like the latter two groups, mammals lack functional extraocular photoreceptors (28); thus, redundancy in photoreception is confined to the retina. One challenge is to determine the relative contributions of melanopsin, rod/cone opsins, cryptochromes, and other currently uncharacterized photopigments in communicating photic information to the circadian system. Fluorescent lights (Philips F32T8/TL741 Hi-Vision, 4100 K) were used both for light pulses and for overhead lighting in the activity recording room. Light intensity on the cage floor ranged from 20 to 60 W⅐cm Ϫ2 or 70 to 280 lux for light pulses and from 10 to 30 W⅐cm Ϫ2 or 30 to 120 lux for LD and LL conditions, depending on the location of the sensor in the cage. Light was measured in both radiometric (International Light, model IL-1405 Radiometer System) and photometric (Iso-Tech, ILM 350) units of measure to facilitate comparison between traditional photoreception and circadian studies. Mice used for in situ hybridization were sacrificed at the end of the light or dark pulse and their brains were frozen on dry ice. In situ experiments were performed as described in (16). Small sample sizes precluded statistical evaluation of c-fos levels
