Parameters affecting the maintenance of negatively reinforced key pecking

Three negative reinforcement experiments employing a key-peck response are described. In Experiment I, pigeons shocked on the average of twice per minute (imposed condition) could produce, by pecking a key, an alternate condition with correlated stimuli. Delayed shocks were added, across sessions, to the alternate condition until pecking stopped. Two of three pigeons continued to peck despite a 100% increase in shock frequency. In Experiment II, pigeons were shocked in the imposed condition four times per minute. The postresponse delay to shock was held constant by delivering, in the alternate condition, the next shock, or the next two, three, or four shocks from the imposed-condition shock schedule. All three subjects continued to peck with no change in delay to the first two postresponse shocks but with a 75% reduction in shock frequency. In Experiment III, a response produced an immediate shock followed by a shock-free period. Three of four subjects continued to respond despite reduced delay to shock. Delay-to-shock or shock-frequency reduction was sufficient to maintain key pecking, but neither was necessary. The conditions that negatively reinforce the pigeon's key peck were similar to conditions that negatively reinforce the rat's bar press.     

Key-peck durations under behavioral contrast and differential reinforcement

Pigeons were maintained on a multiple schedule in which both components were variable-interval one-minute schedules. When they were switched to a condition in which one component was extinction, behavioral contrast was observed. The median durations of the key pecks in the unchanged component did not decrease in size. The results are incompatible with a theory of behavioral contrast which considers the added pecks to be short-duration responses. In a second experiment, pigeons were required to emit short-duration key pecks in one component of a multiple schedule, and long-duration pecks in the other. Two of three pigeons learned to emit responses appropriate to the requirements of the component in effect, suggesting that the duration of the key-peck response is sensitive to differential reinforcement.     

The influence of ultraviolet radiation on the pigeon's color discrimination

Two experiments demonstrated the pigeon's sensitivity to ultraviolet light. In Experiment I, pigeons' responses were reinforced on a multiple schedule with a variable-interval reinforcement schedule in one component and extinction in the other component. Response rates were quite different in the two components where the 520-nm stimuli signalling each component differed only in that one of them contained a 366-nm ultraviolet component. In Experiment II, pigeons were trained to peck one side key when two halves of a split field were of different wavelength and to peck another side key when they were of the same wavelength. Initially, field halves contained both "visible" and ultraviolet components of energy. Discrimination performance improved when the ultraviolet component was removed from one field half. It was argued that the critical change in the stimulus was a color change, rather than a brightness one, or a fluorescence of structures in the pigeon's eye.     

Aggression during the fixed-ratio and extinction components of a multiple schedule of reinforcement

Pigeons were trained to key peck for food on multiple reinforcement schedules including components of continuous and fixed-ratio reinforcement and extinction. At the end of the chamber opposite the response key was a restrained target pigeon. The target restraining equipment was designed to record automatically blows struck against the target. When the experimental pigeons were paired with restrained target pigeons they attacked the target. Attack occurred during extinction after both continuous and fixed-ratio reinforcement. Attack also occurred occasionally during fixed-ratio 25 and fixed-ratio 40 and frequently during fixed-ratio 60 and fixed-ratio 120. No attack occurred during fixed-ratio 15 and continuous reinforcement. After a history of stable responding without a target bird present, the introduction of a target bird resulted in severely strained key-peck responding characterized by long periods of neither key pecking nor aggressing.     

Failure to obtain positive contrast when pigeons press a bar

In Experiment 1, two groups of pigeons were exposed to multiple variable-interval 1-min variable-interval 1-min schedules of reinforcement. The required response was a key peck for one group, and a foot press of a bar for the other. When the procedure was shifted to multiple variable-interval 1-min extinction, positive contrast occurred with pigeons that key pecked, whereas negative induction occurred with those that bar pressed. Moreover, the absence of contrast could not be ascribed to the lack of inhibition, since negative generalization gradients after bar press training were U-shaped in Experiment 2. The results are discussed in terms of possible relationships between positive contrast and the phenomenon of autoshaping.     

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.     

The development of stimulus control with and without a lighted key

In two experiments, the effect of an illuminated response key on the acquisition of stimulus control by an airflow stimulus was assessed. In the first experiment, pigeons were given nondifferential training with airflow emerging from behind the response key in one of three conditions of illumination: trained to peck a lighted key, trained to peck an unlighted key with a houselight present, trained to peck a key in total darkness. After 10 days of training on a variable-interval schedule of reinforcement, all subjects were given a generalization test on airflow velocity. The gradients for subjects trained in the dark were sharp, while those for subjects trained in lighted conditions were shallow. In the second experiment, the effect of an irrelevant keylight on the acquisition of an airflow velocity discrimination was assessed. Two groups of pigeons were trained to discriminate two airflow velocities. One group was trained with a lighted response key and the other was trained to peck the response key in total darkness. The dark-trained subjects acquired the discrimination more rapidly. The results demonstrate that the acquisition of stimulus control by airflow with either a differential or nondifferential training procedure can be overshadowed by keylight.     

Behavioral contrast without response-rate reduction

Behavioral contrast was obtained in two experiments, which both employed a standard free-operant successive discrimination (a multiple variable-interval extinction schedule), without the occurrence of reductions of response rate in the extinction component. In Experiment I, one group of four pigeons was trained on a multiple schedule in which one stimulus was associated with a variable-interval schedule and the second stimulus with response-independent reinforcement on a free variable-interval schedule. Though by the end of this training three pigeons were responding very little to the second stimulus, when this stimulus was associated with extinction all subjects showed a contrast effect. In Experiment II, eight pigeons were trained extensively to respond to a single stimulus on a variable-interval schedule, before a second stimulus associated with extinction was introduced. This second stimulus was dissimilar to the initial stimulus and five pigeons never responded in its presence. Nevertheless, all pigeons showed a contrast effect and there was no evidence that the effect was smaller in errorless subjects or smaller than in a subsequent discrimination where all subjects made many errors. Both experiments indicated that response reduction in one component of a multiple schedule is not a necessary condition for the occurrence of contrast.     

Contrast and autoshaping in multiple schedules varying reinforcer rate and duration

Thirteen master pigeons were exposed to multiple schedules in which reinforcement frequency (Experiment I) or duration (Experiment II) was varied. In Phases 1 and 3 of Experiment I, the values of the first and second components' random-interval schedules were 33 and 99 seconds, respectively. In Phase 2, these values were 99 seconds for both components. In Experiment II, a random-interval 33-second schedule was associated with each component. During Phases 1 and 3, the first and second components had hopper durations of 7.5 and 2.5 seconds respectively. During Phase 2, both components' hopper durations were 2.5 seconds. In each experiment, positive contrast obtained for about half the master subjects. The rest showed a rate increase in both components (positive induction). Each master subject's key colors and reinforcers were synchronously presented on a response-independent basis to a yoked control. Richer component key-pecking occurred during each experiment's Phases 1 and 3 among half these subjects. However, none responded during the contrast condition (unchanged component of each experiment's Phase 2). From this it is inferred that autoshaping did not contribute to the contrast and induction findings among master birds. Little evidence of local contrast (highest rate at beginning of richer component) was found in any subject. These data show that (a) contrast can occur independently from autoshaping, (b) contrast assays during equal-valued components may produce induction, (c) local contrast in multiple schedules often does not occur, and (d) differential hopper durations can produce autoshaping and contrast.     

Conditional discrimination performance by pigeons on a response-independent procedure

Pigeons were trained on a differential autoshaping procedure in which both components of two-stimulus sequences predicted delivery or nondelivery of food. All birds acquired the conditional discrimination. When the subjects were exposed to an extinction procedure, the stimuli maintained conditional control as long as the birds continued to peck the key. When a delay interval was imposed between the two components of a stimulus sequence using a titration procedure, the stimuli maintained conditional control up to delay values of 7 to 10 sec. These data are consistent with the view that the controlling stimuli in conditional discrimination situations are compounds of stimulus elements.     

Positive and negative contrast as a function of component duration for key pecking and treadle pressing

Pigeons responded on several multiple schedules for food reinforcers. The duration of the components varied from four seconds to 16 minutes. The absolute size of positive (Experiment 1) and negative (Experiment 2) behavioral contrast varied inversely with component duration when key pecks produced the reinforcers. The absolute size of negative contrast varied directly with component duration, when treadle presses produced the reinforcers (Experiment 3). These results conform to theories that suggest that positive and negative contrast are symmetrical when pigeons peck keys. They also conform to theories that suggest that the same principles do not govern contrast when pigeons peck keys as when they press treadles. Finally, the results support the measurement of behavioral contrast by the differences between baseline rates of responding and the rates emitted when contrast is present.     

Contrast effects in response rate and accuracy of delayed matching to sample

Behavioural contrast is an inverse relation between the response rate in one component of a multiple schedule and the reinforcer rate in an alternated component. To explore possible contrast effects in accuracy as well as response rate, four pigeons were trained in multiple schedules where key pecking produced delayed matching-to-sample trials on a variable-interval schedule. Reinforcer probability for correct matches was constant at .3 in one component, and the conditions of reinforcement were varied in the second component. In Experiment 1, the varied component arranged the same contingencies as the constant component but with reinforcer probabilities of .9 or .1 across conditions. In the varied component, both response rate and accuracy of delayed matching were directly related to reinforcer probability; in the constant component, however, contrast effects on response rate were weak, and there was no evidence of contrast in accuracy of matching. In Experiment 2, the varied component was either variable interval with immediate food reinforcement or extinction. Reliable contrast effects were obtained in both response rate and in accuracy of matching in the constant component, and their magnitudes were correlated within and between subjects. The results of Experiment 2 join previous findings of covariation in the effects of reinforcement on free-operant responding and accuracy of discrimination.     

The role of autopecking in behavioral contrast

Four groups of four pigeons each were studied on two different multiple schedules. The cues correlated with the schedule components were localized on the response key for two groups and were not localized for the others. Two groups worked on multiple schedules with variable interval 15-sec in both components, and variable interval 15-sec in one component and extinction in the other. The other two groups had identical procedures except that food was presented on a response-independent variable-time schedule. Variable-interval birds with localized stimuli showed marked behavioral contrast; variable-interval birds with non-localized stimuli showed no behavioral contrast. Variable-time birds with key-light stimuli acquired high rates of autopecking, which changed as treatment changed in a manner that paralleled rate changes, resulting in behavioral contrast for variable-interval birds. Variable-time birds with non-localized stimuli key pecked only at a low rate. The findings indicate that behavioral contrast in pigeons may result from the autopecking that is obtained with stimulus-contingent food presentation.     

Eccentric stimuli on multiple fixed-interval schedules

The effects of presenting a different (“eccentric”) stimulus for one interval during either or both components of a cyclic multiple fixed-interval fixed-interval schedule, with 12 short and four long intervals per cycle, were studied in three experiments. Eccentric stimuli in the short-interval component reliably produced a persistent, substantial elevation in key-peck rate. The effect appears to depend on schedule context and an initial “disinhibiting” effect of the eccentric stimulus.     

Temporal inhibition: effects of changes in rate of reinforcement and rate of responding

Pigeons were trained to key peck on several multiple schedules in which the first of two components was always a simple fixed-interval schedule. The rate of responding at the beginning of the constant fixed-interval schedule was found to decrease with increases in the rate of reinforcement associated with the other component of the multiple schedule, but remained unchanged with decreases in the rate of responding associated with the other component. These results were interpreted as being consistent with the view that the presence and magnitude of the temporal inhibitory effects observed in a given fixed-interval schedule are a function of the properties of reinforcing stimuli, rather than of changes in the rate of responding associated with the time interval immediately preceding the fixed interval in question.     

Behavioral control by an imprinted stimulus: long-term effects

Newly hatched ducklings were exposed to imprinting procedures and subsequently trained to peck a key by presenting the imprinting stimulus as the reinforcing (response-contingent) event. Individual ducklings then lived in the apparatus under an arrangement in which each peck produced a 15-sec stimulus presentation. For all ducklings, key-pecks tended to occur in bursts, and as the duckling matured, burst length decreased and the interval between bursts increased. However, even when subjects were 60 days old, some responses still occurred.     

Notes on fixed-ratio and fixed-interval escape responding in the pigeon

After learning to peck a key when each peck removed a slowly increasing series of electric shocks, pigeons were placed on fixed-ratio and fixed-interval escape schedules. The resulting behavior was comparable to that of other species on ratio and interval escape schedules. Thus, while the pigeon apparently requires special techniques for the initial shaping of a key-peck response with negative reinforcement, this response, once obtained, can be subjected to intermittent schedules of negative reinforcement with no great difficulty.     

Effects of barbiturates and other sedative hypnotics in pigeons trained to discriminate phencyclidine from saline.

Pigeons were trained to peck the center key (lighted white) of three response keys to turn off the center keylight and to light one of the side keys with a red keylight and the other side key with a green keylight. Five responses (fixed-ratio component) on either side key relighted the center key. Food was delivered following 10 fixed-ratio components on the red key if 1.5 mg/kg phencyclidine had been given before the session. The position of the red and green keylights on the side keys varied randomly each time they were lighted by a peck on the center key. Subsequently, increasing doses of phencyclidine, barbital, amobarbital, phenobarbital, methaqualone, methyprylon, diazepam, oxazepam, and d-amphetamine were substituted for the training dose of phencyclidine, using a cumulative dosing procedure. At low doses of the sedative hypnotics, birds pecked the keylight color associated with saline. At higher doses, birds pecked both key colors. At the highest doses of pentobarbital and amobarbital, some birds responded almost exclusively On he color associated with phencyclidine. When responding on keys of both colors occurred following administration of phencyclidine or other sedative hypnotics, this responding was controlled by key position rather than by key color.     

Punishment-specific effects of pentobarbital: dependency on the type of punisher.

Pigeons were trained to peck a key under a multiple random-interval 1-minute, random-interval 6-minute schedule of food presentation. Subsequently, over three phases, additions were made during the random-interval 1-minute component as follows: pecks during the component occasionally were punished by timeout presentation (Phase 1), timeouts were presented independently of responding during the component (Phase 2), pecks during the component occasionally were punished by electric-shock presentation (Phase 3). In Phases 1 and 3, response-dependent timeout and shock suppressed responding and established equivalent rates in both components of the multiple schedule. Intermediate doses of pentobarbital increased responding suppressed by electric-shock punishment but had little or no effect on responding suppressed by timeout punishment. Response-independent presentation of timeouts did not result in suppression of responding (thus showing that response-dependent timeout acted as a punisher), and pentobarbital did not reliably increase unpunished responding. Pentobarbital's selective "punishment-attenuating" properties depend on the nature of the punisher.     

Delayed reinforcement in a multiple schedule

Three rats and a pigeon were first trained on a two-component multiple schedule in which reinforcement in the two components occurred immediately after a response. Later, reinforcement in one component was delayed by a few seconds. During both stages of the experiment, reinforcement was scheduled by equal variable- (pigeon) or random-interval (rats) schedules in the two components. The main effect of the delayed reinforcement was to increase the rate of responding in the unchanged (non-delay) component. This behavioral contrast effect did not appear in all cases to be dependent upon a reduction in the rate of responding or the frequency of reinforcement in the delay component. This finding suggests that a reduction in response rate and/or reinforcement frequency in one component of a multiple schedule may not be a necessary prerequisite for the occurrence of behavioral contrast. This finding is, however, consistent with an explanation that suggests that behavioral contrast results from the introduction of a less-preferred condition in one component of a multiple schedule, since it is known that animals “prefer” immediate to delayed reinforcement.