Choice as a function of local versus molar reinforcement contingencies.

Rats were trained on a discrete-trial probability learning task. In Experiment 1, the molar reinforcement probabilities for the two response alternatives were equal, and the local contingencies of reinforcement differentially reinforced a win-stay, lose-shift response pattern. The win-stay portion was learned substantially more easily and appeared from the outset of training, suggesting that its occurrence did not depend upon discrimination of the local contingencies but rather only upon simple strengthening effects of individual reinforcements. Control by both types of local contingencies decreased with increases in the intertrial interval, although some control remained with intertrial intervals as long as 30 s. In Experiment 2, the local contingencies always favored win-shift and lose-shift response patterns but were asymmetrical for the two responses, causing the molar reinforcement rates for the two responses to differ. Some learning of the alternation pattern occurred with short intertrial intervals, although win-stay behavior occurred for some subjects. The local reinforcement contingencies were discriminated poorly with longer intertrial intervals. In the absence of control by the local contingencies, choice proportion was determined by the molar contingencies, as indicated by high exponent values for the generalized matching law with long intertrial intervals, and lower values with short intertrial intervals. The results show that when molar contingencies of reinforcement and local contingencies are in opposition, both may have independent roles. Control by molar contingencies cannot generally be explained by local contingencies.     

Matching, maximizing, and the behavioral unit: concurrent reinforcement of response sequences.

Pigeons pecked two keys in a probability matching situation in which four two-peck sequences were intermittently reinforced: left-left, left-right, right-left and right-right. In Phase 1, relative reinforcement rate was varied with respect to the first response of a sequence: reinforcers were differentially assigned for left-left and left-right sequences as opposed to right-left and right-right sequences. The second response of reinforced sequences occurred equally on the left and right keys across conditions. In Phase II, relative reinforcement rate was varied for sequences that involve an alternation as opposed to those that did not. The relative outputs of the different sequences matched the relative reinforcement rates for the different sequences in both phases. Relative response rates for key pecks did not always match relative reinforcement rates. The intertrial interval separating responses was varied in both phases; increases in the intertrial interval affected the relative frequency of different sequences. The results demonstrate that response sequences acted as functional units influencing choice and thus support a structural account of choice. At the same time, the matching of relative sequence proportion and relative reinforcement rate supports a matching account.     

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.     

Unsignalled delay of reinforcement in variable-interval schedules

Three pigeons responded on several tandem variable-interval fixed-time schedules in which the value of the fixed-time component was varied to assess the effects of different unsignalled delays of reinforcement. Actual (obtained) delays between the last key peck in an interval and reinforcement were consistently shorter than the nominal (programmed) delay. When nominal delays were relatively short, response rates were higher during the delay condition than during the corresponding nondelay condition. At longer nominal delay intervals, response rates decreased monotonically with increasing delays. The results were consistent with those obtained from delay-of-reinforcement procedures that impose either a stimulus change (signal) or a no-response requirement during the delay interval.     

Timeout from concurrent schedules.

Response-contingent timeouts of equal duration and frequency were added to both alternatives of unequal concurrent schedules of reinforcement. For each of 4 pigeons in Experiment 1, relative response rates generally became less extreme as the frequency of timeout increased. In Experiment 2, relative response rates consistently approached indifference as the duration of timeout was increased. Variation in time allocation was less consistent in both experiments. Absolute response rates did not vary with the timeout contingency in either experiment. In a third experiment, neither measure of choice varied systematically when the duration of a postreinforcement blackout was varied. In contrast to the present results, preference has been shown to vary directly with the parameters of shock delivery in related procedures. The pattern of results in the first two experiments follows that obtained with other manipulations of the overall rate of reinforcement in concurrent schedules. The results of the third experiment suggest that an intertrial interval following reinforcement is not a critical feature of the overall rate of reinforcement.     

Discrimination and generalization by rats of temporal stimuli lasting for minutes

In three experiments, rats learned to respond differentially to reinforced and nonreinforced trials when the length of the intertrial interval (ITI) predicted the trial outcome (reinforcement or nonreinforcement). Rats in control groups, for whom the length of the ITI did not predict the outcome, did not show differential performance on nonreinforced and reinforced trials. Generalization gradients obtained following discrimination training were comparable to those obtained following discrimination training with other types of discriminative stimuli. That is, groups which had shown differential performance in discrimination training yielded generalization gradients with fastest speeds at the previously reinforced ITIs, slowest speeds at the previously nonreinforced ITIs, and intermediate speeds at ITIs of intermediate length. Control groups yielded flat gradients across all ITIs tested. These effects were shown for relative time discrimination (short time = reinforcement, long time = nonreinforcement, or the reverse) and also for an absolute time discrimination (long and short time = reinforcement, middle time = nonreinforcement or the reverse). This method was effective for time duration measured in minutes rather than seconds, as is more commonly the case.     

Response requirements as constraints on output

Two experiments studied how added response requirements affected fixed-interval schedule performance. Experiment 1 involved tandem fixed-interval fixed-ratio schedules, and Experiment 2 studied conjunctive fixed-interval fixed-ratio schedules. In both, pigeons' output, defined as overall response rate or as responses during the interval, first increased and then decreased as the ratio was raised. With small ratio requirements, the frequency of reinforcement in time either did not change or decreased slightly. With progressively larger ratios, reinforcement frequency decreased consistently. Alternative explanations were discussed. The first, a reinforcement theory account, was that response strength is an increasing monotonic function of both the response requirement and reinforcement frequency, and the bitonic output function represents interacting effects. Increases in the response requirement accompanied by small changes in reinforcement frequency enhance output, but further increases result in large enough decrements in reinforcement frequency so that output is lowered. The second explanation does not view reinforcement as a basic process but, instead, derives from concepts of economics and conservation. Organisms allocate their behavior among alternatives so as to maximize value, where value is a function of the responses that can occur in a given situation under the set of restrictions imposed by particular schedules. One form of this theory explicitly predicts that output is a bitonic function of ratio requirements in simple ratio schedules. However, it was not clear that this model could explain the present effects involving joint ratio and interval schedule restrictions.     

Molar optimization versus delayed reinforcement as explanations of choice between fixed-ratio and progressive-ratio schedules.

In a discrete-trials procedure, pigeons chose between a fixed-ratio 81 schedule and a progressive-ratio schedule by making a single peck at the key correlated with one or the other of these schedules. The response requirement on the progressive-ratio schedule began at 1 and increased by 10 each time the progressive-ratio schedule was chosen. Each time the fixed-ratio schedule was chosen, the requirement on the progressive-ratio schedule was reset to 1 response. In conditions where there was no intertrial interval, subjects chose the progressive-ratio schedule for an average of about five consecutive trials (during which the response requirement increased to 41), and then chose the fixed-ratio schedule. This ratio was larger than that predicted by an optimality analysis that assumes that subjects respond in a pattern that minimizes the response-reinforcer ratio or one that assumes that subjects respond in a pattern that maximizes the overall rate of reinforcement. In conditions with a 25-s or 50-s intertrial interval, subjects chose the progressive-ratio schedule for an average of about eight consecutive trials before choosing the fixed-ratio schedule. This change in performance with the addition of an intertrial interval was also not predicted by an optimality analysis. On the other hand, the results were consistent with the theory that choice is determined by the delays to the reinforcers delivered on the present trial and on subsequent trials.     

Intertrial effects of same-different judgements

Specific intertrial effects (repetition effects) and general intertrial effects (refractoriness or persisting attention to the preceding trial) were studied with the same-different judgment task, which dissociates the effects of response repetition and stimulus repetition. Response repetition alone did not facilitate performance. Stimulus repetition did aid performance, but mainly when accompanied by response repetition. Subjects tended to avoid the normal comparison process by using the (invalid!) “bypass rule” (Fletcher and Rabbitt, 1978): repeat the response if the stimulus or some aspect thereof (letter contents, size, position) is repeated from the preceding trial, otherwise change the response. As to general effects, partial refractoriness was evident at response execution, but not at earlier processing stages. Mean RT increased, but errors decreased, as the response-stimulus interval (RSI) between trials decreased. Presenting a new letter pair immediately after the preceding response produced a delay, but subjects used the waiting time, while the response system recovered or was redirected to the present trial, to improve the accuracy of their decision.     

Conditioned reinforcement and discrimination performance

Pigeons were trained in a three-key chamber to peck one side key in the presence of a vertical line on the center key and to peck the other side key in the presence of a horizontal line. Correct choice responses were reinforced with food according to fixed- and variable-ratio, fixed-interval, and differential-reinforcement-of-long-latency schedules of reinforcement. For each schedule, the birds performed under each of two conditions: (1) each correct choice response produced a brief presentation of stimuli intermittently paired with food, then the next trial; (2) each correct choice response produced an intertrial interval only. For all schedules except one long latency schedule, response rates were higher under the condition of brief stimulus presentation than under the comparable control condition. Presentation of brief magazine stimuli increased choice accuracy. The amount of change in accuracy was correlated with the rate of food presentation. Performance under the schedules with highest food reinforcement rates showed no enhancement; performance under the schedules with the lowest reinforcement rates showed the greatest enhancement.     

Influences of delay and rate of reinforcement on discrete-trial choice

An adjusting procedure was used to measure pigeons' preferences among alternatives that differed in the duration of a delay before reinforcement and of an intertrial interval (ITI) after reinforcement. In most conditions, a peck at a red key led to a fixed delay, followed by reinforcement, a fixed ITI, and then the beginning of the next trial. A peck at a green key led to an adjustable delay, reinforcement, and then the next trial began without an ITI. The purpose of the adjusting delay was to estimate an indifference point, or a delay that made a subject approximately indifferent between the two alternatives. As the ITI for the red key increased from 0 s to 60 s, the green-key delay at the indifference point increased systematically but only slightly. The fact that there was some increase showed that pigeons' choices were controlled by more than simply the delay to the next reinforcer. One interpretation of these results is that besides delay of reinforcement, rate of reinforcement also influenced choice. However, an analysis that ignored reinforcement rate, but considered the delays between a choice response and the reinforcers on subsequent trials, was able to account for most of the obtained increases in green-key delays. It was concluded that in this type of discrete-trial situation, rate of reinforcement exerts little control over choice behavior, and perhaps none at all.     

Context effects on choice.

Four pigeons responded on a concurrent-chains schedule in four experiments that examined whether the effectiveness of a stimulus as a conditioned reinforcer is best described by a global approach, as measured by the average interreinforcement interval, or by a local contextual approach, as measured by the onset of the stimulus preceding the conditioned reinforcer. The interreinforcement interval was manipulated by the inclusion of an intertrial interval, which increased the overall time to reinforcement but did not change the local contingencies on a given trial A global analysis predicted choice for the richer alternative to decrease with the inclusion of an intertrial interval, whereas a local analysis predicted no change in preference. Experiment 1 examined sensitivity to intertrial intervals when each was signaled by the same houselight that operated throughout the session. In Experiment 2, the intertrial interval always was signaled by the stimulus correlated with the richer terminal link. In Experiment 3, the intertrial interval was signaled by the keylights correlated with the initial links and two novel houselights. Experiment 4 provided free food pseudorandomly during the intertrial interval. In all experiments, subjects' preferences were consistent with a local analysis of choice in concurrent chains. These results are discussed in terms of delay-reduction theory, which traditionally has failed to distinguish global and local contexts.     

Discrimination learning by macaca mulatta with option to switch between S+ and S−

Three rhesus monkeys were trained to press either of two response keys. A response on the reinforcement key during presentation of the reinforced stimulus produced a sucrose pellet followed by an intertrial interval, but during presentation of the unreinforced stimulus produced only the intertrial interval. A response on the switching key changed the discriminative stimulus from reinforced to unreinforced or from unreinforced to reinforced. The reinforced stimulus was presented automatically on half the trials, but could be produced only by a switching response on the other half. Switching tended to occur in three distinct stages during acquisition of discriminative behavior. The first stage was identified as "nondiscriminative switching"; the second as "nonswitching"; and the third as "discriminative switching".     

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.     

Alternative reinforcement effects on fixed-interval performance

Pigeons' key pecks were reinforced with food on a fixed-interval schedule. Food also was available at variable time periods either independently of responding or for not key pecking (a differential-reinforcement-of-other-behavior schedule). The latter condition arranged reinforcement following the first pause of t seconds after it became available according to a variable-time schedule. This schedule allowed separation of the effects of pause requirements ≤ five-seconds and reinforcement frequency. The time spent pausing increased as the duration of the pause required for reinforcement increased from 0 to 30 seconds and as the frequency of reinforcement for pausing increased from 0 to 2 reinforcers per minute. Key pecking was more evenly distributed within each fixed interval with shorter required pauses and with more frequent reinforcement for pausing. The results complement those obtained with other concurrent schedules in which the same operant response was reinforced in both components.     

Short-term memory in the pigeon: the previously reinforced response

Eighteen pigeons served in a discrete-trials short-term memory experiment in which the reinforcement probability for a peck on one of two keys depended on the response reinforced on the previous trial: either the probability of reinforcement on a trial was 0.8 for the same response reinforced on the previous trial and was 0.2 for the other response (Group A), or, it was 0 or 0.2 for the same response and 1.0 or 0.8 for the other response (Group B). A correction procedure ensured that over all trials reinforcement was distributed equally across the left and right keys. The optimal strategy was either a winstay, lose-shift strategy (Group A) or a win-shift, lose-stay strategy (Group B). The retention interval, that is the intertrial interval, was varied. The average probability of choosing the optimal alternative reinforced 80% of the time was 0.96, 0.84, and 0.74 after delays of 2.5, 4.0, and 6.0 sec, respectively for Group A, and was 0.87, 0.81, and 0.55 after delays of 2.5, 4.0, and 6.0 sec, respectively, for Group B. This outcome is consistent with the view that behavior approximated the optimal response strategy but only to an extent permitted by a subject's short-term memory for the cue correlated with reinforcement, that is, its own most-recently reinforced response. More generally, this result is consistent with “molecular” analyses of operant behavior, but is inconsistent with traditional “molar” analyses holding that fundamental controlling relations may be discovered by routinely averaging over different local reinforcement contingencies. In the present experiment, the molar results were byproducts of local reinforcement contingencies involving an organism's own recent behavior.     

Pigeons' spatial memory: III. Effect of distractors on delayed matching of key location.

The effect of distractors on pigeons' delayed matching of key location was investigated. Baseline trials began with a "ready" stimulus (brief operation of the grain feeder). Then one (randomly chosen) key from a three-by-three matrix was lit briefly as the sample. After a short delay (retention interval) the sample key was lit again along with one of the other eight keys. A peck at the key that had served as the sample (correct comparison) produced grain reinforcement, whereas a peck to the other key (incorrect comparison) produced only the intertrial interval. In Experiment 1, a houselight distractor, presented during either the sample, retention interval, or choice phases of the trial, had little if any effect on accuracy of matching key location. In Experiment 2, one of three types of spatial stimuli was interpolated during the retention interval, or the interval was blank as during baseline trials. The three stimuli were: the sample (correct comparison) location for that trial, the incorrect comparison location for that trial, or one of the seven unused locations for that trial. Relative to blank trials, accuracy improved slightly on sample-interpolated trials, decreased slightly on unused location-interpolated trials, and decreased considerably on incorrect comparison-interpolated trials. In Experiment 3, retention intervals were blank or had one of six types of interpolation: the sample, the incorrect comparison, two presentations of the sample, two presentations of the incorrect comparison, the sample followed by the incorrect comparison, or the incorrect comparison followed by the sample.(ABSTRACT TRUNCATED AT 250 WORDS)     

The effects of extinction,noncontingent reinforcement and differential reinforcement of other behavior as control procedures

Several techniques have been used in applied research as controls for the introduction of a reinforcement contingency, including extinction, noncontingent reinforcement (NCR), and differential reinforcement of other behavior (DRO). Little research, however, has examined the relative strengths and limitations of these "reversal" controls. We compared the effects of extinction with those of NCR and DRO in both multi-element and reversal designs, with respect to (a) rate and amount of response decrement, (b) rate of response recovery following reintroduction of reinforcement, and (c) any positive or negative side effects associated with transitions. Results indicated that extinction generally produced the most consistent and rapid reversal effects, with few observed negative side effects.     

Some variables affecting rate of key pecking during response-independent procedures (autoshaping)

Rate of key pecking by pigeons subjected to response-independent procedures in which a stimulus on the response key preceded food presentation was investigated in eight experiments. Color and shape of the stimulus, duration of the stimulus, probability of food following the stimulus, duration of the intertrial interval, and duration of food presentation were varied separately and in combination. All variables studied, except color and shape of the stimulus, had a reliable effect on pecking rate, but some variables had stronger effects than others. Rapid key pecking may be obtained with a variety of response-independent procedures, as well as by response-dependent reinforcement. The results of experiments in which food is both dependent on key pecking and correlated with stimulus conditions are not representative of simple operant effects. Key pecking is an ideal response for studying the simultaneous operation of response-reinforcer and stimulus-reinforcer effects.     

On the form of the relation between response rates in a multiple schedule

Three pigeons received training on multiple variable-interval schedules with brief alternating components, concurrently with a fixed-interval schedule of food reinforcement on a second key. Fixed-interval performance exhibited typical increases in rate within the interval, and was independent of multiple-schedule responding. Responding on the multiple-schedule key decreased as a function of proximity to reinforcement on the fixed-interval key. The overall relative rate of responding in one component of the multiple schedule roughly matched the overall relative rate of reinforcement. Within the fixed interval, response rate during one multiple-schedule component was a monotonic, negatively accelerated function of response rate during the other component. To a first approximation, the data were described by a power function, where the exponent depended on the relative rate of reinforcement obtained in the two components. The relative rate of responding in one component of the multiple schedule increased as a function of proximity to fixed-interval reinforcement, and often exceeded the overall obtained relative rate of reinforcement. The form of the function relating response rates is discussed in relation to findings on rate-dependent effects of drugs, chaining, and the relation between response rate and reinforcement rate in single-schedule conditions.