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.     

Concurrent performances: rate constancies without changeover delays

Pigeons' pecks on two keys were maintained, without changeover delays, by independent variable-interval schedules of food reinforcement. Four regularly cycling 2-min components scheduled reinforcement respectively for both keys, left key only, both keys, and right key only. Initially, reinforcement scheduled for one key alone produced more responding on that key than reinforcement scheduled concurrently for both keys. Continued sessions reduced this difference; response rate on a given key approached constancy, or invariance with respect to the performance on and schedule for the other key. When extinction replaced the reinforcement schedule on either key, responding on that key decreased more during components that scheduled reinforcement for the other key than during those that did not. This demonstration that responses on one key were not supported by reinforcers on the other key suggested that the alternation of concurrent responding and either-key-alone responding prevented concurrent superstitions from developing.     

Resurgence and downshifts in alternative reinforcement rate

Resurgence refers to an increase in a previously suppressed target behavior with a relative worsening of conditions for a more recently reinforced alternative behavior. This experiment examined the relation between resurgence and the magnitude of a reduction in the rate of reinforcement for the alternative behavior. Groups of both male and female rats initially pressed a target lever for food on a variable-interval (VI) 30-s schedule. In a second phase, responding to the target lever was extinguished for all groups and pressing an alternative lever was reinforced on a VI 10-s schedule. Next, the rate of reinforcement for alternative behavior was reduced differentially across groups by arranging extinction, VI 80-s, VI 40-s, VI 20-s, or continued VI 10-s reinforcement. Target responding increased as an exponential function of the magnitude of the reduction in alternative reinforcement rates. With the exception that males appeared to show higher rates of target responding in baseline and higher rates of alternative responding in other phases, the overall pattern of responding across phases was not meaningfully different between sexes. The pattern of both target and alternative response rates across sessions and phases was well described quantitatively by the Resurgence as Choice in Context model.     

Remote effects of aversive contingencies: Disruption of appetitive behavior by adjacent avoidance sessions

Disruption of ongoing appetitive behavior before and after daily avoidance sessions was examined. After baselines of appetitive responding were established under a fixed-interval 180-s schedule of food presentation, 4 rats were exposed to 40-min sessions of the appetitive schedule just prior to 100-min sessions of electric shock postponement, while another 4 rats received the 40-min appetitive sessions just following daily sessions of shock postponement. In all 8 subjects, fixed-interval response rates decreased relative to baseline levels, the effect being somewhat more pronounced when the avoidance sessions immediately followed. The disruption of fixed-interval responding was only partially reversed when avoidance sessions were discontinued. During the initial exposure to the avoidance sessions, patterns of responding under the fixed-interval schedule were differentially sensitive to disruption, with high baseline response rates generally more disturbed than low rates. These disruptions were not systematically related to changes in reinforcement frequency, which remained fairly high and invariant across all conditions of the experiment; they were also not systematically related to the response rates or to the shock rates of the adjacent avoidance sessions. The results, while qualitatively resembling patterns of conditioned suppression as typically studied, occurred on a greatly expanded time scale. As disruption of behavior extending over time, the present data suggest that some forms of conditioned suppression are perhaps best viewed within a larger temporal context.     

Response specificity in animal timing.

The stimuli that control responding in the peak procedure were investigated by training rats, in separate sessions, to make two different responses for food reinforcement. During one type of session, lever pressing was normally reinforced 32 s after the onset of a light. During the other type of session, chain pulling was normally reinforced either 8 s after the onset of one auditory cue or 128 s after the onset of a different auditory cue. For both types of sessions, only the appropriate manipulandum was available, and 20% of the trials lasted 240 s and involved no response-contingent consequences. Rats were then tested with the auditory cues in the presence of the lever and the light in the presence of the chain. If the time of reinforcement associated with each stimulus was learned, response rates should peak at these times during transfer testing. However, if a specific response pattern was learned for each stimulus, little transfer should occur. The results did not clearly support either prediction, leading to the conclusion that both a representation of the time of reinforcement and the rat's own behavior may control responding in this situation.     

Repeated,within‐session resurgence

Resurgence is a reliable, transient effect that only occasionally is replicated more than once within a single experiment or subject. In the present experiments, within‐session resurgence was generated repeatedly by dividing individual sessions into three phases (Training, Alternative‐Reinforcement, and Resurgence‐Test). In Experiments 1 and 2, resurgence reliably occurred in most of the 22‐30 daily sessions when responding was reinforced on, respectively, fixed‐ and variable‐interval schedules. Resurgence magnitude and duration did decrease across replications for some subjects, but not for others. To examine the utility of the procedure in studying the effects of an independent variable on resurgence, in Experiment 3 the effects of rich and lean baseline and alternative reinforcement rates on resurgence were compared. The target response was eliminated more rapidly, resurgence occurred more often, and usually was greater following rich alternative reinforcement rates. Resurgence was of greater magnitude when the baseline reinforcement rate was relatively lean compared to the alternative reinforcement rate. These experiments provide a reliable method for generating resurgence within individual sessions, instead of across multiple‐session conditions, that can be repeated over many successive sessions.     

The role of the response-reinforcer relation in delay-of-reinforcement effects

The role of the response-reinforcer relation in maintaining operant behavior under conditions of delayed reinforcement was investigated by using a two-operandum (i.e., two-key) procedure with pigeons. Responding on one key was reinforced under a tandem variable-interval differential-reinforcement-of-other-behavior (tandem VI DRO) schedule. The schedule defined a resetting unsignaled delay-of-reinforcement procedure in that a response was required when the interfood interval of the VI schedule lapsed, but further responding during the DRO component on either key reset the time interval. This ensured a fixed delay duration between any response and reinforcement. Responding on another key, physically identical to the first one except for spatial location, otherwise was without consequence. The location of the key correlated with the delay-of-reinforcement procedure varied between sessions according to a semirandom sequence. Differences in response rates between the two keys were greater, with proportionally higher rates on the key correlated with the delay-of-reinforcement procedure, the longer the delay-of-reinforcement procedure remained correlated with the same key. Differences in responding on the two keys also increased within individual sessions. These results suggest that the response-reinforcer relation is the primary determinant of responding when responding is acquired and maintained with delayed reinforcement.     

Effects of reinforcement rate and delay on the acquisition of lever pressing by rats.

The acquisition of lever pressing by naive rats, in the absence of shaping, was studied as a function of different rates and unsignaled delays of reinforcement. Groups of 3 rats were each exposed to tandem schedules that differed in either the first or the second component. First-component schedules were either continuous reinforcement or random-interval 15, 30, 60 or 120 s; second-component schedules were fixed-time 0, 1, 3, 6, 12, or 24 s. Rate of responding was low under continuous immediate reinforcement and higher under random-interval 15 s. Random interval 30-s and 60-s schedules produced lower rates that were similar to each other. Random-interval 120 s controlled the lowest rate in the immediate-reinforcement condition. Adding a constant 12-s delay to each of the first-component schedule parameters controlled lower response rates that did not vary systematically with reinforcement rate. The continuous and random-interval 60-s schedules of immediate reinforcement controlled higher global and first-component response rates than did the same schedules combined with longer delays, and first-component rates showed some graded effects of delay duration. In addition, the same schedules controlled higher second-component response rates in combination with a 1-s delay than in combination with longer delays. These results were related to those from previous studies on acquisition with delayed reinforcement as well as to those from similar reinforcement procedures used during steady-state responding.     

Effects of food deprivation and reinforcement magnitude on conditioned suppression

In Experiment I, the responding of rats lever pressing on a variable-interval schedule for sucrose solution was partially suppressed by a variable duration conditioned stimulus followed by shock. When food deprivation was increased, response rates during and before the conditioned stimulus increased monotonically. Varying the concentration of sucrose across blocks of sessions or from session to session in a semi-random sequence had little effect on response rates either before or during the conditioned stimulus. With a fixed sequence of increasing concentrations across a five-session block, increased concentration produced much more rapid increases in response rates before than during the conditioned stimulus. In Experiment II, rats were presented with the same sequence of increasing concentrations across a five-session block. When tested at 80% body weight, response rates increased rapidly as concentration increased, but at 100%, body-weight rates increased only slightly. The effect of a change in body weight in Experiment II thus mimicked the effect of the conditioned stimulus in the latter part of Experiment I. These findings support the view that the effect of a pre-aversive conditioned stimulus is similar to that of a change in food deprivation, but unlike that of a change in reinforcement magnitude.     

Extinction of responding maintained by timeout from avoidance

The resistance to extinction of lever pressing maintained by timeout from avoidance was examined. Rats were trained under a concurrent schedule in which responses on one lever postponed shock on a free-operant avoidance (Sidman) schedule (response-shock interval = 30 s) and responses on another lever produced 2 min of signaled timeout from avoidance on a variable-ratio 15 schedule. Following extended training (106 to 363 2-hr sessions), two experiments were conducted. In Experiment 1 two different methods of extinction were compared. In one session, all shocks were omitted, and there was some weakening of avoidance but little change in timeout responding. In another session, responding on the timeout lever was ineffective, and under these conditions timeout responding showed rapid extinction. The within-session patterns produced by extinction manipulations were different than the effects of drugs such as morphine, which also reduces timeout responding. In Experiment 2 shock was omitted for many consecutive sessions. Response rates on the avoidance lever declined relatively rapidly, with noticeable reductions within 5 to 10 sessions. Extinction of the timeout lever response was much slower than extinction of avoidance in all 4 rats, and 2 rats continued responding at baseline levels for more than 20 extinction sessions. These results show that lever pressing maintained by negative reinforcement can be highly resistant to extinction. The persistence of responding on the timeout lever after avoidance extinction is not readily explained by current theories.     

Response rate and changeover performance on concurrent variable-interval schedules

Six pigeons were exposed to variable-interval schedules arranged on one, two, three, and four response keys. The reinforcement rate was also varied across conditions. Numbers of responses, the time spent responding, the number of reinforcements, and the number of changeovers between keys were recorded. Response rates on each key were an increasing function of reinforcement rate on that key and a decreasing function of the reinforcement rate on other keys. Response and time-allocation ratios under-matched ratios of obtained reinforcements. Three sets of equations were developed to express changeover rate as a function of response rate, time allocation, and reinforcement rate respectively. These functions were then applied to a broad range of experiments in the literature in order to test their generality. Further expressions were developed to account for changeover rates reported in experiments where changeover delays were varied.     

Simple and conditional visual discrimination with wheel running as reinforcement in rats.

Three experiments explored whether access to wheel running is sufficient as reinforcement to establish and maintain simple and conditional visual discriminations in nondeprived rats. In Experiment 1, 2 rats learned to press a lit key to produce access to running; responding was virtually absent when the key was dark, but latencies to respond were longer than for customary food and water reinforcers. Increases in the intertrial interval did not improve the discrimination performance. In Experiment 2, 3 rats acquired a go-left/go-right discrimination with a trial-initiating response and reached an accuracy that exceeded 80%; when two keys showed a steady light, pressing the left key produced access to running whereas pressing the right key produced access to running when both keys showed blinking light. Latencies to respond to the lights shortened when the trial-initiation response was introduced and became much shorter than in Experiment 1. In Experiment 3, 1 rat acquired a conditional discrimination task (matching to sample) with steady versus blinking lights at an accuracy exceeding 80%. A trial-initiation response allowed self-paced trials as in Experiment 2. When the rat was exposed to the task for 19 successive 24-hr periods with access to food and water, the discrimination performance settled in a typical circadian pattern and peak accuracy exceeded 90%. When the trial-initiation response was under extinction, without access to running, the circadian activity pattern determined the time of spontaneous recovery. The experiments demonstrate that wheel-running reinforcement can be used to establish and maintain simple and conditional visual discriminations in nondeprived rats.     

Precurrent contingencies: Behavior reinforced by altering reinforcement probability for other behavior

The present study explored the effects of a precurrent contingency in which one (precurrent) activity increased the reinforcement probability for another (current) activity. Four human subjects responded on a two-key computer mouse. Each right-key press was reinforced (points exchangeable for money) with .02 probability. In one condition (no precurrent contingency), pressing the left key had no scheduled consequence; in another condition (precurrent contingency), pressing the left key increased the reinforcement probability for right-key responding to .08 for 15 s. Initial exposure to the precurrent contingency resulted in acquisition of precurrent left-key responding for 3 subjects, but for the 4th subject a special contingency was required. Right-key responding occurred at a high stable rate across the conditions. Changeovers to left-key responding dropped to near zero when the precurrent contingency was absent and were maintained at enhanced levels when the precurrent contingency was present. Contacts with the left key consisted of short response runs. Right-key responses were more frequently emitted within 15 s of a left-key response when the precurrent contingency was present, an efficient adaptation to the contingency. Continued research on precurrent behavior may produce insights into complex phenomena such as autoclitics and self-control.     

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.     

d-amphetamine and fixed-interval performance: effects of operant history.

Sixteen rats were initially exposed for 50 sessions to either a fixed-ratio 40 or an interresponse-time-greater-than-11-second food reinforcement schedule, then shifted to a fixed-interval 15-second food reinforcement schedule. Animals with fixed-ratio 40 histories lever pressed at much higher rates under the fixed-interval schedule than did animals with inter-response-time-greater-than-11-second histories. This difference persisted across 93 sessions of fixed-interval exposure. The effects of d=amphetamine were assessed after 15 and 59 sessions of fixed-interval exposure. On both occasions, the low-rate responding of animals with interresponse-time-greater-than-11-second histories was typically increased by all doses of the drug, while the high-rate responding of animals with fixed-ratio 40 histories was typically decreased by all doses of the drug. These results suggest that control response rate under the fixed-interval schedule, which may be affected by a history of responding under another schedule, is the primary determinant of the relative effects of d-amphetamine.     

Matching, contrast, and equalizing in the concurrent lever-press responding of rats

Five rats pressed levers for food reinforces delivered by several concurrent variable-interval variable-interval schedules. The rate of reinforcement available for responding on one component schedule was held constant at 60 reinforcers per hour. The rate of reinforcement available for responding on the other schedule varied from 30 to 240 reinforcers per hour. The behavior of the rats resembled the behavior of pigeons pecking keys for food reinforcers. The ratio of the overall rates of responding emitted under, and the ratio of the time spent responding under, the two components of each concurrent schedule were approximately equal to the ratio of the overall rates of reinforcement obtained from the components. The overall rate of responding emitted under, and the time spent responding under, the variable component schedule varied directly with the overall rate of reinforcement from that schedule. The overall rate of responding emitted under, and the time spent responding under, the constant component schedule varied inversely with the overall rate of reinforcement obtained from the variable component. The local rates of responding emitted under, and the local rates of reinforcement obtained from, the two components did not differ consistently across subjects. But they were not exactly equal either.     

Reinforcer magnitude (sucrose concentration) and the matching law theory of response strength

This experiment investigated the relationship between reinforcer magnitude (sucrose concentration) and response rate. The purpose was to evaluate the behavior of two parameters of an equation that predicts absolute response rate as a function of reinforcement rate and two free parameters. According to Herrnstein's (1970) theory of reinforced behavior, one parameter of this "response-strength equation" measures the efficacy of the reinforcer maintaining responding and the other parameter measures motoric components of response rate, such as response duration. Seven rats served as subjects. Experimental sessions consisted of a series of five different variable-interval schedules of reinforcement, each in effect for 5 minutes. Within each session, obtained reinforcement rates varied over more than a 30-fold range, from about 20 per hour to 700 per hour. The reinforcer was sucrose solution, and, between sessions, its concentration was varied from 0.0 to 0.64 molar (0 to 21.9%). For sucrose concentrations of 0.16 to 0.64 m, response rate was a negatively accelerated function of reinforcement rate. Increases in sucrose concentration increased response rates maintained by low but not high reinforcement rates. This pattern of changes corresponds to a change in the reinforcement-efficacy parameter of the response-strength equation. In contrast, the motor-performance parameter did not change as a function of sucrose concentration. These findings are inconsistent with the results of a similar study (Bradshaw, Szabadi, & Bevan, 1978) but support Herrnstein's theory of reinforced behavior.     

Bouts of responding from variable-interval reinforcement of lever pressing by rats

Four rats obtained food pellets by lever pressing. A variable-interval reinforcement schedule assigned reinforcers on average every 2 min during one block of 20 sessions and on average every 8 min during another block. Also, at each variable-interval duration, a block of sessions was conducted with a schedule that imposed a variable-ratio 4 response requirement after each variable interval (i.e., a tandem variable-time variable-ratio 4 schedule). The total rate of lever pressing increased as a function of the rate of reinforcement and as a result of imposing the variable-ratio requirement. Analysis of log survivor plots of interresponse times indicated that lever pressing occurred in bouts that were separated by pauses. Increasing the rate of reinforcement increased total response rate by increasing the rate of initiating bouts and, less reliably, by lengthening bouts. Imposing the variable-ratio component increased response rate mainly by lengthening bouts. This pattern of results is similar to that reported previously with key poking as the response. Also, response rates within bouts were relatively insensitive to either variable.     

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.