Photoperiodic programming of diurnal changes in rabbit visual evoked potentials.

Rabbits which have been exposed for several weeks to the natural cycles of daylight and darkness, or to various fixed 24 hr light-dark alternations, exhibit in constant environmental conditions large diurnal changes in the shape of the Visual Evoked Potential. These changes run a square-wave like time course and seem to reflect a similar diurnal rhythm in the sensitivity of the visual system to photic stimuli. The relation between the durations of the two phases of the rhythm can, within wide limits, be changed by variations in length of the artificial or natural photoperiod. It further appears that after exposure to continuous illumination or darkness rabbits have either a free-running V.E.P. rhythm or no such rhythm at all. The probable importance of the diurnal changes in the visual system for the occurrence of seasonal fluctuations in behaviour is discussed.