Purpose: To investigate the effects of constant or 12-hour cyclic illumination of the pineal gland and the eyes on the growth of the chick eye.
Methods: Chicks (Gallus gallus, Cornell K Strain) were raised either under a 12-hour light-dark cycle of normal light or under constant light, with or without opaque removable hoods that covered the top of the head for 12 hours each day. A second group of chicks was raised under constant light with opaque eye covers that were worn on either both eyes or only the right eye for 12 hours each day. Chicks were placed in the experimental conditions on the third day after hatching and raised for 3 weeks.
Results: Pineal gland hoods and eye covers worn 12 hours a day significantly (P < 0.0001) protected the chicks from hyperopia under constant-light conditions. They also reduced the flattening of the cornea caused by constant light. Most striking was the protection afforded the uncovered eye from constant light's effects by the periodic covering of the opposite eye.
Conclusions: A diurnal light-dark rhythm presented to one of three photosensitive organs (the pineal gland and both eyes) can protect the eyes from the effects of constant light. This is most probably due to the maintenance of a melatonin rhythm in the organ receiving the diurnal light rhythm.