Analysis of the effects of some factors influencing the rabbit's food intake pattern. I. Modification of the circadian pattern by the retinal maintained discharge. II. Entrainment of the rhythm by 24‐h LD alternations. III. Masking by low‐frequency noise

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Abstract
Continuous records of the number of food approaches during ad libitum‐feeding were obtained from 9 rabbits that were exposed, for periods of 3 ‐7 months, to continuous illumination (200–250 Lux), total darkness, and light‐dark regimes of LD1: 23 and LD12: 12. Thereupon, both optic nerves were sectioned and recording was continued for 18 months, with and without the application of periodical low‐frequency noise or regular maintenance schedules. It appears that the circadian pattern of food intake is governed by two loosely coupled circadian oscillators with widely different properties. They are differentially synchronized by the lights on‐ and lights off transients of LD regimes, that seem to be the only ‘entraining’ factors. Other environmental factors, if influencing food intake at all, have a ‘masking’ effect only, such as a suppression of food uptake during noise. Food intake per 24 h was nearly the same in all circumstances. One of the 2 oscillators, the ‘Illumination Insensitive’ one, has highly peculiar properties in that both its effect on the food intake pattern and its free‐running period are practically constant, and in that the latter usually deviates less than 1 min from 24 h.