Effects of d-amphetamine on Self-control Choices under Signaled and Unsignaled Delays

Choosing a larger more delayed reinforcer over a smaller, more immediate reinforcer is said to show “self-control,” whereas choosing the smaller, more immediate reinforcer is said to show “impulsivity.”  Researchers have developed animal models of this and used them to test the effects of drugs on self-control/impulsive choices. However, there are some discrepancies in the literature on the effects of certain drugs, particularly psychomotor stimulants.  One account of these discrepancies relates to the signaling conditions associated with the delay to the larger reinforcer.  In the present study, effects of d-amphetamine on self-control choices in 8 rats were examined.  The larger reinforcer consisted of four dipper presentations of sugar water at various delays which increased across 5 blocks of 10 choice trials. The smaller reinforcer consisted of one dipper presentation of sugar water delivered after a 2-s delay.  For 4 subjects, the delays to both the smaller and larger reinforcers were signaled by a unique stimulus; for the other 4 subjects, there was no unique stimulus associated with the delay. The rats received injections of saline and several doses of d-amphetamine (0.3, 1.0, 1.78, and 3.0 mg/kg) every fifth day. Signaling conditions were reversed after effects of each dose of d-amphetamine had been determined at least twice for all rats.  There were considerable individual differences in the effects of the intermediate doses of d-amphetamine, however, these effects did not differ across signaling conditions, suggesting that delay signals may not play a crucial role in effects of stimulants on self-control choices.