Circadian rhythm of luminance detectability in the rat

Rats lived in a visual detection test environment which allowed for around-the-clock measurement of the detectability of a 500 nm stimulus of low luminance. Daily trends in visual sensitivity were compared with a behavioral activity measure, the rate of trial initiations in the self-paced testing procedure. Both variables showed circadian rhythms with periods exceeding 24 hr in the absence of day-night cues. However, the two oscillations were phase-displaced, such that maximal visual sensitivity occurred earlier than did maximal behavioral activity, each day. The temporal organization of visual sensitivity may underlie well-known photic influence on circadian rhythms and photoperiodic behavior.