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Rat's bar‐press responses were maintained at moderate rates during separate presentations of light and tone by separate but concurrent variable‐interval schedules of food and shock presentation. The relative response rate maintained during light‐out‐no‐tone was alternated in four successive phases: in Phases 1 and 3 responding was maintained at a higher rate than that during light and tone alone by a variable‐interval food schedule, while in Phases 2 and 4 responding was reduced to a lower rate by a differential‐reinforcement‐of‐other‐behavior food schedule. In test presentations of light, tone and a light‐plus‐tone combination, administered at the end of each phase, the proportion of responses emitted during light‐plus‐tone was an inverse function of the relative response rate controlled by light‐out‐no‐tone, indicating that the relative training response rate controlled by the absence of the single stimuli determined the control exerted by the combined stimuli. Different relative response rates maintained in training may also be partly responsible for previously observed differences in the form of generalization gradients following the establishment of multi‐stimulus control.

When offered a choice (Choice Y) between a small immediate reward (2‐sec exposure to grain) and a large reward (4‐sec exposure to grain) delayed by 4 sec, pigeons invariably preferred the small, immediate reward. However, when offered a choice (Choice X) between a delay of T seconds followed by Choice Y and a delay of T seconds followed by restriction to the large delayed reward only, the pigeon's choice depended on T. When T was small, the pigeons chose the alternative leading to Choice Y (and then chose the small, immediate reward). When T was large, the pigeons chose the alternative leading to the large delayed reward only. The reversal of preference as T increases is predicted by several recent models for choice between various amounts and delays of reward. The preference for the large delayed alternative with long durations of T parallels everyday instances of advance commitment to a given course of action. Such commitment may be seen as a prototype for self‐control.

Pigeons were trained on a multiple variable‐interval 30‐sec, variable‐interval 90‐sec schedule with each component presented alternately for an equal (on the average) duration. This average duration of exposure to each component was varied from 5 to 300 sec. The main concern was with rate of response in the variable‐interval 30‐sec component relative to rate of response in the variable‐interval 90‐sec component. In all cases, rate of response was higher in the variable‐interval 30 sec component, but the discrepancy in the rate produced by the two schedules tended to be greatest when the duration of component presentation was brief. The mean proportion of responses emitted during the variable‐interval 30‐sec component (responses in variable‐interval 30‐sec component divided by total responses) varied from about 0.60 to 0.71, where 0.75 would be expected on the basis of a matching rule, and 0.59 was that obtained by Lander and Irwin (1968). These results are in agreement with data reported by Shimp and Wheatley (1971) from a similar experiment.

The performances of three rats were stabilized on a multiple schedule that maintained responding by a free‐operant avoidance schedule during independent presentations of tone and light. The simultaneous absence of these stimuli signalled shock‐free periods and controlled response cessation. Subsequently, test sessions were administered consisting of independent presentations of each stimulus and these stimuli compounded (tone‐plus‐light). During an extinction test, additive summation was observed to the compounded stimuli, i.e., more responses were emitted to the compound than to either tone or light. During a series of 28 maintenance‐test sessions in which the shock schedule remained operative, the compounded stimuli produced a generally enhanced response rate and fewer pauses terminating with shock than either single stimulus condition. These results extend the generality of free‐operant additive summation to responding maintained by aversive control. In addition, a comparison of the present study with previous experiments reporting additive summation of positively reinforced responding indicates that similar variables—rate and aversive differences between training stimulus conditions—should be considered in accounting for response distributions during stimulus compounding when responding is controlled by either positive or negative contingencies.

Three rats were trained to lever press on concurrent random interval 2‐min random interval 2‐min schedules of milk reinforcement. With a 5‐sec changeover delay, relative response rate matched the relative reinforcement duration associated with each lever. A stimulus, during which unavoidable shocks occurred at random intervals, was superimposed on this concurrent baseline, and shifts in preference were found. However, data from this procedure were ambiguous, apparently confounded by shock‐elicited response bursts. Termination of the shocks during the stimulus resulted in a rapid recovery of matching, which was preceded by a brief facilitation of responding on the less‐preferred lever. The procedure was then changed to a conventional conditioned anxiety paradigm with a variable duration pre‐shock stimulus. A marked shift in relative response rate towards the preferred lever was found in all three rats; that is, responding on the preferred lever was far less suppressed during the pre‐shock stimulus than responding on the less‐preferred lever.

In pigeon's oddity performances, maintained by variable‐interval reinforcement of pecks on the odd key of three keys in a triangular array, accuracy and response rate varied inversely with the rate of variable‐interval reinforcement scheduled concurrently for pecks on a fourth, spatially isolated key. But when variable‐interval and extinction components alternated in a multiple schedule for pecks on the spatially isolated key, oddity accuracy was greater during variable‐interval components than during extinction components. Oddity response rate was not affected systematically by the alternating components. Changeovers between the oddity keys and the spatially isolated key were frequent during variable‐interval components; responding occurred almost exclusively on the oddity keys during extinction components. This difference in performance during the two components was eliminated by arranging stimulus‐correlated variable‐interval reinforcement in the multiple schedule on the spatially isolated key: a stimulus was presented in the variable‐interval components only when reinforcement became available, thereby reducing responding on this key to near‐zero levels in both components while maintaining the variable‐interval reinforcement. The effect of the multiple‐schedule components on oddity accuracy was not altered, however, and thus apparently depended directly on concurrent reinforcement and not on differential sequential properties of concurrent responding during the two components.

An earlier experiment scheduled variable‐interval reinforcement for pigeons' pecks on one key, and variable‐interval reinforcement alternating with extinction, in a multiple schedule, for pecks on a second key. During the second key's extinction component, first‐key pecking was relatively slow and continuous, rarely interrupted by second‐key pecking; during the variable‐interval component, first‐key pecking was frequently interrupted by second‐key pecking. When changeover delays operated, so that reinforced pecks on one key could not follow closely upon changeovers from the other key, rapid first‐key pecking between interruptions compensated sufficiently for the time lost in second‐key pecking that the overall rate of first‐key pecking remained roughly constant across the alternating multiple‐schedule components. The present experiments duplicated, on a single key, the temporal pattern of first‐key pecking generated in the earlier experiments: components of continuous key availability were alternated with components of interrupted key availability. Approximately constant overall rates of responding were observed with a single‐key equivalent of a changeover delay scheduled after interruptions and with manipulations of the on‐off durations of the interruption cycle. Rate constancies in the original concurrent situation presumably depended on analogous contingencies that operated upon the concurrent responses, rather than on any constant “reserve” of responses.

Three food‐deprived Long‐Evans rats were exposed to a non‐discriminated shock avoidance procedure. Superimposed upon this operant avoidance baseline were periodic presentations of a conditioned stimulus that was paired with food, the unconditioned stimulus. These pairings resulted in increases in the rate of shock over that recorded when the conditioned stimulus was not present. A traditional suppression ratio failed to reveal any differential effect of the conditioned stimulus on the overall rate of avoidance responding, although all subjects showed a consistent pattern of pausing and postshock response bursts during presentations of the conditioned stimulus. When food was withheld during a final extinction phase, the conditioned stimulus ceased to occasion increases in shock rates and disruptive postshock response bursts were eliminated. An analysis of conditioned suppression procedures is proposed that stresses not only operant‐Pavlovian or appetitive‐aversive incompatibility, but also the manner in which the baseline schedule of reinforcement affects operant behavior changes that are elicited by the superimposed Pavlovian procedure.

Ten naive male albino rats were trained to press a bar under a variable‐interval 30‐sec schedule with water as the reinforcer in two experiments. This behavior was disrupted by chlorpromazine in Experiment I (two rats) and by lysergic acid diethylamide (LSD) in both Experiment I (two rats) and Experiment II (six rats). The administration of the drug was paired with an originally neutral white light. After several pairings with either drug, the light also depressed behavior. When the light was no longer paired with drug, the depression effect extinguished much faster than is usually observed in conditioned suppression studies.

The responses of white rats were maintained on an unsignalled free‐operant avoidance schedule. Superimposed on the avoidance schedule was a blinking white light followed immediately by response‐independent electric shock. Duration of the light stimulus was either 1 or 3 min. Avoidable shock was 1.5 mA; response‐independent shock was 7.5 mA. Suppression of responding during the light stimulus (both durations) developed over sessions. Responding immediately following the response‐independent light‐shock sequence was neither suppressed nor accelerated. The similarity is noted between the present result and findings of “positive conditioned suppression”.