Cocaine and vigilance task performance of rats: effects of delay of reinforcement.

In two experiments rats were food-reinforced for pressing one of two levers in an operant chamber, with the correct lever being indicated by the position of a briefly illuminated light. In Experiment 1 the levers were always in the chamber, whereas in Experiment 2 the levers were inserted into the chamber immediately after cue light termination and withdrawn immediately after a choice response. The rats were tested under four conditions: after an injection (SC) of saline or 2.5 mg/kg cocaine and with delay of reinforcement (DOR) of either 0 or 8 s. In both experiments, cocaine enhanced accuracy under the 0-s DOR condition. However, in neither experiment was there evidence of facilitation with cocaine under 8-s DOR, which by itself increased choice latencies and decreased accuracy when choice latencies exceeded 0.5 s. These results indicate that cocaine may only enhance performance in vigilance tasks under constrained conditions, e.g., those that require minimal levels of information processing.