Effects of illumination on the rabbit's EEG during the two phases of its circadian rhythm.

After exposure to a LD 12:12 regimen for several weeks, rabbits exhibit a programmed circadian rhythm in the RMS values of their occipital and frontal EEG's during 54-hr recordings in constant darkness. Illumination at levels of 80, 160 and 230 Lx raises these RMS values in both phases of the rhythm. The induced rise is large in the phase with low RMS values and small in the other, whereas light-induced changes in spectral composition of the EEG's are slight in either phase. Bilateral optic nerve sectioning results in similar changes in properties of the EEG and in the amplitude of its rhythm. The results are discussed with reference to the influence exerted by steady illumination on the overall-level of retinal maintained "dark" discharge.