Control of responding during stimuli that precede transitions in reinforcement frequency

Pigeons' responses were reinforced on a variant of a mixed variable-interval extinction schedule of reinforcement in which the transition to the higher reinforcement rate was signaled by a trace stimulus projected on the response key prior to the onset of the component correlated with food delivery. In the first of two experiments, the duration of the trace stimulus preceding the component correlated with food delivery was varied from 1.5 to 50.0 s and in the second experiment, the reinforcement frequency in the same component was varied from 10 to 60 reinforcers per hour. Pigeons pecked at the trace stimulus preceding the onset of the component correlated with food delivery even though responding was not reinforced in its presence and only one of the changes in reinforcement rate (i.e., from extinction to reinforcement) was signaled. The rate of pecking during the trace stimulus was a function of its duration but not of the reinforcement frequency in the following component. Higher rates generally occurred at the shorter trace-stimulus durations. Component responding following the offset of the trace stimulus was under discriminative control of the trace stimulus whether or not responding occurred in the presence of the trace stimulus.     

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.     

The effects of fixed‐interval schedules on variability of pigeons' pecking location

Many studies that have investigated performance under reinforcement schedules have measured response rate or interresponse time, which reflect the temporal dimension of responding; however, relatively few studies have examined other dimensions. The present study investigated the effects of fixed‐interval schedules on the location of pigeons' pecking response. A circular response area 22.4 cm in diameter was used so that the pecking responses were effective over a wide range. Pigeons were exposed to a fixed‐interval schedule whose requirement was systematically varied between conditions. Response location moved closer to the location of the last reinforced response as time elapsed in each trial. Additionally, as the fixed‐interval duration requirement increased, response locations shifted to the border of the response area and the variability of response locations increased. These results suggest that fixed‐interval schedules systematically control response location.     

Signalled free-operant avoidance of shock by pigeons pecking a key

Two pigeons were trained to peck a key under a free-operant avoidance schedule. Then, changes in key color signalled the beginning (safe period) and the end (warning period) of the response-shock interval, with a response required to change the key color. Finally, a change in key color signalled the warning period and either a response or a shock reinstated the safe stimulus. During signalled avoidance, response rate was higher during the warning stimulus than during the safe stimulus. More responding tended to occur in the warning stimulus when it was terminated by either a response or a shock than by only a response. In either procedure, response latency during the warning stimulus was a function of the duration of the warning stimulus. In general, response and shock rate were higher during unsignalled than during signalled avoidance. When the warning stimulus was brief, the results were similar to those of unsignalled avoidance. These results confirm previous findings with pigeons, are in general agreement with data provided by other species in studies of signalled avoidance, and thereby indicate the transituationality of the key-pecking operant.     

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.     

Conditional discrimination with ambiguous stimuli.

Five pigeons learned a two-key conditional discrimination. When background color on both keys was red, pecks on the key with a horizontal line produced food. When the color was green, pecks on the key with a vertical line produced food. During part of the experiment, color was presented on only one of the keys. It was found that accuracy was higher when color was combined with the line stimulus correlated with nonreinforcement. In another part of the experiment, color was presented on both keys but a line was present only on one. Accuracy was higher when the line accompanied the nonreinforced option than when the line accompanied the reinforced option. Superior performance when the combined stimuli were displayed on the nonfood key may be explained by the association of different components of the compound stimuli with reinforcement or as the result of rules pigeons follow in solving conditional discriminations.     

Stimulus generalization from feeder to response key in the acquisition of autoshaped pecking

During autoshaping, a 6-second presentation of one stimulus and a variable time 30-second presentation of a second stimulus alternated in appearance on a pigeon key. Grain always was delivered for 3 seconds at the end of the first stimulus interval. In the first experiment, autoshaped pecking of the stimulus preceding grain delivery began much sooner when that stimulus was a black vertical line on a white background and the other stimulus was green than when the opposite stimulus arrangement was used. Because these two stimuli differed in form, hue, brightness, and similarity in hue and brightness to the illumination of the raised feeder, three subsequent experiments examined whether the differential speed of autoshaping in the two groups was due to a feature-positive, feature-negative effect, a preference for brighter over darker stimuli, a simple preference for white over green, or stimulus generalization from the brightness or hue of the illuminated, raised feeder to the stimulus on the key preceding grain delivery. The data from these experiments showed that the first autoshaped key peck was most likely to be made to the stimulus of the same hue as that illuminating the feeder, regardless of whether that stimulus was positively or negatively associated with grain delivery. At least under some conditions, therefore, stimulus-generalization mediated response transfer of pecking grain in the presence of the hue illuminating the feeder to pecking the key illuminated by a similar hue appears to account for the occurrence of autoshaped key pecking.     

Relational discrimination by pigeons in a go/no-go procedure with compound stimuli: a methodological note

A go/no‐go procedure with compound stimuli typically establishes emergent behavior that parallels in structure and typical outcome that of conventional tests for symmetric, transitive, and equivalence relations in normally capable adults. The present study employed a go/no‐go compound stimulus procedure with pigeons. During training, pecks to two‐component compounds A1B1, A2B2, B1C1, and B2C2 were followed by food. Pecks to compounds A1B2, A2B1, B1C2, and B2C1 re‐started the 30‐s stimulus presentation interval. The absence of pecking to those compounds for 30 s ended the trial. Subsequent tests presented these components in new spatial arrangements and/or in recombinative compounds that together corresponded to conventional tests of symmetry, transitivity, and equivalence: B1A1, B2A2, C1B1, C2B2, A1C1, A2C2, C1A1, C2A2 vs. B1A2, B2A1, C1B2, C2B1, A1C2, A2C1, C1A2, C2A1 (positive vs. negative instances of symmetric, transitive, and equivalence relations). On tests for symmetric relations, all pigeons behaved in a manner consistent with training on both positive instances (i.e., by responding) and on negative instances (i.e., by not responding). By contrast, the pigeons' behavior on tests for transitivity and equivalence was inconsistent with baseline training, thus failing to show the recombinative discrimination performance that is typical of normally capable humans when trained and tested using the go/no‐go procedure with compound stimuli.     

Determinants of pigeons' choices in token-based self-control procedures

Four pigeons were exposed to a token-based self-control procedure with stimulus lights serving as token reinforcers. Smaller-reinforcer choices produced one token immediately; larger-reinforcer choices produced three tokens following a delay. Each token could be exchanged for 2-s access to food during a signaled exchange period each trial. The main variables of interest were the exchange delays (delays from the choice to the exchange stimulus) and the food delays (also timed from the choice), which were varied separately and together across blocks of sessions. When exchange delays and food delays were shorter following smaller-reinforcer choices, strong preference for the smaller reinforcer was observed. When exchange delays and food delays were equal for both options, strong preference for the larger reinforcer was observed. When food delays were equal for both options but exchange delays were shorter for smaller-reinforcer choices, preference for the larger reinforcer generally was less extreme than under conditions in which both exchange and food delays were equal. When exchange delays were equal for both options but food delays were shorter for smaller-reinforcer choices, preference for the smaller reinforcer generally was less extreme than under conditions in which both exchange and food delays favored smaller-reinforcer choices. On the whole, the results were consistent with prior research on token-based self-control procedures in showing that choices are governed by reinforcer immediacy when exchange and food delays are unequal and by reinforcer amount when exchange and food delays are equal. Further, by decoupling the exchange delays from food delays, the results tentatively support a role for the exchange stimulus as a conditioned reinforcer.     

Observational learning of two visual discriminations by pigeons: a within-subjects design.

Pigeon's observational learning of successive visual discrimination was studied using within-subject comparisons of data from three experimental conditions. Two pairs of discriminative stimuli were used; each bird was exposed to two of the three experimental conditions, with different pairs of stimuli used in a given bird's two conditions. In one condition, observers were exposed to visual discriminative stimuli only. In a second condition, subjects were exposed to a randomly alternating sequence of two stimuli where the one that would subsequently be used as S+ was paired with the operation of the grain magazine. In a third experimental condition, subjects were exposed to the performance of a conspecific in the operant discrimination procedure. After exposures to conspecific performances, there was facilitation of discriminative learning, relative to that which followed exposures to stimulus and reinforcement sequences or exposures to stimulus sequences alone. Exposure to stimulus and food-delivery sequences enhanced performance relative to exposure to stimulus sequences alone. The differential effects of these three types of exposure were not attributable to order effects or to task difficulty; rather, they clearly were due to the type of exposure.     

Food-paired stimuli as conditioned reinforcers: effects of d-amphetamine.

Seven pigeons were studied in two experiments in which key pecks were reinforced under a second-order schedule wherein satisfaction of variable-interval schedule requirements produced food or a brief stimulus. In the second part of each session, responses produced only the brief stimulus according to a variable-interval schedule (food extinction). For the 4 pigeons in Experiment 1, the response key was red throughout the session. In separate phases, the brief stimulus was either paired with food, not paired with food, or not presented during extinction. d-Amphetamine (0.3 to 10.0 mg/kg) dose-dependently reduced food-maintained responding during the first part of the session and, at intermediate dosages, increased responding during the extinction portion of the session. The magnitude of these increases, however, did not consistently depend on whether the brief stimulus was paired, not paired, or not presented. It was also true that under nondrug conditions, response rates during extinction did not differ reliably depending on pairing operations for the brief stimulus. In Experiment 2, 3 different pigeons responded under a procedure wherein the key was red in the component with food presentations and blue in the extinction component (i.e., multiple schedule). Again, d-amphetamine produced dose-related decreases in responding during the first part of a session and increases in responding in the second part of the session. These increases, however, were related to the pairing operations; larger increases were observed when the brief stimulus was paired with food than when it was not or when it was not presented at all. Under nondrug conditions, the paired brief stimulus controlled higher response rates during extinction than did a nonpaired stimulus or no stimulus. These findings suggest that d-amphetamine can enhance the efficacy of conditioned reinforcers, and that this effect may be more robust if conditioned reinforcers occur in the context of a signaled period of extinction.     

Short-term remembering of discriminative stimuli in pigeons.

Pigeons learned to peck the left or right of two white keys depending on whether a red or a green stimulus was displayed on a third key. The opportunity to peck the white keys was then dealyed for zero to six seconds after the red or green (to-be-remembered) stimulus. On half the trials, the feeder operated during the delay to interrupt behavior that might mediate discriminated responding. No events were scheduled on the remaining trials. In a later condition, the pigeons had the opportunity to peck the white keys during the delay. In general, accuracy decreased as delay increased in all conditions, but performance was least accurate following feeder operations and most accurate when pecking was allowed during the delay. The procedures may be analogous to varying the opportunity for rehearsal in studies of human short-term memory.     

Conditioning of within-trial patterns of key pecking in pigeons

The possibility of conditioning systematic patterns of responding during brief discrete trials was studied by requiring hungry pigeons to key peck and then pause or to pause and then key peck in order to gain access to food. These schedules were highly effective in promoting decelerated and accelerated rates of responding, respectively, within individual trials; indeed, performance was quite similar to that observed when explicit external stimuli were correlated with “peck” and “pause” portions of the daily trials. Finally, schedules of reinforcement that did not selectively reinforce peck-pause or pause-peck patterns neither generated these patterns nor maintained them at the previous high levels. The results, therefore, confirm Shimp's (1976) proposal that organized groupings of discrete responses may function as operants—even in the absence of strict response-reinforcer contiguity.     

Reinforcement contingencies as discriminative stimuli: II. Effects of changes in stimulus probability.

Three pigeons were trained on a matching procedure involving a sample component and a choice component. Responding in the sample component, according to either a differential-reinforcement-of-low-rate schedule on some trials or a differential-reinforcement-of-other-behavior schedule on other trials, produced access to the choice component in which each of two keys was illuminated with a unique color. The correct choice response was defined by the contingency that was met to produce the choice. The food hopper operated for 1.5 seconds following an appropriate sample response and for 3 seconds following a correct choice response. A signal-detection analysis showed that variations in the probability of presentation of the different contingencies systematically affected response bias but not sensitivity to the contingencies as stimuli. Substitution of a blackout for food at the end of the sample component did not differentially affect performance, but elimination of the delay between sample and choice components generally increased the sensitivity measure. The findings suggest a role for reinforcement contingency discrimination in schedule-controlled responding.     

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.     

Effects of variations in the temporal distribution of reinforcements on interval schedule performance

Pigeons were exposed to variable-interval and fixed-interval schedules and schedules approximating variable-interval and fixed-interval schedules. The probabilities of the variable-interval and fixed-interval components in a mixed fixed-interval variable-interval schedule in Experiment I and the minimum and maximum interreinforcement intervals in Experiment II in a variable-interval schedule were manipulated to create intermediate schedule contingencies and contingencies approximating simple variable-interval or fixed-interval contingencies. Maximal control by time as defined by quantitative indices of the temporal pattern of response occurred as fixed-interval contingencies were approximated and minimal control occurred as variable-interval contingencies were approximated. Changes in the temporal pattern of response were systematically related to changes in the temporal distribution of reinforcements with both procedural definitions for manipulating the temporal distribution of reinforcements.     

Control of behavior by an establishing stimulus

Seventeen pigeons were exposed to a three-key discrete-trial procedure in which a peck on the lit center key produced food if, and only if, the left keylight was lit. The center key was illuminated by a peck on the lit right key. Of interest was whether subjects pecked the right key before or after the response-independent onset of the left keylight. Pecks on the right key after left-keylight onset suggest control of behavior by the left keylight—an establishing stimulus. In three experiments, the strength of center-keylight onset as conditioned reinforcer for a response on the right key was manipulated by altering the size of the reduction in time to food delivery correlated with its onset. Control of pigeons' key pecks by onset of the left keylight occurred on more trials per session when the center keylight was a relatively weak conditioned reinforcer and on fewer trials per session when the center keylight was a relatively strong condtioned reinforcer. Differences across conditions in the degree of control by onset of the establishing stimulus were greatest when changes in conditioned reinforcer strength occurred relatively frequently and were signaled. The results provide evidence of the function of an establishing stimulus.     

Acquisition of stimulus control while introducing new stimuli in fading.

After establishing discrimination between a red positive stimulus and a green negative stimulus, the lowest intensity colors that restricted all responding to the positive stimulus were determined. Then, two new white lines differing in terms of line orientation were each superimposed on one of the colors and were increased in intensity. Thereafter, the intensity of the colors was decreased and eventually eliminated. Probe stimuli consisting of the lines presented against dark backgrounds were presented before each change of stimulus intensity, and probe responding was used to assess the control acquired by various dimension of the new stimuli during the course of fading. The lines acquired control of responding while they were being introduced, and control was strengthened as the colors were attenuated. Such a locus of acquisition was attributed to the starting intensity of the original controlling stimuli and was explained in terms of stimulus blocking. Finally, using probes while introducing the new stimuli enhanced the acquisition of control by the new stimuli.