Effects of d-amphetamine on responding under second-order schedules of reinforcement with paired and nonpaired brief stimuli.

Three pigeons were studied under a multiple schedule in which pecks in each component were reinforced according to a variable-interval 120-s second-order schedule with fixed-interval 60-s units. In the first component of the multiple schedule, the completion of a fixed interval produced either food or a 4-s change in key color plus houselight illumination. In the second component an identical schedule was in effect, but the stimulus was a 0.3-s change in key color. Both long and short brief stimuli were not paired with food presentations in Conditions 1 and 3 and were paired with food in Condition 2. There were no consistent differences in response patterns under paired and nonpaired brief-stimulus conditions when the stimulus was a 4-s change in key color accompanied by houselight illumination. However, pairing the 0.3-s key-color change with food presentations resulted in higher indices of curvature and lower response rates in the early segments of the fixed interval than when the stimulus was not paired with food presentations. Low doses of d-amphetamine (0.3 and 1 mg/kg) produced small and inconsistent increases in overall response rates, and higher doses (3 and 10 mg/kg) decreased overall response rates. d-Amphetamine altered response patterns within fixed intervals by decreasing the indices of curvature and increasing response rates in the early segments of the fixed interval. Response rates and patterns under paired and nonpaired brief-stimulus conditions were not differentially affected by d-amphetamine. Thus, evidence for the enhancement of the conditioned reinforcement effects of psychomotor stimulant drugs was not found with the second-order schedules used in the present study.     

Second-order schedules: discrimination of components

Pigeons were exposed to a series of second-order schedules in which the completion of a fixed number of fixed-interval components produced food. In Experiment 1, brief (2 sec) stimulus presentations occurred as each fixed-interval component was completed. During the brief-stimulus presentation terminating the last fixed-interval component, a response was required on a second key, the brief-stimulus key, to produce food. Responses on the brief-stimulus key before the last brief-stimulus presentation had no scheduled consequences, but served as a measure of the extent to which the final component was discriminated from preceding components. Whether there were one, two, four, or eight fixed-interval components, responses on the brief-stimulus key occurred during virtually every brief-stimulus presentation. In Experiment 2, an attempt was made to punish unnecessary responses on the brief-stimulus key, i.e., responses on the brief-stimulus key that occurred before the last component. None of the pigeons learned to withhold these responses, even though they produced a 15-sec timeout and loss of primary reinforcement. In Experiment 3, different key colors were associated with each component of a second-order schedule (a chain schedule). In contrast to Experiment 1, brief-stimulus key responses were confined to the last component. It was concluded that pigeons do not discriminate well between components of second-order schedules unless a unique exteroceptive cue is provided for each component. The relative discriminability of the components may account for the observed differences in initial-component response rates between comparable brief-stimulus, tandem, and chain schedules.     

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.     

Signalled and unsignalled percentage reinforcement of performance under a chained schedule

Pigeons were trained to peck a key under a chained fixed-ratio 15 fixed-interval 25-sec schedule of food presentation. In Experiment 1, blocks of sessions in which 100%, 75%, 50%, and 25% of the sequences ended with food presentation were conducted. When food presentation was omitted, a timeout of equal duration replaced it. As the frequency of food presentation decreased so did the frequency of completing the chained schedule. In Experiment 2, 75%, 50%, or 25% of the sequences terminated with food presentation and outcomes were signalled, i.e., completion of the fixed ratio resulted in either a stimulus correlated with the fixed-interval 25-sec schedule or a stimulus correlated with extinction. As the frequency of food presentation decreased, the number of sequences completed per session increased for two pigeons and remained high for a third. In Experiments 3 and 4, assessments of the effects of signalling the outcome of the chained schedule were made with response-independent presentation of events at the end of the sequence. Again, signalling the outcome of the chained schedule led to more chains being completed per session than did not signalling the outcome. Stimuli differentially paired with food presentation have powerful behavioral effects that may be attributed to the potency of these stimuli as conditioned reinforcers.     

Increased reinforcement when timeout from avoidance includes access to a safe place

Three experiments investigated the reinforcing value of access to a safe place during timeout from an avoidance schedule. Rats were trained on conjoint schedules in which responding both postponed shock on a free-operant avoidance schedule and produced periods of timeout on fixed-ratio schedules. In some conditions, a shelf was inserted into the operant chamber during timeout, enabling subjects to get off the grid floor. The combination of timeout and shelf maintained substantially higher response rates than the baseline avoidance schedule with ratio requirements as high as 90 (Experiment I). Adding the shelf to timeouts in one component of multiple fixed-ratio schedules of timeout resulted in higher response rates in the component where the shelf was included (Experiment II). When timeouts with and without the shelf were arranged on concurrent schedules, the shelf-timeout combination was preferred, even when of shorter duration than timeout alone (Experiment III). In all three experiments, subjects climbed on the shelf, although all shocks were cancelled during timeout periods. The results could not be accounted for solely in terms of the reinforcing properties of changes in shock rates, but required an interpretation that ascribed conditioned reinforcing value to stimuli associated with such changes.     

Relative durations of conditioned stimulus and intertrial interval in conditioned suppression.

The effects of the relative durations of the conditional stimulus and the intertrial interval on bar pressing during a conditioned-suppression procedure were examined as a function of two additional variables--type of operant baseline schedule and rate of shock presentation. In Experiment 1, response suppression was compared across components of a multiple fixed-ratio, random-ratio, fixed-interval, random-interval schedule, at relative conditioned-stimulus/intertrial-interval durations of 1/1, 1/4, and 1/9. In Experiment 2, relative conditioned-stimulus/intertrial-interval duration (1/5, 3/3, or 5/1) was manipulated across groups, while shock frequency (2, 6, or 10 shocks/hr) was manipulated within groups. In both experiments, suppression during the signal was virtually complete at all relative durations. Responding was also suppressed during the intertrial interval, but that suppression varied as a function of experimental manipulations. In Experiment 1, intertrial-interval response rates were higher when relative signal duration was 1/9 than when it was 1/1, although both relative signal duration and shock frequency, which covaried, could have contributed to the difference. In Experiment 2, the patterning of response rates between successive shocks was affected by relative duration, absolute rates during the intertrial interval varied as a function of shock frequency, and differences between suppression during the signal and suppression during the intertrial interval were affected by both relative duration and shock frequency. The data support an analysis based upon relationships between shock-correlated and intertrial-interval stimuli and, as assessed by the relative-delay-to-reinforcement metric, are comparable to results that have been reported from experiments using similar manipulations under the autoshaping paradigm.     

Uncertainty reduction, conditioned reinforcement, and observing

In a concurrent-chains procedure, pigeons chose between equivalent mixed and multiple fixed-interval schedules of reinforcement. In the first experiment, preference for the multiple schedule was higher when the probability of the shorter fixed interval was less than .50 than for complementary points, an outcome consistent with the delay-reduction hypothesis of conditioned reinforcement and observing, but inconsistent with the uncertainty-reduction hypothesis which requires symmetrical preferences with a maximum when the two intervals are equiprobable. A second experiment assessed preference for equivalent mixed and multiple schedules when each choice outcome resulted in two reinforcements, one on the longer and one on the shorter fixed interval. The order of the two fixed intervals was determined probabilistically. Pigeons again preferred multiple to mixed schedules, although multiple-schedule preference did not vary systematically with the likelihood of the shorter fixed interval occurring first. The results from these choice procedures are consistent with those from the observing-response literature in suggesting that the strength of a stimulus cannot be well described as a function of the degree of uncertainty reduction the stimulus provides about reinforcement.     

Reinforcement of human observing behavior by a stimulue correlated with extinction or increased effort

Young men pulled a plunger on mixed and multiple schedules in which periods of variable-interval monetary reinforcement alternated irregularly with periods of extinction (Experiment 1), or in which reinforcement was contingent on different degrees of effort in the two alternating components (Experiment 2). In the baseline conditions, the pair of stimuli correlated with the schedule components could be obtained intermittently by pressing either of two observing keys. In the main conditions, pressing one of the keys continued to produce both discriminative stimuli as appropriate. Pressing the other key produced only the stimulus correlated with variable-interval reinforcement or reduced effort; presses on this key were ineffective during periods of extinction or increased effort. In both experiments, key presses producing both stimuli occurred at higher rates than key presses producing only one, demonstrating enhancement of observing behavior by a stimulus correlated with the less favorable of two contingencies. A control experiment showed that stimulus change alone was not an important factor in the maintenance of the behavior. These findings suggest that negative as well as positive stimuli may play a role in the conditioned reinforcement of human behavior.     

SOME RELATIONS BETWEEN CLASSICALLY CONDITIONED AGGRESSION AND CONDITIONED SUPPRESSION IN SQUIRREL MONKEYS

During three experiments with squirrel monkeys, stimulus and shock pairings were given in the presence of a bite tube. Experiments 1 and 2 used a conditioned-suppression procedure in which bar pressing was reinforced with food. A transparent shield prevented biting of the bar. When the stimulus was paired with shock, bar pressing decreased (conditioned suppression) and tube biting increased during the stimulus (classically conditioned aggression). When the bite tube was removed on alternate sessions in Experiment 2, there was more suppression when the tube was present, thus suggesting that biting competed with bar pressing. However, this simple competing-response interpretation was complicated by the findings of Experiment 3 where, with naive monkeys, bar pressing was never reinforced with food, yet bar pressing was induced during the stimulus and was highest when the bite tube was absent. The fact that stimulus-induced bar pressing developed indicates that bar pressing in conditioned-suppression procedures, suppressed or not, may be maintained by two types of control—the food reinforcer and induced CS control. The higher rate of induced bar pressing during the stimulus with the bite tube absent confounds a simple competing response interpretation of conditioned suppression. It suggests that shock-induced responses during conditioned suppression could be both contributing to and competing with responding maintained by food, with the net effect depending on specific but ill-defined features of the situation.     

Conditioned reinforcement and discrimination in second-order schedules

Pigeons were exposed to multiple second-order schedules in which responding on the “main key” was reinforced according to either a variable-interval or fixed-interval schedule by production of a brief stimulus on the “brief-stimulus key”. A response was required to the brief stimulus during its fourth (final) presentation to produce food; responses to the earlier brief stimuli indicated the extent to which the final brief stimulus was discriminated from preceding ones. Main-key response rates were higher in early components of paired brief-stimulus schedules, in which each brief stimulus was the same as that paired with reinforcement, than in comparable unpaired brief-stimulus or tandem schedules. Poor discrimination occurred between paired brief stimuli (Experiment I). When chain stimuli on the main key induced a discrimination between the first two and second two brief stimuli, the response-rate enhancement in the paired brief-stimulus schedule persisted (Experiment II). Rate enhancement diminished when the initial link of the chain included the first three components (Experiment IV). Eliminating the contingency between responding and brief-stimulus production also diminished rate enhancement (Experiment III). The results show that the discriminative and conditioned reinforcing effects of food-paired brief stimuli may be selectively manipulated and suggest that the reinforcing effects are modulated by other reinforcers in the situation.