PATHWAYS FOR PHOTIC ENTRAINMENT OF MAMMALIAN CIRCADIAN RHYTHMS

Abstract— Photic entrainment of mammalian circadian rhythms is mediated exclusively by the retina. Several retinofugal pathways may be involved in the mediation of entrainment and other effects of light on the circadian system. The retinohypothalamic tract (RHT) is sufficient for entrainment, but other visual projections are capable of mediating either driving or entraining effects of light after RHT interruption in rats and hamsters. Interruption of the primary or accessory optic tracts can modify several features of light entrainment, suggesting their involvement in normal entrainment in hamsters. Retinofugal projections appear to interact in a complex fashion to produce the features of normal photic entrainment.