An economic analysis of "demand" for food in baboons.

Responding of 6 adult male baboons (Papio c. anubis) was maintained under a fixed-ratio schedule of food reinforcement during daily 22-hr experimental sessions. Completion of the ratio requirement resulted in the delivery of a single 1-g food pellet; supplemental feeding was limited to a daily fruit ration. Ratio values were increased on Mondays, Wednesdays, and Fridays according to the following schedule: 2, 4, 8, 16, 32, 64, 96, 128. Responding under each ratio value was examined four times. Under the Fixed-Ratio 2 conditions, food intake ranged between 300 and 600 g. Ratios were increased for each baboon until food intake decreased to about 100 g (20% to 30% of Fixed-Ratio 2 intake). Increasing the response cost increased total time responding and total daily responding in all baboons, but this increase in responding was not sufficient to maintain stable food intake. Baboons responded between 90 and 180 min per day. The highest running response rates were observed under the Fixed-Ratio 2 and Fixed-Ratio 4 schedules. Running rate was similar across the larger ratio values (greater than Fixed-Ratio 8) but was lower than that observed under the Fixed-Ratio 2 and Fixed-Ratio 4 schedules. Similar results were observed the four times that each fixed-ratio value was tested. Intake as a function of cost was analyzed by fitting data to the nonlinear equation proposed by Hursh, Raslear, Shurtleff, Bauman, and Simmons (1988) for "demand" functions. Demand for food was inelastic over most of the ratio values until food intake decreased to 15% to 55% of baseline. The results indicate that demand functions are appropriate for the study of food intake in baboons, but also caution that intake at the cost when demand shifts from inelastic to elastic and its relationship to maximal intake should also be included in analyses of demand for a commodity.     

Behavioral economics of drug self-administration. II. A unit-price analysis of cigarette smoking.

In behavioral economics, the ratio of response requirement to reinforcer magnitude is referred to as unit price. Previous research with nonhuman subjects has demonstrated that (a) comparable amounts of food are consumed at the same unit price even though different response requirements and reinforcer magnitudes comprise that unit price and (b) increases in unit price decrease food consumption in a positively decelerating fashion. The present study assessed the generality of these findings to the cigarette smoking of 5 human volunteers. During approximately 18 3-hr sessions, various combinations of response requirement (fixed-ratio 200, 400, and 1,600) and reinforcer magnitude (1, 2, and 4 puffs per bout) were arranged. Consumption (i.e., the number of puffs) generally was comparable at the same unit price independent of the response requirement and reinforcer magnitude comprising that unit price. In addition, increasing unit price generally decreased consumption in a positively decelerating fashion. These results extend the generality of the unit-price analysis to human cigarette smoking. Moreover, these results further support the position that reinforcer magnitude and response requirement are functionally equivalent and interact to determine consumption. The concept of unit price, by integrating and summarizing the effects of those two operations, provides a more parsimonious explanation of the results than do separate evaluations of the effects of response requirement and reinforcer magnitude.     

Chlordiazepoxide effects on ethanol self-administration: dependence on concurrent conditions.

Experiments examined the effects of acute doses of chlordiazepoxide upon ethanol self-administration in the rat. A concurrent-schedule procedure was used that employed choice between ethanol (5%) and a second fluid (either water or a 1% sucrose solution). When ethanol and water were the available fluids, chlordiazepoxide at doses of 15 and 20 mg/kg reduced ethanol-reinforced responding and intake, with a greater reduction occurring at the 20 mg/kg dose. However, when ethanol and sucrose were concurrently available, in many rats only the 20 mg/kg dose of chlordiazepoxide reduced ethanol-reinforced responding. The differences in dose response function occurred in most animals without large changes in the baseline ethanol-reinforced responding across the two concurrent conditions. Thus the dose-effect curve relating chlordiazepoxide and ethanol self-administration can be altered, dependent upon the nature of the concurrently available reinforcers.     

DOES PACKAGE SIZE MATTER? A UNIT-PRICE ANALYSIS OF “DEMAND” FOR FOOD IN BABOONS

In a study examining “demand” for food, responding of 8 adult male baboons (Papio c. anubis) was maintained under a fixed-ratio schedule of food reinforcement during daily 23-hr experimental sessions. Completion of the ratio requirement resulted in the delivery of one, five, or 10 1-g food pellets. Supplemental feeding was limited to fruit and a dog biscuit daily. Responding increased as “cost” was increased across a wide range of fixed-ratio values before reaching a maximum and then decreasing. Increasing the number of food pellets per delivery decreased total responding and the number of reinforcements per day. A unit-price analysis, in which intake was converted to grams per day and fixed-ratio values were converted to responses per gram, yielded demand functions that overlapped at lower unit prices. Under one or more multiple-pellet conditions, however, intake decreased more quickly than under the one-pellet condition as the fixed-ratio value was increased in all but 1 baboon. This indicates that even when using unit-price conversions, there was variability in total intake. Although unit-price conversions yielded intake data that were more consistent across conditions, conditions differed in response topography even at the same unit prices: Under the multiple-pellet conditions there were longer pauses in responding, running response rate was slower, and the first eating bout (i.e., “meal”) of the session was smaller than under the one-pellet condition. These findings (a) support the heuristic value of a unit-price analysis for studying responding for and consumption of commodities that have similar attributes, and (b) indicate that different response topographies may result in similar intakes of a commodity.     

Behavioral economics of drug self-administration and drug abuse policy.

The concepts of behavioral economics have proven useful for understanding the environmental control of overall levels of responding for a variety of commodities, including reinforcement by drug self-administration. These general concepts are summarized for application to the analysis of drug-reinforced behavior and proposed as the basis for future applications. This behavioral agenda includes the assessment of abuse liability, the assay of drug-reinforcer interactions, the design of drug abuse interventions, and the formulation of drug abuse public policy. These separate domains of investigation are described as part of an overall strategy for designing model projects to control drug use and testing public policy initiatives.     

Food and water intake as functions of resource consumption costs in a closed economy.

In two experiments, rats living in a closed economy were offered continuous, concurrent access to four resources: food, water, a nest, and a running wheel. Costs of consuming food and water were imposed with bar-press requirements, and the price of either one or both resources was raised. As the consumption cost increased, less was consumed in each bout of resource use. Bout frequency increased, but not sufficiently to compensate for the fall in bout size, and total intake fell. Food and water tended to be complementary resources, in that as intake of one fell with its price, intake of the other also decreased. This interaction was accounted for by the defense of the ratio of body water to lean body mass. As amount consumed decreased, increases in feed efficiency (weight gain per unit of food ingested) and the use of stored calories compensated for the reduced energy intake. There was evidence of competition between feeding and drinking at the higher costs: When both commodities were expensive, the decline in the intake of each one was greater than when only one commodity was expensive. Although the time spent nesting, running, and in unmonitored activity was adjusted when feeding or drinking took more of the rat's day, there was no particular activity that was sacrificed.     

Pentobarbital self-administration in rhesus monkeys: drug concentration and fixed-ratio size interactions.

Performances of three rhesus monkeys were reinforced by the oral delivery of pentobarbital and studied as functions of fixed-ratio size and drug concentration. Pentobarbital solutions and water were concurrently available on identical reinforcement schedules from separate liquid-delivery systems during 3-hour sessions. Under a fixed-ratio 16 schedule of drug availability, a descending series of drug concentrations was tested (4, 2, 1, 0.5, 0.25, 0.125, and 0.0625 mg/ml, followed by a retest at 4 mg/ml). Partial concentration series beginning with the highest concentration were repeated with fixed-ratios of 32 and 64, with a fixed-ratio 128 for two monkeys, and with fixed-ratio 256 for one. At each fixed-ratio value, response rate and number of drug deliveries were inverted U-shaped functions of pentobarbital concentration. Drug intake (mg/kg/session) increased directly with drug concentration. As the fixed-ratio size was increased, the number of liquid deliveries decreased. For each drug concentration, when the numbers of drug deliveries at fixed-ratios of 32, 64, and 128 responses were plotted as percentages of those obtained at fixed-ratio 16, the following orderly relationship emerged: the higher the drug concentration, the less that drug deliveries were decreased by increases in fixed-ratio size. This relationship indicates an increase in reinforcing efficacy with increases in pentobarbital concentration.     

Behavioral economics of concurrent ethanol-sucrose and sucrose reinforcement in the rat: effects of altering variable-ratio requirements.

These experiments examined the own-price and cross-price elasticities of a drug (ethanol mixed with 10% sucrose) and a nondrug (10% sucrose) reinforcer. Rats were presented with ethanol-sucrose and sucrose, both available on concurrent independent variable-ratio (VR) 8 schedules of reinforcement. In Experiment 1, the variable ratio for the ethanol mix was systematically raised to 10, 12, 14, 16, 20, and 30, while the variable ratio for sucrose remained at 8. Five of the 6 rats increased ethanol-reinforced responding at some of the increments and defended baseline levels of ethanol intake. However, the rats eventually ceased ethanol-reinforced responding at the highest variable ratios. Sucrose-reinforced responding was not systematically affected by the changes in variable ratio for ethanol mix. In Experiment 2, the variable ratio for sucrose was systematically increased while the ethanol-sucrose response requirement remained constant. The rats decreased sucrose-reinforced responding and increased ethanol-sucrose-reinforced responding, resulting in a two- to 10-fold increase in ethanol intake. Experiment 3 examined the substitutability of qualitatively identical reinforcers: 10% sucrose versus 10% sucrose. Increases in variable-ratio requirements at the preferred lever resulted in a switch in lever preference. Experiment 4 examined whether 10% ethanol mix substituted for 5% ethanol mix, with increasing variable-ratio requirements of the 5% ethanol. All rats eventually responded predominantly for the 10% ethanol mix, but total amount of ethanol consumed per session did not systematically change. In Experiment 5, the variable-ratio requirements for both ethanol and sucrose were simultaneously raised to VR 120; 7 of 8 rats increased ethanol-reinforced responding while decreasing sucrose-reinforced responding. These data suggest that, within this ethanol-induction procedure and within certain parameters, demand for ethanol-sucrose was relatively inelastic, and sucrose consumption was independent of ethanol-sucrose consumption. Demand for sucrose, on the other hand, was relatively elastic, and ethanol-sucrose readily substituted for it. The results are discussed in terms of applying a behavioral economic approach to relationships between drug and nondrug reinforcers.     

Oral self-administration of pentobarbital by rhesus monkeys: relative reinforcing effects under concurrent fixed-ratio schedules.

During daily 3-hr sessions, orally delivered pentobarbital solutions and water, or two separate pentobarbital solutions, were concurrently available to rhesus monkeys according to fixed-ratio schedules of mouth contacts with a spout. First water, and then each of four "comparison-concentration" pentobarbital solutions (0.0625, 0.25, 1, and 4 mg/mL), was successively available from one spout for a block of sessions under a fixed-ratio-64 (three monkeys) or fixed-ratio-16 (one monkey) schedule. Under an identically sized fixed-ratio schedule, deliveries of a "standard-concentration" pentobarbital solution were concurrently available from a second spout. The concentration of the standard solution remained unchanged throughout testing of the series of comparison solutions. Each of three pentobarbital concentrations (4, 1, and 0.25 mg/mL) in turn served as the standard concentration. Within each pair of concurrently available solutions, the higher drug concentration maintained more behavior than the lower concentration. Thus when monkeys were provided with concurrent access to different pentobarbital concentrations, relative reinforcing effects were directly related to drug concentration. Further, the amount of behavior maintained by a particular drug concentration was dependent on the concentration of the concurrently available drug solution. Thus, the relative effectiveness of a reinforcer in maintaining behavior is a function of both the reinforcer's magnitude and the availability of alternative reinforcers in the environment.     

Assessing reinforcers under progressive schedule requirements.

Recent research findings suggest that reinforcing stimuli may be differentially effective as response requirements increase. We extended this line of research by evaluating responding under increasing schedule requirements via progressive‐ratio schedules and behavioral economic analyses. The differential effectiveness of preferred stimuli in treating destructive behavior maintained by automatic reinforcement also was examined. Results showed that one of two stimuli was associated with more responding under increasing schedule requirements for the 4 participants. Furthermore, stimuli associated with more responding under increasing schedule requirements generally were more effective in treating destructive behavior than stimuli associated with less responding. These data suggest that progressive‐ratio schedules and behavioral economic analyses may be useful for developing a new technology for reinforcer identification. From a clinical perspective, these results suggest that two reinforcers may be similarly effective for low‐effort tasks and differentially effective for high‐effort tasks.     

A parametric evaluation of the hedonic and motoric effects of drugs: pimozide and amphetamine

This study uses a curve-fitting approach to evaluate the effects of drugs on reinforced responding in rats. The subjects obtained reinforcement according to a series of five different variable-interval schedules (a five-component multiple schedule). For each rat, pimozide, a neuroleptic, decreased response rate, and the decrease was associated with (1) a decrease in the estimated asymptotic response rate and (2) an increase in the rate of reinforcement necessary for half-asymptotic responding. That is, pimozide decreased the proportion of responding maintained by a given rate of reinforcement. In contrast, intermediate doses of amphetamine increased response rate and increased the proportion of responding maintained by a given rate of reinforcement. It was proposed that the response rate asymptote indexes motor capacity, and the rate of reinforcement necessary for half-asymptotic responding indexes reinforcement efficacy; accordingly, pimozide decreased motor capacity and reinforcement strength and amphetamine increased reinforcement strength.     

Cocaine and food as reinforcers: effects of reinforcer magnitude and response requirement under second-order fixed-ratio and progressive-ratio schedules.

Reinforcer magnitude and fixed-ratio requirement were varied under two second-order schedules. Under one, the first sequence of a fixed number of responses completed after the lapse of a 10-min fixed interval produced reinforcement. Under the second, a second-order progressive-ratio schedule, the fixed number of responses increased after each reinforcement. Either cocaine (0 to 300 micrograms/kg/inj) or food (0 to 5,700 mg/delivery) reinforcers were delivered. Under some conditions, a 2-s illumination of stimulus lights occurred on completion of each ratio sequence. Under the second-order schedule, as cocaine dose or amount of food increased, rates of responding increased; at the highest values, rates of responding decreased. Increases in the ratio requirement from 10 to 170 responses minimally decreased overall response rates. Under the second-order progressive-ratio schedule, increases in dose of cocaine or amount of food increased rates of responding; at the highest amounts of food, rates of responding decreased but response rates at the highest dose of cocaine remained relatively high. The highest ratio requirement that was completed (breaking point) depended on the dose of cocaine but was less dependent on the amount of food. Removing brief-stimulus presentations had a greater effect on completion of ratio requirements with cocaine compared to food.     

Oral self-administration of pentobarbital by rhesus monkeys: maintenance of behavior by different concurrently available volumes of drug solution.

For 4 rhesus monkeys, mouth-contact responses with either of two brass spouts were reinforced according to fixed-ratio schedules by 0.65-mL liquid deliveries during daily 3-hr sessions. Three experiments were conducted. In each experiment, independent fixed-ratio schedules were concurrently in effect at the two spouts. Following completion of each fixed ratio on a spout, a specified number of liquid deliveries were available from that spout under a continuous-reinforcement schedule. The number of such deliveries available at each spout was manipulated independently. In Experiment 1, a 1-mg/mL pentobarbital solution was simultaneously available with water (the drug vehicle) under concurrent fixed-ratio schedules of 32 responses for 3 subjects and 64 responses for the remaining subject. The number (N) of liquid deliveries that were available after completion of each fixed ratio was varied in the following order: 8, 4, 2, 1, and 8 (retest). For each subject at each condition, drug maintained more responding than water. The number of drug deliveries obtained per session was directly related to the amount of drug available per fixed ratio (i.e., to N), whereas the number of fixed ratios completed per session generally was inversely related to the value of N. In Experiment 2, fixed-ratio size was the same for each subject as in Experiment 1, but deliveries of a 1-mg/mL pentobarbital solution were available at both spouts. The number of drug deliveries available under one fixed-ratio schedule (Ns, the "standard" reinforcer amount) was held at eight, and the number of drug deliveries available under the second schedule (Nc, the "comparison" reinforcer amount) was changed across blocks of six sessions of stable responding in the following order: 1, 2, 4, 8, 4, 2, and 1. The identical series of comparison reinforcer amounts (Nc) was then tested twice more, but with the standard reinforcer (Ns) held first at four and then at two deliveries. Across the three choice series, reinforcing effects were directly related to reinforcer magnitude. In Experiment 3, deliveries of a 1-mg/mL pentobarbital solution again were available at both spouts. However, the two reinforcer amounts were held constant at N = 8 deliveries under one schedule and N = 4 deliveries under the second schedule, and fixed-ratio size was systematically varied. Across the range of fixed-ratio sizes from low to high, the degree to which behavior was better maintained by the larger of the two drug quantities was an inverted U-shaped function of fixed-ratio size.(ABSTRACT TRUNCATED AT 400 WORDS)     

Biobehavioral effects of extended salt loading and conflict stress in intact baboons.

Behavioral stressors may inhibit sodium excretion, potentially increasing plasma volume and elevating blood pressure during chronic exposure. Blood pressure regulation may be especially deranged during manipulations that further challenge the kidney, such as a diet high in salt content. The effects on blood pressure and other variables of combined behavioral stress (food/shock conflict) and dietary salt (12 g NaCl per day; 218 mEq Na+ per day) were examined in adult male baboons over the course of 1 year. Mean arterial pressure was not significantly elevated over baseline after 5 months of high dietary salt alone (6 +/- 5 mmHg) but was maximally elevated by an average of 17 (+/- 3 SEM) mmHg after 5 months of combined salt and conflict stress. Control baboons showed no significant trends in mean arterial pressure across the same time period. Individual subjects whose blood pressure was "salt+stress resistant" or "salt+stress sensitive" were differentiated by their degree of pressure diuresis and natriuresis, urinary free cortisol, and a behavioral index of stress sensitivity. The data indicate additive effects of chronic high dietary salt intake and behavioral stressors on blood pressure in nonhuman primates that are dependent on renal function and pituitary-adrenocortical activity.     

Concurrent access to two concentrations of orally delivered phencyclidine: effects of feeding conditions.

Two experiments addressed the effects of food satiation and deprivation on oral self-administration of two concurrently available phencyclidine concentrations. In the first experiment, 8 rhesus monkeys self-administered either of two concentrations of phencyclidine ("PCP, angel dust") and water under concurrent fixed-ratio 16 schedules. One concentration was always held constant (0.25 mg/mL) while a series of other phencyclidine concentrations, ranging from 0 (water) to 1.0 mg/mL, was presented in a nonsystematic order. Initially the monkeys were tested while food satiated, and the procedure was then repeated during food deprivation. The monkeys usually selected the higher concentration within the first few minutes of the session, indicating that taste and/or other immediate postingestional effects were important factors. Contrary to a number of previous reports, there were no consistent differences across subjects in the mean number of liquid deliveries or mean drug intake (mg/kg) during food satiation and deprivation. However, for all monkeys the within-session time course of responding during food satiation consistently differed from that during deprivation. A second experiment assessed whether the failure to find consistent differences in drug intake during food satiation and deprivation had been due to the history of concurrent access to different phencyclidine concentrations or to the extended experience with phencyclidine under food-satiation conditions. Six additional monkeys (Group 2) were exposed to the phencyclidine self-administration procedure (during food satiation and deprivation) for the same length of time as the monkeys in Experiment 1 (Group 1), except they received only concurrent access to phencyclidine (0.25 mg/mL) and water. Both groups then received concurrent access to phencyclidine and water during five repeated cycles of food deprivation and satiation. There were also marked individual differences in Group 2: During food satiation, 2 of the monkeys' responding increased, 1 showed no change, and 3 decreased. Examination of a number of historical variables indicated that the greater the percentage of total sessions spent during food satiation with phencyclidine available (before these experiments began), the greater the amounts of phencyclidine consumed during food satiation and the smaller the differences in phencyclidine intake when the two feeding conditions were compared.     

Unit price as a useful metric in analyzing effects of reinforcer magnitude.

In this paper, we applied the behavioral-economic concept of unit price to the study of reinforcer magnitude in an attempt to provide a consistent account of the effects of reinforcer magnitude on behavior. Recent research in the experimental analysis of behavior and in behavioral pharmacology suggests that reinforcer magnitude interacts with the schedule of reinforcement to determine response rate and total consumption. The utility of the unit-price concept thus stems from its ability to quantify this interaction as a cost-benefit ratio (i.e., unit price = characteristics of the schedule of reinforcement divided by magnitude of reinforcement). Research employing the unit-price concept has shown that as unit price increases, a positively decelerating function exists for consumption (i.e., a function with an increasingly negative slope, when plotted on log coordinates) and a bitonic function exists for response rate. Based on these findings, the present analysis applied the unit-price concept to those studies of reinforcer magnitude and drug self-administration that examined the effects of reinforcer magnitude on response rate using simple schedules of reinforcement (e.g., fixed-ratio schedule). This resulted in three findings: (a) Reinforcer-magnitude manipulations and schedule manipulations interact in a manner that can be quantified in terms of unit price as benefit and cost factors, respectively; (b) different reinforcer-magnitude manipulations are functionally interchangeable as benefit factors in the unit-price ratio; and (c) these conclusions appear warranted despite the differences in reinforcers (food or drug), species (dogs, monkeys, or rats), and schedules (interval or ratio), and despite the fact that these studies were not designed for a unit-price analysis. In methodological terms, these results provide further evidence that employing the unit-price concept is a parsimonious method for examining the effects of reinforcer magnitude. In theoretical terms, these results suggest that a single process may underlie the effect of combined reinforcer-magnitude and schedule manipulations.     

Facilitation of human tobacco self-administration by ethanol: a behavioral analysis.

The effect of ethanol on the cigarette smoking of alcoholic subjects was studied in a residential laboratory. During daily 6-hr sessions, cigarettes were obtained either by request to the ward staff or by operation of a lever (fixed-ratio 5 or 10). In a mixed sequence across days, sessions involved ingestion of either vehicle (orange juice or vehicle plus ethanol (133.7 g). During ethanol sessions, the rate of cigarette smoking increased from 26% to 117% of vehicle levels. A series of control studies eliminated a number of potential behavorial mechanisms for the observed effect and indicated that the ethanol-induced increase in cigarette smoking occurred under a variety of experimental conditions: (1) when smoking could not occur concurrently with ethanol or vehicle consumption; (2) when subjects were not allowed to socialize; (3) when ingestion of ethanol or vehicle was scheduled for a number of consecutive days; (4) when various doses of ethanol were administered under blind conditions. In control experiments, weighing unsmoked tobacco and counting the number of puffs per cigarette indicated the effect was not due to smoking less of each cigarette. The effect was not limited to the experimental sessions alone, since total daily smoking was higher on ethanol days than vehicle days.     

Behavior controlled by scheduled injections of cocaine in squirrel and rhesus monkeys.

Rates and patterns of key-press responding maintained under schedules in which responding resulted in intravenous injections of cocaine were studied in squirrel monkeys and rhesus monkeys. Each injection was followed by a 60- or 100-sec timeout period. Schedule-controlled behavior was obtained at appropriate cocaine doses in each species. Under FR 10 or FR 30 schedules, performance was characterized by high rates of responding (usually more than one response per second) in each ratio. Under FI 5-min schedules, performance was characterized by an initial pause, followed by acceleration of responding to a final rate that was maintained until the end of the interval. Under multiple fixed-ratio fixed-interval schedules, rates and patterns of responding appropriate to each schedule component were maintained. Responding seldom occurred during timeout periods under any schedule studied. At doses of cocaine above or below those that maintained characteristic schedule-controlled behavior, rates of responding were relatively low and patterns of responding were irregular. Characteristic fixed-interval responding was maintained over a wider range of cocaine doses than characteristic fixed-ratio responding. Complex patterns of responding controlled by discriminative stimuli under fixed-ratio or fixed-interval schedules can be maintained by cocaine injections in squirrel monkeys and rhesus monkeys.