Effort as determinant of intake and patterns of drinking in the guinea pig

Behavior regulation and learned performance: Some misapprehensions and disagreements

The behavior-regulation approach to learned performance has been the subject of misapprehension and disagreement concerning: (1) the nature and importance of behavior regulation, (2) the definition and role of behavioral set-points, (3) the relation of optimal schedule performance to behavioral set-points, and (4) the question of whether deviations from total responding or from response patterns are the primary determinant of molar responding under schedule constraint. After clarifying the nature and role of behavior regulation and set-points, this paper shows that the data used to question optimal schedule performance (Allison, 1981a) actually strongly support the general behavior-regulation approach. These data also indicate a role for response-pattern set-points in determining schedule behavior, but contradict the hypothesis that deviations from response-pattern characteristics are the primary determinant of molar schedule effects.

Quantification of rats' behavior during reinforcement periods

What is treated as a single unit of reinforcement often involves what could be called a reinforcement period during which two or more acts of ingestion may occur, and each of these may have associated with it a series of responses, some reflexive, some learned, that lead up to ingestion. Food-tray presentation to a pigeon is an example of such a "reinforcement period." In order to quantify this behavior, a continuous-reinforcement schedule was used as the reinforcement period and was chained to a fixed-ratio schedule. Both fixed-ratio size and reinforcement-period duration were manipulated. Rats were used as subjects, food as reinforcement, and a lever press as the operant. Major findings included (a) a rapid decline in response rates during the first 15 to 20 seconds of the reinforcement periods, and (b) a strong positive relationship between these response rates and the size of the fixed ratio. Also revealed was a short scallop not normally found in fixed-ratio response patterns, whose length was a function of fixed-ratio size and reinforcement-period duration. It is suggested that rapidly fluctuating excitatory processes can account for many of these findings and that such processes are functionally significant in terms of behavioral compensation.

The economics of daily consumption controlling food- and water-reinforced responding

In the first experiment, two rhesus monkeys earned their entire ration of food and water during daily sessions with no provisions to ensure constant daily intakes. Two variable-interval schedules of food presentations were concurrent with one variable-interval schedule of water presentations; the maximum rate of food presentations arranged by one food schedule was varied. As the rate of food presentations was increased, the absolute level of responding on the two food schedules combined decreased, while responding on the water schedule increased. The preference for the variable food schedule compared to the other food schedule approximately matched the proportion of reinforcers obtained from it. The preference for the variable food schedule compared to the water schedule did not match, but greatly decreased, as the proportion of reinforcers from the food schedule increased. When Experiment I was replicated, with provisions to ensure constant daily intakes of food and water (Experiment II), the absolute response rates under the two food schedules combined and under the water schedule no longer changed with increases in the rate of food during the sessions. On the other hand, choice between the two food schedules remained proportional to the distribution of obtained food pellets. These results were interpreted as indicating that behavior to obtain nonsubstitutable commodities, such as food and water, is strongly controlled by the economic conditions of daily consumption, while choice between substitutable commodities is independent of these factors.

Effects of sarin on the operant behavior of guinea pigs

The present study evaluated the dose-response effects of subacute exposure to sublethal doses of the organophosphorus (OP) chemical warfare nerve agent (CWNA) sarin (GB) on the operant behavior of guinea pigs. Dietary restricted guinea pigs, trained to respond for food under a progressive ratio (PR) schedule of reinforcement, were injected five times per week (Monday-Friday) for 2 weeks with fractions (0.1, 0.2, and 0.4) of the established LD(50) of GB (42 microg/kg). Changes in body weight, whole blood (WB) acetylcholinesterase (AChE) levels, and operant performances were monitored over the 2 weeks of GB exposure and for an additional 2 weeks following the termination of exposures. There were dose-related changes in body weight and WB AChE levels throughout the exposure and post-exposure periods. Several parameters of PR performance were disrupted during exposure to 0.4 LD50 GB, however, concurrent weight loss indicated the presence of overt toxicity. PR performance recovered following the termination of exposures. Lower doses (0.1 and 0.2 LD50) of GB failed to produce reliable effects on operant performance during the exposure period. Overall responding decreased during exposure to 0.4 LD50 GB, resulting in reduced response rates and break points. The decrease in overall response rates was attributed to an increase in pausing since there was no decrease in running rate. Motor effects of 0.4 LD50 GB were evident as an increase in the proportion of lever press durations > or = 1.0 s. In the present study, doses of GB lower than 0.4 LD50 produced no marked alteration of operant performance in guinea pigs, although WB AChE levels were maximally inhibited to 20% of control.

Stock optimizing: maximizing reinforcers per session on a variable-interval schedule

In Experiment 1, 2 monkeys earned their daily food ration by pressing a key that delivered food according to a variable-interval 3-min schedule. In Phases 1 and 4, sessions ended after 3 hr. In Phases 2 and 3, sessions ended after a fixed number of responses that reduced food intake and body weights from levels during Phases 1 and 4. Monkeys responded at higher rates and emitted more responses per food delivery when the food earned in a session was reduced. In Experiment 2, monkeys earned their daily food ration by depositing tokens into the response panel. Deposits delivered food according to a variable-interval 3-min schedule. When the token supply was unlimited (Phases 1, 3, and 5), sessions ended after 3 hr. In Phases 2 and 4, sessions ended after 150 tokens were deposited, resulting in a decrease in food intake and body weight. Both monkeys responded at lower rates and emitted fewer responses per food delivery when the food earned in a session was reduced. Experiment 1's results are consistent with a strength account, according to which the phases that reduced body weights increased food's value and therefore increased subjects' response rates. The results of Experiment 2 are consistent with an optimizing strategy, because lowering response rates when food is restricted defends body weight on variable-interval schedules. These contrasting results may be attributed to the discriminability of the contingency between response number and the end of a session being greater in Experiment 2 than in Experiment 1. In consequence, subjects lowered their response rates in order to increase the number of reinforcers per session (stock optimizing).

The magnitude-of-reinforcement function in closed and open economies

It has been hypothesized that the magnitude-of-reinforcement effect may differ in closed and open experimental economies. We determined the relationship between magnitude of reinforcement and response rate in three feeding conditions: a closed economy in which total intake was unrestricted, a closed economy in which total intake was restricted so as to maintain body weight at 85% of free-feeding weight, and a traditional open economy in which subjects received food outside the experimental session. In the closed economies, regardless of body weight, the rats responded faster for smaller pellets and when the fixed ratio for pellets was higher. In the open economy, there was no reliable effect of pellet size or pellet cost on response rate. It is concluded that although there are circumstances in which response rate is an immediate function of the parameters of reinforcement, rate is not necessarily a measure of response strength. Response rate may instead, or additionally, contribute to a strategy of reducing the costs associated with resource utilization.

Anticipatory drinking in rats: compensatory adjustments in the local rate of intake

The current experiment examined whether dehydrated rats could learn to anticipate disruption of access to water. Previous research showed that rats running in a wheel in order to drink compensated for local disruptions on intake by increasing their rate of intake after, but not prior to the initial disruption. The present experiment exposed rats to either a fixed-ratio (FR) schedule that required rats to run in order to drink or a matched, fixed-time (FT) schedule that allowed the animals intermittent access to water without running. Access to water was disrupted by restricting the quantity of water available per drink bout. Rats increased their local rate of water intake prior to the disruption under the FT schedule but not under the FR schedule. The results suggest that rats can learn to increase their local rate of intake in anticipation of local restrictions on water under response-independent (FT) schedules. It was hypothesized that these anticipatory increases in the local rate of intake resulted from a priming effect due to the motivating effects of stimuli associated with water and/or frustration resulting from attempts to drink prior to water availability.

Effect of procurement cost on the drinking of a saccharin-sucrose solution by non-deprived rats

Four non-deprived female rats were required to run in a wheel to obtain 20 min unconstrained access to a saccharin and sucrose solution. Each was run in a series of conditions in which the requirement was a proportion (0.25, 0.5, 1.0, 1.25 and 1.5) of the amount of running performed in a condition in which the wheel alone was available. A condition in which no running was required to gain access to the solution and one in which the subject was locked in the stationary wheel for the time taken to complete the highest requirement before being allowed access to the solution were also included. The results showed that as the requirement increased the amount of solution consumed also increased, and this relationship did not depend on the time taken to perform the requirement.

Effect of procurement wheel running on the rate of drinking in rats

Four rats, 22.5 hr deprived of water, were tested in 3 conditions, in which they were required to run zero, 5 or 300 one-sixth revolutions in a wheel to gain 30 min access to water in a drinking tube. The number of licks performed in the 30 min increased monotonically with the procurement cost, and was 20% greater following the larger cost than when there was no cost. However, examination of the rates of drinking throughout the 30 min revealed that differences occurred between the conditions only at the beginning of the period. In the first 3 min there was a monotonic relationship between the proportion of time spent drinking and the procurement cost, but no effect of the cost on the rate could be detected after the first 6 min. The results are consistent with the hypothesis that the effect is mediated by a transient elevation of the subject's arousal level.

Neuropeptides and thirst

A number of neuropeptides have been found to affect fluid intake when injected directly into the brain of various vertebrate species. These include: angiotensin II and its peptide precursors; the tachykinins Substance P, eledoisin and physalaemin; the opioid peptides met- and leu-enkephalin and beta-endorphin; bombesin; neurotensin; and vasopressin. Some of these stimulate drinking, some inhibit water intake, and the tachykinins have opposite effects on thirst depending on the species tested. Very little is known about the site or mechamism of action of most of these peptides or if their effects on thirst are physiological. The exception is angiotensin II, a peptide hormone that is synthesized in the blood in response to hypovalaemia or hypotension and is involved in many aspects of the regulation of blood volume and pressure. Angiotensin II injected intravenously or intracranially stimulates drinking in all reptiles, birds and mammals tested. In addition to its role as a hormone, angiotensin II may also function as a neurotransmitter or neuromodulator, since all of the enzymes and precursors necessary for its synthesis have been found in the central nervous system.

Patterns of eating as a function of the cost of the meal

Rats were required to complete fixed ratio schedules (FR 20-FR 2560) of wheel turns to obtain access to food. By decreasing meal frequency and increasing meal size directly as functions of the fixed ratio requirement, animals controlled total daily food intake and body weight relatively constant until the highest ratio requirement was introduced. These functional changes in feeding patterns provide experimental support for theoretical models of optimal feeding strategies. At the highest ratio requirement, as animals lost weight, they increased running and therefore opportunities to feed, however, food intake continued to decrease with increasing exposure to this schedule. As rats on this schedule initiated feeding each time food became available, but did not eat large enough to this schedule. As rats on this schedule initiated feeding each time food became available, but did not eat large enough meals to maintain body weight, it is suggested that activity may interest with satiety mechanisms to produce termination of meals.