Aug. 23, 2001 Like a field of sunflowers nodding toward the sun, photoreceptors - the light-sensitive cells in the retina of the eye - can apparently swiftly reorient themselves towards the brightest points of light after cataract surgery, according to a research paper published August 10 in Nature.
The research findings, presented in the "Realignment of Cones After Cataract Removal," by University of California, San Diego psychologists Harvey Smallman and Don MacLeod and Dartmouth mathematician Peter Doyle, represent the strongest evidence to date that photoreceptors have this reorienting capability.
According to Smallman, the lead author who is also a senior scientist at the San Diego-based Pacific Science and Engineering Group, the findings are based on a unique case study of Peter Doyle, who for 40 years lived with asymmetrical congenital cataracts until age 43, when he decided to have them removed.
"These findings offer the firmest evidence to date that the photoreceptors in the back of the eye are capable of reorienting themselves towards light," said Smallman. "While we do not know what factors are at play in prompting this realignment, it appears that photoreceptors may be phototropic in the same way that plants are."
In normal adults, the eye's photoreceptors -- about 130 million rods and cones which enable us to see light and color -- are aligned towards the centers of the pupil, where light is brightest. For many years, Doyle treated his cataracts by using a medication called atropine, which dilated his pupils to produce ring-shaped, clear regions around the dense cataracts. As a result, his photoreceptors were aligned with these larger, clear regions in each eye.
His decision to have the cataracts removed, said Smallman, provided the researchers with the unique opportunity to test the theory that the eye's photoreceptors are able to actively reorient themselves to light because the surgery would shift the brightest region within Doyle's pupils bac k to the pupil centers.
After each cataract surgery, the researchers measured the apparent brightness of light in Doyle's pupils, which would reflect the alignment of cone photoreceptors. They found that after no less than ten days, the photoreceptors had migrated to the centers of Doyle's pupils.
According to Smallman, the biophysical processes that underlie phototropism in photoreceptors may be under the control of a simple feedback signal that works similarly to those in plants. While plants tilt towards light by growing more on the shaded side of the stem than the brighter side, the protein skeleton of photoreceptors may extend differentially to accomplish the same function in the eye.
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