Reinforcing behavioral variability: an analysis of dopamine-receptor subtypes and intermittent reinforcement

Variability is a functional and modifiable component of behavior that is necessary for learning or behavior change to occur. An appreciation of drug effects on reinforced variability could contribute to a fuller understanding of potential drug effects on behavior change. Dopamine agonists were examined under conditions that produced low and high levels of variability. In Experiment 1, D-amphetamine and dopamine-receptor-specific agonists (quinpirole and SKF 38393) were examined with rats that lever-pressed under a Multiple VARY 8:4 Fixed Ratio 4 (FR 4) schedule. In the VARY 8:4 condition, reinforcement followed every four-response sequence that differed from the previous eight sequences. Any four-response sequence was reinforced in the FR 4 component. Experiment 2 was an attempt to hold overall reinforcement rate constant by reinforcing criterion sequences under a Variable Interval 60" schedule of reinforcement. In Experiment 1, D-amphetamine and quinpirole (D₂ agonist) increased variability in the FR 4 component while having no effect on variability in the VARY 8:4 component. SKF 38393 (D₁ agonist) had little effect on variability, even at doses that lowered total responding. In Experiment 2, intermittent reinforcement elevated variability in the FR 4 component and attenuated differential effects of D-amphetamine. Thus, D-amphetamine and quinpirole increased variability when it was low under the FR 4 component in Experiment 1. The high variability under the VARY 8:4 schedule was unaffected by D-amphetamine, except for modest effects at high doses. These observations suggest that dopamine and specifically D₂-type receptors are involved in the production of behavioral variability, and that the drug effects depend upon the baseline level of variability.