Timeout Periods Research Articles

Strain differences in maintenance of cocaine self-administration and their relationship to novelty activity responses.

The authors previously demonstrated that Fischer 344 (F344) and Lewis inbred rats differ in acquisition of cocaine self-administration. Other studies show that acquisition and maintenance of drug self-administration are predicted by locomotor activity in a novel environment among outbred Sprague-Dawley rats. The present study was designed to determine whether this relationship extended to F344 and Lewis rats. In Experiment 1, F344, Lewis, and Sprague-Dawley rats were trained to self-administer cocaine and tested with several doses under fixed- and progressive-ratio schedules of reinforcement. Self-administered infusions and ineffective active lever presses--those emitted during infusion and time-out periods--were assessed. In Experiment 2, separate sets of rats of each strain were examined for locomotor responses (distance traveled and center time) under novelty conditions. Results show that F344 rats self-administer more cocaine than Lewis or Sprague-Dawley rats under both schedules and emit more ineffective lever presses--a possible measure of craving. Strain comparisons of locomotor responses suggest that center time, not activity, relates to self-administration behavior. Maintenance studies of cocaine self-administration rather than acquisition may better reflect vulnerability to addiction.

Reinforcement omission in concurrent fixed-interval and random-interval schedules

The present experiment examined overall and local effects of omission of reinforcers in a choice situation. Pigeons' key-pecking responses were reinforced under concurrent fixed-interval and random-interval schedules of food presentation. After some weeks of baseline sessions in which the probability of reinforcement was 1.00, approximately 25% of food presentations from the fixed-interval schedule were omitted and replaced by timeout periods. In such omission sessions, the overall relative rates of responding to the fixed-interval schedule became lower than those in the baseline sessions. On the other hand, when relative rates of responding to the fixed-interval schedule in the omission sessions were calculated separately for fixed-interval cycles preceded by timeout periods and those preceded by food presentations, the relative rates in the former type of fixed-interval cycles were higher than those in the latter type for three out of four pigeons. These results mean that relative rates of responding cannot always be regarded as reflecting a relative value of an alternative, and that the overall effect of the omission of fixed-interval reinforcers is not reducible to the local effect of omission.

Price limits on a call auction market: Evidence from the Warsaw Stock Exchange

We empirically investigate the impact of price limits on volatility and autocorrelation in the call auction segment of the Warsaw Stock Exchange (WSE). Because call auctions offer time-out periods to investors, we do not expect price limits to counter overreaction and panic in this market structure. Indeed, our empirical findings show that price limits result in excess volatility on the next trading day and strong continuation of price movements, which indicates that price limits only delay the adjustment of prices to equilibrium levels. Our results question the necessity of price limits in the call auction system of the WSE.

Price limits on a call auction market: Evidence from the Warsaw Stock Exchange

We empirically investigate the impact of price limits on volatility and autocorrelation in the call auction segment of the Warsaw Stock Exchange (WSE). Because call auctions offer time-out periods to investors, we do not expect price limits to counter overreaction and panic in this market structure. Indeed, our empirical findings show that price limits result in excess volatility on the next trading day and strong continuation of price movements, which indicates that price limits only delay the adjustment of prices to equilibrium levels. Our results question the necessity of price limits in the call auction system of the WSE.

Double-ended queues with impatience

The effect of “impatient” behaviour is studied primarily in the context of “double-ended” queues where each demands service from the other, typically taxis and passengers. Related models, single queue, and double, with a variety of mechanisms are considered. “Impatience” is to be understood in a wider context than simply becoming tired of waiting: it can arise because the customer, for some reason, runs out of time (inventory and organ transplantation), or because an alternative service becomes available (communication applications). The emphasis in this paper is theoretical but a brief numerical assessment of operational consequences is given. Scope and purpose The “double-ended (or synchronization) queue” is a model for a variety of service demanding/providing systems. In an orderly taxi rank at a railway station or airport, on one side a queue is formed by the arrival of stream of passengers who wait for taxis to their destinations while on the other side a queue of taxis waiting for passengers. Obviously, the two queues can never coexist. The concept of “impatience” enters when a taxi or passenger leaves the queue before receiving service. This concept of “reneging” is widely applicable. In health care, for example, organs are stored for transplantation for needful patients. Both the organs and the demands for them have limited lifetime. A similar scenario applies to perishable inventory systems. In a similar manner, the real-time communication networks admit impatient behaviour. A typical example is a processor-shared queue in data networks with random time-out periods or deadlines. The paper sets out the basics in a variety of theoretical model settings with the common feature of exponential arrival, service and impatience mechanisms. A brief discussion based on numerical calculation is given of some operational features of the models but the thrust is on the theoretical techniques needed to make meaningful operational assessments.

Preference and resistance to change in concurrent variable-interval schedules.

Pigeons were trained on a multiple schedule in which separate concurrent schedules were presented in the two components of the schedule. During one component, concurrent variable-interval 40-sec variable-interval 80-sec schedules operated. In the second component, concurrent variable-interval 40-sec variable-interval 20-sec schedules operated. After stable baseline performance was obtained in both components, extinction probe choice tests were presented to assess preference between the variable-interval 40-sec schedules from the two components. The variable-interval 40-sec schedule paired with the variable-interval 80-sec schedule was preferred over the variable-interval 40-sec schedule paired with the variable-interval 20-sec schedule. The subjects were also exposed to several resistance-to-change manipulations: (1) prefeeding prior to the experimental session, (2) a free-food schedule added to timeout periods separating components, and (3) extinction. The results indicated that preference and resistance to change do not necessarily covary.

Evidence for impairment of behavioural inhibition in performance of operant tasks in tPA−/− mice

We have previously shown that mice that lack the serine protease, tissue plasminogen activator (tPA), show over-responding on the active lever during time-out periods in an I.V. cocaine self-administration task. To investigate this effect further, tPA knockout mice (tPA−/−) were tested in a number of operant paradigms for a liquid food reinforcer. tPA−/− and wild-type (WT) control mice acquired a fixed ratio (FR) and a fixed interval (FI) task equally. However, extinction from the FR schedule resulted in a significant decrease in responses on the active and inactive levers in the WT mice whilst responding on the inactive lever remained high in the tPA−/− animals. In a differential reinforcement of low rate (DRL) task, tPA−/− mice acquired the task at a slower rate than WT animals. This was characterised by high levels of responding on the active lever during the first 15 sessions in the tPA−/− mice. Burst responding on the active lever (lever press rate with an inter-response time of less than 3 s) was especially high in these animals. This behaviour pattern resulted in the animals obtaining less reinforcers than the WT controls. Acute cocaine dose-dependently shifted the pattern of behaviour on the active lever towards shorter inter-response times. However, there was no difference between the tPA−/− and WT mice in their sensitivity to cocaine on this task. Repeated administration of a low dose of cocaine did not alter performance on this task in either set of animals. When the DRL task was modified to allow the tPA−/− and WT mice an equal number of reinforced trials per session there was no difference in the ability of the animals to perform the task. This would suggest that the tPA−/− mice have a tendency to over-respond but that this can be overcome when the task is modified to allow equal opportunity to learn.

Dose-dependent effects but not sensitization of DRL 45-s performance by oral d-amphetamine with cumulative- and repeated-dosing regimens.

The effects of d-amphetamine (AMPH) on a food-reinforced DRL 45-s schedule were evaluated using both cumulative- and repeated-dosing drug regimens. These two dosing regimens were designed to evaluate sensitization as a shift in the dose-response relationship, inasmuch as a range of doses was imposed within each dosing session. Daily 190-min sessions were composed of five 35-min subsessions separated by 3-min time-out periods. For selected sessions, five cumulative or repeated oral doses of AMPH were administered across the session, with a dose given during each of the time-out periods prior to the start of each subsession. Drug sessions were separated by intervals of 7-l0 days of non-drug sessions. Four cumulative dose-effect functions for AMPH were determined: for each dose-effect determination session, increasing doses (0.5, 1, 2, 4, 8 mg/kg gavage) were successively administered prior to each subsession. Four dose-effect functions were then determined in which a 0.5 mg/kg AMPH dose was repeated for each subsession (repetitive-dose regimen) rather than escalating subsession dose size. Then, four additional functions were determined using a larger repetitive dose of 1 mg/kg AMPH. Cumulative doses resulted in a leftward shift in the inter-response times (IRT) distribution accompanied by dose-dependent increases in subcriterion responses (< 45 s) and decreases in reinforced responses. The repeated doses of 0.5 or 1 mg/kg AMPH also resulted in progressive intrasession increases in subcriterion responses and decreases in reinforced responses. Although intrasession, accumulating dose effects were evident and statistically significant, there was no statistical significance or trend supporting sensitization of differential reinforcement of low-rate (DRL) responding with either cumulative- or repeated-dosing regimens across drugging sessions, unlike a previous, similar study in which oral cocaine resulted in robust sensitization.

Disinhibitory effects of LY354740, a new mGluR2 agonist, on behaviors suppressed by electric shock in rats and pigeons

Compounds which affect glutamate transmission are reported as anxiolytic in a number of animal models. In the present studies, we evaluated the anxiolytic effect of a new mGluR2 agonist, LY354740, in rats and pigeons. LY354740 was evaluated in three different rat punished responding assays, using different levels of shock intensity. LY354740 and its racemate, LY314582, were generally ineffective at increasing rates of responding suppressed by electric shock. However, both compounds did increase responding during time-out periods between scheduled components, and both increased responding during a punishment extinction assay. LY354740 was also evaluated in two separate pigeon punished responding procedures. LY354740 was ineffective at increasing punished responding under one schedule, but increased punished responding in the second assay, when shock was not delivered coincident with reinforcement. Clearly, LY354740 was effective in attenuating suppressed responding in some cases, but not all. LY354740 appears to produce a disinhibitory effect on behaviors with low baseline rates of responding (in rats), and a disinhibitory effect on punished behaviors in schedules with less stringent punishment conditions (in rats and pigeons). These results suggest that LY354740 may be useful as a clinically effective anxiolytic. Drug Dev. Res. 47:37–44, 1999. © 1999 Wiley-Liss, Inc.

Humans' choice in a self-control choice situation: sensitivity to reinforcer amount, reinforcer delay, and overall reinforcement density.

Human subjects were exposed to a concurrent-chains schedule in which reinforcer amounts, delays, or both were varied in the terminal links, and consummatory responses were required to receive points that were later exchangeable for money. Two independent variable-interval 30-s schedules were in effect during the initial links, and delay periods were defined by fixed-time schedules. In Experiment 1, subjects were exposed to three different pairs of reinforcer amounts and delays, and sensitivity to reinforcer amount and delay was determined based on the generalized matching law. The relative responding (choice) of most subjects was more sensitive to reinforcer amount than to reinforcer delay. In Experiment 2, subjects chose between immediate smaller reinforcers and delayed larger reinforcers in five conditions with and without timeout periods that followed a shorter delay, in which reinforcer amounts and delays were combined to make different predictions based on local reinforcement density (i.e., points per delay) or overall reinforcement density (i.e., points per total time). In most conditions, subjects' choices were qualitatively in accord with the predictions from the overall reinforcement density calculated by the ratio of reinforcer amount and total time. Therefore, the overall reinforcement density appears to influence the preference of humans in the present self-control choice situation.

Choice of timeout from fixed-time schedules: Comparison of two procedures

Four rats obtained food on fixed-time (FT) 1-, 4-, and 16-min schedules. During FT schedules, a lever press produced a timeout period during which food could not be delivered. When timeout was in effect, a lever press ended the timeout period and reinstated the FT schedule. In different conditions, the timing of the FT interval either stopped for the duration of the timeout period (Stop condition) or the timing of the FT interval continued during timeout (No Stop condition). The percentage of session time spent in timeout periods increased as the time between food deliveries increased regardless of whether or not timing of the FT interval stopped during timeout. The timeout percentage was larger on Stop than No Stop conditions if the obtained interfood interval was also longer. The rate of initiation of timeout periods tended to increase from FT 1-min to FT 4-min schedules, and then not increase further or decrease on FT 16-min. The mean time per timeout period tended to be similar on FT 1- and FT 4-min schedules and then increase on FT 16-min. A given percentage of session time spent in timeout periods appears to be due to interactions between rate of timeout, duration of timeout periods and interfood interval. The data are not consistent with the suggestion that timeout initiation represents escape from aversive aspects of intermittent reinforcement. Instead, choice of timeout shares some of the properties of facultative behaviors.

Effects of exposure regimen on changes in sensitivity to the effects of cocaine on schedule-controlled behavior in rhesus monkeys

It has been reported that conditions of drug exposure can influence changes in sensitivity to cocaine upon repeated administration. In the present experiment, the behavioral effects of prolonged exposure to continuous or intermittent infusion of cocaine were compared in rhesus monkeys responding under a multiple component fixed-ratio (FR) schedule of food presentation. In order to quantify changes in sensitivity to cocaine, cumulative dose-response functions for acute cocaine were determined using a multiple schedule comprised of six 10-minute components separated by time-out periods of 3 minutes. Initially, cocaine decreased responding in a dose-related manner. Continuous infusion of cocaine (4 mg/kg per day) for a period of 4 weeks resulted in a 2- to 4-fold shift to the right in the cocaine dose-response function, i.e., tolerance developed. In contrast, when the same daily dose of cocaine was injected intermittently (1.0 mg/kg per injection) four times/day to different monkeys, there was no change in the effects of cocaine on responding. The present results support the notion that the dosing regimen is an important determinant of changes in sensitivity to the behavioral effects of cocaine. Additionally, since sensitization failed to develop upon intermittent administration of cocaine, behavioral baseline may play a role in changes in sensitivity to cocaine.

WITHIN‐SESSION CHANGES IN THE VI RESPONSE FUNCTION: SEPARATING FOOD DENSITY FROM ELAPSED SESSION TIME

Previous studies examining the relationship between response rate and reinforcement rate on variable-interval schedules (the variable-interval response function) have confounded elapsed session time with within-session changes in food density. The present experiments attempted to manipulate these factors independently and thus isolate their effects on responding. In Experiment 1, 7 rats pressed a bar for food on a series of four variable-interval schedules (7.5 s, 15 s, 30 s, and 480 s). Elapsed session time was held constant while food density was manipulated via a presession feeding. Changes in food density altered the form of the variable-interval response function, independently of elapsed session time. In Experiment 2, 8 rats responded on the same series of variable-interval schedules as in Experiment 1, but food density was held constant and elapsed session time was manipulated via the use of timeout periods. The results revealed no evidence for an effect of elapsed session time independent of food density. The present results extend a recent analysis of the variable-interval response function by Dougan, Kuh, and Vink (1993) by identifying food density as an important factor determining the form of the function. The present results also help clarify the controversy over the correct empirical form of the variable-interval response function by further defining the variables responsible for differences in the form of that function.

Effects of ifenprodil on stimulatory, discriminative stimulus, and convulsant effects of cocaine

Ifenprodil, like cocaine, binds to the dopamine transporter and blocks uptake of dopamine. In this study, the ability of ifenprodil to mimic, enhance or block behavioral and toxic effects of cocaine was evaluated. Fixed-interval responding maintained by food presentation in rats was increased by cocaine but decreased by ifenprodil. Low rates of responding during timeout periods were also increased by cocaine but not ifenprodil. Ifenprodil neither increased locomotor activity nor augmented the stimulatory effect of cocaine; however, ifenprodil attenuated the stimulant effects of cocaine at doses 0.5 log unit lower than those required to reduce spontaneous activity when given alone. Ifenprodil neither substituted for nor augmented the discriminative stimulus effects of cocaine in rats discriminating 10mg/kg cocaine from saline. In contrast to cocaine, ifenprodil did not produce convulsions or signs of proconvulsant activity, and ifenprodil reduced the percentage of mice convulsing in the presence of cocaine. These results suggest that pharmacological actions in addition to blockade of dopamine uptake or novel interactions with the dopamine transporter may contribute to the non-stimulant behavioral profile and cocaine-blocking actions of ifenprodil.

Schedule-induced timeout: Effects of timeout-contingent delayed reinforcement

Rats' lever pressing turned off stimuli associated with various response-independent fixed-time schedules of food delivery and produced a timeout period during which food delivery could not occur. A lever press during timeout turned on the schedule-associated stimuli and reinstated the fixed-time schedule. Every response that produced timeout ended the timing of fixed time intervals; timeout terminating responses started the timing of fixed-time schedules over again. The percentage of session time spent in timeout periods generally increased as the rate of food delivery decreased. When food delivery could not occur, about half of the session time was spent in the stimulus condition formerly associated with timeout. The majority of timeout periods were initiated after 15 seconds or more had lapsed since the last food delivery. When timeout periods could not be produced, lever pressing rates either decreased or were not affected. The rate of food delivery appeared to be a primary determinant of time spent in timeout periods. The results may be inconsistent with interpretations emphasizing escape from aversive aspects of schedules of positive reinforcement.

Effects of selective attention on spatial form processing in monkey primary and secondary somatosensory cortex.

1. The effects of selective attention were studied in SI and SII cortex of a rhesus monkey trained to perform two tasks, a tactile discrimination task and a visual detection task. In the tactile task, a letter was displayed on a video screen in front of the monkey and the animal was rewarded for responding when the raised letter (6.0 mm letter height) scanning across its finger (15 mm/s) matched the letter on the screen. In the visual task, three illuminated squares were displayed on the screen, and the animal was rewarded for detecting when one of the squares dimmed. The neural responses evoked by the raised letters were recorded continuously while the animal's focus of attention was switched back and forth between the two tasks. 2. Significant differences between the discharge rates evoked by raised letters in the two tasks were observed in approximately 50% of neurons in SI cortex and 80% of neurons in SII cortex. The effects in SII cortex were divided between increased (58%) and decreased (22%) rates. In SI cortex only increased rates were observed. 3. The attentional effects were expressed not only as changes in overall neuronal activity but also as modifications of the form of the responses evoked by the letters. 4. Whether attentional effects were observed depended upon the behavioral relevance of individual letters. During brief periods in the tactile task when a behavioral response could not yield a reward (time-out and reward periods) the neuronal responses were not significantly different from the responses evoked by the same letters during the visual task.(ABSTRACT TRUNCATED AT 250 WORDS)

Timeout induced by differential-reinforcement-of-low-rate schedules

Three rats pressed a lever for food on differential-reinforcement-of-low-rate (DRL) schedules ranging from 16 s to 96 s. During DRL performance, a response to a second lever turned off chamber illumination and produced a timeout period during which food could not be obtained. During timeout periods, a response to the second lever reinstated the DRL schedule and associated chamber illumination. The percentage of session time spent in timeout periods increased as the DRL schedule was lengthened and decreased when the schedule was shortened. Changes in timeout percentage were primarily due to changes in the mean time per timeout rather than to changes in the rate of initiation of timeout periods. Extinction increased the timeout percentage. Elimination of the timeout contingency decreased the response rate to the timeout lever, but did not systematically affect DRL lever pressing. The rate of reinforcement appeared to be a primary determinant of the percent of session time spent in timeout periods.

Conjoint schedules of timeout deletion in pigeons.

This experiment attempted to bring behavior under joint control of two distinct contingencies, one that provided food and a second that extended the periods during which that food was available. Pigeons' responses on each of two keys were reinforced according to a single random-interval schedule of food presentation except during signaled timeout periods during which the schedule was temporarily disabled. By means of a conjoint schedule, responses on the initially less preferred key not only produced food but also canceled impending timeouts. When behavior came to predominate on this conjoint alternative, the consequences of responding on the two keys were reversed. Responding in 3 of 4 pigeons proved sensitive to the conjoint scheduled consequences, as evidenced by systematic shifts in response rates favoring the conjoint key. In 2 of these 3 pigeons, sensitivity to the conjoint contingency was evident under time-in:timeout ratios of 2:1 (time-in = 120 s, timeout = 60 s) and 1:5 (time-in = 30 s, timeout = 150 s), whereas for the other pigeon preference for the conjoint key was observed only under the latter sequence of conditions. There was only weak evidence of control by the conjoint scheduled consequences in the 4th subject, despite extended training and forced exposure to the conjoint alternative. The overall pattern of results is consistent with studies of timeout avoidance but also shares features in common with positively reinforced behavior.

Real-time Concurrent C: A language for programming dynamic real-time systems

Concurrent C, is a parallel superset of C (and of C++) that provides facilities such as specifying timeouts during process interactions, delaying program execution, accepting messages in a user-specified order, and asynchronous messages that can be used for writing real-time programs. However, Concurrent C does not provide facilities for specifying strict timing constraints, e.g., Concurrent C only ensures that the lower bounds on the specified delay and timeout periods are satisfied.

Dynamic adaptation of real-time software

In large, dynamic, real-time computer systems, it is frequently most cost effective to employ different software performance and reliability techniques at different levels of granularity, at different times, or within different subsystems. These techniques may include regulation of redundancy and resource allocation, multiversion and multipath execution, adjustments of program attributes such as time-out periods and others. The management of software in such systems is a difficult task. Software that may be adapted to meet varying performance and reliability requirements offers a solution. A REal-time Software Adaptation System (RESAS) includes a uniform model of adaptable software and provides the tool necessary for programmers to implement algorithms that choose and enact adaptations in real time. RESAS has been implemented on a testbed consisting of a multiprocessor and an attached workstation, and adaptation algorithms have been developed that address the problem of adapting software to achieve two goals: software execution within specified time constraints and software resiliency with respect to computer hardware failures.