University of Rochester scientists have taken the sharpest pictures yet of the inside of the living human eye, making visible for the first time the individual cells called cones. Such precise images may one day help doctors treat several types of eye disease.
Scientists took pictures of the cones -- the photoreceptors that allow us to detect color and see clearly during the daytime -- in six people by shining a low-power laser through the pupil and onto the retina. The retina reflected the light back to an electronic camera outside the eye.
Graduate student Donald Miller will describe the work at the annual meeting of the Optical Society of America Monday, Oct. 3 in Dallas. Miller worked with David Williams, professor of psychology and director of the University's Center for Visual Science, and G. Michael Morris, professor at the Institute of Optics.
The retina is like a screen inside the eyeball that captures light signals and transmits them to the brain. It's also the only part of the living brain that scientists can view directly. Though the retina is about the same consistency as wet single- ply tissue paper, it is packed with photoreceptors (both cones, for color and resolution, and rods, for night vision) that allow us to see. Light absorbed by our cones and rods is converted into electrical signals that the brain puts together to form images.
The team saw individual cones, which are about 3 microns wide. The instruments that ophthalmologists currently use see structures no smaller than about 10 microns.
Seeing individual photoreceptors inside human eyes could help doctors detect or treat several types of eye disease, including age-related macular degeneration (AMD), which affects hundreds of thousands of Americans and is the leading cause of blindness in the elderly, and retinitis pigmentosa, which causes the photoreceptors to grow abnormally long and afflicts thousands of people in the U.S., causing gradual deterioration of the retina and eventual blindness.
A key to the experiment was a special electronic camera that is many times more sensitive than photographic film. The camera also has high resolution, dividing a square area on the retina just one-third of a millimeter wide into more than a quarter of a million pixels. The team dilated each subject's pupils with eye drops and carefully corrected the major optical defects that everyone's eyes -- even people with 20/20 vision -- contain.
With corrective lenses in place, Miller and colleagues shot a low-power (.3 milliwatts, well below safety level) beam of yellow light from a dye laser into the eyeball for just a fraction of a second.
"It's a lot like taking a flash picture," says Miller. "You need the flash to illuminate the subject, which in this case is the retina. The effect on the subject is the same as that of a powerful flash from a camera -- they see a bright flash that causes an after-image for a few seconds."
The team hopes to get even better images by using the same "Star Wars" technology that helps telescopes and missiles see through the atmosphere to correct for more complex aberrations present inside everyone's eyes.
"The conventional wisdom is that the eye's optics are too poor to let you see individual receptors," says Williams. "We've shown that this is not true. These pictures offer considerable promise for what we might see in the future with even better technology." tr