Conditioned Jump-Out Avoidance Acquisition and “Incubation of Fear” in a Large Sample of Rats

Over 3000 naive rats received 40 avoidance conditioning trials in a jump-out box. A 2- to 3-hr. break followed Trial 20; otherwise, all trials were closely massed with an intertrial interval of 3 to 5 sec. Acquisition was orderly, with steady-state behavior demonstrated by Markov chain analysis from Trials 31 to 40. Response probabilities on all trials depended strongly on the behavior emitted on 1, 2 or 3 consecutive preceding trials. Although behavior was disrupted on Trial 21, the trial that followed the break in training, post-break performance by Trial 23 was significantly improved over the performance predicted by pre-break behavior. This enhancement of performance could not be attributed solely to results of avoidance failure on Trial 21 and may provide unique evidence for “incubation of fear.”