Rates of responding in the pigeon generated by simple and complex schedules which provide the same rates of reinforcement

Four pigeons pecked for food reinforcement on variable interval 1-min schedules and on the variable-interval 1-min components of multiple, concurrent, and pseudoconcurrent schedules. The pseudoconcurrent schedule provided only one schedule of reinforcement; but, any reinforcer could be collected by responding on either of two keys. The rate of responding generated by the variable interval schedule was not greater than the rates of responding generated by the components of the complex schedules. But, the rate of reinforcement obtained from the variable interval schedule was greater than the rates of reinforcement obtained from the components of the multiple schedule. These results may contradict the equation proposed by Herrnstein (1970). The equation predicts that the rate of responding generated by a schedule of reinforcement will be greater when the schedule appears alone, than when it appears as one component of a complex schedule.     

Differences between rates of responding emitted during simple and multiple schedules

Pigeons pecked keys for food reinforcers delivered by several variable-interval and multiple variable-interval schedules. The rates of responding emitted during the simple schedules were not systematically different from the rates emitted during the multiple schedules when the components of the multiple schedule were identical. The rates of responding emitted during the components were usually greater than the rates emitted during comparable simple schedules when the components were more favorable than the added components of the multiple schedules. Response rates during the components were not significantly lower than those during comparable simple schedules when the components were less favorable. The observation of higher rates of responding during the more favorable components conforms to a prediction of several additive theories (e.g., Rachlin, 1973) but violates a prediction of Herrnstein’s (1970) theory. However, the additive theories are brought into question by the fact that changing the location of the discriminative stimuli did not change the pattern of results.     

Positive behavioral contrast as a function of time-out duration when pigeons peck keys on a within-session procedure

Three pigeons pecked keys for food reinforcers delivered by multiple variable interval variable interval schedules in the first part of each session (baseline) and by multiple variable interval extinction schedules in the second part of each session (contrast). The variable interval schedules delivered reinforcers after an average of 4 min or 30 sec in different conditions. The duration of a time-out between the components varied in five steps from 5 to 120 sec. Positive contrast occurred for all time-out durations in both experiments. That is, the rate of responding emitted during the constant, variable interval component was greater during the contrast than during the baseline schedules. The size of contrast did not change systematically with changes in timeout duration. These results violate most theories of contrast. They are compatible with the idea that animals integrate reinforcers over intervals longer than 2 min.     

Rate of reinforcement and session duration as determinants of within-session patterns of responding

Rats pressed levers for Noyes pellets or keys for sweetened condensed milk reinforcers delivered by multiple schedules. Session length and baseline rates of reinforcement were varied in two experiments. Rates of responding increased during the early part of the session and then decreased for both responses and reinforcers, as well as for all subjects and values of the independent variables. Changes in response rates across the session sometimes exceeded 500%. Respoiise rates peaked approximately 20 min after the beginning of the session, regardless of session duration, when subjects responded on a multiple variable interval 1-min variable interval 1-min schedule. The function was flatter for longer sessions than it was for shorter sessions. The function was flatter, more symmetrical, and peaked later for lower rates of reinforcement than for higher rates of reinforcement. The function appeared early in training, and further experience moved and reduced its peak. Variables related to reinforcement exerted more control over some aspects of this function than did variables related to responding. These within-session patterns of responding may have fundamental implications for experimental design and theorizing.     

Prediction of concurrent keypeck treadle-press responding from simple schedule performance

Four pigeons pecked keys and pressed treadles for food reinforcers delivered by several variable-interval schedules of reinforcement. Then the subjects responded on several concurrent schedules. Keypecking produced reinforcers in one component, and treadle-pressing produced reinforcers in the other. The changeover delay, which prevented reinforcement after all switches from one response to the other, was 0, 5, or 20 sec long. An equation proposed by Kerrnstein (1970) described the rates of treadle-pressing and keypecking emitted during the variable-interval schedules. The k parameter of this equation was larger for keypecking than for treadle-pressing. The R₀ parameters were not systematically different for the two responses. The rates of keypecking and treadle-pressing emitted during the components of the concurrent schedules correlated with, but were not equal to, the rates of responding predicted by Herrnstein’s equation and the subject’s simple schedule responding. The ratios of the rates of responding emitted during, and the ratios of the time spent responding on, the components of the concurrent schedules conformed to an equation proposed by Baum (1974), but not to Herrnstein’s equation.     

Within-session changes in responding during concurrent schedules that employ two different operanda

Five rats responded on several concurrent schedules in which pressing a key produced reinforcers in one component and pressing a lever produced reinforcers in the other component (Experiment 1). Four pigeons responded on several concurrent keypeck treadlepress schedules (Experiment 2). The programmed rates of reinforcement varied from 15 to 240 reinforcers per hour in different conditions. Rates of responding usually changed systematically within experimental sessions, and the changes were similar for the two components of a concurrent schedule. These results imply that within-session changes in responding may not confound the predictions of theories that describe the ratio of the rates of responding during the two components of concurrent schedules. Instead, within-session changes may be controlled by a mechanism that integrates the reinforcers obtained from the two components.     

The relation of multiple-schedule behavioral contrast to deprivation,time in session,and within-session changes in responding

Pigeons' keypecking was reinforced by food on baseline schedules of multiple variable interval (VI) x VI x and on contrast schedules of multiple VI x VI y. Deprivation of food was varied by maintaining subjects at 75%, 85%, and 95% (+/- 2%) of their free-feeding weights. Positive and negative behavioral contrast were observed. The size of the contrast was not systematically altered by changes in deprivation. Positive and negative contrast were both larger later in the session than they were earlier. Within-session decreases in responding were steeper for the baseline than for the contrast schedules for positive contrast. Within-session decreases were steeper for the contrast than for the baseline schedules for negative contrast. These results were predicted by the idea that different amounts of habituation to the reinforcer during the baseline and contrast schedules contribute to behavioral contrast. The results show that contrast occurs under conditions that reduce the effect of the following component. The results support the assumption that positive and negative contrast are produced by symmetrical theoretical variables.     

Responding on a multiple variable-time variable-time schedule as a function of component duration

Five pigeons pecked lighted keys for food reinforcers delivered by a multiple variable-time 30-sec variable-time 2-min schedule. The duration of the components varied from 5 sec to 16 min. The rate of responding generated by the more favorable component schedule decreased as component duration increased to an intermediate value and then increased with additional increases in duration. The decrease confirmed a prediction of additive theories of behavioral contrast. The rate of responding generated by the less favorable component did not increase as component duration increased. This decrease may represent a floor effect or it may violate a prediction of one additive theory of contrast.     

Sum of responding as a function of sum of reinforcement on two-key concurrent schedules

Four pigeons pecked for food reinforcers delivered by several two-key concurrent schedules. The sum of the rates of reinforcement provided by the component schedules varied from 25 to 300 reinforcers/h. The ratio of the rates of reinforcement remained constant at 1:4. The sum of the rates of responding generated by the component schedules increased with increases in the sum of the rates of reinforcement which the components provided. The increase in response rate was predicted by equations proposed by Catania (1963) and by Herrnstein (1970). But the data conformed more closely to Herrnstein’s equation.     

Effect on subsequent fixed-interval schedule performance of prior exposure to ratio and interval schedules of reinforcement

In three experiments, we examined the effect on the patterns of responding noted on fixed interval (FI) schedules of prior exposure to a range of interval and ratio schedules. Rats leverpressed for food reinforcement on random ratio (RR), random interval (RI), or variable interval (VI) schedules prior to transfer to FI schedules. In Experiment 1, prior exposure to an RR schedule retarded the development of typical FI patterns of responding. Exposure to a yoked RI schedule produced even greater retardation of typical FI performance. This effect was replicated in Experiment 2, using a within-subjects design. Rats responded on a multiple RR-RI schedule prior to a multiple FI-FI schedule. Typical FI performance emerged more slowly in the component previously associated with the RI than with that associated with the RR. In Experiment 3, exposure to an RR schedule retarded the development of FI performance to a greater extent than did exposure to a VR schedule. The latter schedule was programmed to allow the possibility that inhibitory control would develop after reinforcement. These results confirm that ratio schedules independently result in the disruption of FI responding. This effect was not long lasting and cannot be used plausibly to explain species differences in responding to FI schedules. However, it does suggest that temporal control—as manifested by the transfer of inhibitory control from one schedule to another—could facilitate movement between interval schedules.     

Behavioral contrast as a function of component duration for leverpressing using a within-session procedure

Ten rats pressed levers for food reinforcers delivered by multiple schedules. Behavioral contrast was measured using a within-session procedure that presented the baseline and contrast schedules within single sessions. The absolute sizes of both positive and negative contrast increased and then decreased as components lengthened. Negative induction occurred when components were very short. These results question theories that predict that the size of contrast will vary inversely with component duration. They support theories that attribute positive and negative contrast to similar theoretical mechanisms. A comparison of the present results with those of past studies indicates that keypecking by pigeons and leverpressing by rats change as different functions of component duration. Treadlepressing by pigeons and leverpressing by rats change as similar functions. These findings challenge general process theories that argue that all responses obey the same behavioral laws.     

Rats show no preference between free and earned water in an advance-response procedure

Although an arbitrarily specified instrumental response may persist when free reinforcers are concurrently available, the interpretation that earned reinforcers are preferred is tenuous. The present advance-response procedure used both time allocation and advance response rates as indices of preference between free and earned water in rats. When multiple schedule components were two response-dependent schedules with different overall reinforcement rates, higher rates of reinforcement were preferred. However, when the multiple schedule consisted of response-dependent and response-independent components equated for overall rates of reinforcement, no consistent preference for free or earned reinforcers was evident. That a preference for free reinforcers was not obtained is difficult to reconcile with concepts of least effort.     

Preference for the interval schedule following multiple variable-ratio yoked-variable-interval schedule training

Pigeons were studied on multiple variable-ratio yoked-variable-interval schedules in which components had equal rates of food reinforcement and appeared equally often on each of two keys. Interpolated between component changes on the final multiple schedule were 10-sec probes in which both schedule stimuli were present, one on each key. During multiple schedule training, variable-ratio response rates were greater than yoked-variable-interval rates; however, response rate differences in the components were not a function of the mean ratio value for the 40-to-320-ratio range studied. During the choice probes, subjects responded more to the stimulus associated with the interval schedule than to the one associated with the ratio schedule. It was concluded that pigeons prefer interval schedules over equal reinforcement rate ratio schedules, because the former generate fewer responses per reinforcement.     

The effect of rate of reinforcement and time in session on preference for variability

Pigeons pecked keys on concurrent-chains schedules that provided a variable interval 30-sec schedule in the initial link. One terminal link provided reinforcers in a fixed manner; the other provided reinforcers in a variable manner with the same arithmetic mean as the fixed alternative. In Experiment 1, the terminal links provided fixed and variable interval schedules. In Experiment 2, the terminal links provided reinforcers after a fixed or a variable delay following the response that produced them. In Experiment 3, the terminal links provided reinforcers that were fixed or variable in size. Rate of reinforcement was varied by changing the scheduled interreinforcer interval in the terminal link from 5 to 225 sec. The subjects usually preferred the variable option in Experiments 1 and 2 but differed in preference in Experiment 3. The preference for variability was usually stronger for lower (longer terminal links) than for higher (shorter terminal links) rates of reinforcement. Preference did not change systematically with time in the session. Some aspects of these results are inconsistent with explanations for the preference for variability in terms of scaling factors, scalar expectancy theory, risk-sensitive models of optimal foraging theory, and habituation to the reinforcer. Initial-link response rates also changed within sessions when the schedules provided high, but not low, rates of reinforcement. Within-session changes in responding were similar for the two initial links. These similarities imply that habituation to the reinforcer is represented differently in theories of choice than are other variables related to reinforcement.     

Behavioral contrast and type of reward: Role of elicited response topography

Groups of pigeons were exposed to multiple variable-interval variable-interval and multiple variable-interval extinction schedules of either food or water reinforcement for keypecking. Discriminative stimuli associated with component schedules were located either on the operant key or on a second “signal” key. When the stimuli were projected on the operant key, positive contrast appeared during discrimination conditions with either food or water as the reinforcer. When the stimuli were projected on the signal key, overall responding to the operant and signal keys showed contrast with food, but negative induction with water as the reinforcer. In the latter condition, the signal for the variable-interval shcedule of water reinforcement elicited a variety of water-related behavior, only some of which was directed at the signal. Thus, the type of reward and location of discriminative stimuli interacted to determine the presence or absence of behavioral contrast effects. In large part, these results support and extend the autoshaping view of contrast.     

Conjunctive schedules of reinforcement IV: Effects on the pattern of responding of changes in requirement at reinforcement

The effects of different schedule requirements at reinforcement on patterns of responding by pigeons were assessed under conjunctive schedules with comparable response-number requirements, Under one conjunctive schedule (conjunctive fixed-interval fixed-ratio schedule), a response was reinforced after a 6-min interval had elapsedand a specific minimum number of responses had been emitted, Under a second conjunctive schedule, a response was reinforced after the 6-min fixed interval and upon completion of a tandem schedule requirement (conjunctive fixed-interval tandem schedule), This schedule retained the same required minimum number of responses as the first conjunctive schedule, but responses were never reinforced according to a fixed-ratio schedule; the tandem schedule was comprised of a fixed-ratio and a small (.1 to 10.0 sec) fixed-interval schedule, Under the conjunctive fixed-interval fixed-ratio schedule, responding was characterized by an initial pause, an abrupt transition to a high response rate, and a second transition to a lower rate that prevailed or slightly increased up to reinforcement, Under the conjunctive fixed-interval tandem schedule, pauses were extended, response rates were lower, and the initial high rate of responding was generally absent, The above effects depended upon the size of the fixed interval of the tandem schedule, The distinct pattern of responding generated by conjunctive fixed-interval fixed-ratio schedules depends upon occasional reinforcement of fixed-ratio responding and not merely on the addition of a minimum number of required responses.     

Excitatory and inhibitory effects of free reinforcers

Rats’ responding was maintained on a random-interval 1-min food schedule. In addition, non-contingent pellets were delivered, independently of the animals’ behavior, at either fixed intervals (Experiment 1) or at random intervals (Experiment 2). As the rate of delivery of the periodic and aperiodic free reinforcers increased, the rate of responding decreased. But these free reinforcers, in addition to having this inhibitory effect, had also a local excitatory effect upon responding: lever-pressing increased to a level above its mean rate following the delivery of a free food pellet. The time course of this behavioral aftereffect of free reinforcers, for both fixed and random intervals, was dependent upon the proportion of the interval between successive free food deliveries. The relation of these results to those obtained with response-contingent reinforcement, free punishment, and in schedule-induced phenomena is discussed.     

Stimulus control in multiple variable-ratio schedules of reinforcement

One component of a multiple variable-ratio 150 variable-ratio 150 schedule remained unchanged while the reinforcement schedule of the other component was periodically changed to extinction and then returned to variable ratio 150. When the reinforcement schedule of the changed component was an unmodified variable ratio 150 schedule, the magnitude of negative contrast during baseline recovery was equal to the magnitude of positive contrast observed during the previous change from multiple variable-ratio 150 variable-ratio 150 to multiple variable-ratio 150 extinction. When the schedule of the changed component was modified during baseline recovery so that responses terminating interresponse times greater than the average baseline interresponse time in the unchanged component were not reinforced, the magnitude of the unchanged-component response rate decrease was reduced. The magnitude of negative contrast was attenuated even though response and reinforcement rates in the variable component increased to levels at or above their prior multiple variable-ratio 150 variable-ratio 150 baseline levels. The results support a general theory that negative contrast results from both (1) the removal of certain positive-contrast-producing operations and (2) changes in the interresponse time-reinforcer relations that occur as a byproduct of schedule manipulations.     

Contrast during multiple schedules with different component response requirements

Positive and negative behavioral contrast were examined when pigeons were required to keypeck in one component and treadle press in the other component of a series of multiple schedules. The experimental conditions were constructed so that the positive and negative contrast groups were exposed to complementary conditions. Positive keypeck and negative treadle-press contrast occurred in all subjects. Positive treadle-press contrast seemed to depend on the order of schedule presentation. Only one subject exhibited strong negative keypeck contrast. The results indicate that symmetrical conditions are not sufficient to produce positive and negative contrast for a given response when topographically different responses are used in the components of a multiple schedule. The results are globally consistent with the competition theory of behavioral contrast.