Changeover behavior and preference in concurrent schedules

Pigeons were trained on a multiple schedule of reinforcement in which separate concurrent schedules occurred in each of two components. Key pecking was reinforced with milo. During one component, a variable-interval 40-s schedule was concurrent with a variable-interval 20-s schedule; during the other component, a variable-interval 40-s schedule was concurrent with a variable-interval 80-s schedule. During probe tests, the stimuli correlated with the two variable-interval 40-s schedules were presented simultaneously to assess preference, measured by the relative response rates to the two stimuli. In Experiment 1, the concurrently available variable-interval 20-s schedule operated normally; that is, reinforcer availability was not signaled. Following this baseline training, relative response rate during the probes favored the variable-interval 40-s alternative that had been paired with the lower valued schedule (i.e., with the variable-interval 80-s schedule). In Experiment 2, a signal for reinforcer availability was added to the high-value alternative (i.e., to the variable-interval 20-s schedule), thus reducing the rate of key pecking maintained by that schedule but leaving the reinforcement rate unchanged. Following that baseline training, relative response rates during probes favored the variable-interval 40-s alternative that had been paired with the higher valued schedule. The reversal in the pattern of preference implies that the pattern of changeover behavior established during training, and not reinforcement rate, determined the preference patterns obtained on the probe tests.     

Temporal control of behavior: schedule interactions

In Experiment I the response that terminated the postreinforcement pauses occurring under a fixed-interval 60-second schedule was reinforced, if the pause duration exceeded 30 seconds. The percentage of such pauses, rather than increasing, decreased. There were complex effects on the discriminative control of the pause by the reinforcer terminating the previous fixed interval, depending on whether the fixed interval and the added reinforcer were the same or different. In Experiments II(a) and II(b), each reinforcement initiated an alternative fixed-interval interresponse-time-greater-than-t-sec schedule, the schedule values being systematically varied. When the response following a pause exceeding a given duration was reinforced, fewer such pauses occurred than when they were not reinforced, i.e., on the comparable simple fixed-interval schedule. There was no systematic relationship between mean interrinforcement interval and duration of the postreinforcement pause. The pause duration initiated by reinforcement was directly related to the dependency controlling the shortest pause at that time, regardless of changes in mean interreinforcement interval.     

Responding maintained under fixed-interval and fixed-time schedules of electric shock presentation

Following initial histories under a schedule of electric shock postponement, lever pressing in squirrel monkeys was maintained under fixed-interval and fixed-time schedules of electric shock presentation. No difference in either rate or pattern of responding was obtained when these schedules were presented as components of a multiple schedule. When they were presented singly for long periods of time, the fixed-interval schedule consistently maintained a higher response rate than the fixed-time schedule. The pattern of responding under both schedules was similar, typically consisting of a pause at the beginning of each interval followed by either a steady or a positively accelerating rate of responding. The results suggest that the response-shock dependency is of critical importance in the maintenance of high rates of responding under schedules of electric shock presentation, and support the general view that such responding may be conceptualized as operant behavior under control of many of the same variables that control responding under comparable schedules of food or water reinforcement.     

The relative law of effect: effects of shock intensity on response strength in multiple schedules

Key pecking of four birds was reinforced with food according to a two-component multiple variable-interval 1-minute variable-interval 4-minute schedule. In addition, key pecking was punished by a brief shock according to a variable-interval 30-second schedule during both components of the multiple schedule. The intensity of the shock was varied. For all birds, punishment had a stronger suppressive effect on the responding maintained by the leaner food schedule, and the ratio of responding during the two components of the multiple schedule became closer to the ratio of reinforcement as shock intensity was increased, as the relative law of effect predicts. At the higher shock intensity, there was some evidence that the ratio of responses overmatched the ratio of reinforcements.     

Interactions in multiple schedules: negative induction with squirrel monkeys

In Experiment I, lever pressing by squirrel monkeys was maintained under a sequence of variable-interval, multiple variable-interval variable-interval, and multiple variable-interval extinction schedules of food presentation. Negative induction (decreased responding in the unchanged component) occurred when one component of the multiple variable-interval variable-interval schedule was changed to extinction. Negative induction was transient over sessions; responding in the unchanged component usually recovered to a rate similar to that under the multiple variable-interval variable-interval schedule. Negative induction was not accompanied by consistent changes in the patterns of local responding within the unchanged component, and did not depend on whether component schedules were associated with localized (lever lights) or diffuse visual stimuli (houselights), or on whether the unchanged component was a 60- or 180-sec variable-interval schedule. In Experiment II, responding was maintained under a sequence of variable-interval and multiple variable-interval timeout schedules of food presentation. Negative induction occurred when responding declined gradually in the timeout component but not when responding declined abruptly. The nature of interactions in multiple schedules may depend on the species; negative induction was observed with squirrel monkeys under conditions similar to those that produce positive contrast with pigeons.     

Drug effects on responding maintained by stimulus-reinforcer and response-reinforcer contingencies.

The effects of pentobarbital and d-amphetamine were assessed on key pecking by pigeons under conventional single-key multiple schedules and under two-key multiple schedules in which discriminative stimuli appeared on one key (stimulus key) while pecks on a second key (constant key) produced food. Pecks on the stimulus key had no scheduled consequences. A 60-second variable-interval schedule operated in one component of each multiple schedule: either extinction or a 60-second variable-time schedule operated in the alternate component. When the alternate-component schedule was extinction, a high rate of responding was maintained in the variable-interval component of the single-key schedule; responding on both keys was maintained in the variable-interval component of the two-key schedule. Pentobarbital increased responding in the variable-interval component of the single-key schedule and increased stimulus-key, but not constant-key responding in that component of the two-key schedule. When the alternate-component schedule was changed to variable time, responding declined in the variable-interval component of the single-key schedule; stimulus-key responding was no longer maintained under the two-key schedule. Pentobarbital decreased responding in the variable-interval component of both schedules. With an exception, d-amphetamine only decreased responding in the variable-interval component of the single- and two-key schedules both when the alternate-component schedule was extinction and when it was variable time. The results suggest that the effects of pentobarbital, but not d-amphetamine, depend on the nature of the contingency (stimulus-reinforcer, response-reinforcer) that maintains responding.     

A molar theory of reinforcement schedules

Behavior of subjects exposed to concurrent and individual interval and ratio schedules of reinforcement may be described in terms of a set of expressions relating the value of responses to their durations, a feedback equation relating reinforcement to response duration, and the assumption that subjects allocate their time among various responses so as to maximize value.     

Delayed reinforcement of fixed-ratio performance without mediating exteroceptive conditioned reinforcement

The performance of pigeons was studied under conditions in which the completion of a fixed-ratio requirement was not contiguous with the presentation of a reinforcer. Timein and timeout periods alternated throughout the experimental sessions. Responses made by an experimental bird during the timein period were accumulated, and when a fixed-ratio requirement had been met, grain was presented to the experimental bird and a yoked control following their first response in the next timein period. Across most manipulations of the fixed-ratio requirement and of the duration of the timeout period, the response rates of the experimental birds were considerably higher than those of their controls, suggesting that the response-reinforcer dependency controlled the behavior of the experimental bird in the absence of a close temporal association between responding on the ratio schedule and reinforcer presentations.     

Variability of response location on fixed-ratio and fixed-interval schedules of reinforcement

Variability of response location was studied in monkeys performing in a six-lever chamber. Fixed-ratio schedules, ranging from FR 1 to FR 300, generated a high degree of stereotypy of response location. In contrast, fixed-interval schedules of comparable reinforcement frequencies (0.06 to 4 minutes) generated much greater variability. These results failed to confirm any simple relationship between response variability and intermittence of reinforcement. Rather, variability seems to be determined by the particular characteristics of the reinforcement schedule.