Diurnal Variation in Performance of Free-Operant Avoidance Behavior of Rats

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Abstract
Possible daily periodicity in experiments involving conditioned behaviors has not usually been investigated or controlled. The present experiment argues for such studies in that variation in the performance of a free-operant avoidance task by 30 rats was detected as a function of when during the 24-hr. day behavioral testing occurred. In terms of shocks avoided, performance was best in the middle of the dark phase of the day and worst late in the light phase. The absence of a similar rhythm in the operant response rate suggests the 24-hr. periodic change in efficacy of avoidance was not related to diurnal variation in general activity levels. Implications of daily periodicity are discussed in terms of experimental procedures and controls.