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Institutionalized retardates were exposed to a multiple variable‐interval: extinction schedule of reinforcement in which 5‐min periods of variable‐interval reinforcement and 5‐min periods of extinction were presented in a random order. This schedule was found to generate sequential contrast effects: response rates during variable‐interval reinforcement were higher when a variable‐interval period followed an extinction period than when it followed another variable‐interval period. The rate of responding within a variable‐interval period also was affected by the number of extinction periods preceding a variable‐interval period. As the number of successive extinction periods that preceded a variable‐interval period increased, the rate of responding during that variable‐interval period increased. The sequential contrast effects were transient, being most evident during the early sessions and generally disappearing by the tenth session.

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Three experiments, using a total of 13 pigeons, examined the stimulus control acquired by the separate components of a compound visual stimulus transilluminating the pecking key. Experiment I measured the control acquired by components of compound discriminative stimuli used in discrimination training. Experiment II sought to demonstrate the effect of pretraining a single stimulus discrimination on control acquired by each component in a compound stimulus discrimination. It also investigated the effect of training the compound stimulus discrimination before the single stimulus discrimination. Experiment III sought a continuous stimulus control function when pretraining stimulus intensities were varied. The results suggest that the extent to which a bird “pays attention” to a stimulus, defined in terms of the degree of stimulus control acquired by that stimulus, is determined by how well it previously learned to discriminate that stimulus from other stimuli.

Pigeons were exposed to a multiple schedule which provided equally frequent reinforcement in the presence of two stimuli but which produced markedly different rates of key‐pecking. Generalization gradients were displaced away from the stimulus associated with the lower rate of key‐pecking. Another group of pigeons had similar training, except that a low rate of key‐pecking was established in a stimulus with a much higher frequency of food reinforcement. In this case, the generalization gradients were not affected by the training on the schedule producing a low response rate.

Reinforcements were arranged independently of the pigeon's behavior by concurrent variable‐interval schedules. The reinforcements arranged by one of the schedules occurred when the chamber was illuminated with amber light, and the reinforcements arranged by the other schedule occurred when the chamber was illuminated with blue light. Both schedules functioned concurrently, but reinforcers were delivered by each only in the presence of the appropriate stimulus condition. A response on a white key, the only key in the chamber, alternated the stimulus condition and the effective schedule. The results of this procedure were similar to those obtained with concurrent response‐dependent variable‐interval schedules of reinforcement. The proportion of the total session time spent in the presence of a schedule component approximated the proportion of the total number of reinforcements in the component. Changeover rate was a decreasing function of the changeover delay and of the difference between the relative rates of reinforcement for each pair of concurrent schedules.

Three experiments were conducted with rats to determine the effects of electric shock on responding during an operant discrimination. In two of these experiments, a conditioned suppression procedure was superimposed upon a stimulus signalling the availability of food reinforcement (SD). Response rates were greatly suppressed, not only in the warning signal periods which preceded each shock, but in the presence of SD, and the stimulus signalling the unavailability of reinforcement (SΔ) as well. A third experiment, in which a very mild shock was used without a warning signal, demonstrated an increased response rate in SD and SΔ, although this effect was rather unsystematic. In a similar study, Hearst (1965) found an increased rate in SΔ independent of any change in the SD rate. The present study failed to obtain Hearst's effect but illustrated a suppressive effect with a similar procedure.

A retarded child was taught to imitate diverse demonstrations made by an experimenter, until new demonstrations were imitated correctly upon first presentation without direct training. These imitations could be maintained without reinforcement, when they were distributed among other reinforced imitations. Factors responsible for the continued performance of these unreinforced imitations were examined. When subjected to massed extinction trials, unreinforced imitations eventually disappeared; they reappeared when again interspersed among reinforced imitations. In addition, the stimulus function of “similarity of response between subject and experimenter” was examined. The subject was taught a set of non‐imitative responses, through discriminative stimuli controlled by the experimenter, and a comparable imitative set. Unreinforced non‐imitations, like reinforced imitations, were maintained only when interspersed among reinforced imitations. When all reinforcement was discontinued, all responses extinguished similarly, indicating that reinforcement was necessary to maintain the response‐class organization, but not confirming an essential role for “similarity” as such.

A multiple schedule of food reinforcement for key‐pecking was arranged which consisted of nine fixed‐ratios, each of which operated in the presence of a different stimulus. Pigeons could complete a given fixed‐ratio within the multiple schedule or, by pecking a second key, could switch from the fixed‐ratio schedule to a variable‐ratio schedule consisting of the same nine ratios. Stable switching behavior was established which did not maximize simple probability or rate of reinforcement. Instead, the subjects showed a stable preference for the variable‐ratio schedule of food reinforcement. Increasing the number of responses required to switch, and removing the occasions on which reinforcement was delivered after a single response in the variable schedule, decreased the number of switches to the variable schedule. Periods of delay interposed between a completed switch and the availability of reinforcement after one response in the variable schedule also decreased switching to the variable schedule, particularly at long delay intervals.

Pigeons learned to respond to the middle‐sized member of six or seven sets of three stimuli differing in size. The sets were used successively, each serving as the discrimination problem from 10 to 16 times. After attaining criterion with one set, the birds received the others as probes. The number of responses in probes was related to the similarity of the probes to the prevailing discrimination problem. The birds responded either to the probe stimulus to which responding had most recently been reinforced, or to the probe stimulus closest in size to the positive member (S+) of the prevailing discrimination problem. Responses to a middle‐sized probe‐set stimulus occurred when it was the probe‐set member most recently correlated with reinforcement, when it was one of two stimuli closest in size to S+, and when the stimulus closest in size to S+ was a negative member of the discrimination problem. All of the behavior could be explained in terms of control by the absolute sizes of the various stimuli.

Pigeons were exposed to reinforcement both for short (2 < IRT < 3 sec) and long (10 < IRT < 11 sec) interresponse times. They developed bimodal interresponse‐time distributions, which were decomposable into two independent component distributions under the control of the short and long contingencies respectively. The birds' allocation of responses between these two distributions was determined by a simple power‐law relationship between reinforcement ratios, and response ratios derived from the component distributions. Comparison between this situation and concurrent choice situations raises the possibility that the power‐law relation between ratios may be a more general law pf choice than the matching of relative frequencies (probabilities).