Figure 1.

The experimental protocol consisted of 6 groups 3 of which received Fear Conditioning and Extinction Learning in the evening (black) and 3 in the morning (grey). During Fear Conditioning, conditioned fear responses were established to 2 different colors (CS+) by a mild electric shock. Immediately afterward, during Extinction Learning, conditioned responses to one CS+ (CS+E) but not the other (CS+U) were extinguished. Within the 3 groups trained in the morning and the 3 trained in the evening, one in each then underwent a 3, 12 or 24-hr delay during which they retained fear and extinction memories. After this delay, they were then tested for Extinction Recall and, immediately afterward, for contextual Fear Renewal. The 3 and 24-hr delays following morning and evening training allowed both training and testing to occur at the same approximate time of day (“time-congruent” participant grouping) and these are indicated in the figure by bars of the same shading as their training session placed above the phases of Session 2.