Discrimination and emission of temporal intervals by pigeons

Because the frequency distribution of IRTs showed little or no control by a DRL schedule, the schedule was modified so that the pigeon's behavior after each IRT would indicate whether or not it had discriminated the duration of the IRT. After every two pecks on a red key, the key changed to blue for 30 sec. Then it automatically became red again. Pecks on the blue key were reinforced with food on a VI schedule only when the preceding IRT on the red key had been longer than 18 sec. The birds did not selectively emit longer IRTs on the red key: the value of IRTs/op did not increase with IRT duration. However, they did discriminate the duration of the IRT emitted on the red key: the rate of pecking on the blue key was an increasing function of the duration of the preceding IRT on the red key.     

Selective sensitivity of schedule-induced activity to an operant suppression contingency.

The sensitivity of pigeons' schedule-induced activity to operant consequences was studied in two experiments. During a 30-s interval between food presentations, a keylight stimulus brightened incrementally. Stable terminal key pecking and interim locomotor activity developed. An operant "setback" contingency was applied to activity. The contingency arranged for locomotor movements (detected by a nine-panel floorboard) to be followed by a resetting of the keylight brightness to a dimmer value and a 1-s delay of reinforcement (for individual responses). Experiment 1 showed that activity patterns were highly sensitive to their operant consequences. Accompanying key-peck rates were only transiently affected. In Experiment 2, the setback contingency was imposed during restricted portions of the trial, and differential operant control of activity was demonstrated. However, birds in this study produced higher rates of key pecking as activity rates were reduced. These results suggest that although schedule-induced activity arises in response to the temporal arrangement of stimulus events, this behavior may retain considerable sensitivity to response-consequence relations.     

Topographical variations in behavior during autoshaping, automaintenance, and omission training

Three pigeons were exposed to an autoshaping and automaintenance procedure while a computer-controlled tracking system continuously recorded the position of the bird's head as it moved freely in the experimental chamber. Although only 2 birds pecked the key during the conditional stimulus (red keylight), all 3 birds exhibited stable patterns of approaching the conditional stimulus and withdrawing from the intertrial stimulus (white keylight). Subsequent exposure to an omission procedure, in which pecks on the red key cancelled the presentation of food upon the termination of the red keylight, greatly reduced key pecking, but approaching and pecking in the vicinity of the conditional stimulus were maintained at high levels. When the omission contingency was removed key pecking increased. During all phases the birds withdrew from the area of the white key and engaged in repetitive back-and-forth or circuiting movements during this intertrial stimulus. The data document (a) the strong control the conditional stimulus in autoshaping and automaintenance exerts over approach to the key and pecking motions whether or not the conditional stimulus elicits key pecking at a high level; and (b) withdrawal from the vicinity of the key and the occurrence of stereotypic behavior during the intertrial interval.     

Key pecking in pigeons produced by pairing keylight with inaccessible grain

In Experiment I, keylight was paired with inaccessible grain delivery (under two conditions of keylight intensity) to determine if autoshaping would occur in the absence of primary reinforcement. In Experiment II, the procedure was repeated with accessible grain, for comparison. In Experiment III, the procedures were repeated with explicitly unpaired presentations of keylight and either inaccessible or accessible grain. The results indicated that key pecking occurred as quickly in the presence of keylight pairings with inaccessible grain as with accessible grain, though (except for one bird) key pecking was not maintained with inaccessible grain. Furthermore, compared to the dim keylight, the bright keylight greatly suppressed key pecking when paired with inaccessible grain, and reduced the rate of key pecking when paired with accessible grain. Little key pecking occurred in groups exposed to explicitly unpaired presentations of keylight (whether bright or dim) and grain (whether accessible or inaccessible). When the birds in Experiment III were retested with explicitly paired presentations of keylight and grain, little key pecking was observed, suggesting suppressive effects of prior explicitly unpaired presentations. It is suggested that the effects of key-brightness manipulation were produced by the association of grain with cues other than the response key, or by distraction produced by partial illumination of the grain hopper.     

Automaintenance without stimulus-change reinforcement: Temporal control of key pecks

Yoked pairs of experimentally naive pigeons were exposed to a modified autoshaping procedure in which key pecking by the leader birds postponed both keylight termination and access to grain for the leader and the follower bird. Key pecking developed and was maintained in all birds and continued through two reversals of roles in the yoked procedure. Although temporal control developed more slowly in follower birds, asymptotic temporal distributions of key pecking were similar for all birds in both leader and follower roles; maximum responding occurred soon after keylight onset and decreased to a minimum prior to reinforcement. Response distributions for both leader and follower birds were described by Killeen's (1975) mathematical model of temporal control. Follower birds received response-independent reinforcement, and the development by these birds of temporal distributions which are minimal immediately prior to reinforcement is without precedent in Pavlovian appetitive conditioning. However, maintenance of key pecking by the leader birds, whose responses postponed both stimulus-change and food reinforcement, supports an interpretation of autoshaped and automaintained key pecking as responding elicited by signaled grain presentation.     

Second-order autoshaped key pecking based on an auditory stimulus.

In Experiment 1, pigeons were exposed either to paired or to unpaired presentations of a tone and grain, and then to paired presentations of a keylight with the tone. Substantial second-order conditioned pecking to the keylight was produced in the birds that had received paired presentations of tone and grain. In Experiment 2, second-order pecking to the keylight increased in probability across four groups that had received, respectively, 20, 80, 140, or 200 paired presentations of tone and grain. In Experiment 3, the amount of pecking directed towards a keylight which predicted the first-order, tone CS was as substantial in birds without a prior history of key pecking as in birds with such a history. A further experiment failed to discover any significant differences in the levels of second-order pecking to a keylight paired with a first-order tone CS or with a first-order keylight CS. Thus, an auditory signal that does not itself support pecking may enable a localized visual stimulus to evoke key pecking.     

The effects of morphine on the production and discrimination of interresponse times

Recent experiments suggest that the effects of drugs of abuse on the discrimination of the passage of time may differ for experimenter-imposed and subject-produced events. The current experiment examined this suggestion by determining the effects of morphine on the discrimination of interresponse times (IRTs). Pigeons pecked a center key on a random-interval 20-s schedule of matching-to-sample trials. Once the interval had timed out, a choice trial randomly followed either a short (2- to 3-s) or long (6- to 9-s) IRT on the center key. Pecking the side key lit one color produced food after a short IRT, and pecking the side key lit the other color produced food after a long IRT. Two experimental phases differed in the functional role of the different key colors. Under control conditions, the IRT distributions had two modes, one at the lower bound of the short category and a smaller one at the lower bound of the long category. Pigeons accurately categorized the duration of the IRTs: One key color was pecked following short IRTs and the other key color was pecked following long IRTs. Morphine flattened the IRT distribution and reduced the accuracy of categorizing IRTs. Categorization of long IRTs was particularly disrupted. Morphine did not produce overestimation of time as assessed by the production or categorization of IRTs. These results are similar to those obtained previously for the effects of morphine on the discrimination of the duration of experimenter-imposed events.     

Key pecking under response-independent food presentation after long simple and compound stimuli

Sixteen pigeons were trained to peck a key using a response-independent (auto-shaping) procedure of food presentation. The 4-sec grain presentations were independent of responding but a keylight stimulus preceded each, with a 4-min interval between the grain presentation and the next stimulus. Subjects were divided into four groups, with two durations of the keylight (30 or 120 sec) and either one or four successive colors on the response key preceding food delivery. In Phase 2, the birds were continued with the same keylight duration but were presented the alternative number of key colors. All pigeons pecked the key during the stimulus. Birds in the two groups with the 30-sec stimulus duration began to respond significantly sooner than birds with the 120-sec duration. There were no significant differences in rate of pecking between groups by the last five days of Phase 1. In Phase 1, the pigeons exposed to the four stimulus components showed an increase in rate of pecking over the four components as grain presentation approached. The pigeons with one stimulus component did not exhibit this regularity. Analogous conditions in Phase 2 had similar results except for one group. The implications of the occurrence of key pecking due to response-independent food delivery for multiple and chained schedules were pointed out.     

Generalization peak shift for autoshaped and operant key pecks.

Pigeons acquired discriminated key pecking between 528- and 540-nm stimuli by either a response-reinforcer (operant group) or a stimulus-reinforcer (autoshaped group) contingency, with other training-schedule parameters comparable over groups. For the birds in the operant group, key pecks intermittently produced grain in the presence of one hue on the key (positive stimulus) but not in the other (negative stimulus). For the birds in the autoshaped group, pecking emerged when grain was intermittently presented independently of key pecking during one key color but was not presented during the other key color. Two independent contingency assays, peck-location comparisons and elimination of differences in reinforcement rate, confirmed the effectiveness of the two training procedures in establishing operant or respondent control of key pecking. After reaching a 10:1, or better, discrimination ratio between key pecks during the two key colors, the birds received a wavelength generalization test. Criterion baseline key-peck rates were comparable for operant and autoshaped groups prior to testing. On the generalization test, performed in extinction, all birds pecked most at a stimulus removed from the positive training stimulus in the direction away from the negative stimulus. In testing, autoshaped "peak" rates (24.5 to 64.9 pecks per minute) were from 33% to 80% higher than rates in the presence of the training stimuli. Respondent peak shift rarely has been reported heretofore, and never this consistently and robustly. These results further confirm the similarity of perceptual processing in classical and operant learning. They are discussed in terms of Spence's gradient-interaction theory and Weiss' (1978) two-process model of stimulus control.     

Fixed-ratio performance with and without a postreinforcement timeout

Pigeons pecked a key, producing food reinforcement on fixed-ratio (FR) schedules requiring 50, 100, or 150 responses. In each session, 30-second timeouts were inserted before a random half of the FR trials, whereas the other trials began immediately after reinforcement. In general, preratio pauses were shorter on trials preceded by timeouts. On these trials, the probability of a first response tended to be highest in the first 20 seconds of the trials, suggesting that the shorter pauses were the result of transient behavioral contrast. Direct observations and analyses of interresponse times (IRTs) after the preratio pause indicated that IRTs could be grouped into three categories: (1) IRTs of about .1 second, which were produced by small head movements in the vicinity of the key; (2) IRTs of about .3 second, which were produced by distinct pecking motions; and (3) IRTs greater than .5 second, which were accompanied by pausing or movements away from the key. At all ratio sizes, as a subject progressed through a trial, the probability of a long IRT decreased, whereas the probability of an intermediate IRT usually increased at first and then decreased. The probability of a short IRT increased monotonically across a trial. The results show that responding changes systematically as a subject progresses through a ratio on an FR schedule. Some characteristics of performance varied as functions of the absolute size of the response requirement, whereas others appeared to depend on the relative location within a ratio (i.e., the proportion of the ratio completed at a given moment).     

Topography of the food-reinforced key peck and the source of 30-millisecond interresponse times

High-speed photography of key pecking revealed that the arc described by the upper bill as a pigeon closes its beak is capable of operating a Lehigh Valley pigeon key set at 8 to 14 g. Arc-produced switch closure follows initial switch closure in less than 50 msec. When birds were trained on ratio schedules, the probability of interresponse times (IRTs) shorter than 50 msec exceeded 0.30. Interval-trained birds produced a much lower probability of short-IRTs. When the schedules were reversed, there was only weak evidence of a reversal in the probability of short IRTs. A temporal analysis of topographic features observed in the original photographs failed to reveal differences between ratio and interval pecking topography. It appeared that only the point of contact with the key differed between subjects trained on the two schedules. It was concluded that only the locus, but not the topography, of the food-reinforced key peck was modified by the schedule of reinforcement.     

Higher order autoshaping

A series of experiments is reported on appetitive higher order conditioning in the pigeon. Experiment I showed that second order autoshaping can be produced by pairing a neutral keylight with a keylight of another colour, previously paired with food. Experiment II employed an omission procedure to show that second order autoshaping is a consequence of the contingency between first and second order stimuli. In Experiment III, extinction of responding to the first order stimulus was shown to reduce responding to the second order stimulus. Experiments IV and V showed firstly that this reduction is not due to generalization of extinction, and secondly that second order key pecks may be produced in the absence of any pecking to the first order stimulus. The results suggest that second order autoshaping is based largely on a direct association between the first and second order stimuli.     

Controls for and constraints on auto-shaping

Auto-shaping the pigeon's key-peck response was examined as a respondent conditioning procedure with the use of Rescorla's truly-random control procedure. In the first experiment, pigeons received presentations of brief light on the response key and brief presentations of food where the light and the food were independently presented. All birds failed to key peck after many light and food presentations, but explicit pairing of the light and food rapidly conditioned pecking to the light. Experiment 2 showed that even when an independent light/food presentation schedule was reduced to variable-time 30 sec, additional naive birds would not key peck and only one bird pecked when the schedules were variable-time 15 sec. A third experiment examined an explicit-unpairing control procedure, where the light and food were not only presented on independent schedules but were also separated by a minimum time, and found that auto-shaping did not occur. A fourth experiment investigated a number of control procedures and found them ineffective. A fifth experiment investigated the effects of a physical separation of the locus of the response key and the food dispenser, and a sixth experiment investigated using a tone in place of the light. It was concluded that pecking is generated by auto-shaping procedures only when an intermittently presented keylight is regularly paired with food.     

Delayed temporal discrimination in pigeons: A comparison of two procedures

A within-subjects comparison was made of pigeons' performance on two temporal discrimination procedures that were signaled by differently colored keylight samples. During stimulus trials, a peck on the key displaying a slanted line was reinforced following short keylight samples, and a peck on the key displaying a horizontal line was reinforced following long keylight samples, regardless of the location of the stimuli on those two choice keys. During position trials, a peck on the left key was reinforced following short keylight samples and a peck on the right key was reinforced following long keylight samples, regardless of which line stimulus appeared on the correct key. Thus, on stimulus trials, the correct choice key could not be discriminated prior to the presentation of the test stimuli, whereas on position trials, the correct choice key could be discriminated during the presentation of the sample stimulus. During Phase 1, with a 0-s delay between sample and choice stimuli, discrimination learning was faster on position trials than on stimulus trials for all 4 birds. During Phase 2, 0-, 0.5-, and 1.0-s delays produced differential loss of stimulus control under the two tasks for 2 birds. Response patterns during the delay intervals provided some evidence for differential mediation of the two delayed discriminations. These between-task differences suggest that the same processes may not mediate performance in each.     

Responding under positive and negative response contingencies in pigeons and crows

Four crows were trained to key peck for food. Then, they were exposed to a positive response contingency that required them to peck the key when it was illuminated briefly (the trial) in order to receive food. This procedure resulted in consistent within-trial pecking. When the contingency changed so that food was presented at the end of a trial when no response occurred, but the trial terminated immediately and food was omitted when a response occurred (negative response contingency), responding decreased markedly. Eight pigeons were studied under the same change in contingencies. These birds varied in their response histories from naive to having several years' experience. The previously naive pigeons also showed rapid declines in responding under the negative contingency; the responding of the birds with extended training histories declined much more slowly. Eventually, however, six of the eight pigeons showed little or no responding under the negative contingency, while they responded consistently when re-exposed to the positive contingency. These findings question the power and the generality of the negative automaintenance phenomenon.     

Effects of d-amphetamine, diazepam, and pentobarbital on the schedule-controlled pecking and locomotor activity of pigeons

Pigeons were trained on a variant of the autoshaping procedure devised by Matthews and Lerer (1987) in which a keylight stimulus ramp of increasing brightness signaled the passing of a 30-s interfood interval. This procedure generates two distinct behavioral components: key pecking and locomotor activity. The effects of three psychoactive drugs on these behavior classes were measured. d-Amphetamine had negligible effects on both types of behavior, whereas diazepam and pentobartital increased key pecking and decreased activity in a dose-dependent fashion. In Experiment 2, the possibility that drug effects were suppressed by excessively strong stimulus control exerted in Experiment 1 was tested by decreasing the discriminability of the stimulus ramp. The direction of the effects of diazepam and pentobarbital was the same as in Experiment 1 but the magnitude of the effects tended to be larger. The effects of d-amphetamine, however, remained quite small, suggesting that, under these conditions, locomotor activity and key pecking are less sensitive to d-amphetamine. In Experiments 3 and 4, key pecking was eliminated by removing the keylight. Reinforcers were presented at fixed intervals in Experiment 3 and at variable intervals in Experiment 4. The drug effects on activity observed in Experiments 1 and 2 disappeared in both Experiments 3 and 4. The results suggest that diazepam and pentobarbital affect activity indirectly by increasing key-pecking behavior, which, in turn, competitively decreases activity.     

Categorization of natural movements by pigeons: visual concept discrimination and biological motion

In three experiments, pigeons were exposed to a discriminated autoshaping procedure in which categories of moving stimuli, presented on videotape, were differentially associated with reinforcement. All stimuli depicted pigeons making defined responses. In Experiment 1, one category consisted of several different scenes of pecking and the other consisted of scenes of walking, flying, head movements, or standing still. Four of the 4 birds for which pecking scenes were positive stimuli discriminated successfully, whereas only 1 of the 4 for which pecking was the negative category did so. In the pecking-positive group, there were differences between the pecking rates in the presence of the four negative actions, and these differences were consistent across subjects. In Experiment 2, only the categories of walking and pecking were used; some but not all birds learned this discrimination, whichever category was positive, and these birds showed some transfer to new stimuli in which the same movements were represented only by a small number of point lights (Johansson's “biological motion” displays). In Experiment 3, discriminations between pecking and walking movement categories using point-light displays were trained. Four of the 8 birds discriminated successfully, but transfer to fully detailed displays could not be demonstrated. Pseudoconcept control groups, in which scenes from the same categories of motion were used in both the positive and negative stimulus sets, were used in Experiments 1 and 3. None of the 8 pigeons trained under these conditions showed discriminative responding. The results suggest that pigeons can respond differentially to moving stimuli on the basis of movement cues alone.     

The effects of the stimulus-reinforcer correlation in a discrete-trials IRT>t procedure

The correlation between a keylight and food in a discrete-trials, interresponse-time-greater-than 6-sec (IRT>6-sec) procedure was varied by manipulating the rate of response-independent food presentation in the intertrial interval. When the correlation was positive, the rates of pecking in the IRT>6-sec condition were high and food was obtained on only about 5% of the trials. Likewise, responding was maintained at a high rate in yoked birds that received the same presentations of the light and food as the birds in the IRT>6-sec condition. When the rate of reinforcement between trials was equated to or made greater than the rate of reinforcement within trials, the response rate decreased for all birds, and those decreases were considerably larger for the yoked birds. However, the percentage of trials in which reinforced responses occurred under the IRT>6-sec procedure did not increase substantially when the light and food were either uncorrelated or negatively correlated. The percentage of trials in which a reinforcer was obtained increased when the keylight was left on continuously and the discriminative stimulus was not presented on the key. The results show that the stimulus-reinforcer correlation affects responding in the discrete-trials IRT>6-sec procedure, but that the effects of the stimulus-reinforcer correlation vary as a function of whether reinforcement is response-dependent or response-independent. The differences between the effects of response-independent and response-dependent pairings and nonpairings of the light and food are best accounted for in terms of differences in the control of responding by background stimuli.     

Autoshaped key pecking maintained by access to a social space.

When four experimentally naive pigeons were exposed to occasional forward pairings of a keylight followed by a doorlight (that signaled access to a large social space), all subjects began to peck the lit key. In a second experiment, where the keylight either preceded the presentation of the doorlight or was presented independently of it, key pecking was maintained only in the former circumstance. The unconditioned stimulus in these experiments--arrival in the social space--did not elicit pecking. Hence, the conditioned response of key pecking and the unconditioned response of entering the social space differed. This demonstration of autoshaping with a social-space unconditioned stimulus argues against a stimulus-substitution account of the findings.     

Contributions of elicitation to measures of self-control

Pigeons' not pecking or pecking constituted choice between a delayed, large reinforcer and an immediate, small reinforcer (self-control) and at other times between a delayed reinforcer and no reinforcer (omission). Both a tone and a keylight were tested as choice signals, and the delayed reinforcer was either response independent or response dependent. Pigeons pecked during the choice signals on over 95% of the trials in the self-control procedure, and pecked during the choice signals on over 75% of the trials in the omission procedure. Consistent pecking was observed with either the tone or the keylight as a choice signal, with the exception that a tone paired with a response-independent delayed reinforcer did not maintain pecking in the omission procedure. Pigeons pecked during more choice signals when delayed reinforcers were response dependent than when the delayed reinforcers were response independent. These results indicate that Pavlovian conditioning influences self-control experiments, especially in single-key procedures.