Interstimulus interval as a discriminative stimulus: evidence of the generality of a novel asymmetry in temporal discrimination learning.

Three appetitive conditioning experiments with rats examined temporal discrimination learning within Pavlovian conditioning trials. In all experiments, the duration of a feature white noise stimulus signaled whether or not a subsequent 10-s target tone would be reinforced. In Experiment 1, the feature durations were 4 and 1min. For one group of rats (Group 4+/1-), 4min of noise signaled that the tone would be reinforced and 1min of noise signaled that the tone would not be reinforced. A second group (Group 1+/4-) was trained with the reverse contingency. The results showed a clear asymmetry in temporal discrimination learning: rats trained with 4+/1- (Long+/Short-) learned the discrimination readily (responding more in the tone on reinforced than on nonreinforced trials), whereas rats trained with 1+/4- (Short+/Long) did not. In Experiment 2, the feature durations were shortened to 60 and 15s. Due to strong excitatory conditioning of the 15-s feature, the reverse asymmetry was observed, with the Short+/Long- discrimination learned more readily than the Long+/Short- discrimination. However, Experiment 3 demonstrated that the original Long+/Short- advantage could be recovered while using 60- and 15-s feature durations if the excitatory conditioning of the feature was reduced by including nonreinforced feature trials. The results support previous research involving the timing of intertrial intervals and are consistent with the temporal elements hypothesis which holds that the passage of time is encoded as a series of hypothetical stimulus elements.