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Pigeons sometimes peck a key at different rates in the presence of different stimuli, even when the same schedule of reinforcement is correlated with each of these stimuli. The possibility that the occurrence of such stimulus bias is dependent on adventitious effects of food reinforcement in the presence of the stimuli was evaluated by correlating extinction with two stimuli. Both pigeons showed stimulus bias, indicating that the occurrence of this phenomenon is not critically dependent upon any effects of scheduled food reinforcements.

The cooperative responses of pairs of human subjects were reinforced under several stimulus conditions in two settings designed to require a “social” response, i.e., where at least one of the two persons is responding to the behavior of the other. The first task, designed by Lindsley and Cohen, required individual responses within 0.5 sec of one another for reinforcement. The second (modified) task required a delay of 3 sec between individual responses. To determine dependence of cooperation on social stimuli, rates of cooperative behavior on these tasks were compared in the presence and absence of a stimulus indicating to each subject the other's response and a stimulus which indicated the duration of the timeout after reinforcement. The results indicated that only in the modified task was a high rate of cooperation always contingent upon the presence of the social stimuli.

Pigeons were trained on a three‐stimulus simultaneous discrimination with reinforcements given on fixed‐ratio schedules ranging from one response per reinforcement to 205 responses per reinforcement. Responses to any of the three stimuli advanced the ratio, but, when it was completed, a reinforcement followed a response to one designated stimulus. Throughout the ratio, the birds responded mainly to the designated stimulus. The relatively few responses to the other stimuli usually occurred immediately after postreinforcement pauses. The distribution of responses to the various stimuli was not affected by omitting reinforcements.

In discrimination learning, the negativity of the stimulus correlated with nonreinforcement (S–) declines after 100 training trials while the stimulus correlated with reinforcement (S+) is paradoxically more positive with lesser amounts of discrimination training. Training subjects on two simultaneous discrimination tasks revealed a within‐subjects overlearning reversal effect, where a more‐frequently presented discrimination problem was better learned in reversal than was a discrimination problem presented less frequently during training.

Rats were trained to criterion in successive reversals of a classically conditioned heart‐rate discrimination. Criterion was attained more rapidly in the later reversals in the series than in the early reversals and the inter‐problem learning curve was of the same general form as that found with successive instrumental discrimination reversal in the same species.

In Experiment I, a four‐ply multiple schedule was used to study the effects on rate of responding in rats of food, water, and food and/or water reinforcement under different deprivation conditions. Food and water were associated separately with different stimuli, the combination of which was associated with food and water together, or with food or water randomly. Rates in the presence of the combined stimuli were consistently intermediate to the rates generated by the separate stimuli, a result seemingly incompatible with a “summation” hypothesis. Experiment II was a simplified systematic replication of Experiment I, verifying the major findings.

This investigation, using rats as subjects and punishment by timeout for responses maintained on a ratio schedule, sought to determine whether behavior would be suppressed by timeout punishment when such suppression also reduced reinforcement density or frequency. A series of experiments indicated that timeout punishment suppressed responding, with the degree of suppression increasing as a function of the duration of the timeout period. Suppressive effects were found to decrease as a function of increases in deprivation (body weight) and were eliminated when the punished response also was reinforced. It was concluded that timeout can produce aversive effects even when loss of reinforcement results. An alternative interpretation of the findings, based on the effects of extinction periods and delay of reinforcement on chained behavior, was discussed.

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