[Diurnal axial eye growth rhythm in chicks is modified by diurnal light-dark cycle]

In chicks, the distance from the cornea to the posterior sclera (defined as axial eye length) grows in a diurnal rhythm: growth during the day is greater than during the night. The purpose of this study was to investigate whether this diurnal, axial eye growth rhythm is influenced by the diurnal light/dark illumination cycle.

One day after hatching, 18 White Leghorn chicks were reared under a 12 hour light/dark illumination cycle. After seven days, 12 chicks were switched to continuous light; the other 6 chicks remained under the 12 hour light/dark cycle and served as controls. Every 12 hours, at times corresponding to the onset of former light and dark phases over a period of 2 days, axial eye length was measured in the right eyes by laser Doppler interferometry with a precision of 20 microns.

In group 1, axial eye growth during time intervals corresponding to former light phases (0.063 +/- 0.024 mm; mean +/- 95% confidence limit) and dark phases (0.040 +/- 0.017 mm) were not significantly different (one-factor ANOVA). In group 2, axial eye growth during light phases (0.065 +/- 0.028 mm) was significantly greater than during dark phases (0.015 +/- 0.023 mm; p = 0.015). Significantly, total axial eye growth during the two-day period was greater in group 1 (0.206 +/- 0.015 mm) than in group 2 (0.160 +/- 0.021 mm; p = 0.02; unpaired, two-tailed Student's t-test).

The diurnal, axial eye growth rhythm is strongly attenuated following an acute switch from a 12 hour light/dark cycle to continuous light. Control chicks exhibit the normal rhythm. This observation demonstrates that the diurnal, axial eye growth rhythm is influenced by the light/dark illumination cycle. Whether the alteration of the rhythm and the increased axial growth during continuous light is caused by the increased amount of light alone or by increased light-induced form vision remains to be investigated.