Three tests of 'anticipatory responding' as an account for induction produced by upcoming food-pellet reinforcement.

Previous research has demonstrated that rats' rates of lever pressing for low-concentration liquid-sucrose reinforcers are increased when food-pellet, rather than sucrose, reinforcement will be upcoming in the same session (i.e. induction). The present experiments were designed to determine whether this induction was the product of 'anticipatory responses' for the upcoming food pellets being added to the responses being made for the currently available sucrose reinforcement. Experiment 1 tested this idea by summing sucrose-reinforced responding and 'anticipatory responding' from different conditions and comparing the sum to responding from a third condition in which subjects responded for sucrose when food-pellet reinforcement was upcoming. The comparison yielded similar response rates. Experiment 2 employed a blackout, of different durations in different conditions, to delay the upcoming food-pellet reinforcement. Consistent with the anticipatory-responding account, the delay decreased the size of the induction. However, results from the blackouts were not entirely consistent with the anticipatory-responding explanation. Experiment 3 provided, in some conditions, sucrose and food-pellet reinforcement in the first and second halves of the session, respectively, for responding on separate levers. These conditions separated 'anticipatory responses' for the food pellets from responses for the sucrose reinforcers. However, induction in responding for sucrose was still present. Together, these experiments demonstrate that, although anticipatory responses may contribute to induction in some instances, they are not solely responsible for the effect.

The effect of food deprivation on within-session patterns of wheel running.

In the present study, rats' wheel running was reinforced when they responded at different food deprivations. In experiment 1, fourths of a wheel turn were reinforced on a variable interval (VI) 15- or 60-s schedule during 50-min sessions. Subjects responded at 75, 85 or 95% of their free-feeding body weights, across conditions. Within-session decreases in responding were steepest at subjects' 75% weights for the VI 60-s schedule, but were similar at different weights for the VI 15-s schedule. In experiment 2, subjects responded on a VI 60-s schedule at 75 or 95% of their free-feeding body weights. Reinforcer size was one or four food pellets. Steeper within-session decreases in responding were observed at subjects' 75% weight than at their 95% weight, with no effect of reinforcer amount. The present results cannot disconfirm either leading theory for within-session changes because the terms involved (i.e. habituation and satiation) are not adequately defined. However, the present results seem to pose problems for both explanations.

Within-session responding when different reinforcers are delivered in each half of the session.

The present study investigated whether within-session responding was specific to the reinforcer currently being delivered and whether it was determined solely by retrospective factors. In four separate experiments, four rats pressed a lever on a multiple variable interval 60-s variable interval 60-s schedule during 60-min sessions. A different reinforcer (5% liquid sucrose or food pellets) was delivered in each half of the session. Rate of reinforcement in one half of the session varied across conditions. Response patterns in the second half of the session were unaffected by changes in the conditions of reinforcement of the other reinforcer in the first half of the session (specificity). Rate of responding was affected, however. The upcoming reinforcer influenced responding when sucrose was delivered in the first half of the session and food pellets were delivered in the second half, but not when their order was reversed. This effect makes contact with several other areas of research (e.g. behavioral contrast). They also suggest that the leading explanations for within-session changes in responding may be limited or incomplete.

Exposure to context may contribute to within-session changes in responding.

Rats and pigeons responded on multiple variable interval 30-s variable interval 30-s and multiple variable interval 60-s variable interval 60-s schedules. The 60-min sessions began 0, 5, 10, 15 or 30 min after the subject was placed in the experimental enclosure, determined randomly. Early-session response rates were usually higher, and the early-session increases in responding were usually smaller, when the beginning of the session was delayed than when it was immediate. These results show that factors related to reinforcement (e.g. satiation, sensitization-habituation to the reinforcers) do not provide a complete explanation for within-session changes in operant responding. Instead, an additional factor, possibly arousal or sensitization to the experimental context, also contributes. The results suggest an explanation for the spontaneous recovery of extinguished behavior.

Behavioral contrast using different reinforcers: effect of baseline rate of reinforcement.

The present study determined whether behavioral contrast would occur when different reinforcers were delivered in the different components and whether its size would vary at different baseline rates of reinforcement. Pigeons pecked keys on a multiple variable-interval schedule. Mixed grain was the reinforcer in one component and wheat was the reinforcer in the other component. In contrast conditions, the rate of wheat reinforcement was increased or decreased, from the baseline delivery rate, by a factor of four. Contrast was studied at four different baseline rates of reinforcement. Contrast was usually observed and its size almost always varied directly with the programmed baseline rate of reinforcement. The present results indicate that changes in the condition of reinforcement of a different reinforcer can produce contrast. They also broaden the potential implications of behavioral contrast.

Previous- and following-component contrast effects using a three-component multiple schedule.

Behavioral contrast can be defined as an inverse relation between the rate of behavior in one component of a multiple schedule of reinforcement and the conditions of reinforcement in another component. In the present experiment, four pigeons pecked a key for mixed-grain reinforcers delivered by a three-component multiple schedule. During baseline conditions, the same variable-interval schedule was in effect during each component. In contrast conditions, the rate of reinforcement in the second component was increased or decreased. Behavioral contrast was usually observed in both the first and third components. Across-component contrast effects were neither larger nor more reliable when the change in reinforcement occurred in the following component than when it occurred in the previous component. Furthermore, in both the first and third components, the size of contrast was almost always largest early in the component and decreased as the component progressed. Both across- and within-component data were analyzed using the model proposed by Williams and Wixted (1986). The results question the adequacy of the model. The results also pose difficulties for several existing theories of contrast.

Within-session patterns of responding with changes in the variability and probability of food delivery.

The present study was an attempt to determine the factors to which subjects sensitize and/or habituate within experimental sessions. Rats pressed a lever and pigeons pecked a key for food reinforcers delivered during a 60 min session. In experiment 1, subjects initially responded on a simple variable-interval 30 s schedule that consisted of 25 intervals. In subsequent conditions, the number of intervals in the series was decreased until subjects responded on a simple fixed-interval 30 s schedule. In experiment 2, subjects always responded on a variable-interval 15 s schedule that resulted in a stimulus change. The probability that a food reinforcer would accompany the stimulus change varied across conditions. Results showed that within-session patterns of responding did not change for either species with changes in the temporal pattern of reinforcer delivery (experiment 1). The within-session response patterns generally became flatter with decreases in the probability of food delivery for both species (experiment 2). The present results indicate that subjects are sensitizing and/or habituating to the reinforcer itself and/or to some aspect of its delivery. They also help to highlight the strength of sensitization-habituation as an explanation for within-session changes in responding.

Behavioral contrast with changes in duration and rate of reinforcement.

Behavioral contrast can be defined as an inverse relation between rate of responding in one component of a multiple schedule and conditions of reinforcement in the other components. The present study was an attempt to produce contrast with changes in duration of reinforcement. Pigeons pecked a key for mixed grain delivered by a multiple variable-interval variable-interval schedule. In Experiment 1, the reinforcer duration in the second component was decreased or increased, for different subjects, by a factor of two, four, and six from that delivered during baseline. These changes usually produced contrast. In Experiment 2, rate, duration, or rate and duration of reinforcement varied in the second component. Contrast was usually observed when only one variable was manipulated. When both variables were manipulated in the same condition, but in opposite directions, responding in the constant component usually changed inversely with the change in rate of reinforcement, not the change in reinforcer duration. The results demonstrate that changes in duration of reinforcement can produce contrast. They also increase the empirical base for which a successful theoretical account of contrast must encompass and the generality of contrast, which increases its potential practical implications.

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.

Reinforcer value may change within experimental sessions.

Large and systematic changes in response rates often occur within sessions during operant conditioning procedures. In the present experiment, we asked whether the value of the reinforcer that supports responding also changes within sessions. Pigeons pecked a key for mixed grain available throughout the session. Occasionally, wheat was also provided for pecking a second key. The ratio of the rates of responding for mixed grain and wheat, a frequently used measure of relative reinforcer value, changed significantly within sessions when mixed grain was provided at high, but not at low, rates. Habituation to the reinforcer provides the most likely explanation for these changes in reinforcer value. Eventually, habituation may provide a unified explanation for the within-session changes in behavior that occur when many species of subjects respond on a wide variety of tasks.

Within-session changes in responding during variable interval schedules.

Four rats and four pigeons responded for food delivered by variable interval schedules that provided programmed rates of reinforcement ranging from 15 to 480 reinforcers per hour. Rate of responding increased, decreased, or increased and then decreased within sessions. The within-session pattern of responding changed with changes in the programmed rate of reinforcement and with the species of subject. Finding within-session changes in responding during variable interval schedules extends the generality of these changes to another schedule. It implies that variable interval schedules should be used cautiously as baselines for assessing the effects of other variables, such as drugs. Finally, the results suggest that systematic pauses in responding during the session may contribute to the decreases in the average rates of responding with increases in the rates of reinforcement that are sometimes observed when subjects respond on variable interval schedules that deliver high rates of reinforcement.

Within-session patterns of responding when rats run in a T-maze.

Free-operant responding changes systematically within experimental sessions, usually increasing and then decreasing, even when the contingencies of reinforcement remain constant within the session. The present study extended this finding to running times in a T-maze. In Experiment 1, running times changed significantly within the session. Furthermore, the within-session pattern of running times changed with changes in the amount or rate of reinforcement. In Experiment 2, the within-session change in running times was found to be independent of the accuracy of responding. In Experiment 3, manipulating whether responding and/or reinforcement occurred early in the session did not affect the pattern of running times later in the session. The present results increase the generality of within-session changes in responding to a discrete-trials procedure. Furthermore, within-session patterns of running times usually changed in a similar manner as within-session patterns of free-operant responding when similar factors were manipulated. Thus, the same factors may be producing the changes in both situations.

Within-session response rates when reinforcement rate is changed within each session.

Three pigeons pecked keys and 5 rats pressed levers for food delivered on variable-interval schedules. During baseline conditions, subjects responded on a variable-interval 40-s schedule throughout the session. During experimental conditions, the programmed rate of reinforcement changed every 10 min in the 50-min sessions. When rats served as subjects, Herrnstein's (1970) hyperbolic equation provided a good description of the relation between rate of responding during a 10-min interval and the rate of reinforcement obtained during that interval. Responding, measured over 10-min blocks, was also approximately equally sensitive to changes in the programmed rate of reinforcement at all times in the session. Herrnstein's equation provided a poorer description of the changes in responding when pigeons served as subjects. Differences in experimental experience or differences in the absolute rates at which subjects responded may have contributed to the differences in results for these different species.

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.

Prospective factors contribute little to within-session changes in responding.

Five rats pressed levers for food delivered by a multiple variable interval 1-min variable interval 1-min schedule. In theunpredictable conditions, sessions were 20, 40, 60, 80, or 100 min long, determined randomly at the beginning of each session. In thepredictable conditions, each of these session durations was presented for 15 consecutive sessions. Rate of responding changed systematically within the session even when the end of the session was unpredictable. This implies that prospective factors related to anticipation of the end of the session are not necessary for producing withinsession changes in responding. Within-session patterns of responding were also similar for the predictable and unpredictable conditions. This suggests that prospective factors contributed little to the form of the within-session patterns under the present conditions.

Within-session response patterns when rats press levers for water: Effects of component stimuli and experimental environment.

Rats pressed levers for water reinforcers delivered by multiple variable interval one-minute variable interval one-minute schedules. Experiment 1 manipulated the stimuli signalling the components of the multiple schedule. Experiment 2 varied the experimental environment. Responding changed significantly within the session during every condition of both experiments. Manipulating the component stimuli significantly altered the within-session pattern of responding; varying the environment did not. These results show within-session patterns of responding occur when subjects respond for water reinforcers. They also imply that contextually-based explanations for within-sessions patterns of responding are unlikely.