Drug Effects on Behavior in Transition: Does Context Matter?

Abstract

The generalized matching law has been used extensively to characterize choice under concurrent schedules of reinforcement. This model uses data obtained during periods of stable responding to describe response allocation among two or more response alternatives. Much less research has been conducted on the acquisition of choice. Here, we describe a procedure to study choice and its acquisition over the course of a single, 2-hour experimental session and suggest a mathematical model for quantifying the rate and magnitude of behavior change. Further, we examined the role of external stimuli and d-amphetamine on transitional and steady-state behavior.

Rats were trained to respond under concurrent schedules of reinforcement during 2-hour experimental sessions. Each session began with a scheduled reinforcement ratio of 1:1 (baseline). Thirty minutes into the session, the reinforcement ratio either remained 1:1 or changed to one of the following (transition): 32:1, 16:1, 8:1, 4:1, 1:4, 1:8, 1:16, or 1:32. A 10-second time out occurred between baseline and transition periods, during which responses had no programmed consequences. Subjects were divided into three experimental groups based on the degree to which this transition was signaled. For one-third of the animals, the time out was not accompanied by any changes in external stimuli. For one-third of the animals, stimulus and house lights were extinguished at the beginning of the time out and were re-illuminated at the end of the time out. For the remaining third, stimulus and house lights were also extinguished at the beginning of the time out, and all stimulus lights were re-illuminated at the end of the time out with the exception that the stimulus light above the newly-lean lever remained extinguished for the duration of the transition. In addition, subjects in all groups were tested in the 8:1 and 32:1 conditions following administration of 0.3, 0.56, and 1.0 mg/kg d-amphetamine. Results showed that the generalized matching equation and the logistic equation described by Newland and Reile (1999) were good descriptors of choice and acquisition, respectively. d-Amphetamine increased both sensitivity to changes in the reinforcement ratio and the rate at which preference was acquired. Effects of stimulus group were only observed on the rate of changing over between response alternatives. These data demonstrate the usefulness of single-session transitions in examining the effects of drugs on choice and its acquisition, and support the hypothesis that the dopamine agonist d-amphetamine increases behavioral sensitivity to reinforcement.