Effects of the interval between exposure to a novel environment and the occurrence of shock on the freezing responses of rats.

Rats were exposed to a novel environment (E1) at time T1, given a footshock at time T2, and tested for freezing in E1 or in a second environment (E2). The function relating freezing to the T1-T2 interval among rats tested in E1 was an inverted U-shape. Rats exposed to short T1-T2 intervals displayed just as much freezing in E2 as in E1, whereas rats exposed to longer intervals froze less in E2 than in E1. These differences between the freezing responses in E1 and in E2 were obtained when the T1-T2 intervals were varied, but time spent in the shocked E1 was equated. Rats given two shocks in E1 differentiated between E1 and E2 when the initial shock occurred some time after exposure to E1, but not when the initial shock was presented shortly after that exposure. Rats shocked some time after exposure to E1 on Day 1 and shortly after exposure to that environment on Day 2 differentiated between E1 and E2 more than did rats exposed to the reverse sequence of T1-T2 intervals. The results were attributed to the formation of a network of connections among the E1 cues in rats exposed to moderate or long T1-T2 intervals, and to an impairment in the formation of this network as a result of the conditioning of a subset of cues in rats exposed to short T1-T2 intervals.