BEHAVIORAL CORRELATES OF CIRCADIAN RHYTHMS IN THELIMULUSVISUAL SYSTEM

Abstract

A clock in the brain of Limulus generates circadian rhythms in retinal sensitivity. We examined the relation between behavioral responses to light and circadian changes within the visual system. Our first experiment recorded unconditioned movements of the telson (tail) elicited by a constant level of illumination of the lateral eyes at different times of the day while an animal remained in the dark. Under these conditions, tail movements followed the same pattern of response as the electroretinogram (ERG) recorded from the lateral eyes. That is, the probability of tail movement was directly proportional to the amplitude of the ERG and both exhibited a circadian rhythm. In the second experiment we conditioned the reflexive movements of the tail and gills by pairing illumination of the lateral eyes with an aversive stimulus, and then measured the level of illumination necessary to elicit responses at different times of the day. Results show that animals maintained in darkness are about 10 times more sensitive at night than during the day. The day-night change in visual sensitivity measured behaviorally is consistent with that measured physiologically (Barlow et al., 1980). The daily rhythm of visual performance could thus be attributed to the known rhythm in retinal sensitivity generated by a circadian clock located in the brain.