Circadian rhythms in vertebrates (including man) have been conjectured to help control changes in sensitivity of visual systems (which must operate over some 10 orders of magnitude in the course of night and day), by anticipating the changes in light intensity which occur at dusk and dawn (Cahill and Besharse, 1995). Diurnal rhythms in melatonin and dopamine in the retina have been shown to be affected both by a circadian oscillator as well as by changes in local light levels (Cahill and Besharse, 1995). In an experiment conducted in the laboratory of one of the authors (HCH), the growth of the eyes of baby chicks in their first two weeks of life has been shown to be strongly affected by exposure to 20 hours or more of light per day (Li et al., 2000). A descriptive model of the nature of the biochemistry of retinal dynamics has been presented (Morgan and Boelen, 1996). In this work we offer a mathematical model of the retinal oscillator based on the descriptive model given in (Morgan and Boelen, 1996). Using this model we simulate the experiment described in (Li et al., 2000).

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