Drug discrimination: Stimulus control during repeated testing in extinction

Rats were trained, under a two-lever drug-discrimination procedure, to resp ond differentially depending upon whether lorazepam (1.0 mg/kg) or no injec tion had been administered before the session. Responses on the appropriate lever produced a food pellet under a modified fixed-ratio (FR) 10 schedule , in which the 10 responses had to be emitted consecutively. In reinforceme nt tests, completing an FR 10 on either lever produced a pellet. In extinct ion tests, stimulus changes paired with reinforcement occurred but no pelle t was delivered. Training sessions were conducted between test sessions. Ea ch of four extinction phases consisted of six tests preceded by one stimulu s (e.g., lorazepam). Repeated exposures to extinction reduced response rate s for all rats, but stimulus control, as inferred from either percentage of total responses or percentage of total FR 10s on the drug-appropriate leve r, remained high. The percentage of total FR 10s measure was less subject t o skewing under low-rate conditions than was the percentage of total respon ses measure and provided an evaluation of stimulus control in terms of meet ing the consecutive response contingency. These results demonstrate a level of independence between response rate and stimulus control in drug discrim ination, which has positive implications for the validity of interpreting d iscriminative effects of novel test conditions in well-trained animals, eve n when overall response rates are low.