Within-session changes in responding during concurrent schedules with different reinforcers in the components.

Rats and pigeons responded on several concurrent schedules that provided different reinforcers in the two components (food and water for rats, Experiment 1; wheat and mixed grain for pigeons, Experiment 2). The rate of responding and the time spent responding on each component usually changed within the session. The within-session changes in response rates and time spent responding usually followed different patterns for the two components of a concurrent schedule. For most subjects, the bias and sensitivity to reinforcement parameters of the generalized matching law, as well as the percentage of the variance accounted for, decreased within the session. Negative sensitivity parameters were sometimes found late in the session for the concurrent food-water schedules. These results imply that within-session changes in responding could cause problems for assessing the validity of quantitative theories of concurrent-schedule responding when the components provide different reinforcers. They question changes in a general motivational state, such as arousal, as a complete explanation for within-session changes in responding. The results are compatible with satiation for, or sensitization-habituation to, the reinforcers as explanations.     

Satiation, capacity, and within-session responding

Responding may change substantially over the course of a session (McSweeney, Hinson, & Cannon, 1996). The role of satiation in this effect was investigated in three experiments. Experiment 1 showed that the capacity of pigeons to consume milo over a 1-hr period was relatively stable across three different methods of measurement. In Experiment 2, pigeons were divided into two groups that differed in their capacity based on one of those measures. Key pecking was then reinforced under a variable-interval 30-s schedule with hopper durations of 2 or 5 s. According to the satiation hypothesis, subjects with small capacities should satiate faster and therefore show greater decreases in food-reinforced responding than would subjects with larger capacities. The results confirmed this prediction and showed that the magnitudes of within-session decreases were better predicted by the amount an animal consumed relative to its capacity than by absolute amount alone. In Experiment 3, each pigeon was prefed 0, 5, 15, or 25 g of milo prior to each session. Consistent with the satiation hypothesis, increases in prefeeding produced lower overall response rates in the smaller capacity subjects than in the larger capacity subjects at each level of prefeeding. These experiments demonstrate the importance of a new variable in the control of behavior, and provide a recommended technique for its measurement.     

Within-session Response Patterns On Conjoint Variable-interval Variable-time Schedules

Operant responding often changes within sessions, even when factors such as rate of reinforcement remain constant. The present study was designed to determine whether within-session response patterns are determined by the total number of reinforcers delivered during the session or only by the reinforcers earned by the operant response. Four rats pressed a lever and 3 pigeons pecked a key for food reinforcers delivered by a conjoint variable-interval variable-time schedule. The overall rate of reinforcement of the conjoint schedule varied across conditions from 15 to 480 reinforcers per hour. When fewer than 120 reinforcers were delivered per hour, the within-session patterns of responding on conjoint schedules were similar to those previously observed when subjects received the same total number of reinforcers by responding on simple variable-interval schedules. Response patterns were less similar to those observed on simple variable-interval schedules when the overall rate of reinforcement exceeded 120 reinforcers per hour. These results suggest that response-independent reinforcers can affect the within-session pattern of responding on a response-dependent schedule. The results are incompatible with a response-based explanation of within-session changes in responding (e.g., fatigue), but are consistent with both reinforcer-based (e.g., satiation) and stimulus-based (e.g., habituation) explanations.     

Within-session Changes In Responding During Concurrent Variable-interval Schedules

Five rats and 4 pigeons responded for food delivered by several concurrent variable-interval schedules. The sum of the rates of reinforcement programmed for the two components varied from 15 to 480 reinforcers per hour in different conditions. Rates of responding usually changed within the experimental session in a similar manner for the two components of each concurrent schedule. The within-session changes were similar to previously reported changes during simple schedules that provided rates of reinforcement equal to the sum of all reinforcers obtained from the concurrent schedules. The number of changeovers also changed within sessions in a manner similar to the changes in instrumental responding. These results suggest that changeovers are governed by the same variables that govern instrumental responding. They also suggest that the within-session change in responding during each component of a concurrent schedule is determined by approximately the sum of the reinforcers obtained from both components when both components provide the same type of reinforcer.     

Representing within-session response rates proportionally and entirely.

In this technical article, methods for collecting and representing response rates maintained by schedules of reinforcement are presented. First, the time in a session that each important event (e.g., responses, reinforcers) occurs is collected and stored by a computer. Another computer program is used, then, to convert each response to a percentage of the total responses in a session and to plot these percentages cumulatively as a function of the time in the session that they occurred. In this manner, response rates may be expressed proportionally (i.e., using the same y-axis scale regardless of absolute response rate) without requiring the arbitrary selection of an interval over which responses are aggregated and expressed relative to the entire-session rate. A property of these records is that deviations in the slope of the obtained record from the diagonal, which connects (x, y) = (start of session, 0%) to (x, y) = (end of session, 100%), occurring at any point and for any duration, represent changes in the local response rate from the entire-session rate. This method of representing ongoing responding is illustrated by several records of key pecking of a pigeon on a variable-interval 60-s schedule of food reinforcement. Relative local response rates were also computed from these data at several levels of resolution (i.e., the time over which responses were aggregated), including the level typically employed by those interested in within-session changes in response rates.     

Behavioral economics and behavioral momentum

Some relations between elasticity of demand and the conditions of reinforcement are reanalyzed in terms of resistance to change, in ways suggested by the metaphor of behavioral momentum; some relations between resistance to change and the conditions of reinforcement are reanalyzed in terms of elasticity of demand, in ways suggested by behavioral economics. In addition, some data on labor supply in relation to variable-ratio schedules and alternative reinforcement are reanalyzed in terms of resistance to change and compared with steady-state resistance data for performance on multiple and concurrent interval schedules. The results of these studies can be summarized by two functions based on the behavioral momentum approach, relating relative behavioral mass to relative reinforcement per response or per unit time. The former is a relation between relative unit price and relative behavioral mass, suggesting the possibility of convergent measurement of a theoretical construct common to both approaches. However, the momentum and economic approaches differ fundamentally on whether it is preferable to construe discriminated operant behavior as selected and strengthened by its consequences or as part of a behavior–consequence bundle that maximizes utility.     

Within-session changes in key and lever pressing for water during several multiple variable-interval schedules

Rats pressed keys or levers for water reinforcers delivered by several multiple variable-interval schedules. The programmed rate of reinforcement varied from 15 to 240 reinforcers per hour in different conditions. Responding usually increased and then decreased within experimental sessions. As for food reinforcers, the within-session changes in both lever and key pressing were smaller, peaked later, and were more symmetrical around the middle of the session for lower than for higher rates of reinforcement. When schedules provided high rates of reinforcement, some quantitative differences appeared in the within-session changes for lever and key pressing and for food and water. These results imply that basically similar factors produce within-session changes in responding for lever and key pressing and for food and water. The nature of the reinforcer and the choice of response can also influence the quantitative properties of within-session changes at high rates of reinforcement. Finally, the results show that the application of Herrnstein's (1970) equation to rates of responding averaged over the session requires careful consideration.     

Low-response-rate conditioning history and fixed-interval responding in rats.

Bar presses by one group of rats were conditioned under a differential-reinforcement-of-low-rate reinforcement schedule immediately prior to conditioning under a fixed-interval schedule. In a second group of rats, bar presses were conditioned first under a differential-reinforcement-of-low-rate schedule and then under a fixed-ratio schedule prior to conditioning under a fixed-interval schedule. Low response rates occurred under the fixed-interval schedule only when it was immediately preceded by low-rate conditioning. Otherwise, fixed-interval responding was similar to responding under the fixed-ratio schedule. This finding suggests that responses of laboratory animals are sensitive to immediate history, and, unlike human responses, are relatively insensitive to a history of low-rate conditioning when it is followed by high-rate conditioning.     

Patterns of responding within sessions.

Rates of responding changed systematically across sessions for rats pressing levers and keys and for pigeons pressing treadles and pecking keys. A bitonic function in which response rates increased and then decreased across sessions was the most common finding, although an increase in responding also occurred alone. The change in response rate was usually large. The function relating responding to time in session had the following general characteristics: It appeared early in training, and further experience moved and reduced its peak; it was flatter for longer sessions; and it was flatter, more symmetrical, and peaked later for lower than for higher rates of reinforcement. Factors related to reinforcement exerted more control over the location of the peak rate of responding and the steepness of the decline in response rates than did factors related to responding. These within-session changes in response rates have fundamental theoretical and methodological implications.     

Within-session changes in responding during several simple schedules

Pigeons' key pecking was reinforced by food delivered by several fixed-interval, variable-ratio, and differential-reinforcement-of-low-rate schedules. Rate of responding, number of responses per reinforcer, length of postreinforcement pause, running response rate, and the time required to collect an available reinforcer changed systematically within sessions when the schedules provided high rates of reinforcement, but usually not when they provided low rates. These results suggest that the factors that produce within-session changes in responding are generally similar for different schedules of reinforcement. However, a separate factor may also contribute during variable-ratio schedules. The results question explanations for within-session changes that are related solely to the passage of time, to responding, and to one interpretation of attention. They support the idea that one or more factors related to reinforcement play a role.     

The contribution of an added counter to a fixed-ratio schedule

Although previous research showed that a visual counter increased the rate of responding on a large fixed-ratio schedule, a theoretical analysis of the factors responsible for fixed-ratio performance suggests that the primary control by number of responses since reinforcement is to weaken the performance. The present experiment employed a multiple schedule in which the same fixed-ratio value alternated with and without an added counter. It tested the hypothesis that the differential reinforcement of high-rate responding masked the attenuation of the fixed-ratio performance from the unoptimal discriminative control produced by the fixed relation between number of responses and reinforcement. In the present experiment the postreinforcement pause was consistently longer in the components with the added counter, while running rates remained comparable between the components of the multiple schedule. Both components of the multiple schedule involved differential reinforcement of high-rate responding while only the components with the added counter amplified the discriminative control by number of pecks since reinforcement.     

Resurgence of temporal patterns of responding

The resurgence of temporal patterns of key pecking by pigeons was investigated in two experiments. In Experiment 1, positively accelerated and linear patterns of responding were established on one key under a discrete-trial multiple fixed-interval variable-interval schedule. Subsequently, only responses on a second key produced reinforcers according to a variable-interval schedule. When reinforcement on the second key was discontinued, positively accelerated and linear response patterns resurged on the first key, in the presence of the stimuli previously correlated with the fixed- and variable-interval schedules, respectively. In Experiment 2, resurgence was assessed after temporal patterns were directly reinforced. Initially, responding was reinforced if it approximated an algorithm-defined temporal pattern during trials. Subsequently, reinforcement depended on pausing during trials and, when it was discontinued, resurgence of previously reinforced patterns occurred for each pigeon and for 2 of 3 pigeons during a replication. The results of both experiments demonstrate the resurgence of temporally organized responding and replicate and extend previous findings on resurgence of discrete responses and spatial response sequences.     

The effects of differing response-force requirements on fixed-ratio responding of rats.

Rats were exposed to two-component multiple schedules of food delivery. In the first experiment, 15 responses were required to produce food in both components. A downward force of 0.25 N (25 g) was always required to operate the response lever in one component. In the other, the required force was 0.25, 0.50, 1.00, or 2.00 N (25, 50, 100, or 200 g). In the second experiment, 0.25 N of force operated the lever in one component, but in the other, the force requirement for five consecutive responses at the beginning, middle, or end of each ratio was increased from 0.25 to 2.00 N. In the third experiment, the number of responses required to produce food was reduced from 15 to 5, and then to 1. Again, the effects of altering response force from 0.25 to 2.00 N were examined. In general, as response force increased in all experiments, mean response rates decreased and mean interresponse times increased.     

Within-session Changes In Responding During Autoshaping And Automaintenance Procedures

Four pigeons were exposed to autoshaping procedures in which an 8-second light on a response key was followed by food. Pecks on the key had no scheduled consequences. Subjects were also exposed to negative automaintenance procedures in which a peck on the illuminated key canceled the following food. The intertrial interval varied from an average of 7 seconds to an average of 232 seconds in different conditions. Rate of responding usually changed within sessions during autoshaping. Responding also changed within sessions for the 1 subject that responded during negative automaintenance. The within-session patterns of responding were flatter, peaked later, and were more symmetrical around the middle of the sessions at lower rates of food presentation, regardless of whether subjects responded on autoshaping, negative automaintenance, or previously reported variable-interval schedules. These results imply that similar variables produce within-session changes in responding during both classical (Pavlovian) and operant conditioning procedures.     

Paired baseline performance as a behavioral ideal

Several recent theories view performance under the constraints of a schedule as an attempt to approach the basepoint, the total amount of the instrumental response and the total amount of the contingent response seen in the absence of schedule constraint. Some new analyses of experiments on concurrent ratio schedules, and simple ratio schedules offering an optional magnitude of contingent reward, tested this view directly. In each of the five experiments examined the organism rejected the chance of a closer approach to the basepoint, and thereby failed in addition to maximize the rate of reinforcement.     

Cocaine and food deprivation: effects on food-reinforced fixed-ratio performance in pigeons.

Key pecking by 6 pigeons was maintained by a fixed-ratio 30 schedule of food presentation while body weights were 80% of free-feeding weights. Acute administration of cocaine (0.3 to 13.0 mg/kg, i.m.) dose-dependently decreased response rates. Dose-effect curves were shifted to the right when 3 of the 6 pigeons were maintained at 70% of free-feeding weights and were shifted to the left when the other 3 pigeons were maintained at 90% of free-feeding weights. Then a dose of cocaine that initially decreased response rates by more than 95% of control rates was administered before each daily session. Comparable degrees of tolerance to these rate-decreasing effects developed in the two groups. The rate at which responding recovered was relatively rapid for pigeons in the 70% free-feeding-weight group and was slower for 2 of the 3 pigeons in the 90% free-feeding-weight group. When body weights were then increased from 70% to 80% or were decreased from 90% to 80% of free-feeding weight, performance was disrupted initially only for pigeons whose weight went from 70% to 80% of free feeding. In the present experiment the degree of deprivation may have indirectly influenced the degree of tolerance that developed to cocaine's response rate-decreasing effects because it directly influenced the dose chosen to be administered chronically. The degree of deprivation appeared to have a more direct influence on the rate at which tolerance developed.     

Response persistence under ratio and interval reinforcement schedules

In Experiment 1, rats were exposed to progressive-ratio schedules of food reinforcement while other rats were exposed simultaneously to yoked-interval schedules that arranged equivalent interreinforcer intervals but required only a single response at the end of the interval for food delivery. In Experiment 2, a within-subject yoked-control procedure was employed in which pigeons were exposed to alternating sessions (one per day) of progressive-ratio schedules and yoked-interval schedules as described above. In both experiments, responding under the yoked-interval schedule persisted beyond the point at which responding under the progressive-ratio schedule had ceased. The progressive-ratio schedules controlled break-and-run distributions, and the yoked-interval schedules controlled more even distributions of responses in time. Response rates decreased and postreinforcement pauses increased over time within individual sessions under both schedules. The results suggest that responding maintained by interval schedules is more persistent than that maintained by ratio schedules. The limitations and implications of this conclusion are discussed in the context of other investigations of response strength and behavioral momentum.     

Temporal control by progressive-interval schedules of reinforcement.

Progressive-interval performances are described using measures that have proven to be successful in the analysis of fixed-interval responding. Five rats were trained with schedules in which the durations of consecutive intervals increased arithmetically as each interval was completed (either 6-s or 12-s steps for different subjects). The response patterns that emerged with extended training (90 sessions) indicated that performances had come under temporal control. Postreinforcement pausing increased as a function of the interval duration, the pauses were proportional to the prevailing duration, and the likelihood of the first response within an interval increased as the interval elapsed. To assess the resistance of these patterns to disruption, subjects were trained with a schedule that generated high response rates and short pauses (variable ratio). When the progressive-interval schedule was reinstated, pausing was attenuated and rates were elevated, but performances reverted to earlier patterns with continued exposure. The results indicated that temporal control by progressive-interval schedules, although slow to develop, is similar in many respects to that for fixed-interval schedules.     

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.