The effect upon simple animal behavior of different frequencies of reinforcement, Part II: separate control of the reinforcement of different IRTs

Rats' responding was stabilized for over 35 days on 4-min variable-interval reinforcement. Reinforcements per hour for 4-sec wide classes of interresponse times were then separately controlled by adjusting those for each class to the variable-interval values that had just prevailed. This produced little or no change in interresponse times, indicating that the new procedure was substantially equivalent to a variable-interval schedule. The variable-interval schedule produced a high and stable conditional probability of interresponse times in the 0- to 4-sec class, associated with a peak in reinforcements per hour for this class. Reducing the reinforcements per hour for this class while raising that for another class (by 3.3 reinforcements per hour) significantly reduced the conditional probability of 0- to 4-sec interresponse times. Restoring the 3.3 reinforcements per hour to the 0- to 4-sec class significantly elevated the conditional probability of interresponse times in this class. Hence, it is concluded that the distribution of interresponse times produced by a subject during some variable-interval schedules is determined partly by the relative reinforcement of different interresponse times that the variable-interval schedule provided.Reprinted from Part II of the Final Report of Research under Contract DA-49-007-MD-408 with the Medical Research and Development Board, Office of the Surgeon General, Department of the Army, 31 December 1954. Edwin B. Newman, Responsible Investigator; Douglas Anger, Research Assistant and author of report. Experimental work done in the Psychological Laboratories of Harvard University.     

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.     

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.     

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 concurrent reinforcement of two interresponse times: absolute rate of reinforcement

Three pigeons obtained food on a one-key schedule of reinforcement for two concurrent, discriminated interresponse times. The overall rate of reinforcement was determined by a family of variable-interval schedules and by a continuous reinforcement schedule. The average frequency of reinforcement varied from 1.1 to 300 reinforcements per hour; the relative frequency of reinforcement for each of the two interresponse times was 0.5 throughout the experiment. The number of responses per minute increased sharply as the number of reinforcements per hour increased from 1 to 20. Beyond 30 reinforcements per hour, the curve was approximately flat, although it sometimes decreased slightly at the highest reinforcement rates. The relative frequency of the shorter interresponse time also increased sharply as the number of reinforcements per hour increased from 1 to 20. The asymptote of the relative frequency function approximately equalled the relative reciprocal of the length of the shorter interresponse time for reinforcement rates greater than 30 or 40 reinforcements per hour. This approximation was obscured by the response-rate function.     

Time allocation and response rate

Food reinforcement for key pecking by three pigeons was arranged by a variable-interval schedule and a device that assigned each reinforcement to one of 10 component response rates corresponding to 10 classes of equally reinforced interresponse times ranging from 1.0 to 6.0 sec in 0.5-sec classes. The overall number of reinforcements per hour was varied from one to more than 60. Overall response rate was a monotonically increasing, negatively accelerated function of the overall number of reinforcements per hour. This function was decomposed into two time-allocation functions: (1) the time allocated to all of the reinforced component response rates as a function of the total reinforcement rate, and, (2) the time allocated to a particular reinforced component response rate as a function of the reinforcement rate for that component. Asymptotic response rate was predicted by combining the asymptotes of the two separate time-allocation functions: virtually all of the time was spent responding, and the percentage of the time spent responding that was allocated to a particular reinforced component response rate roughly equalled the relative reinforcements per hour for that component.     

The reinforcement of four interresponse times in a two-alternative situation

Pigeons pecked for food in a two-key procedure. A concurrent variable-interval variable-interval schedule of reinforcement for two classes of interresponse times was arranged on each key. A visual stimulus set the occasion for potential reinforcement of the four operant classes: shorter and longer interresponse times on left and right keys. In Exp. I, the relative frequency of respones on a key equalled the relative frequency of reinforcement on that key. In Exp. II, the relative frequency of an interresponse time equalled the relative reciprocal of its length. In Exp. III, the relative frequency of an interresponse time was a monotonically increasing function of its relative frequency of reinforcement. These functions relating the relative frequency of an interresponse time to its relative length and to its relative frequency of reinforcement were the same as if there had been no second key. Also, the distribution of responses between keys was independent of the relative frequency of an interresponse time on either key. Experiment IV replicated Exp. I except that choices between keys were controlled by a stimulus that signalled the availability of reinforcement on the right key. A comparison of Exp. I and IV suggested that the relative frequency of an interresponse time on one key generally was independent of behavior on the other key, but that the number of responses per minute on a key did depend on behavior on the other key.     

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.     

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.     

Operant acceleration during a pre-reward stimulus

Stimuli of 20, 40, and 80 sec duration terminated with five non-response-contingent food pellets were superimposed upon lever pressing reinforced with single pellets on a DRL 30-sec schedule. Two rhesus monkeys served as subjects. No change in response frequency was observed during the 20- and 40-sec stimuli. During the 80-sec pre-food stimulus, overall response frequency increased to approximately 150% and 220% of pre-stimulus levels, and the temporal distributions of interresponse times shifted toward the shorter intervals. When the 80-sec stimulus was no longer terminated with food, the response frequency decreased and the temporal distributions of interresponse times gradually approached pre-stimulus levels. An increased frequency of short interresponse times and an increase in response rate was again observed when the pellet termination procedure was reinstituted with the 80-sec stimulus. No change in response frequency or interresponse times was observed in the absence of the conditioning stimulus, and performance efficiency, as reflected in the ratio of responses to reinforcements during non-stimulus periods, remained stable throughout the experiment.     

Time-allocation, matching, and contrast

A variable-interval schedule arranged food reinforcement for key pecking by pigeons on a single operandum at two rates, corresponding to two classes of reinforced interresponse times ranging from 1.5 to 2.5 sec and from 3.5 to 4.5 sec. The scheduled reinforcement rate for the higher component response rate was constant and equivalent to that of a variable-interval 4-min schedule. The scheduled reinforcement rate for the lower component response rate varied from zero to over 100 per hour. The number of occurrences of the constant component response rate varied inversely with the reinforcement rate for the variable component. This result, by definition a concurrent reinforcement interaction, or contrast, was the combined effect of two time-allocation functions, which together determine mean response rate: the time allocated to both component rates as a function of the total reinforcement rate, and the time allocated to a particular component rate as a function of the percentage of reinforcements for that component. The present experiment reveals a further parallel between the controlling relations for free responding on a single operandum and those for choice between two operanda; in each case, a concurrent reinforcement interaction can be found that corresponds to matching.     

The reinforcement of short interresponse times

Five contingencies were superimposed successively on a variable-interval schedule of reinforcement. In each of the resulting conditions, a different short, interresponse time was reinforced and an interresponse-time distribution was obtained from each of three pigeons. The lower bound of the reinforced interresponse times ranged from 0.3 to 2.4 sec. The resulting distributions were combined, according to a rationale based upon concurrent operants, induction, and a property of variable-interval schedules, to describe the interresponse-time distributions from a variable-interval schedule.     

The concurrent reinforcement of two interresponse times: the relative frequency of an interresponse time equals its relative harmonic length

The relative lengths of two concurrently reinforced interresponse times were varied in an experiment in which three pigeons obtained food by pecking on a single key. Visual discriminative stimuli accompanied the two time intervals in which reinforcements were scheduled according to a one-minute variable-interval. The steady-state relative frequency of an interresponse time approximately equalled the complement of its relative length, that is, its relative harmonic length. Thus, lengths of interresponse times and delays of reinforcement have the same effect on the relative frequencies of interresponse times and choices in one-key and two-key concurrent variable-interval schedules, respectively. A second experiment generalized further the functional equivalence between the effects of these one-key and two-key concurrent schedules by revealing that the usual matching-to-relative-immediacy in two-key concurrent schedules is undisturbed if reinforcement depends upon the occurrence of a response at the end of the delay interval, as it does in the one-key schedules. The results of both experiments are consistent with a quantitative theory of concurrent operant behavior.     

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.     

Molecular contingencies in schedules of intermittent punishment

In two experiments, key pecking of pigeons was maintained by a variable-interval 180-s schedule of food presentation. Conjointly, a second schedule delivered response-dependent electric shock. In the first experiment, shocks were presented according to either a variable-interval or a nondifferential interval-percentile schedule. The variable-interval shock schedule differentially delivered shocks following long interresponse times. Although the nondifferential shock schedules delivered shocks less differentially with respect to interresponse times, the two shock schedules equally reduced the relative frequency of long interresponse times. The second experiment differentially shocked long or short interresponse times in different conditions, with resulting decreases in the relative frequency of the targeted interresponse times. These experiments highlight the importance of selecting the appropriate level of analysis for the interaction of behavior and environment. Orderly relations present at one level of analysis (e.g., interresponse times) may not be revealed at other levels of analysis (e.g., overall response rate).     

Response rate, reinforcement frequency, and behavioral contrast

Pigeons responded for food on a multiple schedule in which periods of green-key illumination alternated with periods of red-key illumination. When behavior had stabilized with a variable-interval 2-min schedule of reinforcement operating during both stimuli, low rates of responding (interresponse times greater than 2 sec) were differentially reinforced during the green component. Conditions during the red stimulus were unchanged. Response rates during the green component fell without changing the frequency of reinforcement but there were no unequivocal contrast effects during the red stimulus. The frequency of reinforcement during the green component was then reduced by changing to a variable-interval 8-min schedule without reducing the response rates in that component, which were held at a low level by the spacing requirement. Again, the conditions during the red stimulus were unchanged but response rates during that stimulus increased. These results show that reductions in reinforcement frequency, independently of response rate, can produce interactions in multiple schedules.     

A local-rate-of-response and interresponse-time analysis of behavioral contrast

Three pigeons were exposed to a two-component multiple schedule in which a variable-interval 3-min schedule was always in effect in one component. The schedule in the other component was either variable-interval 3-min or extinction in alternate blocks of sessions. When the schedule was changed from multiple variable-interval 3-min variable-interval 3-min to multiple variable-interval 3-min extinction in the second and fourth phases of the experiment, overall response rates in the unchanged variable-interval 3-min component increased in two pigeons. Response rate declined when the schedule was changed to multiple variable-interval 3-min variable-interval 3-min again. Correlated with increases in overall response rate in the unchanged component were increases in local response rates at the beginning of the unchanged component and immediately after food presentation. Local rates 40 sec after food presentation did not increase greatly in the presence of the multiple variable-interval 3-min extinction schedule. An interresponse time analysis of three local rate samples showed small increases in the relative frequency of short-duration interresponse times at the beginning of the unchanged component and immediately after food presentation. Neither the postreinforcement pause nor the latency to the first response in the unchanged component changed systematically.     

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.     

Interresponse-time analysis of stimulus control in multiple schedules

Interresponse-time distributions were recorded in two components of multiple variable-interval schedules that were varied over several conditions. Values of the exponent for power functions relating ratios of interresponse times emitted per opportunity to ratios of reinforcers obtained in the two components varied with interresponse-time class interval. The exponent (sensitivity to reinforcement) afforded a measure of stimulus control exerted by the discriminative stimuli. Exponents were near zero for short interresponse times, consistent with previous conclusions that responses following short interresponse times are controlled by response-produced or proprioceptive stimuli. Values of exponents increased with longer interresponse times, indicating strong control by exteroceptive stimuli over responses following interresponse times of approximately one second or longer.     

Choice behavior and the accessibility of the reinforcer

In Experiment 1, matching of relative response rates to relative rates of reinforcement was obtained in concurrent variable-interval schedules when the absolute values of the two concurrent variable-interval schedules varied from 6 sec and 12 sec to 600 sec and 1200 sec. Increases in the duration of the changeover delay, however, produced decreases in the relative response rates and, consequently, some deviation from matching. In Experiment 2, matching of relative response rates to the relative duration of the reinforcer failed to occur when the equal variable-interval schedules arranging access to the two different reinforcer durations (1.5 and 6 sec) were varied in size from concurrent variable-interval 10-sec schedules to concurrent variable-interval 600-sec schedules.