Positive and negative conditioned suppression in the pigeon: Effects of the locus and modality of the CS

In Experiment I, four pigeons were exposed to trials in which a 12-sec key light illumination was followed by free food. These trials were superimposed upon a baseline of key pecking for food reinforcement on a variable-interval schedule. When the signal for food was on the operant key, response rate was substantially higher during the signal than during the baseline procedure. When the signal was on a second, signal key, operant responding was suppressed during the signal and substantial pecking of the signal key occurred. The sum of signal key and operant key pecks far exceeded the operant baseline rate of responding. An explanation of opposite results obtained with rats and pigeons as subjects in experiments of this type was suggested in terms of the spatial relation between the signal for free food and the operant target which usually characterizes these experiments. Experiment II assessed the importance of signal location when shock rather than food was the US. Suppression of operant key pecking was unaffected by signal location. Experiment III assessed the relative effectiveness of visual and auditory stimuli (clicks) as signals for food and shock, and found that all combinations of signal and US were equally effective in suppressing operant key responding. The three experiments together suggested that the identification of important effects of species—typical behavior in one experimental situation does not imply that there will be like effects in similar situations.     

Facilitation of food-reinforced responding by a signal for response-independent food

Five pigeons whose key pecking was maintained by 4-sec access to grain on a variable-interval 2-min schedule received Pavlovian differential conditioning trials superimposed upon the instrumental baseline. The conditioned stimuli were changes in the stimulus on the key from white to red, or to a white horizontal line against a dark background. The positive conditioned stimulus was 20 sec long, and was followed immediately by 8-sec access to grain. The negative conditioned stimulus, also 20 sec long, was never paired with response-independent food. All pigeons responded more rapidly in the presence of the positive conditioned stimulus than in the presence of the negative one. The positive conditioned stimulus produced an increase in response rate over the pre-conditioned stimulus period. The negative conditioned stimulus had no marked effect upon response rate. When the roles of the positive and negative stimuli were reversed, and the duration of the response-independent reinforcement was reduced to 4 sec, the new positive conditioned stimulus came to facilitate responding, and the new negative conditioned stimulus no longer produced facilitation. A second discrimination reversal produced similar outcomes. When a third reversal was initiated, and the duration of response-independent reinforcement was reduced to 2 sec, the difference between the effects of the positive and negative stimuli diminished.     

Punishment of observing by a stimulus associated with the lower of two reinforcement frequencies

Three pigeons were used to investigate the effects of a stimulus associated with the lower of two reinforcement frequencies on the response producing it. In a three-key chamber, pecking the center key produced grain on alternating variable-interval schedules with mean durations of 2 min or 30 sec. Initially, green illumination of the keys accompanied the more favorable (30-sec) schedule and red accompanied the less favorable (2-min) schedule. Then the keys remained yellow unless the bird pecked one of the side (observing) keys to produce the discriminative stimuli for a 30-sec period. Subsequently, when red was withheld as a possible consequence of pecking a particular side key, the rate on that key increased; when red was restored, the observing rate decreased. Thus the stimulus associated with less frequent reinforcement had a punishing effect on the behavior producing it. When green was withheld on one of the side keys and the other key produced both colors, observing behavior was not maintained on the red-only key, but was maintained on the key that produced both colors.     

Negatively reinforced key pecking

A reinforcement-switching procedure was used to produce negatively reinforced key pecking in pigeons. First, key pecking on a chain schedule (fixed-interval 10-sec variable-interval 60-sec) was conditioned using grain reinforcement. Second, intermittent shock in the initial link was introduced at a low intensity and gradually increased. Third, food reinforcement in the terminal link was eliminated. With shock at 90 V occurring on the average every 3 sec, initial-link pecking was maintained with no terminal-link food. Three of four pigeons responded consistently at shock intensities of 90, 70, and 50 V but not at 30 V. A fourth pigeon responded at but not below 90 V. Rate of response was directly related to shock frequency. Eliminating food deprivation did not affect the negatively reinforced performance.     

Effects of search cost on foraging and feeding: a three-component chain analysis

An experiment determined whether pigeons minimize number of key pecks per food delivery and maintain their baseline intake of food while key pecking on a three-component chain schedule. Pigeons at either 80% or 100% body weight obtained all their food during baseline and contingency sessions. During baseline sessions, pecks on the left and center keys had no consequences; each peck on the right key activated the feeder. During contingency sessions, pigeons key pecked on a three-component chain schedule simulating components of a foraging chain. In the search component either 3, 9 or 15 key pecks (varied parametrically across blocks of sessions) on the left key produced a stimulus on the middle key, indicating an encounter with either the low-cost prey (3 key pecks) or an equally probable high-cost prey (21 key pecks). In the procurement component the pigeon pecked either: (a) the left key once, thus returning to the search component, or (b) the middle key either 3 or 21 times, which activated the right response key. In the handling component one peck on the right key operated the feeder. The pigeons always procured the low-cost prey and minimized the number of key pecks per hopper by procuring the high-cost prey when the search-cost ratio was high (15 key pecks) but not when it was low (3 key pecks). All pigeons maintained their baselines of eating during contingency sessions by key pecking more frequently and eating more efficiently. The 80% body-weight birds produced higher overall rates of key pecking and eating. These results have implications for ecological theories of optimal foraging and for psychological theories of learned performance.     

Effects of response rate, reinforcement frequency, and the duration of a stimulus preceding response-independent food

Food-reinforced key pecking in the pigeon was maintained under a four-component multiple schedule. In two components, responding was maintained at high rates under a random-ratio schedule. In the other two components, responding was maintained at low rates under a schedule that specified a minimum interresponse time. For both high and low response rates, one of the schedule components was associated with a high reinforcement frequency and the other components with a lower reinforcement frequency. During performance under these schedules, a stimulus terminated by access to response-independent food was periodically presented. The duration of this pre-food stimulus was 5, 30, 60, or 120 sec. Changes in rate of key pecking during the pre-food stimulus were systematically related to baseline response rate and the duration of the stimulus. Both high and low response rates were increased during the 5-sec stimulus. At longer stimulus durations, low response rates were unaffected and high response rates were decreased during the stimulus. For two of three pigeons, high response rates maintained under a lower frequency of reinforcement tended to be decreased more than high response rates maintained under a higher reinforcement frequency. In general, the magnitude of decrease in high response rates was inversely related to the duration of the pre-food stimulus.     

A test of the negative discriminative stimulus as a reinforcer of observing

Five pigeons were used to test the hypothesis that the source of reinforcement for observing behavior is the information that it provides concerning the schedule of primary reinforcement. On a variable-interval schedule, pecking the left-hand key produced a 30-sec display of such information. During this 30-sec period, when pecking the right-hand key was reinforced on a random-interval schedule, both keys were green; when no reinforcement was scheduled (extinction) both keys were red. Later, this baseline procedure, in which both red and green were available, was replaced for blocks of sessions by procedures in which either (a) the red was eliminated and only the green could be produced; or (b) the green was eliminated and only the red could be produced. The results were that green maintained rates of pecking on the left key that were as high or higher than when both colors were available and that red maintained no responding. It was concluded that the reinforcing value of a stimulus depends on the positive or negative direction of its correlation with primary reinforcement, rather than upon the amount of information that it conveys.     

Effects of chlordiazepoxide and d-amphetamine on responding suppressed by conditioned punishment

Key pecking by two pigeons, maintained under a variable-interval two-minute schedule of food presentation, was suppressed when each response produced a five-second visual stimulus that was occasionally paired with shock. Stimulus-shock pairings occurred independently of responding according to a variable-time six-minute schedule for one bird or once per session for the other bird. The effects of chlordiazepoxide and d-amphetamine were assessed on this baseline of behavior suppressed by conditioned punishment. Chlordiazepoxide produced dose-related increases in the rate of punished responding, whereas d-amphetamine produced only decreases. In addition, chlordiazepoxide produced dose-related increases in response rate during the response-produced and response-independent stimulus presentations; d-amphetamine, however, only decreased responding during stimulus presentations.     

Food-deprivation level alters the effects of morphine on pigeons'' key pecking.

Four pigeons pecked response keys under a multiple fixed-ratio 30 fixed-interval 5-min schedule of food presentation. Components alternated separated by 15-s timeouts; each was presented six times. Pigeons were maintained at 70%, 85%, and greater than 90% of their free-feeding weights across experimental conditions. When response rates were stable, the effects of morphine (0.56 to 10.0 mg/kg) and saline were investigated. Morphine reduced response rates in a dose-dependent manner under the fixed-ratio schedule and at high doses under the fixed-interval schedule. In some cases, low doses of morphine increased rates under the fixed-interval schedule. When pigeons were less food deprived, reductions in pecking rates occurred at lower doses under both schedules for 3 of 4 birds compared to when they were more food deprived. When pigeons were more food deprived, low doses of morphine increased rates of pecking in the initial portions of fixed intervals by a greater magnitude. Thus, food-deprivation levels altered both the rate-decreasing and rate-increasing effects of morphine. These effects may share a common mechanism with increased locomotor activity produced by drugs and with increased drug self-administration under conditions of more severe food deprivation.     

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.     

Choice performance in several concurrent key-peck treadle-press reinforcement schedules

Five pigeons were exposed to several concurrent variable-interval food reinforcement schedules. For three subjects, one component of the schedule required a key-pecking response, the other a treadle-pressing response. For the other two subjects, both schedule components required treadle-pressing responses. The relative probability of reinforcement associated with the manipulanda was varied from 0 to 1.0 in 13 experimental conditions for the Key-Treadle subjects and nine conditions for the Treadle-Treadle subjects. The results indicated that the logarithms of relative time spent responding, and the logarithms of relative number of responses emitted on a manipulandum, approximated direct linear functions of logarithms of the relative frequencies of reinforcement associated with that manipulandum. No systematic bias in favor of time spent key pecking over time spent treadle pressing was apparent for the Key-Treadle subjects. All subjects exhibited undermatching, in that the ratios of time and response allocation at the alternatives systematically differed from the ratios of reinforcers obtained from the alternatives in the direction of indifference. Key pecking appeared to have no special link to food beyond treadle pressing or what would be expected on the basis of the reinforcement dependencies alone.     

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.     

Conjunctive schedules of reinforcement: I. Rate-dependent effects of pentobarbital and d-amphetamine

Key pecking of pigeons was maintained under conjunctive schedules of food presentation in which both a fixed-interval and a fixed-ratio schedule had to be completed before a peck produced food. For two pigeons, pecks on a single key completed both schedule requirements (fixed-interval 3-min, fixed-ratio 50 for one bird, fixed-interval 5-min, fixed-ratio 50 for the second). For two other pigeons, each requirement was scheduled on a separate key. On the two-key schedule, a peck after 5 min on the key scheduling the fixed-interval requirement produced food if at least 10 pecks had occurred on the ratio key (conjunctive fixed-interval 5-min, fixed-ratio 10). When each requirement was scheduled on a separate key, response rates on the fixed-ratio key were generally higher in the early portion of the interval and declined as the interval progressed; responding on the fixed-interval key, once initiated, typically remained at a constant rate throughout the interval. Responding under the single-key schedule was characterized by a high rate early in the interval; this then changed to a lower rate that continued until a peck produced food. For all pigeons, increases in response rates with pentobarbital and d-amphetamine were inversely related to the control rate of responding. When equivalent rates on each key of the two-key schedule were compared, both drugs increased rates on the fixed-ratio key less. Although the effects of both drugs were rate dependent, each drug differentially modified the pattern of responding under the single-key schedule.     

Reinforcement of behavioral patterns: shaping a scallop

Temporal patterns of key pecking by pigeons were shaped by a schedule in which the delivery of food was contingent upon a measure of the overall extent to which the temporal pattern of behavior within a 5-sec trial conformed to a required pattern. This pattern approximated a constant rate of change in the rate of key pecking throughout the 5-sec trial. In comparison with behavior maintained by a classical fixed-interval 5-sec schedule, the new schedule controlled a better approximation to a "scallop" within individual trials and greatly reduced intersubject variability. These results are consistent with the view that the delivery of a reinforcer after a behavioral pattern a few seconds in duration may strengthen the entire pattern as a unit, or operant. The response topography contiguous with reinforcement may be a negligible fraction of the strengthened operant. One implication of this view is that mean response rate for such brief responses as key pecks and lever presses is a byproduct of whatever patterns are strengthened, and generally will not reveal fundamental controlling relationships, whenever a reinforcer is not contiguous with all the behavior on which it is contingent.     

The differentiation of response numerosities in the pigeon

Two experiments examined how pigeons differentiate response patterns along the dimension of number. In Experiment 1, 5 pigeons received food after pecking the left key at least N times and then switching to the right key (Mechner's Fixed Consecutive Number schedule). Parameter N varied across conditions from 4 to 32. Results showed that run length on the left key followed a normal distribution whose mean and standard deviation increased linearly with N; the coefficient of variation approached a constant value (the scalar property). In Experiment 2, 4 pigeons received food with probability p for pecking the left key exactly four times and then switching. If that did not happen, the pigeons still could receive food by returning to the left key and pecking it for a total of at least 16 times and then switching. Parameter p varied across conditions from 1.0 to .25. Results showed that when p= 1.0 or p=.5, pigeons learned two response numerosities within the same condition. When p=.25, each pigeon adapted to the schedule differently. Two of them emitted first runs well described by a mixture of two normal distributions, one with mean close to 4 and the other with mean close to 16 pecks. A mathematical model for the differentiation of response numerosity in Fixed Consecutive Number schedules is proposed.     

Conditioned suppression with extinction as the signalled stimulus

The key pecking of pigeons that was maintained by a 60-sec random-interval schedule of food reinforcement was suppressed during a variable-duration warning stimulus that signalled a 5-min extinction period. The onset of the extinction period immediately followed the termination of the warning signal and was independent of the subject's responses. All subjects eventually showed nearly complete suppression of responding during the warning stimulus.     

Mechanisms underlying the effects of unsignaled delayed reinforcement on key pecking of pigeons under variable-interval schedules.

Three experiments were conducted to test an interpretation of the response-rate-reducing effects of unsignaled nonresetting delays to reinforcement in pigeons. According to this interpretation, rates of key pecking decrease under these conditions because key pecks alternate with hopper-observing behavior. In Experiment 1, 4 pigeons pecked a food key that raised the hopper provided that pecks on a different variable-interval-schedule key met the requirements of a variable-interval 60-s schedule. The stimuli associated with the availability of the hopper (i.e., houselight and keylight off, food key illuminated, feedback following food-key pecks) were gradually removed across phases while the dependent relation between hopper availability and variable-interval-schedule key pecks was maintained. Rates of pecking the variable-interval-schedule key decreased to low levels and rates of food-key pecks increased when variable-interval-schedule key pecks did not produce hopper-correlated stimuli. In Experiment 2, pigeons initially pecked a single key under a variable-interval 60-s schedule. Then the dependent relation between hopper presentation and key pecks was eliminated by arranging a variable-time 60-s schedule. When rates of pecking had decreased to low levels, conditions were changed so that pecks during the final 5 s of each interval changed the keylight color from green to amber. When pecking produced these hopper-correlated stimuli, pecking occurred at high rates, despite the absence of a peck-food dependency. When peck-produced changes in keylight color were uncorrelated with food, rates of pecking fell to low levels. In Experiment 3, details (obtained delays, interresponse-time distributions, eating times) of the transition from high to low response rates produced by the introduction of a 3-s unsignaled delay were tracked from session to session in 3 pigeons that had been initially trained to peck under a conventional variable-interval 60-s schedule. Decreases in response rates soon after the transition to delayed reinforcement were accompanied by decreases in eating times and alterations in interresponse-time distributions. As response rates decreased and became stable, eating times increased and their variability decreased. These findings support an interpretation of the effects of delayed reinforcement that emphasizes the importance of hopper-observing behavior.     

Changing the response unit from a single peck to a fixed number of pecks in fixed-interval schedules

Each of three pigeons was studied first under a standard fixed-interval schedule. With the fixed interval held constant, the schedule was changed to a second-order schedule in which the response unit was the behavior on a small fixed-ratio schedule (first a fixed-ratio 10 and then a fixed-ratio 20 schedule). That is, every completion of the fixed-ratio schedule produced a 0.7-sec darkening of the key and reset the response count to zero for the next ratio. The first fixed-ratio completed after the fixed-interval schedule elapsed produced the 0.7-sec blackout followed immediately by food. These manipulations were carried out under two different fixed-interval durations for each bird ranging from 3 min to 12 min. The standard fixed-interval schedules produced the typical pause after reinforcement followed by responding at a moderate rate until the next reinforcement. The second-order schedules also engendered a pause after reinforcement, but responding occurred in bursts separated by brief pauses after each blackout. For a particular fixed-interval duration, post-reinforcement pauses increased slightly as the number of pecks in the response unit increased despite large differences in the rate and pattern of key pecking. Post-reinforcement pause increased with the fixed-interval duration under all response units. These data confirm that the allocation of time between pausing and responding is relatively independent of the rate and topography of responding after the pause.     

Maintenance of key pecking by response-independent food presentation: the role of the modality of the signal for food

Three pigeons were exposed to a series of procedures in which periods of response-independent food presentation, on a variable-time schedule, alternated with periods in which food was never presented. The stimuli that signalled periods of food availability or non-availability varied from one procedure to the next, and were sometimes key colors, sometimes tones, and sometimes compounds of both. Key pecking was initiated and maintained when key color was a signal for food; key pecking was not initiated when a tone was the signal for food. However, control of key pecking that was already established could be transferred from key color to tone, and subsequently, initiated by the tone. It is suggested that for pigeons, pre-experimental relationships exist among food, visual stimuli, and pecking, and that a similar relationship, which includes auditory stimuli, must be induced in the laboratory.     

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.