Runway acquisition and extinction as a joint function of magnitude of reward and percentage of rewarded acquisition trials

To determine the joint effects of partial reward and reward magnitude on acquisition and extinction rates, and on acquisition and extinction asymptotes, 215 Wistar albino rats were trained in a Hunter straight runway. The experimental design was a 4 × 4 × 2 factorial combining four reward magnitudes, four reward percentages, and two experimenters. The data revealed that the acquisition rate was an increasing function of both percentage and magnitude of reward and that neither reward magnitude nor percentage of reward significantly affected acquisition asymptote. For extinction, it was found that, for continuous schedules, the larger the reward magnitude the less the resistance to extinction and, for partial schedules, the larger the reward magnitude the greater the resistance to extinction. These results were interpreted within the framework of the sequential effects hypothesis (Capaldi, 1966).     

Schedule-induced drinking as a function of percentage reinforcement

Drinking was recorded in rats while lever pressing was maintained on a series of percentage reinforcement schedules in which the per cent of 1-min fixed intervals terminating with food was 100, 90, 30, 70, 10, 50, and 100%. Intervals in which a pellet was omitted were terminated by brief light flash and click stimuli that were also correlated with food presentations. Drinking failed to develop in five of six subjects following intervals in which the brief stimuli were presented regardless of percentage reinforcement. Postpellet drinking, which followed intervals terminated with pellet delivery, however, increased in both duration and amount ingested per interval as percentage reinforcement was systematically decreased. The increase in postpellet drinking above that produced by 100% reinforcement was interpreted as an analogue of the positive-contrast effect observed with food-reinforced operants.     

The conditioned reinforcement of repeated acquisition

Three monkeys were trained to emit a chain of three responses on three separate levers in a set of six levers to obtain food. The chain producing food (correct chain) was changed each day. During a trial, a press on any lever produced a feedback stimulus; a press on a correct lever produced an additional distinctive stimulus; the third correct press produced a food pellet. Test sessions in which either the food or the distinctive stimuli were removed were interspersed with baseline sessions. In tests without food presentations, the subjects acquired the correct chain rapidly, with a level of accuracy comparable to baseline. Removing the distintive stimuli for either the first or second member of the correct chain greatly retarded acquisition of that member of the chain. Removing all distinctive stimuli often reduced accuracy throughout the chain to chance level, even though food was presented following each correct chain. These results were interpreted as evidence that the distinctive stimuli presented after correct responses functioned as conditioned reinforcers. Reductions in accuracy following an omitted distinctive stimulus indicated that they were also discriminative stimuli for correct responding in their presence.