studies have been done in excised eyes, not in living people, Howland adds. To take a picture of the intact retina, a researcher has to send a flash of light into the eye, then record the light that bounces off the retina with a camera.

The same imperfections in the eye's cornea and lens that reduce visual acuity, however, also warp retinal images obtained in this way, limiting their resolution. "If you had a perfect eye, the light would come out parallel," says Williams, and could be focused into an undistorted image of the retina.

Many common vision problems, such as myopia and astigmatism, are caused by aberrations in, the shape of the eye that are easy to correct with eyeglass lenses. Other aberrations are harder to deal with, both in improving a person's vision and in examining the eye.

Several years ago, the Rochester group found a solution for dealing with such troublesome distortions when they examine eyes: They do a trick with mirrors. The researchers can direct the light leaving an eye onto a deformable mirror--the key to adaptive optics--and correct for any aberration, thereby producing the highest quality images of the retina ever seen (SN: 10/8/94, p. …