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Two undergraduate males worked for money on a button‐pressing task associated with concurrent fixed‐ratio fixed‐interval schedules of reinforcement. Manipulations of the fixed‐ratio requirement produced an interaction between the various fixed‐ratio and fixed‐interval performances. When the fixed ratio was small, more fixed‐interval responding occurred per interval than when the fixed ratio was large. In general, the data were similar to those obtained with lower organisms except that no post‐reinforcement pause or ratio strain was seen.

In an analysis of interactions between concurrent performances, variable‐interval reinforcement was scheduled, in various sequences, for both keys, for only one key, or for neither key of a two‐key pigeon chamber. With changeover delays of 0.5 or 1.0 sec, and with each key's reinforcements discriminated on the basis of key‐correlated feeder stimuli, reinforcement of pecks on one key reduced the pecking maintained by reinforcement on the other key. The decrease in pecking early after reinforcement was discontinued on one key was not substantially affected by whether pecks on the other key were reinforced, but after reinforcement was discontinued on both keys, reinstatement of reinforcement for one key sometimes produced transient increases in pecking on the other key. Correlating the availability of right‐key reinforcements with a stimulus, which maintained right‐key reinforcement while reducing right‐key pecking to negligible levels, demonstrated that these interactions depended on concurrent reinforcement, not concurrent responding. Thus, reinforcement of a response, but not necessarily the occurrence of the response, inhibits other reinforced responses. Compared with accounts in terms of excitatory effects of extinction, often invoked in treatments of behavioral contrast, this inhibitory account has the advantage of dealing only with observed dimensions of behavior.

Responding by pigeons on one key of a two‐key chamber alternated the color of the second key, on which responding produced food according to a variable‐interval schedule of reinforcement. From time to time, reinforcement would be available for a response, but in the presence of a particular stimulus, either red or green light on the key. Red or green was chosen irregularly from reinforcement to reinforcement, so that a proportion of the total number of reinforcements could be specified for each color. Experimental manipulations involved variations of (1) the proportions for each color, (2) changeover delay, or, alternatively, (3) a fixed‐ratio changeover requirement. The main findings were: (1) relative overall rates of responding and relative times in the presence of a key color approximated the proportions of reinforcements obtained in the presence of that color, while relative local rates of responding changed little; (2) changeover rate decreased as the proportions diverged from 0.50; (3) relative overall rate of responding and relative time remained constant as the changeover delay was increased from 2 to 32 sec, with reinforcement proportions for red and green of 0.75 and 0.25, but they increased above 0.90 when a fixed‐ratio changeover of 20 responses replaced the changeover delay; (4) changeover rate decreased as the delay or fixed‐ratio was increased.

Rats responded on a fixed‐interval schedule during which a 3‐sec stimulus preceded each water reinforcement. The stimulus was then scheduled concurrently for responses on the same lever according to either a variable interval or a variable ratio. Although water reinforcement. continued on a fixed‐interval schedule, the pattern of responding became typical of a variable‐interval or variable‐ratio schedule. When the 3‐sec stimulus was presented on a variable‐interval or variable‐ratio schedule, but was omitted on the fixed‐interval schedule, the response rate decreased. When the stimulus occurred after the same time periods as those of the variable‐interval schedule, but at least 7‐sec after the last response, the rate decreased. The rate became higher when the fixed‐interval schedule was discontinued and each presentation of the 3‐sec stimulus was followed by water on a variable‐interval schedule. When both water and the 3‐sec stimulus were discontinued for a period of time, resulting in extinction of the lever response, and the 3‐sec stimulus alone then presented on a variable‐interval or variable‐ratio schedule after lever responses, rate increased and then gradually decreased.

Four pigeons received conditional discrimination training in which reinforcement contingencies were related to specific combinations of color and form, but were unrelated to either color or form considered separately. During discrete‐trial training, each response in the presence of two of four color‐form displays produced reinforcement and terminated the trial; responding to the other two displays was never reinforced, and each such response prolonged the particular trial on which it occurred. Subsequently, the subjects received multiple‐schedule training in which responding to either of the displays previously associated with reinforcement was now reinforced on a variable‐interval schedule, and extinction was the schedule again correlated with the other two displays. After differential responding to the stimuli was clearly evident, intensity of the combination displays was changed in subsequent training sessions. Complex stimulus control was generally maintained across variation in intensity, although there were temporary disruptions in performance associated with onset of some of the intensity changes. Finally, a component‐stimulus test revealed considerably more responding to the forms than to the colors.

The key pecking of pigeons maintained on a variable‐interval schedule of food reinforcement was suppressed during occasional presentations of a warning stimulus paired with electric shock. On alternate sessions, a co‐actor pigeon was visible in an adjoining chamber where it emitted the same food‐reinforced key peck during the warning stimulus that signalled shock for the subject. With no shock and at low shock intensities, where the subject's responding was not suppressed or suppressed only slightly, the co‐actor had little effect. At the higher shock intensities, where the subject's responding was reduced by at least 40%, the response rate during the warning stimulus was consistently higher when the co‐actor was present. One explanation of these results assumes a special relationship between social stimuli and aversive stimuli in which the presence of another animal reduces emotional reactions and thereby allows operant responses to increase. This was not the case here because the mere presence of the co‐actor did not maintain social facilitation. Rather, the present results, taken in conjunction with previous findings, suggest that changes in social and non‐social variables which affect the rate of food‐reinforced responding may produce proportionately larger changes in responding when that responding is suppressed by aversive stimulation than when it is not.

The conditioned suppression technique was employed to establish criterion discrimination of an amyl acetate concentration of 3% of vapor saturation, and to generate differential response rates in the presence of equal concentrations of amyl acetate and butyl acetate. The magnitude of suppression was also recorded as a function of amyl acetate concentration, with the concentrations presented in descending, ascending, and irregular series. The three stimulus presentation procedures generated approximately equivalent suppression versus concentration functions. Amyl acetate suppression thresholds were 0.16%, 0.50%, and 0.73% of vapor saturation for three subjects. Amyl acetate, butyl acetate, and butyric acid thresholds for two additional subjects were approximately 0.10% of vapor saturation. No suppression was recorded during control trials.

Using a conditioned suppression procedure, the effects of three contingent relationships between conditioned (CS) and unconditioned (US) stimuli were investigated. A traditional positive (if CS—then US) contingency suppressed response rate during the CS relative to responding during stimulus‐free minutes of the session. A negative (if CS—then no US) contingency resulted in suppressed responding during CS‐off minutes, and rate increases during the CS. A no‐contingency control procedure, during which CS and US were randomly related, almost totally suppressed responding throughout the session and showed no differential effects of the CS on response rate. An analysis of changes in response rate during the minute after US‐offset revealed acceleration under the no‐contingency condition and, to a somewhat lesser degree, under the negative contingency. Both conditioned suppression and non‐suppression are analyzed in terms of the temporal relationship between CS and US.

Inter‐subject differences in response rates under free‐operant avoidance and escape schedules are commonly obtained from humans. Data are presented which demonstrate that such differences can be controlled experimentally by giving subjects different conditioning histories. Subjects given a fixed‐ratio history avoided and/or escaped from “point‐loss periods” at higher rates than subjects given a differential‐reinforcement‐of‐low‐rates history. History related differences in response rates were maintained during 40 hr of escape responding. For low‐rates history subjects, response rates under escape contingencies decreased as the rate of point‐loss periods decreased.

Pigeons performed on second‐order schedules of reinforcement consisting of four fixed‐interval components. Only the terminal component ended with food. Performance was studied both when a brief stimulus followed the completion of each of the first three fixed intervals (brief‐stimulus schedule) and when the stimulus was omitted (tandem schedule). Variations in the temporal contiguity of the last presentation of the stimulus and the presentation of food indicated that the shorter the delay, the greater was the enhancement of rate of responding in comparison with tandem performance. A positively accelerated pattern of responding within fixed‐interval components was a function of the contiguity of the brief stimulus and reinforcement; this pattern was absent for all tandem‐schedule performance.