Stimulus control of respondent and operant key pecking: A single key procedure

Pigeons' responses to a uniformly illuminated response key were either reinforced on a variable-interval one-minute schedule of reinforcement or extinguished for one-minute periods. When 1.5 second signals were presented at the beginning of each component, so as to differentially predict reinforcement, the pigeons pecked at the signals, at rates higher than rates during the remainder of the component. When the brief signals were not differentially predictive of reinforcement, pecking in their presence decreased to near zero levels. Similar results were obtained with signals based upon colors and upon line orientations. Changes in rates of (unreinforced) pecking occurred during the signal whether pigeons responded differentially during the remainder of the component or not. Experiment II demonstrated that the presence of the signal correlated with extinction was not necessary for pecking to develop at the signal which preceded the component in which responding was intermittently reinforced. The experiments demonstrated a clear dissociation of respondent control from operant control of a response. In addition, operant behavior was shown to be relatively insensitive to differing rates of reinforcement, as compared to the sensitivity of respondent behavior to differing rates of reinforcement produced by the very same operant behavior.     

Self-control in pigeons under the Mischel paradigm

Walter Mischel studied self-control in preschool children in the following manner: if the child waited for an interval to end, he or she received the more preferred of two reinforcers; if the child responded to terminate the interval by ringing a bell, the less preferred reinforcer was given. We used an analogous procedure to study self-control in pigeons: if the bird waited for a trial to end, it received the more preferred reinforcer; if the bird terminated the trial by pecking a key, the less preferred reinforcer was given. We explored the effects on self-control of a number of variables analogous to those studied by Mischel and co-workers, e.g., presence versus absence of reinforcers, of alternative responses, and of stimuli during the wait interval; prior experience of the subjects; and test paradigm. The results obtained with pigeons paralleled the results obtained by Mischel with human children.     

Stimulus control of delayed matching in pigeons: Directed forgetting

Pigeons were trained in delayed matching-to-sample with two postsample stimuli. A postsample R-cue signaled that a matching choice phase would follow. A postsample F-cue signaled that a matching choice phase would not follow. Previous research found reduced matching accuracy on F-cued probe trials when comparison stimuli were presented in the choice phase. The present four experiments systematically varied the events following an F-cue to determine the conditions under which the F-cue reduces delayed-matching accuracy. When F-cues and R-cues controlled different behavior, matching on probe trials was poor. When both cues controlled the same behavior, matching on probe trials was good. This result is best explained by the theory that comparison stimuli retrieve the sample representation, but only in the behavioral context established by the R-cue. The present research supports the view that response-produced stimuli serve a contextual role in animal short-term memory.     

Drug effects on repeated acquisition: comparison of cumulative and non-cumulative dosing

Pigeons acquired a different four-response chain each session by responding sequentially on three keys in the presence of a sequence of four colors. The response chain was maintained by food presentation under a fixed-ratio schedule. Errors produced a brief timeout but did not reset the chain. Each day there were four 15-minute sessions, with a 10-minute inter-session interval. Cumulative dose-effect curves for phencyclidine, pentobarbital, and d-amphetamine were obtained by giving an injection before each of the four sessions; successive injections increased the cumulative dose in equally spaced logarithmic steps. For comparison, non-cumulative doses of each drug (i.e., doses not preceded by other doses on the same day) were also tested. As the cumulative dose of each drug increased, the overall response rate decreased, the percent errors increased, and there was less within-session error reduction (acquisition). With phencyclidine and pentobarbital, the rate-decreasing and error-increasing effects tended to be greater with a non-cumulative dose than with the corresponding cumulative dose. In contrast, with d-amphetamine, the effects were considerably greater with the cumulative doses. The results indicate that although the cumulative-dosing procedure saved a substantial amount of time in determining dose-effect curves, there were quantitative differences in effects between cumulative and non-cumulative doses.     

Hill-climbing by pigeons

Pigeons were exposed to two types of concurrent operant-reinforcement schedules in order to determine what choice rules determine behavior on these schedules. In the first set of experiments, concurrent variable-interval, variable-interval schedules, key-peck responses to either of two alternative schedules produced food reinforcement after a random time interval. The frequency of food-reinforcement availability for the two schedules was varied over different ranges for different birds. In the second series of experiments, concurrent variable-ratio, variable-interval schedules, key-peck responses to one schedule produced food reinforcement after a random time interval, whereas food reinforcement occurred for an alternative schedule only after a random number of responses. Results from both experiments showed that pigeons consistently follow a behavioral strategy in which the alternative schedule chosen at any time is the one which offers the highest momentary reinforcement probability (momentary maximizing). The quality of momentary maximizing was somewhat higher and more consistent when both alternative reinforcement schedules were time-based than when one schedule was time-based and the alternative response-count based. Previous attempts to provide evidence for the existence of momentary maximizing were shown to be based upon faulty assumptions about the behavior implied by momentary maximizing and resultant inappropriate measures of behavior.     

Concurrent schedules: Spatial separation of response alternatives

Four pigeons were exposed to independent concurrent variable-interval 20-second variable-interval 60-second schedules of reinforcement. A transparent partition was inserted midway between the two response keys. The length of the partition was systematically manipulated. Increasing partition length produced a decrease in changeover rate in Experiment 1. Over-matching was observed with a partition length of 20 centimeters. In Experiment 2 a four-second limited hold was added to the schedules. Increasing partition length produced a decrease in changeover rate that exceeded the decrease observed in Experiment 1. This manipulation produced nearly exclusive choice of the variable-interval 20-second component. The present results, together with results obtained in related research, suggest that deviation from matching is a function of procedural variables that determine the consequences of a changeover response.     

Stimulus control of Pavlovian facilitation

Two experiments were conducted using an autoshaping procedure with pigeons to examine whether dimensional stimulus control by a Pavlovian facilitator parallels the control established following operant discrimination training. Facilitation training consisted of the presentation of a black vertical line on a white background as the B stimulus in a feature-positive discrimination in which the A stimulus (white keylight) was followed by grain presentation only if preceded by B. In this way, B facilitates or sets the occasion for pecking at A. Subsequent testing for generalization along the line-orientation dimension produced decremental gradients when the facilitation paradigm incorporated an explicit feature-negative stimulus (B−). These results parallel the decremental control obtained following operant discrimination training and suggest that Pavlovian facilitators and instrumental discriminative stimuli are functionally equivalent.     

Concurrent schedules: Effects of time- and response-allocation constraints

Five pigeons were trained on concurrent variable-interval schedules arranged on two keys. In Part 1 of the experiment, the subjects responded under no constraints, and the ratios of reinforcers obtainable were varied over five levels. In Part 2, the conditions of the experiment were changed such that the time spent responding on the left key before a subsequent changeover to the right key determined the minimum time that must be spent responding on the right key before a changeover to the left key could occur. When the left key provided a higher reinforcer rate than the right key, this procedure ensured that the time allocated to the two keys was approximately equal. The data showed that such a time-allocation constraint only marginally constrained response allocation. In Part 3, the numbers of responses emitted on the left key before a changeover to the right key determined the minimum number of responses that had to be emitted on the right key before a changeover to the left key could occur. This response constraint completely constrained time allocation. These data are consistent with the view that response allocation is a fundamental process (and time allocation a derivative process), or that response and time allocation are independently controlled, in concurrent-schedule performance.     

The interaction between stimulus and reinforcer control on remembering.

In a symbolic matching-to-sample task, 6 pigeons obtained food by pecking a red side key when the brighter of two white lights had been presented on the center key and by pecking a green side key when the dimmer of two white lights had been presented on the center key. Across Part 1 and Parts 6 to 10, the delay between sample-stimulus presentation and the availability of the choice keys was varied between 0 s and 25 s. Across Parts 1 to 5, the delay between the emission of a correct choice and the delivery of a reinforcer was varied between 0 s and 30 s. Although increasing both types of delay decreased stimulus discriminability, lengthening the stimulus-choice delay produced a greater decrement in choice accuracy than did lengthening the choice-reinforcer delay. Additionally, the relative reinforcer rate for correct choice was varied across both types of delay. The sensitivity of behavior to the distribution of reinforcers decreased as discriminability decreased under both procedures. These data are consistent with the view, based on the generalized matching law, that sample stimuli and reinforcers interact in their control over remembering.     

CONDITIONAL DISCRIMINATION VS. MATCHING TO SAMPLE: AN EXPANSION OF THE TESTING PARADIGM

A subject's performance under a conditional-discrimination procedure defines conditional relations between stimuli: “If A1, then B1; if A2, then B2.” The procedure may also generate matching to sample. If so, the stimuli will be related not only by conditionality, but by equivalence: A1 and B1 will become equivalent members of one stimulus class, A2 and B2 of another. One paradigm for testing whether a conditional-discrimination procedure has generated equivalence relations uses three sets of stimuli, A, B, and C, three stimuli per set. Subjects learn to select Set-B and Set-C comparisons conditionally upon Set-A samples. Having been explicitly taught six sample-comparison relations, A1B1, A1C1, A2B2, A2C2, A3B3, and A3C3, subjects prove immediately capable of matching the B- and C-stimuli; six new relations emerge (B1C1, B2C2, B3C3, C1B1, C2B2, C3B3). The 12 stimulus relations, six taught and six emergent, define the existence of three three-member stimulus classes, A1B1C1, A2B2C2, and A3B3C3. This paradigm was expanded by introducing three more stimuli (Set D), and teaching eight children not only the AB and AC relations but DC relations also—selecting Set-C comparisons conditionally upon Set-D samples. Six of the children proved immediately capable of matching the B- and D-stimuli to each other. By selecting appropriate Set-B comparisons conditionally upon Set-D samples, and Set-D comparisons conditionally upon Set-B samples, they demonstrated the existence of three four-member stimulus classes, A1B1C1D1, A2B2C2D2, and A3B3C3D3. These larger classes were confirmed by the subjects' success with the prerequisite lower-level conditional relations; they were also able to select Set-D comparisons conditionally upon samples from Sets A and C, and to do the BC and CB matching that defined the original three-member classes. Adding the three DC relations therefore generated 12 more, three each in BD, DB, AD, and CD. Enlarging each class by one member brought about a disproportionate increase in the number of emergent relations. Ancillary oral naming tests suggested that the subject's application of the same name to each stimulus was neither necessary nor sufficient to establish classes of equivalent stimuli.