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H.C. Howland; Are Circadian Signals Between Chick Eyes Carried by Direct Light Transmission? . Invest. Ophthalmol. Vis. Sci. 2006;47(13):3335.
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It is known that the effects of continuous illumination (CL) of an eye (hyperopia, shallow anterior chamber, flattened cornea) can be mitigated by patching the other eye in a 12 light/12 dark rhythm. Likewise an eye kept in constant darkness can be protected from constant dark effects (similar to those of CL) by exposing the other eye to a circadian light rhythm. We wished to determine if the protecting signal could be carried by light leakage between the eyes.
We measured the light transmission between the eyes in live chicks by illuminating one eye with a fiber optic light guide and measuring the light emitted from the other eye using calibrated digital photography. We also measured the optical density of freshly excised retinas using digital photo–microscopy. We constructed optical models of the two eyes and the transmission between them to provide a theoretical framework for our observations in the living chick.
The optical density of the 4 day old chick retina is approx 3.3 log units of attenuation.. Thus light passing through the two retinas is attenuated by a factor of 1 / (4*10^6). However our computed attenuation of the light passing through one eye, into the other and emerging from the pupil is about 2.45 log units. Hence the light must not be passing through the retinas. We believe that the light enters one optic nerve and passes from that into the other optic nerve to emerge in the other eye from the margins of the optic disc that are not covered by the pectin (a pigmented capillary tuft on the optic nerve head.
Due to the measured attenuation of transmitted light, we do not believe that direct trans–illumination of the eyes is responsible for protection against constant illumination by a fellow eye covered in a 12L/12D rhythm. However it is possible that trans–illumination provides the circadian signal, or part of the signal in protection against constant dark effects.
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