Effects of reinforcer probability, delay, and response requirements on the choices of rats and pigeons: possible species differences

In Experiment 1 with rats, a left lever press led to a 5-s delay and then a possible reinforcer. A right lever press led to an adjusting delay and then a certain reinforcer. This delay was adjusted over trials to estimate an indifference point, or a delay at which the two alternatives were chosen about equally often. Indifference points increased as the probability of reinforcement for the left lever decreased. In some conditions with a 20% chance of food, a light above the left lever was lit during the 5-s delay on all trials, but in other conditions, the light was only lit on those trials that ended with food. Unlike previous results with pigeons, the presence or absence of the delay light on no-food trials had no effect on the rats' indifference points. In other conditions, the rats showed less preference for the 20% alternative when the time between trials was longer. In Experiment 2 with rats, fixed-interval schedules were used instead of simple delays, and the presence or absence of the fixed-interval requirement on no-food trials had no effect on the indifference points. In Experiment 3 with rats and Experiment 4 with pigeons, the animals chose between a fixed-ratio 8 schedule that led to food on 33% of the trials and an adjusting-ratio schedule with food on 100% of the trials. Surprisingly, the rats showed less preference for the 33% alternative in conditions in which the ratio requirement was omitted on no-food trials. For the pigeons, the presence or absence of the ratio requirement on no-food trials had little effect. The results suggest that there may be differences between rats and pigeons in how they respond in choice situations involving delayed and probabilistic reinforcers.     

Effects of Interspersing Additional Brief Math Problems on Student Performance and Perception of Math Assignments: Getting Students to Prefer to Do More Work

Three experiments were conducted to assess the effects of the interspersing procedure on students' mathematics performance, perceptions of mathematics assignments, and preference for mathematics assignments when more work was added. In Experiment I, college students worked computation problems from two assignments. The control assignment contained 15 three-digit by two-digit problems (3 × 2) and the experimental assignments contained 18 3 × 2 problems and 6 additional interspersed one-digit by one-digit problems (1 × 1). Students then rated the assignments and chose one for homework. Results showed significantly more students preferred (i.e., chose it for homework) the experimental assignment with the additional target (i.e., 3 × 2) and interspersed problems. Furthermore, significantly more students rated this experiment as requiring less effort and being less difficult. Moreover, students' mathematics performance was not affected by the procedure. Experiment II strengthened the internal validity by showing that students did not prefer the experimental assignment because it contained more target problems. Experiment III showed that the interspersing procedure was not powerful enough to extend findings beyond the 20% level (i.e., 40 and 60% more conditions). Results showed how the interspersing procedure could be used to encourage students to choose to do more target computation tasks. Discussion focuses on the applied value of the interspersing procedure and plausible causal mechanisms that may account for current and past findings related to student choice and interspersing brief tasks.     

An investigation of hierarchical Bayes procedures in item response theory

Hierarchical Bayes procedures for the two-parameter logistic item response model were compared for estimating item and ability parameters. Simulated data sets were analyzed via two joint and two marginal Bayesian estimation procedures. The marginal Bayesian estimation procedures yielded consistently smaller root mean square differences than the joint Bayesian estimation procedures for item and ability estimates. As the sample size and test length increased, the four Bayes procedures yielded essentially the same result.The authors wish to thank the Editor and anonymous reviewers for their insightful comments and suggestions.     

TIME DELAY: A TECHNIQUE TO INCREASE LANGUAGE USE AND FACILITATE GENERALIZATION IN RETARDED CHILDREN

Institutional breakfast-serving procedures were manipulated to assess what effect changes in that aspect of the environment would have on requests for food. During baseline, six severely retarded children were required to pick up their food trays and return to their seats. The first manipulation, delaying the giving of the food tray for 15 seconds, served as a cue to evoke meal requests by three of the six children. Two of the remaining three required a model of an appropriate meal request (i.e., “Tray, please.”) at the end of the 15-second delay before they began requesting their meals. To evoke meal requests from the sixth child, an intensive training procedure, consisting of massed trials of delay and modeling, was required. Three different probes were administered to assess generalization across the people serving the meals, across mealtimes, and across both people and mealtimes. Typically, generalized responding in these new situations could be prompted by use of the 15-second delay procedure. Functional aspects of the delay procedure and its potential usefulness for evoking speech and facilitating generalization are discussed.     

Choice in a "self-control" paradigm: effects of a fading procedure

Pigeons chose between an immediate 2-second reinforcer (access to grain) and a 6-second reinforcer delayed 6 seconds. The four pigeons in the control group were exposed to this condition initially. The four experimental subjects first received a condition where both reinforcers were delayed 6 seconds. The small reinforcer delay was then gradually reduced to zero over more than 11,000 trials. Control subjects almost never chose the large delayed reinforcer. Experimental subjects chose the large delayed reinforcer significantly more often. Two experimental subjects showed preference for the large reinforcer even when the consequences for pecking the two keys were switched. The results indicate that fading procedures can lead to increased “self-control” in pigeons in a choice between a large delayed reinforcer and a small immediate reinforcer.     

Performance of rats under concurrent variable-interval schedules of negative reinforcement

The behavior of rats under concurrent variable-interval schedules of negative reinforcement was examined. A single one-minute variable-interval programmer determined the availability of 30-second timeouts from electric shock. These were assigned to one or the other of the two component schedules with a probability of 0 to 1.0. The response requirement for the component schedules was standing to the right or left of the center of the experimental chamber. With a six-second changeover delay, the relative time spent under one component schedule varied directly and linearly with the relative number of timeouts earned under that component schedule. The absolute number of changeovers was highest when a similar number of timeouts was earned under each component schedule, and lowest when all or nearly all timeouts were earned under one component schedule. In general, these relations are similar to those reported with concurrent variable-interval schedules of positive reinforcement.     

Effects on choice of reinforcement delay and conditioned reinforcement

Pigeons chose between fixed-interval schedules of different durations presented in the terminal links of concurrent-chains schedules. The pair of schedules was always in the ratio of 2:1, but the absolute duration of the fixed intervals varied. In one set of conditions, the different terminal-link schedules were associated with different keylight stimuli (cued conditions). In a second set of conditions, the different terminal-link schedules were associated with the same stimulus (uncued conditions). Results from the cued conditions replicated previous findings that preference for the shorter fixed-interval schedule increased with fixed-interval duration. Preferences in the uncued conditions were lower than in the corresponding cued conditions but also increased with fixed-interval length. In addition, the degree of control under the uncued conditions was correlated with the extent to which the schedule during the terminal link was discriminated immediately upon entry into the terminal link. The pattern of results in both conditions was inconsistent with the notion that choice behavior matches relative immediacy of reinforcement. Reanalysis of previous evidence for matching (Chung and Herrnstein, 1967) showed that matching in fact did not occur, as the preferences of their subjects for the shorter of two delays also increased with the absolute size of the delays.     

Local response-rate constancy on concurrent variable-interval schedules of reinforcement

Concurrent variable-interval schedules were arranged with a main key that alternated in color and schedule assignment, along with a changeover key on which a small fixed ratio was required to changeover. Acceptable matching was observed with pigeons in two replications, but there was a tendency toward overmatching. Local response rates were found to differ for unequal schedules of a concurrent pair: local response rate was greater for the variable-interval schedule with the smaller average interreinforcement interval, but qualifications based on an interresponse-time analysis were discussed. In a second experiment, two 3-minute variable-interval schedules were arranged concurrently, and the experimental variable was the changeover procedure: either a changeover delay was incurred by each changeover or a small fixed ratio on a changeover key was required to complete a changeover. Changeover delays of 2 and 5 seconds were compared with a fixed-ratio changeover of five responses. The response output on the main key (associated with the variable-interval schedules) was greater when a changeover delay was arranged than when a fixed ratio was required to changeover. A detailed analysis of stripchart records showed that a 2-second delay generated an increased response rate for 3 seconds after a changeover, while the fixed-ratio requirement generated an increased rate during the first second only, followed by a depressed response rate for 2 seconds.     

A comparison of signaled and unsignaled delay of reinforcement

Pigeons were trained on either a variable-interval 60-second schedule, or on a schedule that differentially reinforced responses that were spaced at least 20 seconds apart. The birds were then exposed to several durations of reinforcement delay, with comparisons between signaled and unsignaled delays. Although unsignaled delays of 5 and 10 seconds produced large decreases in response rate, signaled delays of up to 10 seconds produced only moderate decreases in response rates. In addition, some subjects responded more rapidly with a .5 or 1.0 second duration of unsignaled delay than with immediate reinforcement. These response rate changes occurred regardless of whether the rate of reinforcement concomitantly decreased or increased.