Carlos F. Aparicio

Dynamics of choice: relative rate and amount affect local preference at three different time scales.

To examine extended control over local choice, the present study investigated preference in transition as food-rate ratio provided by two levers changed across seven components within daily sessions, and food-amount ratio changed across phases. Phase 1 arranged a food-amount ratio of 4:1 (i.e., the left lever delivered four pellets and the right lever one pellet); Phase 2 reversed the food-amount ratio to 1:4, and in Phase 3 the food-amount ratio was 3:2. At a relatively extended time scale, preference was described well by a linear relation between log response ratio and log rate ratio (the generalized matching law). A small amount of carryover occurred from one rate ratio to the next but disappeared after four food deliveries. Estimates of sensitivity to food-amount ratio were around 1.0 and were independent of rate ratio. Analysis across food deliveries within rate-ratio components showed that the effect of a small amount was diminished by the presence of a large amount-that is, when a larger amount was present in the situation (three or four pellets), the value of a small amount (one or two pellets) became paltry. More local analysis of visits to the levers between food deliveries showed that postfood visits following a large amount were disproportionately longer than following a small amount. Continuing food deliveries from the same source tended to make visits less dependent on relative amount, but a discontinuation (i.e., food from the other lever) reinstated dependence on relative amount. Analysis at a still smaller time scale revealed preference pulses following food deliveries that confirmed the tendency toward dependence on absolute amount with continuing deliveries, and toward dependence on relative amount following discontinuations. A mathematical model based on a linear-operator equation accounts for many of the results. The larger and longer preference following a switch to a larger amount is consistent with the idea that local preference depends on relatively extended variables even on short time scales.

Effects of reinforcer quality and step size on rats’ performance under progressive ratio schedules ☆

The Mathematical Principles of Reinforcement (MPR) model accurately predicts performance in fixed ratio (FR) schedules of reinforcement. The aim of the present study was to assess the generality of MPR with rats responding under progressive ratio (PR) schedules of different step sizes (PR1 or PR3) that provided either food or saccharin pellets. The results showed that the rats responding for saccharin pellets produced higher breakpoints (i.e., completed higher ratios) than those responding for food pellets. In terms of theoretical parameters, one finding unanticipated on the basis of MPR was that the a parameter (specific activation) was higher for the PR3 schedules. This finding suggests that specific activation may be affected indirectly by motor parameters of the task.

Sensitivity, changeover responses, and choice in transition☆

Studies of choice in steady state have shown that sensitivity to reinforcement increases with increasing fixed-ratio changeover (FR CO) requirements. We assessed the generality of this finding with choice in transition. Food deliveries were programmed according to concurrent variable-interval (VI) schedules. Seven different VI pairs arranged ratios of food deliveries (left/right) of 27:1, 9:1, 3:1, 1:1, 1:3, 1:9, and 1:27 at a constant overall rate across components. Within sessions, all seven ratios were presented in random order. Each component lasted for 10 food deliveries; components were separated by 60-s blackouts. A changeover lever required 1, 2, 4, 8, 16, 32, and 64 responses to alternate between two main levers. Redeterminations to all FR COs, but 64 responses, were obtained in descending order. Choice adjusted rapidly to rapid changes in the reinforcer ratio, tracking the lever associated with the highest probability of reinforcer. Sensitivity to reinforcement increased with increasing FR CO, replicating the negatively accelerated function found in our earlier study. With successive reinforcers in components, however, sensitivity reached asymptote values sooner with the largest (8, 16, and 32 responses), than with the smallest (1, 2, and 4 responses), FR CO requirements.

Spontaneously hypertensive (SHR) rats choose more impulsively than Wistar-Kyoto (WKY) rats on a delay discounting task

HighlightsConcurrent‐chains procedures detect differences in impulsive choice between SHRs and WKYs.The SHR is a suitable model for ADHD.The hyperbolic‐decay model and the generalized matching law fit delay discounting data.Positive correlations between k and s suggest compatibility between these models.Impulsivity increases with training. &NA; Indications of Attention Deficit Hyperactivity Disorder (ADHD) in the spontaneously hypertensive rat (SHR) are not consistent across different tests of impulsivity, questioning the SHRs validity as a rodent model of ADHD. This study used a concurrent‐chains procedure to examine possible differences in impulsive choice between SHRs and control‐normotensive Wistar Kyoto (WKY) rats. The aim was to extend the generality of findings showing regularities between the hyperbolic‐decay model and the generalized matching law fitting delay discounting data from nonhuman animals. The objectives were to: (1) examine differences in impulsive choice between SHRs and WKYs; (2) add evidence suggesting that the SHR is a suitable model of ADHD; (3) demonstrate that concurrent‐chains procedures requiring locomotion detect differences in impulsive choice between SHRs and WKYs; (4) support the idea that impulsivity in nonhuman animals increases with training. The initial link used two non‐independent random interval schedules arranging entries to the terminal links, where one fixed‐time (FT) schedule delayed 1‐food pellet and the other FT 4‐food pellets. The FT delaying the former was kept constant at 0.1 s and that delaying the latter changed after every 10 food deliveries, defining six delay components (0.1, 5, 10, 20, 40, and 80 s) presented in random order each session. Results showed that the SHRs choose more impulsively than the WKYs, adding to the body of evidence suggesting that the SHR may be a suitable model of ADHD. Both models of choice fitted the impulsive choices of the SHRs and WKYs well; positive correlations between estimates of parameters k and s suggested compatibility between models of choice showing that impulsivity increases with training.