One-trial context fear conditioning as a function of the interstimulus interval

In two experiments, we examined the effects of a wide range of interstimulus intervals (2.5, 15, 45, 120, 135, and 405 sec) on one-trial context fear conditioning with rats. Here, the interstimulus interval (ISI) denotes the time between placement in a conditioning chamber and the onset of a single footshock. On the conditioning day, we observed that the rats' behavior at the time of shock onset varied systematically across ISI values. On the subsequent test day, we used context-evoked freezing as a measure of context conditioning and found the well-known inverted U-shaped ISI function. We also found that conditioned freezing for the shortest ISI values was concentrated early in the test session, whereas freezing at longer ISIs was distributed more evenly throughout the test session. The freezing results found here are more consistent with the literature on conditioning with punctate cues than are previously described results from one-trial context fear-conditioning procedures.