Response acquisition with signaled delayed reinforcement in a rodent model of ADHD

Previous research by Hand et al. [10] showed that acquisition of lever pressing was retarded in spontaneously hypertensive rats (SHRs) relative to Wistar-Kyoto rats (WKYs) when unsignaled delays of 15s separated lever presses from food delivery. The SHRs took longer to begin responding, exhibited a slower increase in response rates, responded at a lower asymptotic response rate and earned fewer reinforcers than the WKYs. The present experiment examined whether similar strain differences in acquisition would be observed if the same delay to reinforcement was signaled. Signaled delays of reinforcement typically result in lesser disruption of steady-state operant behavior than unsignaled delays, presumably because the signals function as conditioned reinforcers. Under a response-acquisition procedure, signals might be expected to facilitate acquisition which could minimize SHR-WKY strain differences. The present study exposed SHR and WKY rats to a procedure where a single lever press illuminated the houselight and delivered a food pellet 15s later. Response acquisition was similar between SHR and WKY rats under 15-s signaled delays of reinforcement; the responses emitted, delay resets and pellets earned by both strains were similar. Removal of the delay signal immediately decreased responding for both strains with the SHRs showing a significantly slower recovery over time. Overall the results suggest that signals occurring during response-reinforcer delays can mitigate the response-weakening effects of delayed reinforcement in a rodent model of ADHD.