Stimulus generalization and delay of reinforcement during one component of a multiple schedule

The key pecking of six pigeons was reinforced according to a variable-interval 1-min schedule during each of two successively presented stimuli. When the key was illuminated by a black line on a white background, reinforcement was delayed for 10 sec. When the key was illuminated by a plain white light, reinforcement was not delayed. All subjects responded at a lower rate during the presentation of the black line. A subsequent generalization test along the line-orientation dimension produced a U-shaped gradient, with the nadir located at or near the training stimulus, for each subject. These gradients suggested that the lower rate of response during the stimulus associated with delayed reinforcement may have been due to an inhibition of responding.     

Stimulus control and delayed reinforcement

Pigeons were tested for generalization along the line-orientation dimension, after being trained on various two-component multiple schedules. The first component contained either a variable-interval 1-min schedule of immediate reinforcement or an extinction schedule and was associated with a plain white key (S1). The second component contained a variable-interval 1-min schedule of delayed reinforcement and was associated with a black line on a white background (S2). The major results showed that (a) decremental gradients were obtained around the stimulus associated with the delayed reinforcement component when S1 was associated with extinction, but that incremental gradients were obtained when S1 was associated with immediate reinforcement, (b) the subjects' pretraining did not affect the generalization gradients if sufficient training on the terminal multiple schedule was provided, and (c) changing the S1 schedule from immediate reinforcement to extinction produced behavioral contrast if reinforcement was delayed for 10 sec during S2, but not if it was delayed for 20 sec.     

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.     

Choice behavior and the accessibility of the reinforcer

In Experiment 1, matching of relative response rates to relative rates of reinforcement was obtained in concurrent variable-interval schedules when the absolute values of the two concurrent variable-interval schedules varied from 6 sec and 12 sec to 600 sec and 1200 sec. Increases in the duration of the changeover delay, however, produced decreases in the relative response rates and, consequently, some deviation from matching. In Experiment 2, matching of relative response rates to the relative duration of the reinforcer failed to occur when the equal variable-interval schedules arranging access to the two different reinforcer durations (1.5 and 6 sec) were varied in size from concurrent variable-interval 10-sec schedules to concurrent variable-interval 600-sec schedules.     

Dimensional stimulus control following brief wavelength training

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

Enhancement of conditioned reinforcement by uncertainty

Pigeons were trained in three conditions. In the baseline condition, the birds responded on a fixed-interval schedule with the response key white. When the interval was completed, the key turned either red or green for a delay interval that was terminated by a grain presentation dependent on no key pecks during the final 2 sec. In the uncertainty condition, no grain was presented at the end of the delay periods when the key was red. In the certainty condition, the white light appeared only on occasions when pecking would turn the key green and produce food. Otherwise, the key was illuminated red throughout the total time period. The highest response rate in white occurred in the uncertainty condition, the next highest in the certainty condition, and the lowest in baseline. The results suggest that uncertainty facilitated responding, although uncertainty is not a necessary condition for conditioned reinforcement.     

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 stimulus duration on observing behavior maintained by differential reinforcement magnitude

Pigeons made observing responses for stimuli signalling the availability of either 10-sec or 2-sec access to grain on fixed-interval 1-min schedules. If observing responses did not occur, food-producing responses occurred to a stimulus common to both reinforcement magnitudes. When the stimuli remained on for the duration of the components and signalled differential reinforcement magnitudes, observing responses were maintained; however, when the stimuli remained on for 10 sec, observing responses decreased markedly. In addition, it was shown that the occasional presentation of the stimulus signalling 10-sec access to grain was necessary for the maintenance of observing behavior. A control condition demonstrated that when all the available stimuli signalled 6-sec access to grain, observing responses declined. Taken together, the results demonstrated that the occasional presentation of the stimulus that remained on for the duration of the component and signalled the larger reinforcement magnitude was necessary for the maintenance of observing behavior.     

Varying temporal placement of an added stimulus in a fixed-interval schedule

One paradigm for exploring stimulus effects on behavior is defined for steady state experiments. The paradigm is illustrated by a 60-sec fixed-interval reinforcement schedule wherein a 6-sec light is introduced into each interval. The temporal relation of this stimulus to the reinforcer is the independent variable that is systematically explored. Two experiments studied this temporal relation under two parametric conditions: (a) when the 6-sec light occurs once in each 60-sec interval, (b) when the 6-sec light occurs twice in each interval, the second time always during the 6 sec immediately preceding the reinforcer. Functions are presented showing the effect of the 6-sec light on responding at all points in the fixed-interval.     

Rate changes after unscheduled omission and presentation of reinforcement

Changes in response rate similar to frustration effects were studied in a two-lever situation. Responding on one lever on a fixed-interval schedule produced access to water for 5 sec and an exteroceptive stimulus. In the presence of this stimulus, responding on another lever on a fixed-interval schedule produced access to water for 5 sec and terminated the stimulus. Occasional omission of a previously scheduled reinforcer after responding on the first lever resulted consistently in increases in rate on the second lever during the immediately succeeding interval. In another procedure, occasional presentation of a previously unscheduled reinforcer after responding on the first lever resulted consistently in decreases in rate on the second lever during the immediately succeeding interval. Changes occurred after the first omissions or presentations and were about the same in magnitude as the procedure continued over several sessions. Typically, an increase or decrease in rate was maintained throughout an entire 100-sec interval. Changes in rate on the second lever of approximately the same magnitude also occurred when rate on the first lever was near-zero under a schedule that differentially reinforced behavior other than lever pressing.     

Conditioned suppression and conditioned enhancement with the same positive UCS: an effect of CS duration

Previous experiments have shown that positively reinforced operant responding is suppressed during a conditioned stimulus terminated with an electric shock (conditioned suppression). In the present experiment, the conditioned stimulus was terminated with a positive unconditioned stimulus, and it was found that the duration of the conditioned stimulus was a key factor in determining whether response suppression or response enhancement was observed during the stimulus. The lever-pressing responses of rats were maintained by a variable-interval schedule of food reinforcement. While the rats were pressing the lever, a light was occasionally turned on, its offset coincident with a brief period of access to a sucrose solution. In consecutive blocks of sessions, the light duration was 40 sec, 12 sec, or 120 sec. Results showed that the rate of lever pressing was substantially suppressed during the 12-sec stimulus, slightly suppressed during the 40-sec stimulus, and enhanced during the 120-sec stimulus.     

Information integration and compounding of discriminative stimuli in pigeons

Keypecks were reinforced with food in three pigeons according to a multiple schedule with variable interval schedules of reinforcement associated with the components. If the key was green (G) or white with three horizontal black lines (H), variable interval 30 sec was in effect. A red key (R) or three black vertical lines on a white surround (V) indicated that variable interval 60 sec was in effect. Following this training, a single test session was conducted in which the reinforcer was not available and in which the single stimuli and the compounds HG, HR, VG, and VR were presented. Response rates to the compounds were generally less than to the elements alone. This unexpected result was apparently due to degrading of the stimuli during superimposition. However, the compound rates aligned well with a linear model that assumes no interaction between orientation and color stimuli. This initial agreement with Anderson’s information integration approach suggests further application of the model to stimulus compounding in infrahuman animals.

     

Relative delay of reinforcement and choice

Pigeons' responses on either of two concurrently available keys each associated with variable-interval 60-sec schedules occasionally changed the schedule on that key to a terminal-link interval schedule providing access to gain while the other key became inoperative. Experiment I compared simple fixed- and mixed-interval schedules in the terminal links, and showed that for all subjects and schedules the distribution of responses during the concurrent initial links was accurately described by the relative inverse delay of reinforcement squared. Experiment II extended the generality of this formulation to a situation in which rate of reinforcement was constant and delay alone was varied.     

The effects upon food-reinforced pecking and treadle-pressing of auditory and visual signals for response-independent food

Seven pigeons whose key-pecking was maintained by food reinforcement on a differential-reinforcement-of-low-rates 12-sec limited-hold 4-sec schedule and 12 other pigeons whose treadle-pressing was maintained by the same schedule received appetitive Pavlovian conditioning trials superimposed upon the instrumental baseline. Half the birds in each group received a tone as the CS, and the other half received a stimulus change on the key. Each CS was 20 sec long, and was immediately followed by 10-sec access to grain. The visual CS markedly facilitated the rate of pecking on the key for the birds whose baseline response was pecking. The visual CS produced auto-shaping of the key-peck and tended to produce suppression of treadle-pressing for the birds whose baseline response was treadle-pressing. The auditory CS produced inconsistent effects across birds regardless of the baseline response. In all cases the conditioned effects extinguished when response-independent food was omitted.     

An auto-adjusting shaping procedure

A variable interval (VI) schedule is described that automatically adjusts the programmed rates of reinforcement in accordance with the rates of responding of subjects during the two immediately preceding 30-sec time intervals. The schedule prescribes that as rate of responding decreases, programmed reinforcement rate increases, and that when rate of responding increases, reinforcement rate decreases. Thus, programmed reinforcement rate is adjusted continuously until some target value is reached. Ten rats were exposed to this procedure five times a day at 1-h intervals. The target, set at VI 120 sec, was reached by most subjects within 4 days of training. Subsequently, all subjects responded consistently during five daily 1-h sessions with VI 120 sec. This procedure speeds up the training of subjects on long VI schedules and substantially reduces the time and effort spent observing the subjects and adjusting the schedule parameter value during the early development of responding.     

Choice and number of reinforcers

Pigeons were exposed to the concurrent-chains procedure in two experiments designed to investigate the effects of unequal numbers of reinforcers on choice. In Experiment 1, the pigeons were indifferent between long and short durations of access to variable-interval schedules of equal reinforcement density, but preferred a short high-density terminal link over a longer, lower density terminal link, even though in both sets of comparisons there were many more reinforcers per cycle in the longer terminal link. In Experiment 2, the pigeons preferred five reinforcers, the first of which was available after 30 sec, over a single reinforcer available at 30 sec, but only when the local interval between successive reinforcers was short. The pigeons were indifferent when this local interval was sufficiently long. The pigeons' behavior appeared to be under the control of local terminal-link variables, such as the intervals to the first reinforcer and between successive reinforcers, and was not well described in terms of transformed delays of reinforcement or reductions in average delay to reinforcement.     

Behaviour of ‘impulsive’ and ‘non-impulsive’ humans in a temporal differentiation schedule of reinforcement

The operant behaviour of psychometrically defined ‘impulsive’ and ‘non-impulsive’ subjects, under a temporal differentiation schedule of reinforcement, was examined. Four impulsive and four non-impulsive females were selected on the basis of their scores on the Matching Familiar Figures Test. The subjects participated in fifteen 45-min sessions in which they were exposed to an inter-response-time-greater-than-10-sec schedule of monetary reinforcement. During Phase I (sessions 1–5) no information was provided about the reinforcement contingency. During Phase II (sessions 6–10) a light on the response panel was illuminated whenever a reinforcer became available. At the start of Phase III (session 11) the subjects were given explicit information about the reinforcement contingency. At the start of Phase IV (sessions 12–15) the subjects were told that the light would no longer be operative although the contingency would remain unaltered. During Phase I the impulsive subjects earned fewer reinforcers, and emitted a greater proportion of non-reinforced responses (inter-response times less than 10 sec) than the non-impulsive subjects. During Phase II both groups increased their earnings, although the performance of the non-impulsive group remained superior to that of the impulsive group. In Phase III both groups performed equally well. In Phase IV the performance of both groups deteriorated, the impulsive group performing more poorly than the non-impulsive group. The results demonstrate the sensitivity of operant performance maintained under temporal differentiation schedules to personality dimensions such as ‘impulsiveness/non-impulsiveness’.     

The effect of multiple S periods on responding on a fixed-interval schedule: IV. Effect of continuous S with only short S probes

Pigeons were studied under FI 500 sec in which an S^Δ was present throughout the interval except during the terminal 50-sec segment and one earlier 50-sec segment. Very little responding occurred during the presence of S^Δ. The rate of responding in the earlier 50-sec S^D segments was lower than in the terminal S^D segment. There was a clear trend for the rate of responding in the earlier S^D segment to be progressively higher the later it occurred in the course of the FI 500 sec. This trend was shown roughly to parallel the increasing rate of responding in a conventional FI 500 sec with no interruption by S^Δ. Since the changing tendency to respond through the FI survives massive disruption by S^Δ, it is concluded that the control of responding through the FI does not require continuous mediating behavior. It is suggested that it is the decaying retroactive influence of the reinforcer on responses that occurred longer and longer before the reinforcer occurred which produces the familiar scalloped pattern of responding under FI schedules.     

Preference and resistance to change with constant- and variable-duration terminal links: independence of reinforcement rate and magnitude

Pigeons responded in a three-component multiple concurrent-chains procedure in which the variable-interval reinforcement schedules were the same across components but magnitudes differed across components. The terminal links were arranged either as a variable delay followed by presentation of a reinforcer ("variable duration") or as a fixed period of access to the schedule during which a variable number of reinforcers could be earned ("constant duration"). Relative reinforcement rate was varied parametrically across both types of conditions. After baseline training in each condition, resistance to change of terminal-link responding was assessed by delivering food during the initial links according to a variable-time schedule. Both preference and resistance to change were more sensitive to reinforcement-rate differences in the constant-duration conditions. Sensitivities of preference and resistance to change to relative reinforcement rate did not change depending on relative reinforcement magnitude. Taken together, these results confirm and extend those of prior studies, and suggest that reinforcement rate and magnitude combine additively to determine preference and resistance to change. A single structural relation linking preference and resistance to change describes all the data from this and several related studies.     

The role of instrumental responding and contiguity of stimuli in the development of infant secondary reinforcement

Ten-month-old infants received contingent pairings of a tone (T+) and food reinforcer. Groups S^r and S^D received the food on an FI 23-sec schedule for target touching, the former group receiving T+ immediately after the response and 1.5 sec prior to food and the latter group receiving T+ at the end of the intertrial interval. Group SC received food reinforcers 1.5 sec after T+ with no response required. A second tone (Tⁿ) was heard by all groups once during each intertrial interval, at randomly determined points. All groups subsequently were given a spatial discrimination task, receiving T+ for one alternative and Tⁿ for the other. Group S^r gave significantly more responses for T+ than for Tⁿ, but neither of the other two groups produced a superiority for T+. Thus, both contiguity with a primary reinforcer and the presence of an operant during training appear to be necessary for a neutral signal to acquire the ability to enhance responding.