Additive summation following intradimensional discrimination training

Rats were exposed to intradimensional composite stimuli presented on the response lever that varied in both light intensity and flicker rate. For all subjects, pressing the lever was reinforced when it was illuminated at a high intensity and flickered at a low rate (I + f) or when it was illuminated at a low intensity and flickered at a high rate (i + F). For half the subjects, lever responding was not reinforced when it was illuminated at a low intensity and flickering at a low rate (i + f). For the remaining subjects, lever presses were not reinforced when the lever was illuminated at a high intensity and flickered at a high rate (I + F). When the composite stimulus composed of the light intensity and flicker rates that had been associated only with reinforced responding was displayed (I + F for half the subjects and i + f for the remaining subjects), it controlled the highest response rate of all stimuli (additive summation). The results demonstrated that similar attentional processes control intra- and interdimensional composite-stimulus discriminations in a manner consistent with Weiss' (1972) analysis of summation.     

Effects of d-amphetamine and pentobarbital under concurrent fixed-ratio schedules.

Pigeons were studied under a two-key concurrent fixed-ratio schedule of food presentation. During the first five sessions, the fixed-ratio requirements were 30 responses on one key (major key) and 120 responses on the other key (minor key): responding occurred almost exclusively on the major key. When the fixed-ratio requirements were then made equal at 30 responses on both keys, responding continued to predominate on the major key. The asymmetric distribution of responses persisted when the concurrent fixed-ratio fixed-ratio schedule was interrupted with periods during which the major key was associated with extinction while the other key remained associated with a fixed-ratio schedule. Additionally, in some subjects the fixed-ratio requirements were increased. These schedule modifications decreased the asymmetry in responding but did not eliminate it. d-Amphetamine decreased rates on both keys and slightly increased the asymmetric distribution of responses, while pentobarbital reversed the distribution of responses by increasing low rates and decreasing high rates. The pigeons maintained their original asymmetric distribution of responses during the 1 1/2-year-long study, despite schedule alterations and drug administrations.     

Within-session meal-size effects on induced drinking.

As a control for the effects of session duration and hunger on the relation between food magnitude and induced drinking, four food-deprived rats were exposed to a variable-time 50-s schedule of food delivery in which the size of each food delivery varied randomly within sessions. Food-related behavior and schedule-induced drinking per opportunity were examined as functions of meal size and postfood time. All rats showed an inverted-U-shaped relation between drinking per opportunity and meal size. This relation was caused by variation in the percentage of intervals that contained drinking and by variation in the number of drinking bouts per interval, rather than by bout duration or by the amount of drinking within those intervals that actually contained drinking. Head-in-feeder time increased linearly with meal size. Schedule-induced drinking was entrained by food delivery in 3 of 4 subjects; the entrainment was due to regulation of the starting time of each drinking bout rather than to regulation of bout duration.     

Interpersonal contingencies: Performance differences and cost-effectiveness

Three reinforcement contingencies were compared with regard to performance differences and cost-effectiveness (i.e., responses per unit reinforcer). Pairs of college students were studied under individual, cooperative, or competitive contingencies using a concurrent setting that included one of these three contingencies as one alternative and a lower paying individual contingency as the other alternative. With the individual and the cooperative contingencies, overall response rates were typically high; under competitive contingencies the overall response rates were substantially lower. Subjects responded at very high rates when competing, but chose not to compete most of the time. Competition and cooperation produced the most cost-effective responding, assessed as the number of responses made per $.01 of reinforcer. High overall rates of competitive responding were obtained when the contests were longer and the lower paying alternative contingency was not available.     

Mirror pecking and timeout under a multiple fixed-ratio schedule of food delivery

Pigeons were trained to peck a key under a multiple fixed-ratio 25 fixed-ratio 175 schedule of food presentation. In the first condition, either a mirror or the opportunity to produce a 30-second timeout were available. In a second condition, mirror and timeout availability were reversed for the two groups. Following a return to the initial condition, mirror and timeout keys were presented together for all birds. Mirror and timeout responses occurred predominantly in the pause in the larger fixed-ratio component, regardless of whether the opportunities for the two responses were available singly or together. Mirror responding occurred in a greater proportion of the pauses than did timeouts. When the opportunities for both mirror pecking and timeout were available concurrently, they occurred with probabilities similar to those under the single conditions. Within the pause itself, mirror responses most frequently occurred immediately after reinforcement. Timeouts occurred most frequently toward the end of the pause, and some timeouts occurred in the early part of the run. Longer preratio pausing occurred in the larger fixed-ratio component in the conditions in which the mirror was present, whether or not any mirror pecks were recorded.     

Human Sensitivity To Concurrent Schedules Of Reinforcement: Effects Of Observing Schedule-correlated Stimuli

The determinants of human sensitivity to concurrent variable-interval variable-interval schedules of reinforcement have been difficult to identify, in part because of procedural differences separating published experiments. This experiment investigated vigilance to stimuli correlated with concurrent schedules. Across phases, 3 college students were provided with either no schedule-correlated stimuli, an observing response that provided brief access to the stimuli, or a contingency that required the subject to identify the stimulus correlated with the source of each obtained reinforcer. Sensitivity, as quantified by the generalized matching equation, was low when no stimuli were available. When the stimuli were response contingent, 1 subject observed them, and her behavior became more sensitive to the distribution of reinforcers across the concurrent schedules. When the procedure required discrimination of the stimulus correlated with each reinforcer, the other 2 subjects also observed the stimuli, and their schedule sensitivity was increased as well. These results implicate procedural differences, rather than inherent behavioral differences, as the source of differences in sensitivity to schedules of reinforcement between humans and nonhumans.     

Choice with probabilistic reinforcement: effects of delay and conditioned reinforcers.

Two experiments measured pigeons' choices between probabilistic reinforcers and certain but delayed reinforcers. In Experiment 1, a peck on a red key led to a 5-s delay and then a possible reinforcer (with a probability of .2). A peck on a green key led to a certain reinforcer after an adjusting delay. This delay was adjusted over trials so as to estimate an indifference point, or a duration at which the two alternatives were chosen about equally often. In all conditions, red houselights were present during the 5-s delay on reinforced trials with the probabilistic alternative, but the houselight colors on nonreinforced trials differed across conditions. Subjects showed a stronger preference for the probabilistic alternative when the houselights were a different color (white or blue) during the delay on nonreinforced trials than when they were red on both reinforced and nonreinforced trials. These results supported the hypothesis that the value or effectiveness of a probabilistic reinforcer is inversely related to the cumulative time per reinforcer spent in the presence of stimuli associated with the probabilistic alternative. Experiment 2 tested some quantitative versions of this hypothesis by varying the delay for the probabilistic alternative (either 0 s or 2 s) and the probability of reinforcement (from .1 to 1.0). The results were best described by an equation that took into account both the cumulative durations of stimuli associated with the probabilistic reinforcer and the variability in these durations from one reinforcer to the next.     

Effects of different accessibility of reinforcement schedules on choice in humans

Based on the delay-reduction hypothesis, a less profitable schedule should be rejected if its duration exceeds the mean delay to reinforcement. It should be accepted if its duration is shorter than the mean delay. This was tested for humans, using a successive-choice schedule. The accessibility of the less profitable (variable-interval 18 s) schedule was varied by changing the duration (in terms of a fixed interval) of the waiting-time component preceding its presentation. Forty-eight students were randomly assigned to three groups. In Phase 1, the duration of the less profitable schedule equaled the mean delay to reinforcement in all groups. In Phase 2, waiting time preceding the less profitable schedule was reduced in Group 1 and increased in Group 2. Thus, the schedule was correlated either with a relative delay increase (Group 1) or a delay reduction (Group 2). In Group 3, conditions remained unchanged. As predicted, acceptance of the less profitable schedule decreased in Group 1 and increased in Group 2. The increased acceptance in Group 2 was accompanied by a decreased acceptance of the more profitable (variable-interval 3 s) schedule, resembling a pattern of negative contrast. Response rates were higher under the component preceding (a) the more profitable schedule in Group 1 and (b) the less profitable schedule in Group 2. Implications for the modification of human choice behavior are discussed.     

Earning and obtaining reinforcers under concurrent interval scheduling

Contingencies of reinforcement specify how reinforcers are earned and how they are obtained. Ratio contingencies specify the number of responses that earn a reinforcer, and the response satisfying the ratio requirement obtains the earned reinforcer. Simple interval schedules specify that a certain time earns a reinforcer, which is obtained by the first response after the interval. The earning of reinforcers has been overlooked, perhaps because simple schedules confound the rates of earning reinforcers with the rates of obtaining reinforcers. In concurrent variable-interval schedules, however, spending time at one alternative earns reinforcers not only at that alternative, but at the other alternative as well. Reinforcers earned for delivery at the other alternative are obtained after changing over. Thus the rates of earning reinforcers are not confounded with the rate of obtaining reinforcers, but the rates of earning reinforcers are the same at both alternatives, which masks their possibly differing effects on preference. Two experiments examined the separate effects of earning reinforcers and of obtaining reinforcers on preference by using concurrent interval schedules composed of two pairs of stay and switch schedules (MacDonall, 2000). In both experiments, the generalized matching law, which is based on rates of obtaining reinforcers, described responding only when rates of earning reinforcers were the same at each alternative. An equation that included both the ratio of the rates of obtaining reinforcers and the ratio of the rates of earning reinforcers described the results from all conditions from each experiment.     

Drug discrimination using a conditioned taste-aversion paradigm in rhesus monkeys.

The development of drug discrimination was assessed in rhesus monkeys using the conditioned taste-aversion paradigm. Monkeys were initially trained to respond under a fixed-ratio 30-response schedule of food-pellet delivery to assess the rate-decreasing effects of alprazolam (0.03 to 3 mg/kg, i.m., 60 min presession). Alprazolam decreased responding at doses greater than 0.1 mg/kg. Discriminative stimulus effects of alprazolam were then assessed by giving 0.03 mg/kg before sessions in which 1.8 mEq/kg lithium chloride was given immediately after the session (alprazolam/lithium session). On intervening days, saline was given before and after the session (saline/saline session). Rates of responding decreased over successive alprazolam/lithium sessions and also during the saline/saline session that immediately followed an alprazolam/lithium session. During subsequent saline/saline sessions, rates of responding returned to levels near baseline rates within two to four sessions. The discriminative stimulus effects of alprazolam were then assessed by giving 0.1 mg/kg before sessions in which 1 mg/kg d-amphetamine was given immediately after the session (alprazolam/d-amphetamine session). Rates of responding decreased during subsequent alprazolam/d-amphetamine sessions in drug-experienced monkeys, but did not decrease during intervening saline/saline sessions. These findings demonstrate that drug stimuli associated with postsession drug injections can rapidly develop control over behavior and suggest that similar methods be explored in the assessment of drug discrimination.