Differential effects of training intertrial interval on acquisition of trace and long-delay fear conditioning in rats

Many factors govern conditioning effectiveness, including the intertrial interval (ITI) used during training. The present study systematically varied the training ITI during both trace and long-delay fear conditioning. Rats were trained using one of six different ITIs and subsequently tested for conditioning to the white noise conditioned stimulus (CS) and the training context. After trace conditioning, percent freezing to the CS was positively correlated with training ITI, whereas percent freezing to the context was negatively correlated with training ITI. In contrast, when rats were trained using a long-delay paradigm, freezing during the CS test session did not vary as a function of training ITI; rats exhibited robust freezing at all ITIs. The long-delay conditioned rats exhibited relatively low levels of freezing during the context test. Thus, trace is more sensitive than long-delay fear conditioning to variations in the training ITI. These data suggest that training ITI is an important variable to consider when evaluating age or treatment effects, where the optimal ITI may vary with advancing age or pharmacological treatment.