Control of pigeons' pecking by trace stimuli

In Experiment I, pigeons' pecking a white key was reinforced with grain when white was immediately preceded by a vertical white line on a green surround, but not by green alone. This procedure produced control of pecking by the line. Next, pecking white was reinforced after vertical line on green, but not after green alone or other orientations of the white line on green. The line-tilt dimension initially did not control pecking, a result that showed that interdimensional (line versus no line) training does not always result in dimensional control. Line-tilt control was eventually established but was accompanied by a decrease in interdimensional control. In Experiment II, interdimensional training, with or without a trace interval intervening between line on green or green alone and white, was followed by tests for line-tilt control. While interdimensional control was unaffected by the trace interval, line-tilt control tended to be less with the trace interval. This dissociation of interdimensional and dimensional control, as well as the failure of interdimensional training to produce dimensional control in Experiment I, suggests that the line stimulus is multidimensional.     

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.     

Generalization gradients of inhibition after different amounts of training

Five groups of pigeons received seven sessions of variable-interval reinforcement for pecking a blank white key, followed by either 1, 2, 4, 8, or 16 sessions of training on a successive discrimination in which the positive stimulus was the blank white key and the negative stimulus was a black vertical line on the white key. After training, a generalization test was administered along the line-tilt continuum. Relative gradients of inhibition became steeper with increased amounts of training, and reliably nonhorizontal absolute gradients were obtained only from groups of subjects with at least four days of training. Therefore, inhibitory stimulus control improves with added training. Several problems with the concept of “inhibition” are examined and some implications of the results for theoretical analyses of operant discrimination learning are discussed.     

Concurrent performances: stimulus-control gradients during schedules of signalled and unsignalled concurrent reinforcement

On one key, pigeons' pecks were reinforced according to a variable-interval schedule in the presence of vertical lines, and were not reinforced in the presence of oblique lines. On a second key, pecks were reinforced according to a variable-interval schedule in the presence of blue, according to a signalled variable-interval schedule in the presence of red, and were not reinforced in the presence of white. Subsequently, during extinction, stimulus-control gradients were obtained by presenting eight different line orientations on the first key concurrent with each of the three colors on the second key. On the first key, line-orientation gradients tended to be lower, narrower, and less shifted in peak or area when the second-key stimulus was blue or red, the stimuli respectively correlated with unsignalled and signalled reinforcement, than when it was white, the stimulus correlated with extinction. Thus, the effect on first-key line-orientation gradients depended on second-key stimuli correlated with concurrent reinforcement, whether or not these stimuli were also correlated with concurrent responding. As a function of first-key line orientation, an inverted gradient was obtained on the second key during blue; during both red and white, rates of pecking on the second key were near zero.     

Control of responding by stimulus duration

Pigeons were trained on a procedure in which the key was white for 30 sec, alternating with periods of darkness, or timeout. In a nondifferential training procedure, timeout duration was held constant at either 9 or 21 sec for different animals, and pecks on the white key were reinforced on a variable-interval 36-sec schedule. After 30 sessions an extinction generalization test was conducted where the duration of the timeout was varied from 3 to 27 sec. This test showed no differences in responding following timeouts of different durations. In a differential training procedure, timeout durations of either 9 or 21 sec were randomly scheduled for each animal. The variable-internal schedule was in effect following the same timeout duration as in the prior nondifferential procedure. No pecks were reinforced after the other timeout duration. In 40 sessions, differences in response rates following the two durations gradually developed. A maintained generalization procedure was then imposed in which timeout durations were varied from 3 to 27 sec, with the variable-interval schedule in effect following only the same duration as in the previous procedures. The first maintained generalization session showed that the prior differential training had established control of the animals' behavior by the timeout duration. In continued training on the maintained generalization procedure, control by the timeout duration decreased.     

The role of preliminary magazine training in acquisition of the autoshaped key peck

A series of experiments tested the hypothesis that initial key pecks in the autoshaping procedure are generalized pecks at the illuminated grain hopper. Experiment I found that autoshaping readily occurred when the chamber was continuously illuminated by a house-light. In Experiment II, pigeons given magazine training and autoshaping with an unlighted grain hopper failed to autoshape in 200 trials. Acquisition of autoshaped key pecking was retarded in Experiment III when stimulus control by the magazine light was reduced. In the fourth study, pigeons were given magazine training with either a red or white magazine light and then given autoshaping with concurrently presented red and white keys. For all pigeons in this experiment, the first key peck occurred on the key of the same color as that pigeon's magazine light. The results of these experiments were interpreted as supporting an account of autoshaping that identifies initial key pecks as arising due to generalization of pecking at the lighted grain hopper to pecking at the lighted key.     

Alternative reinforcement effects on fixed-interval performance

Pigeons' key pecks were reinforced with food on a fixed-interval schedule. Food also was available at variable time periods either independently of responding or for not key pecking (a differential-reinforcement-of-other-behavior schedule). The latter condition arranged reinforcement following the first pause of t seconds after it became available according to a variable-time schedule. This schedule allowed separation of the effects of pause requirements ≤ five-seconds and reinforcement frequency. The time spent pausing increased as the duration of the pause required for reinforcement increased from 0 to 30 seconds and as the frequency of reinforcement for pausing increased from 0 to 2 reinforcers per minute. Key pecking was more evenly distributed within each fixed interval with shorter required pauses and with more frequent reinforcement for pausing. The results complement those obtained with other concurrent schedules in which the same operant response was reinforced in both components.     

Local contrast and maintained generalization.

Pigeons received variable-interval reinforcement for key pecking during presentations of horizontal and vertical line-orientation stimuli, while pecks during five intermediate orientations were extinguished. Lowest peck rates were observed during presentations of negative stimuli adjacent to the positive orientations while peck rate during 45 degrees (the intermediate negative orientation) was relatively high, i.e., there were negative contrast shoulders. When peck rates were manipulated in the positive orientations, peck rate in neithboring orientations changed in the opposite direction. Contrast shoulders faded after prolonged training. A second type of contrast, local contrast, was correlated with similarity of preceding stimulus and different average peck rates during different stages of the discrimination process. The data suggest that sequential local contrast accompanying the formation of a discrimination contributes to the form of generalization gradients. Blough's model of stimulus control predicts the changes in gradient form described here, but may not accurately depict the underlying process responsible for gradient form.     

Inhibitory stimulus control and the magnitude of delayed reinforcement

The key pecking of pigeons was reinforced according to a variable-interval 1-min schedule during each of two successively presented stimuli. When the key was illuminated by a black line on a white background, reinforcement was delayed for 10 sec. When the key was illuminated by a plain white light, reinforcement was not delayed. For half of the subjects, the delayed reinforcer was 4.0-sec access to mixed grain, and for the remaining subjects it was 1.5-sec access. The immediate reinforcer was 1.5-sec access for all subjects. All subjects responded at a lower rate during the presentation of the black line; no between-group difference in terms of terminal response rate during the presentation of the line was found. However, subjects that received 4.0 sec of delayed reinforcement responded at a lower terminal rate during presentation of the plain white light than subjects that received 1.5 sec of delayed reinforcement. A subsequent generalization test along the line-orientation dimension produced flatter U-shaped gradients for subjects that received 4.0-sec of delayed reinforcement.     

Stimulus control during conditional discrimination

Pigeons were used to assess stimulus control during the development of a conditional discrimination. The training consisted of three stages. In Stage 1, key pecks were reinforced in the presence of a white line tilted 40 degrees to the right of vertical on a green background and non-reinforced when the same line appeared on a red background. In Stage 2, key pecks were reinforced when a white vertical line appeared on a red background and were non-reinforced in the presence of a 40 degrees slanted line on a red background. In Stage 3, key pecks were reinforced in the presence of the green background regardless of the line tilt, but were differentially reinforced in the presence of the red background (as in Stage 2). Generalization tests were conducted after each stage of training and consisted of five white lines on backgrounds that were green, red, or dark. The effects of the differential reinforcement contingencies on control by line orientation were restricted to the condition in which the red light appeared and resulted in behavioral control that could be characterized as: if red, pay closer attention to line tilt than if not red.     

EFFECTS OF ALTERNATIVE REINFORCEMENT: DOES THE SOURCE MATTER?1

In a chamber with a single response key, pigeon's key pecks were reinforced with food according to a variable-interval schedule. In addition, extra reinforcements occurred concurrently according to an independent schedule. In one condition, availability of the extra reinforcements was signalled by a change in key color from white to red. The extra reinforcements occurred after a peck on the red key. In a second condition, the extra reinforcements were unsignalled and occurred only after a 2-sec pause in pecking for one group of subjects and were unsignalled and occurred freely as scheduled for another group of subjects. In the first two conditions, duration of reinforcement was varied. A third condition duplicated the second but varied rate rather than duration of reinforcement. The rate of pecking varied inversely with the amount of extra reinforcement per unit time according to the same function, regardless of the condition regulating occurrence of the extra reinforcements, and regardless of whether or not a 2-sec pause was required for their occurrence. The shape of this function was predicted by Herrnstein's (1970) matching law.     

Differential reinstatement predicted by preextinction response rate

Reinstatement refers to the recovery of previously extinguished responding by the responseindependent delivery of a stimulus that was a reinforcer in training. Two experiments were conducted to examine relative reinstatement following the training of differential preextinction response rates, either with equal (Experiment 1) or unequal (Experiment 2) preextinction reinforcement rates. In Experiment 1, each of 3 pigeons first pecked at relatively high rates in the tandem variable-time 117-sec fixed-interval 3-sec component of a multiple schedule and at lower rates in a separate tandem variableinterval 117-sec fixed-time 3-sec component. Reinforcement rates were equal between components. Pecking then was extinguished in each component, before being reinstated under a multiple variabletime 120-sec variable-time 120-sec schedule. Greater reinstatement occurred in the component previously correlated with higher rates of pecking. In Experiment 2, in an initial condition, the mean rate of lever pressing for one group of 8 rats was significantly higher under a fixed-ratio 3 schedule than for another group of 8 rats under a fixed-ratio 1 schedule. Mean reinforcement rate was significantly higher for the group exposed to the fixed-ratio 1 schedule. For each group, lever pressing then was extinguished, before being reinstated under a variable-time 30-sec schedule. Significantly greater mean reinstatement occurred for the group previously exposed to the fixed-ratio 3 schedule. These results suggest that differential reinstatement may be predicted by preextinction response rate, perhaps independently of preextinction reinforcement rate.     

Required pecking alters judgments of the passage of time by pigeons

There is evidence that how humans perceive time is affected by the activity in which they are engaged when they are judging time. In humans, typically, the more demanding the task, the faster time seems to pass. We asked whether a similar effect could be found in pigeons. Pigeons were trained to discriminate between a short-(2-sec) and a long-(10-sec) duration stimulus. Depending on the color of the stimulus (white or blue), the pigeons were required to peck (at least once per second or the trial was aborted) or to refrain from pecking (pecks aborted the trial). Once these tasks had been acquired to a high degree, probe trials involving white and blue stimuli were presented at durations between 2 and 10 sec. On trials in which the pigeons were required to peck, the point of subjective equality (i.e., the point at which pigeons are equally likely to choose the stimulus associated with long stimuli as the stimulus associated with short stimuli) was almost 1 sec longer than on trials in which the pigeons were required to refrain from pecking. In other words, on trials that required pecking, more time passed before the pigeons indicated that the probe duration was at the subjective midpoint between 2 and 10 sec than on trials that did not require pecking. This result suggests that like humans, the pigeons underestimated the passage of time when they were active or when attention to time-related cues had to be shared with attention to satisfying the response rate requirement.     

Peak shift in concurrent schedules

Pigeons were exposed to two keys, a main key and a changeover key. Initially non-differential training was given in which pecking the main key was reinforced on a variable-interval 2-min schedule when the key displayed the first stimulus, a black line on a blue background, and was reinforced on an identical but independent variable-interval 2-min schedule when the key displayed a plain blue stimulus. Later, differential training was given in which pecking the main key was reinforced on a variable-interval 2-min schedule when the first stimulus was displayed; and was reinforced on a variable-interval 10-min schedule when a second stimulus, a black line of another orientation on a blue background, was displayed. During non-differential and differential training, each peck on the changeover key changed the stimulus on the main key. Generalization tests were given before and after the differential training. These consisted of presentations on the main key of seven orientations of the black line on the blue background, including the first and second stimuli, with no reinforcements being given. Changeover-key pecks changed the stimuli on the main key. Generalization gradients were obtained using three measures: time spent, responses, and response rate in the presence of each test stimulus. Typically, maximum values on these measures occurred to stimuli away from the first in a direction opposite the second stimulus, and minimum values occurred to stimuli away from the second in a direction opposite the first.     

Control of key pecking by the duration of a visual stimulus

Pigeons' key pecks were brought under the control of the duration of a visual stimulus in one-key and two-key procedures. In the one-key procedure, pecks were reinforced after presentations of a long-duration stimulus but not after presentations of a short-duration stimulus. In the two-key procedure, left-key pecks were reinforced after the long-duration stimulus and right-key pecks after the short-duration stimulus. In both procedures, the long-duration stimulus was 10 sec, and the short-duration stimulus was increased from 1 to 8 sec in 1-sec steps. Discriminative control developed with both procedures, but with greater accuracy in the two-key procedure, in which a difference threshold was obtained at short-duration values between 7 and 8 sec, or about 2.5 sec shorter than the long-duration stimulus.     

Stimulus generalization and the response-reinforcement contingency

Generalization gradients along a line-tilt continuum were obtained from groups of pigeons that had been trained to peck a key on different schedules of reinforcement. In Exp. I, gradients following training on a differential-reinforcement-of-low-rate (DRL) schedule proved to be much flatter than gradients following the usual 1-min variable interval (VI) training. In Exp. II, the value of the VI schedule itself was parametrically studied; Ss trained on long VI schedules (e.g., 4-min) produced much flatter gradients than Ss trained on short VI schedules (30-sec; 1-min). The results were interpreted mainly in terms of the relative control exerted by internal, proprioceptive cues on the different reinforcement schedules. Several implications of the results for other problems in the field of stimulus generalization are discussed.     

Discrimination training facilitates pigeons' performance on one-trial-per-day delayed matching of key location

Six pigeons were tested on a one-trial-per-day variant of delayed matching of key location. In one condition, a trial began with the illumination of a pair of quasi-randomly selected pecking keys in a large 10-key test box. Pigeons' pecks to one key (the sample) were reinforced with 8-second access to grain on a variable-interval 30-second schedule, whereas pecks to the other key (the distractor) had no scheduled consequences. In the second condition, the nonreinforced distractor was not presented. In both conditions, subjects were removed from the apparatus after 15 minutes and placed in a holding cage. Subjects were subsequently replaced in the box after a delay (retention interval) of 30 seconds and were reexposed to the illuminated sample and distractor keys for 1 minute. If a pigeon made more pecks to the sample during this interval, the distractor was extinguished and subsequent pecks to the sample were reinforced on the previous schedule for an additional 15 minutes. If, however, a pigeon made more pecks to the distractor, both keys were extinguished and the subject was returned to its home cage. For all subjects, matching-to-sample accuracy was higher in the first condition. In a second experiment, the retention interval was increased to 5, 15, and 30 minutes, and then to 1, 2, 4, 8, 12, and 24 hours. Most subjects remembered the correct key location for up to 4 hours, and in one case, up to 24 hours, demonstrating a spatial-memory proficiency far better than previously reported in this species on delayed matching tasks. The results are discussed in terms of the commonly held distinction between working and reference memory.     

Dimensional stimulus control following brief wavelength training

Pigeons with extensive training pecking a key illuminated by a white line then had brief training with the key illuminated by 555 nanometers. This was immediately followed by a wavelength generalization test in extinction. Dimensional stimulus control about the training wavelength increased with the duration and number of reinforcements given on variable-interval 30-sec and variable-interval 10-sec schedules in Experiment I. In Experiment II, dimensional stimulus control was obtained after only 4 min of wavelength training from birds with prior and independent discrimination training. Experiment III provided groups equated in number of reinforcers with groups in Experiment I and two 8-min duration groups. Analyses, which included results from both Experiments I and III, showed that dimensional stimulus control increased: (a) more rapidly as a function of the duration of variable-interval 10-sec than variable-interval 30-sec reinforcement; (b) at the same rate across variable-interval reinforcement schedules, as a function of the number of reinforcers available during brief wavelength training.     

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.     

Response summation to a compound stimulus in a context of choice

Key pecks by two groups of pigeons were reinforced on concurrent schedules. For group Ē, pecks were reinforced during either a visual or an auditory stimulus; for group E, an additional, extinction component was available, during which both visual and auditory stimuli were absent. After training, both groups were given a compound test to measure preference among four stimuli, the three used in training plus a compound of the visual and auditory stimulus. Group E showed preference for the compound, emitting more pecks and spending more time in this stimulus than in other stimuli. Group Ē showed no preference between the compound and visual stimulus, nor between the auditory stimulus and the absence of both stimuli, but preferred the former pair over the latter pair of stimuli.