Effects of timeout on spaced responding in pigeons

Three pigeons were trained under a differential-reinforcement-of-low-rate schedule of 20 sec, and then exposed to a schedule under which responses terminating interresponse times less than 20 sec produced timeout and responses terminating interresponse times greater than 20 sec produced reinforcement. Response-produced timeouts selectively decreased the probability of short interresponse times and thereby produced a higher frequency of reinforcement. The suppressive effect of timeout was independent of timeout duration, with timeouts of 5, 10, or 20 sec. Similar effects were found when the minimum interresponse time that could be terminated by response-produced reinforcement was increased to 30 sec. The suppressive effects of timeout on responding maintained by these schedules were similar to previous reports in which responding was punished with electric shock.     

Reinforcement contingencies maintaining collateral responding under a DRL schedule

Two-key conjunctive schedules were studied with one key (food key) under a differential-reinforcement-of-low-rate 20-sec schedule, while the consequences of responding on another key (collateral key) were varied. When food depended not only upon a food-key interresponse time in excess of 20 sec, but also upon the occurrence of one or more collateral-key responses during the food-key interresponse time, the rate of collateral-key responding was low and food-key interresponse times rarely exceeded 20 sec. When collateral-key responses could produce a discriminative stimulus correlated with the availability of food under the DRL schedule, the discriminative stimulus functioned as a conditioned reinforcer to maintain higher rates of collateral-key responding, and the spacing of food-key responses increased. If the occurrence of the discriminative stimulus was independent of collateral-key responses, the rate of collateral-key responding was again low, but the spacing of food-key responses was still controlled by the discriminative stimulus. Both the conditioned reinforcer and the explicit reinforcement contingency could maintain collateral-key responding, but the adventitious correlation between collateral-key responses and the delivery of food could not maintain very much collateral-key responding. The pattern of responding on the food-key was determined to a much greater extent by the correlation between the discriminative stimulus and the delivery of food than by the pattern of responding on the collateral key.     

Two-key concurrent paced variable-interval paced variable-interval schedules of reinforcement

Nine pigeons were used in two experiments in which a response was reinforced if a variable-interval schedule had assigned a reinforcement and if the response terminated an interresponse time within a certain interval, or class, of interresponse times. One such class was scheduled on one key, and a second class was scheduled on a second key. The procedure was, therefore, a two-key concurrent paced variable-interval paced variable-interval schedule. In Exp. I, the lengths of the two reinforced interresponse times were varied. The relative frequency of responding on a key approximately equalled the relative reciprocal of the length of the interresponse time reinforced on that key. In Exp. II, the relative frequency and relative magnitude of reinforcement were varied. The relative frequency of responding on the key for which the shorter interresponse time was reinforced was a monotonically increasing, negatively accelerated function of the relative frequency of reinforcement on that key. The relative frequency of responding depended on the relative magnitude of reinforcement in approximately the same way as it depended on the relative frequency of reinforcement. The relative frequency of responding on the key for which the shorter interresponse time was reinforced depended on the lengths of the two reinforced interresponse times and on the relative frequency and relative magnitude of reinforcement in the same way as the relative frequency of the shorter interresponse time depended on these variables in previous one-key concurrent schedules of reinforcement for two interresponse times.     

The role of reinforcement in controlling sequential IRT dependencies

Sequential dependencies were investigated with two rats in a mixed and in a tandem differential-reinforcement-of-low-rate-responding schedule. In each schedule, 5-sec and 15-sec components were presented in fixed alternation. In the mixed schedule, a 5-sec interresponse time followed a 15-sec interresponse time and a 15-sec interresponse time followed a 5-sec interresponse time in predictable sequence. The correlation between prior and subsequent interresponse times, however, existed only when the prior interresponse time resulted in reinforcement. In the tandem schedule, an interresponse time greater than 5 sec in the differential-reinforcement-of-low-rate 5-sec component was not associated directly with reinforcement. One subject demonstrated sequential response patterns similar to those noted in the mixed schedule, even though the prior 5-sec interresponse time was not reinforced in the tandem schedule. The results indicate that the prior interresponse time length alone is not sufficient to influence the subsequent interresponse time length. Implications are, however, that a temporal response pattern arises when an interresponse interacts with schedule contingencies to control the interreinforcement interval.     

Two-component schedules of differential-reinforcement-of-low-rate

Two-component schedules of differential-reinforcement-of-low-rate were presented, where the contingencies specified separately two minimum interresponse times, t₁ and t₂, required for reinforcement, depending on whether the interresponse time was initiated by, in one case, a reinforced response (t₁) or, in the other, a nonreinforced response (t₂). A distinctive pattern of responding developed on each of the two contingencies. Duration of an interresponse time approximated t₁ when the t₁ contingency was in effect, and t₂ when the t₂ contingency was in effect. This relationship persisted even when t₂ was shorter than t₁, and responding at a higher rate on the t₁ contingency would have greatly increased the rate of reinforcement. Increasing the value of t₂ resulted in both longer interresponse times on the t₁ contingency, and a higher probability of a response-burst on those occasions when the contingency switched from t₁ to t₂. The results indicated that both reinforced and nonreinforced responses functioned as discriminative events in determining the duration of following interresponse times.     

An interresponse time analysis of variable-ratio punishment

An interresponse time analysis was used to study the effects of variable-ratio punishment schedules on the temporal pattern of reinforced responding. Twelve pigeons responded on a baseline variable-interval schedule of food reinforcement. A variable-ratio ten schedule of electric shock punishment was then introduced. The shock intensity was systematically increased to the highest intensity at which responding could be maintained. At this intensity, the mean variable-ratio value was increased and then decreased. Variable-ratio punishment resulted in an increased relative frequency of very short unreinforced interresponse times (response bursting). Increased response bursting accounted for instances of response rate facilitation. In addition, shock was followed by interresponse times of decreasing mean length over the first several responses after shock.     

A test of the effectiveness of the differential-reinforcement-of-low-rate schedule

Pigeons and rats were used in a yoked-control design that equated the reinforcement distributions of differential-reinforcement-of-low-rate and variable-interval schedules. Both a between-subjects design and a within-subjects design found response rate higher for the variable-interval schedule than for the differential-reinforcement-of-low-rate schedule, thus demonstrating the effectiveness of the differential-reinforcement-of-low-rate contingency. The interresponse-time distributions were unimodal for all subjects under the variable-interval schedule and bimodal for pigeons under the differential-reinforcement-of-low-rate schedule. The interresponse-time distributions for rats under the differential-reinforcement-of-low-rate schedule were also bimodal in three of four cases but the height of the modes at the shorter interresponse times were small in both absolute value and in relation to the height of the modes at the shorter interresponse times of the pigeons' distributions.     

Molar And Molecular Control In Variable-interval And Variable-ratio Schedules

Response rates are typically higher under variable-ratio than under variable-interval schedules of reinforcement, perhaps because of differences in the dependence of reinforcement rate on response rate or because of differences in the reinforcement of long interresponse times. A variable-interval-with-added-linear-feedback schedule is a variable-interval schedule that provides a response rate/reinforcement rate correlation by permitting the minimum interfood interval to decrease with rapid responding. Four rats were exposed to variable-ratio 15, 30, and 60 food reinforcement schedules, variable-interval 15-, 30-, and 60-s food reinforcement schedules, and two versions of variable-interval-with-added-linear-feedback 15-, 30-, and 60-s food reinforcement schedules. Response rates on the variable-interval-with-added-linear-feedback schedule were similar to those on the variable-interval schedule; all three schedules led to lower response rates than those on the variable-ratio schedules, especially when the schedule values were 30. Also, reinforced interresponse times on the variable-interval-with-added-linear-feedback schedule were similar to those on variable interval and much longer than those produced by variable ratio. The results were interpreted as supporting the hypothesis that response rates on variable-interval schedules in rats are lower than those on comparable variable-ratio schedules, primarily because the former schedules reinforce long interresponse times.     

Temporal patterns of responding in small fixed-ratio schedules

Pigeons were exposed to an ascending series of small fixed-ratio schedules from fixed-ratio 1 to 7. Two of those pigeons were later placed on a fixed-ratio 30 schedule. The two primary dependent variables were the postreinforcement pause and the interresponse time. Changes in these variables under small fixed ratios were sometimes opposite to changes reported with large fixed ratios. For example, postreinforcement pauses decreased in length as the fixed-ratio requirement increased from fixed-ratio 1 to fixed-ratio 3. Also, the interresponse times early in the small fixed-ratio schedule were shorter than those immediately preceding reinforcement. These findings question the role of interresponse-time reinforcement in determining temporal patterns of responding under small fixed-ratio schedules. They also suggest that there may be a limited region in which the independent variable, fixed-ratio size, does not operate as previously described.     

SUPPRESSIVE AND FACILITATIVE EFFECTS OF SHOCK INTENSITY AND INTERRESPONSE TIMES FOLLOWED BY SHOCK

Although response‐dependent shock often suppresses responding, response facilitation can occur. In two experiments, we examined the suppressive and facilitative effects of shock by manipulating shock intensity and the interresponse times that produced shock. Rats' lever presses were reinforced on a variable‐interval 40‐s schedule of food presentation. Shock followed either long or short interresponse times. Shock intensity was raised from 0.05 mA to 0.4 mA or 0.8 mA. Overall, shock contingent on long interresponse times punished long interresponse times and increased response rates. Shock contingent on short interresponse times punished short interresponse times and decreased response rates. In Experiment 1, raising the range of interresponse times that produced shock enhanced these effects. In Experiment 2, the effects of shock intensity depended on the interresponse times that produced shock. When long interresponse times produced shock, low intensities increased response rates. High intensities decreased response rates. When short interresponse times produced shock, high shock intensities punished short interresponse times and decreased response rates more than low intensities. The results may explain why punishment procedures occasionally facilitate responding and establish parameters for future studies of punishment.     

Changes in functional response units with briefly delayed reinforcement

In two experiments, key-peck responding of pigeons was compared under variable-interval schedules that arranged immediate reinforcement and ones that arranged unsignaled delays of reinforcement. Responses during the nominal unsignaled delay periods had no effect on the reinforcer presentations. In Experiment 1, the unsignaled delays were studied using variable-interval schedules as baselines. Relative to the immediate reinforcement condition, 0.5-s unsignaled delays decreased the duration of the reinforced interresponse times and increased the overall frequency of short (<0.5-s) interresponse times. Longer, 5.0-s unsignaled delays increased the duration of the reinforced interresponse times and decreased the overall frequency of the short interresponse times. In Experiment 2, similar effects to those of Experiment 1 were obtained when the 0.5-s unsignaled delays were imposed upon a baseline schedule that explicitly arranged reinforcement of short interresponse times and therefore already generated a large number of short interresponse times. The results support earlier suggestions that the unsignaled 0.5-s delays change the functional response unit from a single key peck to a multiple key-peck unit. These findings are discussed in terms of the mechanisms by which contingencies control response structure in the absence of specific structural requirements.     

Choice and behavioral patterning

Ten pigeons pecked left and right keys in a discrete-trials experiment in which access to food was contingent upon changeovers to the right key after particular runs of left-key pecks. In each of three sets of conditions, two run lengths were reinforced according to a concurrent variable-interval schedule: reinforcement followed runs of either 1 or 2, 1 or 4, or 2 or 4 left-key pecks preceding changeovers. The intertrial interval separating successive pecks was varied from .5 to 10.0 sec, and the relative frequency of reinforcement for the shorter of the two reinforced runs was varied from 0 to .75. The contingencies established local behavioral patterning that roughly approximated that required for reinforcement. For a fixed pair of reinforced run lengths, preference for the shorter of the two frequently increased as the intertrial interval increased and therefore as the minimum temporal durations of both reinforced runs increased. Preference for the shorter of the two also increased as its corresponding relative frequency of reinforcement increased. Both of these effects on preference were qualitatively similar to corresponding effects in previous research with two different kinds of reinforced behavioral patterns, interresponse times and interchangeover times. In all these experiments, analytical units were found in the temporal patterns of behavior, not in the behavior immediately contiguous with a reinforcer. It is suggested that a particular local temporal pattern of behavior is established to the extent to which it is repeatedly remembered when reinforcers are delivered, regardless of whether the delivery of a reinforcer is explicitly contingent upon that pattern.     

Behavior under large values of the differential-reinforcement-of-low-rate schedule

Pigeons pecked a key and rats pressed a lever for food reinforcement under large values of the differential-reinforcement-of-low-rate schedule. Each subject was tested under 10 different schedule values ranging from 1 to 45 min and was exposed to each schedule value at least twice. The mean interresponse time and mean interreinforcement time increased with the schedule value according to power functions. Response-probability functions were computed for schedule values below 20 min and showed an increase in response probability as a function of time since the last response in most cases. Mean responses per reinforcer increased as a function of schedule value for the rats, but decreased as a function of schedule value for the pigeons. The proportion of responses with interresponse times shorter than 1 sec were an increasing function of schedule value for the pigeons, but did not vary as a function of schedule value for the rats.     

Briefly delayed reinforcement: An interresponse time analysis

Key-peck responding of pigeons was compared under VI or DRL schedules arranging immediate reinforcement and briefly (.5 sec) delayed reinforcement. Delays were either signaled by a blackout in the chamber, unsignaled, or unsignaled with an additional requirement that responding not occur during the .5 sec interval immediately preceding reinforcement (response delay). Relative to the immediate reinforcement condition, response rates increased during the unsignaled delay, decreased during the signaled delay, and were inconsistent during the response delay condition. An analysis of interresponse times (IRTs) under the different conditions revealed a substantial increase in the frequency of short (0 to .5 sec) IRTs during the unsignaled condition and generally during the response delay conditions compared to that during the immediate reinforcement baseline. Signaled delays decreased the frequency of short (0 to .5 sec) IRTs relative to the immediate reinforcement condition. The results suggest that brief unsignaled delays and, in many instances, response delays increase the frequency of short IRTs by eliminating constraints on responding.     

The copyist model of response emission

The variety of different performances maintained by schedules of reinforcement complicates comprehensive model creation. The present account assumes the simpler goal of modeling the performances of only variable reinforcement schedules because they tend to maintain steady response rates over time. The model presented assumes that rate is determined by the mean of interresponse times (time between two responses) between successive reinforcers, averaged so that their contribution to that mean diminishes exponentially with distance from reinforcement. To respond, the model randomly selects an interresponse time from the last 300 of these mean interresponse times, the selection likelihood arranged so that the proportion of session time spent emitting each of these 300 interresponse times is the same. This interresponse time defines the mean of an exponential distribution from which one is randomly chosen for emission. The response rates obtained approximated those found on several variable schedules. Furthermore, the model reproduced three effects: (1) the variable ratio maintaining higher response rates than does the variable interval; (2) the finding for variable schedules that when the reinforcement rate varies from low to high, the response rate function has an ascending and then descending limb; and (3) matching on concurrent schedules. Because these results are due to an algorithm that reproduces reinforced interresponse times, responding to single and concurrent schedules is viewed as merely copying what was reinforced before.     

Response-rate differences in variable-interval and variable-ratio schedules: An old problem revisited

In Experiment 1, a variable-ratio 10 schedule became, successively, a variable-interval schedule with only the minimum interreinforcement intervals yoked to the variable ratio, or a variable-interval schedule with both interreinforcement intervals and reinforced interresponse times yoked to the variable ratio. Response rates in the variable-interval schedule with both interreinforcement interval and reinforced interresponse time yoking fell between the higher rates maintained by the variable-ratio schedule and the lower rates maintained by the variable-interval schedule with only interreinforcement interval yoking. In Experiment 2, a tandem variable-interval 15-s variable-ratio 5 schedule became a yoked tandem variable-ratio 5 variable-interval x-s schedule, and a tandem variable-interval 30-s variable-ratio 10 schedule became a yoked tandem variable-ratio 10 variable-interval x-s schedule. In the yoked tandem schedules, the minimum interreinforcement intervals in the variable-interval components were those that equated overall interreinforcement times in the two phases. Response rates did not decline in the yoked schedules even when the reinforced interresponse times became longer. Experiment 1 suggests that both reinforced interresponse times and response rate–reinforcement rate correlations determine response-rate differences in variable-ratio 10 and yoked variable-interval schedules in rats. Experiment 2 suggests a minimal role for the reinforced interresponse time in determining response rates on tandem variable-interval 30-s variable-ratio 10 and yoked tandem variable-ratio 10 variable-interval x-s schedules in rats.     

Local patterns of responding maintained by concurrent and multiple schedules

Local patterns of responding were studied when pigeons pecked for food in concurrent variable-interval schedules (Experiment I) and in multiple variable-interval schedules (Experiment II). In Experiment I, similarities in the distribution of interresponse times on the two keys provided further evidence that responding on concurrent schedules is determined more by allocation of time than by changes in local pattern of responding. Relative responding in local intervals since a preceding reinforcement showed consistent deviations from matching between relative responding and relative reinforcement in various postreinforcement intervals. Response rates in local intervals since a preceding changeover showed that rate of responding is not the same on both keys in all postchangeover intervals. The relative amount of time consumed by interchangeover times of a given duration approximately matched relative frequency of reinforced interchangeover times of that duration. However, computer simulation showed that this matching was probably a necessary artifact of concurrent schedules. In Experiment II, when component durations were 180 sec, the relationship between distribution of interresponse times and rate of reinforcement in the component showed that responding was determined by local pattern of responding in the components. Since responding on concurrent schedules appears to be determined by time allocation, this result would establish a behavioral difference between multiple and concurrent schedules. However, when component durations were 5 sec, local pattern of responding in a component (defined by interresponse times) was less important in determining responding than was amount of time spent responding in a component (defined by latencies). In fact, with 5-sec component durations, the relative amount of time spent responding in a component approximately matched relative frequency of reinforcement in the component. Thus, as component durations in multiple schedules decrease, multiple schedules become more like concurrent schedules, in the sense that responding is affected by allocation of time rather than by local pattern of responding.     

Effects of d-amphetamine and chlordiazepoxide on spaced responding in pigeons

The effects of d-amphetamine and chlordiazepoxide were studied in pigeons on performance (1) under a schedule that reinforced responses on a key (food key) if they were more than 20 sec apart, (2) under the same schedule when responses also were required on a collateral key during the interresponse time on the food key, and (3) under the same schedule when responses were required on a collateral key during the interresponse time on the food key and collateral-key responses could produce a stimulus correlated with the availability of food. Under all three spaced-responding schedules, d-amphetamine and chlordiazepoxide at low dose levels slightly increased the frequency of short interresponse times on the food key for about half the birds, and either did not affect the interresponse time patterns of the other birds, or lengthened the durations slightly. At higher dose levels, d-amphetamine and chlordiazepoxide increased the frequency of long interresponse times or abolished responding in all birds. Changes in the pattern of interresponse times on the food key did not seem to depend on changes in the rate or pattern of collateral-key responses.     

Sequential dependencies in free-responding

Three pigeons pecked for food in an experiment in which reinforcements were arranged for responses terminating sequences of interresponse times. Each reinforced interresponse time belonged to a class extending either from 1.0 to 2.0 sec (class A) or from 3.0 to 4.5 sec (class B). Reinforcements were arranged by a single variable-interval schedule and a random device that assigned each reinforcement to one of four sequences of two successive interresponse times: AA, AB, BA, or BB. Throughout the experiment, half of the reinforcements were delivered for interresponse times in class A and half for those in class B. Over conditions, the interresponse time preceding a reinforced interresponse time always, half of the time, or never, belonged to class A. The duration of the interresponse time preceding a reinforced one had a pronounced effect on response patterning. It also had a pronounced effect on the overall response probability, which was highest, intermediate, and lowest, when the interresponse time preceding a reinforced interresponse time always, half of the time, or never, belonged to class A, respectively. In no case were successive interresponse times independent, so that overall response probability was not representative of momentary response probabilities.     

Magnitude and frequency of reinforcement and frequencies of interresponse times

The relative magnitude and relative frequency of reinforcement for two concurrent interresponse times (1.5 to 2.5 sec and 3.5 to 4.5 sec) were simultaneously varied in an experiment in which pigeons obtained grain by pecking on a single key. Visual discriminative stimuli accompanied the two time intervals in which reinforcements were arranged by a one-minute variable-interval schedule. The resulting interresponse times of each of three pigeons fell into two groups; "short" (1.0 to 2.5 sec) and "long" (3.0 to 4.5 sec). Steady-state relative frequencies of these interresponse times were orderly functions of both reinforcement variables. The combined effects of both independent variables were well summarized by a linear function of one variable, relative access to food. Unlike corresponding two-key concurrent variable-interval schedules, the present schedule did not produce an equality between the relative frequency of an operant and either the relative magnitude or the relative frequency of reinforcement of that operant. A tentative account is provided for this difference between one-key and two-key functions.