Diurnal illumination patterns affect the development of the chick eye

Exposure to continuous illumination disrupts normal ocular development in y oung chicks, causing severe corneal flattening, shallow anterior chambers a nd progressive hyperopia ('constant light (CL) effects'). We have studied t he minimum requirements of a diurnal light cycle to prevent CL effects. (1) Seven groups of 10 chicks were reared under a 0 (constant light, CL), or 1 , 2, 3, 4, 6, or 12/12 h (normal) light-dark cycles. It was found that CL e ffects were prevented if the dark period was 4 h or longer. Below 4 h, the effects were dose-dependent and inversely correlated with the amplitude of the Fourier component of illumination at 1 cycle per day (CPD). (2) Three g roups of 20 chicks were exposed to 4 h of darkness distributed differently over 24 h to vary the amplitude of the Fourier component at 1 CPD. It was f ound that complete suppression of the CL effects required that the 4 h of d arkness were given in one block and at the same time each day. Our results show that normal ocular development in the chick requires a minimum of 4 h darkness per day, provided at the same time of the day without interruption , and suggest that the light-dark cycle interacts with a linear or weakly n onlinear oscillating system. (C) 2000 Elsevier Science Ltd. All rights rese rved.