Effects of illumination level on the rat's rhythmicity of brain self-stimulation behavior

Rhythmic patterns in the rat's brain self-stimulation behavior were analyzed across levels of illumination, including conditions of constant illumination (LL), constant darkness (DD), and light-dark cycles (LD 12:12). LD entrainment was achieved with light intensities ranging from 0.25 to 440 lux, and little or no change was found in the phase-angle difference between the dominant spectral peak and the light transitions. Under constant conditions, the circadian period (tau) increased in proportion to illumination level, with means ranging from 24.10 h (DD) to 25.90 h (LL 440 lux). tau increased linearly as a function of long I within the range of 0.25 to 30 lux, yielding a change of 0.28 h for a 10-fold increment in illumination level, a value which closely matches Aschoff's [3] preliminary estimate of delta tau/delta ILL for the rat. The circadian spectral component was influenced by several factors. (1) Re-entrainment protocol. Given a succession of LL conditions without entrainment segments in between, circadian rhythmicity was obscured at high illumination levels. (2) Duration of LL exposure. Even following an entrainment segment, long-term LL resulted in reduced power or loss of the circadian component. (3) LD vs LL. Spectral power was consistently higher under entrainment than under corresponding LL intensities, and there was a trend toward reduced power as a function of LL intensity. A wide range of ultradian spectral components was found across conditions. Under entrainment, most such components were harmonics of the circadian fundamental; under constant conditions, the frequency relationships were relatively variable.