Retention of reinforcer magnitude.

Pigeons' responses on two keys were recorded before and after the percentage of reinforcers delivered by each key was changed. In each condition of Experiment 1, the reinforcement percentage for one key was 50% for several sessions, then either 70% or 90%for one, two, or three sessions, and then 50% for another few sessions. At the start of the second and third sessions after a change in reinforcement percentages, choice percentages often exhibited spontaneous recovery-a reversion to the response percentages of earlier sessions. The spontaneous recovery consisted of a shift toward a more extreme response percentage in some cases and toward a less extreme response percentage in other cases, depending on what reinforcement percentages were previously in effect. In Experiment 2, some conditions included a 3-day rest period before a change in reinforcement percentages, and other conditions included no such rest days. Slightly less spontaneous recovery was observed in conditions with the rest periods, suggesting that the influence of prior sessions diminished with the passage of time. The results are consistent with the view that choice behavior at the start of a new session is based on a weighted average of the events of the past several sessions.Although the results of several experiments were well described by this model, it is too simple to accommodate where~~is the change in strength of~, and r is a reinforcement parameter that can range from 0 to 1. After each response that is not reinforced, the strength of the response decreases as follows:forcement, the size of a change in probabilities, and the overall rate of reinforcement.The results of these experiments contradicted the predictions of several models oftransitional choice behavior, including Myerson's kinetic model (Myerson & Miezin, 1980; see also Myerson & Hale, 1988), Staddon's ratioinvariance model (Staddon, 1988;Staddon & Horner, 1989), and the linear-operator model (Bush & Mosteller, 1955). However, the results could be described quite well by a simple mathematical model similar to one proposed by Couvillon and Bitterman (1985). The model states that each response alternative, i, has a separate strength, . Response strength increases each time the response is reinforced and decreases each time the response is not reinforced. Each time a response is reinforced,~increases as follows:where n is a nonreinforcement parameter that can range from 0 to 1. A simple matching rule is used to translate from the independent strengths of two responses (~and J-2) to the probability that one will occur:Vi PI = V; + v: .

mentioning

confidence: 92%

Past experience, recency, and spontaneous recovery in choice behavior

Mazur

1996

“…In order to show a contrast effect, the shifted animals must be able' to compare the postshift reward with the memory of the preshift reward (Spear, 1967). As the interval between the preshift period and any given postshift day increases, the memory of the pre shift reward should become more tenuous and, therefore, the comparison process that must underlie contrast more problematical.…”

Section: Methodsmentioning

confidence: 99%

Chlordiazepoxide and the determinants of negative contrast

Flaherty

Grigson

Rowan-Szal

1986

Previous experiments have shown that the negative contrast effect in consummatory behavior that occurs when rats are shifted from 32% to 4% sucrose is alleviated by the tranquilizer chlordiazepoxide (CDP). However, in these experiments, CDP was effective on the second postshift day but not on the first postshift day. The three experiments described in this paper suggest that this differential effectiveness of CDP is not due to the difference in preshift-postshift retention intervals on Day 1 (24 h) and Day 2 (48 h), but is due instead to the necessity of some degree of experience (about 5 min) with the postshift solution. These results, combined with those of an earlier study which showed elevated corticosterone in shifted animals on the second postshift day but not on the first postshift day, suggest that negative contrast may be a dynamic process, involving sequential processes of detection, evaluation, and conflict over the postshift period. It was further suggested that CDP becomes effective in moderating contrast only when the conflict stage is reached.Rats shifted from a 32 % to a 4 % sucrose solution consume substantially less of the 4 % sucrose than do rats that have experienced only the 4 % solution. This negative contrast effect that follows the shift in sucrose solutions is usually largest on the first postshift day and then declines so that, typically, by the fourth or fifth postshift day the shifted and unshifted animals are consuming approximately the same amount of 4% sucrose (e

“…It is possible that the drugs are differentially effective on the first 2 postshift days because contrast is so robust on the lst postshift day that it overrides the effects of the drugs. On the 2nd postshift day, contrast may be weaker, perhaps because of the now more remote comparison with the memory of the preshift solution (Spear, 1967) and/or because of the experience with the postshift solution. It may be when contrast is thus weakened that the drugs become effective.…”

mentioning

confidence: 99%

Correlation of corticosterone elevation and negative contrast varies as a function of postshift day

Flaherty

Becker

Pohorecky

1985

In three experiments, rats shifted from 32% to 4% sucrose consumed less of the 4% sucrose than did rats that had received only the 4% solution. Experiment 1 showed that this negative contrast effect was not accompanied by a corticosterone elevation on the first day subsequent to the shift. Experiments 2 and 2a showed that corticosterone levels were substantially elevated on the 2nd postshift day and that there was a tendency for degree of elevation in corticosterone to be related to degree of lick suppression. These results are discussed in terms of other data suggesting that anxiolytic drugs and disinhibitory stimuli are more effective in alleviating contrast on the 2nd postshift day than on the 1st postshift day. It is suggested that, in the present paradigm, reaction to stimulus change may be the primary determinant of contrast on the 1st postshift day, but emotional processes related to reward loss and/or conflict develop by the 2nd postshift day.Rats shifted from a 32 % sucrose solution to a 4 % sucrose solution consume substantially less of the 4 % solution than animals that have experienced only 4 %sucrose.