Rabbits that have been exposed for several weeks to a L12:D12 regimen exhibit afterwards, under constant circumstances and in steady darkness for 53 h, the presence of "programmed" circadian rhythms in the RMS value and spectral composition of their (computer-analysed) EEG's, in the RMS value of their electromyograms, and in skin temperature. For the occipital and frontal EEG's the RMS value attains its lowest level during a dip occurring about 3 h before the previous light on-moment and its maximum, which is 1.5 times the minimal value, during a peak at some 3 h before the previously occurring light off-transient. Similar changes occur in the power spectral densities of all frequencies between 0.5 cps and 40 cps, with the result that circadian changes in the spectral composition of the EEG, although present and having the same time course, are rather slight. The results are discussed in terms of the two-oscillator model for the central circadian mechanism.
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.
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