Comparison of drug effects on the performance of two timing tasks in rats

Previous evidence suggests that different timing tasks are differentially sensitive to pharmacological manipulation, especially when different values for the temporal parameters are used. The present series of experiments compared the effects of physostigmine, caffeine, pentobarbital, morphine, and naloxone on the performance of a differential reinforcement of low rates with limited hold (DRL-LH) and a temporal response differentiation (TRD) task. In the DRL-LH task, rats were reinforced for responses that occurred 10-14 s from the end of the previous response. In the TRD task, rats were reinforced for responses with a duration of 10-14 s. The peak response time and peak spread of the initiation time distribution (for DRL-LH) or the response duration distribution (for TRD) were used as indices of temporal discrimination. Physostigmine, caffeine, and pentobarbital produced very similar effects on peak response time for both tasks, but the effects of morphine and naloxone were different for the two tasks. Effects on peak spread for the two tasks did not always correspond to changes in peak response time, suggesting that different processes may be measured by these two endpoints. Further, these effects were independent of changes in response rate suggesting that the effects were not due to gross disruptions in motivation or motor control. These results suggest that the effects of drugs on DRL-LH and TRD performance may differ, even when temporal parameters are identical.