REPEATED ACQUISITION OF BEHAVIORAL CHAINS: RESPONSE SEQUENCES OR CONDITIONAL DISCRIMINATIONS?

The purpose of this study was to determine whether pigeons learn a sequence of positional responses or a series of conditional discriminations under a repeated-acquisition-of-behavioral-chains procedure. Three pigeons were trained under a repeated-acquisition procedure in which three different key colors served as stimuli correlated with the three steps in a chain. The order of presentation of the three stimuli was altered during the latter part of each test session after acquisition had occurred. If the pigeons had acquired a response sequence, the pattern of responding should remain the same as in the initial portion of the test session. However, if the pigeons had acquired a conditional discrimination, the response pattern should change in accordance with the changed order of the key colors. Although the results of this study do not rule out the possibility that the subjects acquired, to some degree, a response sequence, the results suggest that the behavior of pigeons under a repeated-acquisition-of-behavioral-chains procedure is controlled primarily by conditional discriminative stimuli.     

Divided stimulus control: Which key did you peck,or what color was it?

Responding on concurrent schedules produced a conditional discrimination (Phases 1 and 2), asking either which peck produced the event, or which color the keys were when the event was produced. In Phases 3 and 4, reinforcer delivery or a delay in blackout was interpolated between responding and the conditional discrimination. In Phase 1, location versus color discrimination accuracy was controlled by the relative reinforcer frequency for correct responses to these questions (divided stimulus control). In Phases 2 to 4, relative reinforcer frequency for correct responses to these questions was .5, and the relative frequency with which concurrent‐schedule responses produced the questions was varied. This variation had no clear effect on the accuracy of reporting Location or Color. These results are consistent with the model of divided control suggested by Davison and Elliffe (2010). Arranging a 3‐s reinforcer between responding and choice decreased both color and location accuracy, but a 3‐s delay only decreased location accuracy. Thus, in concurrent‐schedule performance, both ambient stimuli prior to a reinforcer and the location of the just‐reinforced response are available as discriminative stimuli following the reinforcer. Control of postreinforcer responding is divided between these according to their association with the relative frequency of subsequent reinforcers.     

Development and crossmodal transfer of contextual control of emergent stimulus relations

Six normally capable adults first learned three conditional relations in each of two prospective equivalence classes via match-to-sample training with figures as conditional (sample) and discriminative (comparison) stimuli. Then one trained conditional relation in each prospective class was brought under the control of contextual stimuli, two dictated nonsense syllables. Test performances indicated the emergence of untrained conditional relations, and therefore two equivalence classes, that were conditional on the contextual stimuli. These tests involved untrained combinations of contextual stimuli and stimuli in conditional relations, suggesting that the contextual stimuli functioned independently to control conditional relations rather than forming compound stimuli with samples and comparisons in training. Next, two novel figures were made equivalent to each of the original dictated contextual stimuli by match-to-sample training and testing. On subsequent tests, all subjects demonstrated transfer of conditional control of untrained conditional relations from the original auditory contextual stimuli to equivalent visual stimuli. These outcomes further supported the conclusion that the contextual stimuli exerted true conditional control over conditional relations in the equivalence classes and were not merely elements of compound stimuli.     

On the failure and facilitation of conditional discrimination.

Pigeons acquired a conditional discrimination in an autoshaping procedure in which certain stimulus combinations (form plus color) were followed by food, whereas others were not followed by food. Although the discrimination normally was acquired quickly, it was completely prevented when the color elements of the stimulus compounds were presented during the intertrial intervals preceding the trials in which both stimulus elements were available. This failure of discrimination was then prevented by having the colors serve as houselights rather than being localized on the response key and by pretraining procedures in which the colors were utilized in simpler discriminations. The results suggest that stimulus salience plays a critical role in determining whether conditional discriminations will be acquired, as the effects of all of the different operations could be understood in terms of increasing or decreasing the salience of the color elements, above or below some threshold value.     

Stimulus control of Pavlovian facilitation

Two experiments were conducted using an autoshaping procedure with pigeons to examine whether dimensional stimulus control by a Pavlovian facilitator parallels the control established following operant discrimination training. Facilitation training consisted of the presentation of a black vertical line on a white background as the B stimulus in a feature-positive discrimination in which the A stimulus (white keylight) was followed by grain presentation only if preceded by B. In this way, B facilitates or sets the occasion for pecking at A. Subsequent testing for generalization along the line-orientation dimension produced decremental gradients when the facilitation paradigm incorporated an explicit feature-negative stimulus (B−). These results parallel the decremental control obtained following operant discrimination training and suggest that Pavlovian facilitators and instrumental discriminative stimuli are functionally equivalent.     

Searching for Evidence of Transfer between Drug Facilitators

Four autoshaping experiments with pigeons examined the transfer of facilitation between drug stimuli. In Experiment 1 the presence and absence of methadone or phencyclidine signaled which of two keylight CSs were followed by grain (US). A different keylight CS was reinforced in the absence of the drugs. The drugs facilitated responding to the CS with which they were trained but did not facilitate responding to the CS reinforced in the presence of the other drug. Thus, transfer of facilitation across drug stimuli was not obtained. In Experiment 2 one of the drug facilitators was replaced by a tone. No evidence of transfer of facilitative function was obtained between the tone and the remaining drug facilitator. In Experiment 3 transfer was again not found using a design similar to that used in Experiment 1 except that the drugs did not simultaneously signal nonreinforcement of a keylight CS. In Experiment 4 the procedures of Experiment 1 were employed except that drug facilitators were replaced by distinct visual stimuli surrounding the response key. Clear evidence of transfer between the visual facilitators was obtained. The lack of transfer of modulatory control by drug stimuli in Experiments 1-3, coupled with its occurrence in Experiment 4 and previous studies, suggests that stimulus control by drugs may differ in important ways from stimulus control by conventional stimuli.     

Sample-specific ratio effects in matching to sample

In a symbolic matching-to-sample task, pigeons were trained using sample-specific, fixed-ratio “observing responses.” Subsequently, in a mixed condition, each sample was presented equally often with each ratio requirement, i.e., the ratios were no longer correlated with the samples. In a second experiment, pigeons were trained initially in the mixed condition and subsequently shifted to the sample-specific condition in which the required ratios were correlated with the samples. Results of both experiments suggested joint control of choices by ratio value and by the exteroceptive stimuli. The discriminative properties of the ratios appeared to outweigh absolute ratio-size effects.     

Reflections on stimulus control

The topic of stimulus control is too broad and complex to be traceable here. It would probably take a two-semester course to cover just the highlights of that field's evolution. The more restricted topic of equivalence relations has itself become so broad that even an introductory summary requires more time than we have available. An examination of relations between equivalence and the more general topic of stimulus control, however, may reveal characteristics of both the larger and the more limited field that have not been generally discussed. Consideration of these features may in turn foster future developments within each area. I speak, of course, about aspects of stimulus control that my own experiences have made salient to me; others would surely emphasize different characteristics of the field. It is my hope that cooperative interactions among researchers and theorists who approach stimulus control from different directions will become more common than is currently typical.     

Control by the auditory or the visual element of a compound discriminative stimulus: effects of feedback.

Groups of pigeons were trained to depress a treadle in the presence of a compound stimulus consisting of a tone and a red houselight (a) to avoid electric shock, or (b) to obtain grain. Immediate, exteroceptive feedback was equated for avoidance and appetitive groups within an experiment, but varied across experiments from elevation of a nonilluminated feeder to darkening of the chamber, termination of the tone, and elevation of an illuminated feeder. Responding in the absence of the compound stimulus postponed its next occurrence. After performance had stabilized, the degree to which each element controlled treadle pressing was determined. Generally, in the appetitive tests, the red light controlled much more responding than did the tone, but in the avoidance tests, the tone controlled more responding than did the red light.     

Differential responding in the presence and absence of discriminative stimuli during multielement functional analyses

We evaluated the extent to which discriminative stimuli (S(D)s) facilitate differential responding during multielement functional analyses. Eight individuals, all diagnosed with mental retardation and referred for assessment and treatment of self-injurious behavior (SIB) or aggression, participated. Functional analyses consisted of four or five assessment conditions alternated in multielement designs. Each condition was initially correlated with a specific therapist and a specific room color (S(D)s), and sessions continued until higher rates of target behaviors were consistently observed under a specific test condition. In a subsequent analysis, the programmed S(D)s were removed (i.e., all conditions were now conducted by the same therapist in the same room), and sessions continued until differential responding was observed or until twice as many sessions were conducted with the S(D)s absent (as opposed to present), whichever came first. Results indicated that the inclusion of programmed S(D)s facilitated discrimination among functional analysis conditions for half of the participants. These results suggest that the inclusion of salient cues may increase either the efficiency of functional analyses or the likelihood of obtaining clear assessment outcomes.     

Conditional discrimination and equivalence relations: Control by negative stimuli

Three adult subjects were taught the following two-sample, two-comparison conditional discriminations (each sample is shown with its positive and negative comparison, in that order): A1-B1B2, A2-B2B1; B1-C1C2, B2-C2C1; and C1-D1D2, C2-D2D1. A teaching procedure was designed to encourage control by negative comparisons. Subjects were then tested for emergent performances that would indicate whether the baseline conditional discriminations were reflexive, symmetric, and transitive. The tests documented the emergence of two classes of equivalent stimuli: A1, B2, C1, D2 and A2, B1, C2, D1. These were the classes to be expected if the negative comparisons were the controlling comparisons in the baseline conditional discriminations. The negative comparisons, however, were not the comparisons that subjects were recorded as having chosen in the baseline conditional discriminations. Differential test results confirmed predictions arising from a stimulus-control analysis: In reflexivity tests (AA, BB, CC, DD), subjects chose comparisons that differed from the sample; one-node transitivity (AC, BD) and "equivalence" (CA, DB) tests also yielded results that were the opposite of those to be expected from control by positive comparisons; symmetry tests (BA, CB, DC), two-node transitivity (AD) tests, and two-node "equivalence" (DA) tests yielded results that were to be expected from control by either positive or negative comparisons.     

Effects of spatial rearrangement of object components on picture recognition in pigeons.

Five pigeons were first trained to discriminate among line drawings of four objects: a watering can, an iron, a desk lamp, and a sailboat. The birds were then tested with eight versions of each object, in which the object's components were vertically and horizontally rearranged. The pigeons displayed different degrees of generalization decrement to the different scrambled versions of the objects. Two analyses helped to clarify the nature of the varied accuracy scores. First, cluster analyses disclosed subsets of components that were related to test performance. Second, although the clusters varied somewhat across birds for a given object, there was reliable concordance among the subjects in their rankings of the individual scramblings, suggesting that the pigeons may have attended to common aspects of the drawings.     

A note on the measurement of conditional discrimination.

An analysis of some extreme forms of stimulus control that a simple conditional-discrimination procedure can generate leads to the conclusion that accuracy does not provide an orderly scale of measurement. Dependence on accuracy to evaluate a conditional discrimination, particularly at intermediate levels of accuracy, can generate erroneous conclusions about the extent to which the controlling relations are those specified by the experimenter.