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Behavioral inflexibility is often assessed using reversal learning tasks, which require a relatively low degree of response variability. No studies have assessed sensitivity to reinforcement contingencies that specifically select highly variable response patterns in mice, let alone in models of neurodevelopmental disorders involving limited response variation. Operant variability and incremental repeated acquisition (IRA) were used to assess unique aspects of behavioral variability of two mouse strains: BALB/c, a model of some deficits in ASD, and C57Bl/6. On the operant variability task, BALB/c mice responded more repetitively during adolescence than C57Bl/6 mice when reinforcement did not require variability but responded more variably when reinforcement required variability. During IRA testing in adulthood, both strains acquired an unchanging, performance sequence equally well. Strain differences emerged, however, after novel learning sequences began alternating with the performance sequence: BALB/c mice substantially outperformed C57Bl/6 mice. Using litter-mate controls, it was found that adolescent experience with variability did not affect either learning or performance on the IRA task in adulthood. These findings constrain the use of BALB/c mice as a model of ASD, but once again reveal this strain is highly sensitive to reinforcement contingencies and they are fast and robust learners.

Previously reinforced responses can reappear when reinforcement is withdrawn from current responding. This is known as resurgence. Although resurgence of response topography, spacing, and patterns over time has been demonstrated, there is no evidence of resurgence of response duration. This experiment explored resurgence of response duration in humans. In Phase 1 a multiple schedule of reinforcement with two components was used. In each component a chained variable-interval 30s, variable-ratio 3 schedule was implemented. In the terminal link of the chained schedule, response durations between 0.1 and 0.5s were reinforced during one component, and between 2 and 8s in the other component. In Phase 2, response requirement during the terminal link of the chained schedule was inverted between components relative to Phase 1. In Phase 3 the chained schedule was changed to a variable-interval 30-s, extinction 30s. Resurgence of the durations trained during Phase 1 was observed. It was concluded that duration is a response dimension that reappears during extinction.

Instrumental learning involves both variation and selection: variation of what the animal does, and selection by reward from among the variation. Four experiments with rats suggested a rule about how variation is controlled by recent events. Experiment 1 used the peak procedure. Measurements of bar-press durations showed a sharp increase in mean duration after the time that food was sometimes given. The increase was triggered by the omission of expected food. Our first explanation of the increase was that it was a frustration effect. Experiment 2 tested this explanation with a procedure in which the first response of a trial usually produced food, ending the trial. In Experiment 2, unlike Experiment 1, omission of expected food did not produce a large increase in bar-press duration, which cast doubt on the frustration explanation. Experiments 3 and 4 tested an alternative explanation: a decrease in expectation of reward increases variation. Both used two signals associated with different probabilities of reward. Bar presses were more variable in duration during the signal with the lower probability of reward, supporting this alternative. These experiments show how variation can be studied with ordinary equipment and responses.

Reinforcement was presented contingent upon human subjects simultaneously varying three dimensions of an operant response. The response was drawing rectangles on a computer screen. The dimensions were area of the rectangle, its location on the screen, and its shape. In Experiment 1, an experimental group was reinforced for satisfying the three-part variability contingency. A control group was equally reinforced for drawing rectangles but independently of levels of variability. Results showed that the experimental group varied significantly more along each of the dimensions than did the control group. In Experiment 2, another group of subjects was reinforced for repeating instances along one of the dimensions, e.g. repeatedly draw a rectangle in approximately the same location, while simultaneously varying along the other two dimensions. The subjects learned to satisfy these contingencies as well. These results show reinforcement simultaneously and independently controls the variability of three orthogonal dimensions of a response.

Response variability was examined under five random interval schedules with minimum mean interreinforcement intervals of 15, 30, 60, 120, and 240 sec. Distributions of response force-time integrals showed no systematic variability changes with increases in reinforcement intermittency. Corresponding distributions of interresponse times revealed increases in variability accompanying increases in reinforcement intermittency. The relationship of variability to these two measures, and the potential effect of behavior outside the experimentally measured response class, on recorded variability within the class, are considered.