Some empirical data concerning time of day effects on conditioned freezing in an aversive context-conditioning procedure

The primary purpose of this article is to present some empirical data concerning three different potential time of day effects on conditioned freezing in a commonly-used aversive context-conditioning procedure with an unsignalled electric footshock as the unconditioned stimulus. In Experiment 1, rats were repeatedly placed in a conditioning box in which they received a shock. For one group of rats, these sessions consistently occurred in the morning; for another group in the afternoon. In Experiments 2 and 3, rats received two training sessions per day. One group was consistently shocked in a training box on morning sessions but not on afternoon or evening sessions, whereas another group received the reverse treatment. The pattern of freezing observed during repeated non-shock morning and afternoon/evening test sessions reflected an effect of time of shock delivery on the acquisition of a context-shock association in Experiments 1 and 2, and a time of testing, or non-specific performance, effect in Experiments I and 3. In none of the experiments was there an effect that would reflect differential retrieval of a context-shock association by time cues. These results were discussed in the light of data from previous experiments.