Acquisition of discriminated operant responding: The effects of brief reinforcement delays

In a first series of experiments, 9 groups of rats were exposed to 30 20-minute sessions of successive visual discrimination training (“go/no-go,” or mult FR-1 ext), where components (SD and SA) were equal (1 min) in length. Responses during SD were reinforced with a nonresetting delay (Experiment 1a) or a resetting delay (DRO, Experiment 1b) of 1 s, 2 s, 3 s, or 5 s (a “no delay” control group was also run). The terminal level and rate at which stimulus control developed was inversely related to the obtained delay, irrespective of cumulative reinforcers or reinforcement rate. A second series of experiments explored the role of reinforcer delivery as a discriminative stimulus, and the temporal regularity of the component changes. In Experiment 2a, 4 groups of rats were exposed to a mixed schedule where a delay factor (no delay and 5 s) was crossed with component type factor (fixed and variable lengths), yielding a 2 × 2 design. A 5th group was exposed to a 2-s delay with fixed components in Experiment 2b. Discriminative responding was not observed for any of the delay groups, and only a small effect of reinforcers was seen in the no-delay groups; no effect of component type was found. The results from these experiments demonstrate that delays to reinforcement retard the acquisition of a visual discrimination and may decrease the degree of stimulus control independent of the effects of delays on rate of responding, rate of reinforcement, and reinforcer delivery, per se. In addition, functions relating rate of responding in SD to rate of reinforcement were differentiated on the basis of the type of delay contingency (nonresetting vs. resetting) in contrast to an apparently undifferentiated function relating rate of responding to obtained delay.