Trial and intertrial durations in appetitive conditioning in rats

The effects of trial (T) and intertrial (I) durations were examined in two Pavlovian conditioning experiments with rats, in which a noise conditioned stimulus (CS) was paired with food delivery. In Experiment 1, T was either 10 or 20 sec, and I ranged from 15 to 960 sec, in separate groups of rats. The acquisition rate and final level of conditioned responding showed ratio invariance: They were better predicted by the I/T ratio than by I or T alone. In Experiment 2, the I/T ratio was 6.0 in all the groups, and T was 20, 40, 80, or 160 sec. Ratio invariance was not observed: Despite the common I/T ratio, the rate of acquisition, final level of conditioned responding, and the ability of the CS to block conditioning of another stimulus differed among the groups. At the same time, the temporal distribution of conditioned responding within T was similar in all the groups throughout conditioning and extinction and showed superpositioning when normalized across T. Many but not all aspects of the data were consistent with scalar timing theory.