Adaptive Optics Maps out Retina

Dr. Ricki A. Lewis

Researchers at the University of Rochester have turned Star Wars-era technology designed to clarify images from spy satellites into a method that can image the human retina, with surprising results.
"With this method, we can now go from imaging an invisible, cell-sized structure to seeing all the photoreceptors in a patch of retina," said David Williams, director of the university's Center for Visual Science. The researchers' work is published in the Feb. 11 issue of Nature.
In the human eye, the 7 million cone cells that provide color vision come in three types, distinguished by their photoreceptors and the wavelengths of light that they photobleach. S cones absorb short wavelengths and correspond to blue through violet, but account for only 5 to 10 percent of the total. M cones absorb middle pigments and correspond roughly to green, and L cones absorb long wavelengths and correspond to red. Each cone impinges on a bipolar neuron, which contacts a ganglion neuron, which in turn synapses with the optic nerve, which then passes through the thalamus and stimulates the cerebral cortex.
"The brain takes in a tremendous amount of raw visual information and processes it before providing us with our view of the world," said Austin Roorda, who was a postdoctoral researcher in Williams' lab and is now an assistant professor of optics at the University of Houston College of Optometry.
The university researchers used adaptive optics, using a differential photobleaching step to discriminate the M and L cones. "When light leaves the retina, it passes through the lens and the cornea. All of the rays coming from a single point on the retina should emerge perfectly parallel, but they are bent as they pass through the pupil," Williams said.
"Adaptive optics allows us to measure how each light ray bends as it passes through the pupil. Then we use a deformable mirror that is shaped so that it straightens out light reflecting off it to become parallel. From any point on the retina, when all the light rays are parallel, you get a clear, unblurred image."
Williams and Roorda imaged the retinas of two men with normal color vision, which surprisingly appeared quite different. One man had about equal numbers of M and L cones, but the other had about four times as many L cones. That both men could perceive color reveals an underlying variability and plasticity to the system. Plus, the cones were randomly distributed, and each person had areas devoid of one or the other type.
"This random arrangement is not at all what we expected," Williams said. "If you were to design a digital camera, you'd never choose a random geometry like this. Yet it turns out that that's what the most sophisticated imaging device in the world -- the human eye -- uses."