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Stimulus equivalence and the blocking effect.

Blocking is the effect used to describe the finding that when a conditioning history is established with one stimulus, that history may interfere with or block the conditioning of a second, redundant stimulus when the two stimuli are subsequently presented as a compound. The phenomenon was discovered in a conditioned suppression paradigm, in which it was shown that a stimulus failed to suppress responding when it appeared in a compound with an established conditioned suppressor, despite the contiguous relation between the stimulus compound and an unconditioned stimulus (Kamin, 1968, 1969). Blocking has since been investigated in a variety of classical conditioning procedures, including, for example, simple conditioning experiments (e.g., Bakal, Johnson, & Rescorla, 1974; Marchant & Moore, 1973; Martin & Kevey, 1991; Stickney & Donahoe, 1983), studies of odor aversion (e.g., Cheatle & Rudy, 1978), and further examinations of conditioned suppression (e.g., Dickinson, Hall, & Mackintosh, 1976; Kremer, Specht, & Allen, 1980; Uegaki, 1976). The effect has also been observed in operant preparations, in which the blocking of discriminative (Singh & Solman, 1990; von Saal & Jenkins, 1970; Wu & Solman, 1993) and reinforcement (Williams, 1975; Williams & Heyneman, 1982) control has been demonstrated. The blocking effect has profound implications for the study of learning, for it suggests that the temporal contiguity between stimuli may not always be sufficient for conditioning to occur.

The blocking effect has not been examined within the context of conditional discrimination learning, or more specifically, the study of stimulus equivalence. Several recent investigations of stimulus equivalence have employed sample stimuli consisting of two-element stimulus compounds, in which subjects were trained to match the stimulus compounds to unitary comparisons. When the elements of each compound were later separated during the test for equivalence, both elements were shown to have entered separately into equivalence relations with the other stimuli (Markham & Dougher, 1996; Stromer & Stromer, 1990a, 1990b). Emergent relations between the elements of the compounds themselves have also been observed (Markham & Dougher, 1996). These findings have engendered the speculation that under certain conditions, the contiguous arrangement of stimuli may be sufficient for equivalence classes to develop (Smeets & Barnes, 1997; Stromer, McIlvane, & Sema, 1993). Given this body of results, it seems reasonable to suspect that the blocking effect might also be observed in an equivalence preparation in which stimulus compounds are presented as sample stimuli during training. For example, if subjects first learn to symbolically relate sample stimulus A1 to comparison stimuli B1 and C1, that history is likely to prevent stimulus X1 from entering into equivalence relations with those same stimuli when it subsequently appears in a compound with stimulus A1. Such a demonstration may prove to be an important supplement to the identification of the conditions sufficient for the establishment of equivalence classes. Indeed, the blocking effect may be the result of conditions that are insufficient for some stimuli to become class members.

The reported experiment extends the blocking effect into the domain of stimulus equivalence. We sought to determine whether a prior history of matching unitary samples (A1, A2, and A3) to unitary comparisons (B1, B2, B3, C1, C2, and C3) would block the establishment of equivalence relations between redundant sample stimulus elements (X1, X2, and X3) and the other stimuli, when the "X" and "A" elements were later presented as compounds. In other words, when the "AX" compounds were separated during the test for equivalence, would accuracy on test trials for relations between the "A" stimuli and the "B" and "C" stimuli be substantially higher than accuracy on test trials for relations between the "X" stimuli and the "B" and "C" stimuli?

Two supplemental measures were also used to examine the blocking effect. First, subjects' percentage reinforcement estimates were collected (Pilgrim & Galizio, 1996, pp. 184-186). This measure involves presenting subjects with a series of stimulus pairs at the conclusion of the experiment, with one stimulus presented in the sample stimulus position and a second stimulus presented in the comparison stimulus position. Subjects are asked to estimate, on a scale from 0 to 100, the percentage of trials over the course of the experiment that choosing the presented comparison stimulus given the presented sample stimulus was reinforced. In the present experiment, reinforcement estimates were collected for tested relations between the "A" stimuli and the "B" and "C" stimuli, and for tested relations between the redundant "X" stimuli and the "B" and "C" stimuli. The degree to which subjects' estimates correlated with the stimulus control exerted by the "A" and the "X" elements during the equivalence test was assessed.

Second, the content of subjects' overt verbal behavior was examined. A modified version of the concurrent think-aloud" procedure (Ericsson & Simon, 1984), in which subjects are instructed to "talk-aloud" everything that they are thinking to themselves during the experiment, was utilized. Verbal protocol analyses of this sort have shown that the content of subjects' overt verbal behavior emitted during experiments often correlates with their performance (e.g., Dixon & Hayes, 1998; Wulfert, Dougher, & Greenway, 1991). For example, Wulfert et al. (1991) found that subjects who failed to demonstrate the formation of equivalence classes verbally described sample stimuli and their matching comparisons as unitary stimulus compounds, whereas subjects who did demonstrate the formation of equivalence classes described relations between the matching stimuli (Wulfert et al., 1991). In the present experiment, it was questioned whether subjects who demonstrated the blocking effect would describe relations between the "A" elements and the "B" and "C" stimuli only. Likewise, if the blocking effect was not observed, would those subjects describe relations between both the "A" and "X" elements and the "B" and "C" stimuli? Subjects were instructed to describe the familiar "A" elements throughout the experiment. The purpose of these instructions was to ensure that subjects' verbal behavior pertained to the experimental task, and to directly assess the correspondence between subjects' verbal behavior and the other measures of blocking.

Ten subjects were first exposed to an A-B relation training set, followed by AX-B, B-C, and mixed AX-B and B-C relation training sets. The equivalence test was then conducted, during which the "AX" compounds were separated. At the conclusion of the experiment, percentage reinforcement estimates were recorded. The ongoing verbal behavior of four subjects was recorded.

Method

Subjects

Participating subjects were 10 adults (6 males and 4 females) enrolled in undergraduate psychology courses. Subjects were recruited through in-class announcements and received course credit for their participation. To ensure that subjects did not inform one another of the details of the experiment, subjects were recruited from a variety of courses and were asked to refrain from sharing the details of the experiment with other students. Before the experiment, all subjects signed a statement of informed consent and were told that they could withdraw from the experiment at any time, although none chose to do so. Upon completion of the study, all subjects were thoroughly debriefed.

Apparatus and Stimulus Materials

Data collection and stimulus presentation were computer controlled. The experiment was programmed in Microsoft Visual Basic version 4.0 and was performed on an IBM-compatible personal computer equipped with a color monitor and two-button mouse. The computer was positioned centrally on a 2-ft x 2-ft table. Adjacent to the computer was a tape recorder and microphone. Experimental sessions were conducted in a 12-ft x 12-ft room containing a table and chair. Figure 1 displays the 12 arbitrarily configured stimuli that were displayed on training and test trials. Stimuli were 5 to 6 cm in diameter and were arbitrarily divided into three stimulus classes.

Procedure

All subjects were were given the following instructions before the experiment:

There are several sections to this experiment. Your job is to earn as many points as you can. A figure will appear in the center of the screen. Next, 3 figures will appear below the first figure. It is your job to choose one of the three figures. To choose one of the 3 figures, click the mouse once on the figure that is your choice. In the early sections of the experiment, you will earn a point each time that you have responded correctly, but in later sections you will not always be given feedback for correct responses. However, there will always be a correct response, and it is important that you do your best throughout all sections of the experiment.

In addition, Subjects 7, 8, 9, and 10, whose verbal behavior would be recorded, were given the following additional instructions:

In this experiment we are interested in what people say to themselves as they work through experimental tasks. Therefore, we would like you to THINK ALOUD everything you am thinking to yourself for the entire experiment. Just say EVERYTHING OUT LOUD that you are thinking to yourself. If you are quiet for too long, the experiment will start over again.

Conditional Discrimination Training and Testing

The order of training sets is shown in Figure 2. Nine conditional discriminations involving either unitary samples and unitary comparisons or compound samples and unitary comparisons were trained, and 27 novel conditional discriminations involving unitary samples and unitary comparisons were tested. On all trials, sample stimuli were presented in the top center of the screen, followed 1 s later by the display of three comparison stimuli below the sample stimulus, evenly spaced across the bottom of the screen. Compound samples consisted of stimulus pairs presented 3 mm apart on the computer screen. The left and right position of each sample stimulus element varied randomly across trials, as did the location of each comparison stimulus. The subject selected a comparison stimulus by clicking the computer mouse upon it.

Each sample stimulus presentation marked the onset of a new trial. Following correct matches during training, sample stimuli and matching comparisons became outlined in black for 1.5 s, after which the statement "Good! One point!" was displayed and one point was added to the subject's current point total. Incorrect matches led to a new trial. All trials were separated by a 1-s intertrial interval. Point totals were displayed in the lower left-hand corner of the computer screen throughout training, but not during testing, as testing was conducted in extinction. Data were collected over the course of one session with each subject. Sessions lasted approximately 1 hr for all subjects.

The A-B relations (A1-B1, A2-B2, A3-B3), consisting of unitary samples and unitary comparisons, were trained first. A stability criterion of 30/30, or 100% correct, was required during the A-B training set before subjects could advance to the next set. Each sample stimulus could be presented no more than 10 times per 30-trial block. Prior to the AX-B training set, the 4 subjects who were instructed to talk aloud were given the following instructions:

From now on, sometimes only one figure will be presented in the top of the screen, and sometimes two figures will be presented, one which you will have seen before and one which will be new. You are to MAKE SURE TO DESCRIBE THE FAMILIAR FIGURES that appear at the top of the screen and how they relate to your completion of the task. Remember, if you are quiet for too long, the experiment will start over again. Just make sure to DESCRIBE THE FAMILIAR FIGURES.

The AX-B relations (A1X1-B1, A2X2-B2, A3X3-B3), consisting of compound samples and unitary comparisons, were trained next, followed by the training of the B-C relations (B1-C1, B2-C2, B3-C3), consisting of unitary samples and unitary comparisons. A stability criterion of 14/15, or 93% correct, was required during both the AX-B and B-C training sets before subjects could advance to the next set. Each sample stimulus could be presented no more than five times per 15-trial block during both the AX-B and B-C training sets. The last training set to which subjects were exposed was a mixed training set of AX-B and B-C relations. A stability criterion of 17/18, or 94% correct per 18-trial block, was required before subjects could advance to the equivalence test. Each sample stimulus could be presented no more than six times per 18-trial block. Throughout all four training sets, the order of the sample stimulus presentations was determined randomly.

Testing consisted of 81 trials, in which unitary samples and unitary comparisons were presented. The test trials included the following: Test trials for symmetry (B1-A1, B2-A2, B3-A3; B1-X1, B2-X2, B3-X3; C1-B1, C2-B2, and C3-B3), transitivity (A1-C1, A2-C2, A3-C3; X1-C1, X2-C2, and X3-C3), combined symmetry and transitivity (C1-A1, C2-A2, C3-A3; C1-X1, C2-X2, and C3-X3), and within-compound (A1-X1, A2-X2, A3-X3; X1-A1, X2-A2, and X3-A3) relations. The test trials were presented in a random order, but the emergence of each individual relation was assessed three times each (i.e., the B1-A1 relation was assessed three times, the B2-A2 relation was assessed three times, etc.).

Verbal Protocol Procedure and Coding

The last 4 subjects (Subjects 7, 8, 9, and 10) to participate in the experiment were additionally required to talk aloud. Verbal reports and audible auditory beeps signaling the completion of each trial and each phase were recorded on audio tapes. These data were transcribed and subsequently coded by two independent observers into four categories. These included comments pertaining to (a) relations between sample and comparison stimuli (e.g., "the squiggly lines go with the top hat"); (b) names or descriptions of single stimuli (e.g., "the top hat"); (c) variables unrelated to the experiment (e.g., "it is so hot in this room"); and (d) other, including responses not previously classified (e.g., "I'll keep choosing the center one"). In addition, comments which fell into the first category for training trials on which the "AX" compounds were presented, were further divided into one of two subcategories: (a) descriptions of both elements of the "AX" compounds and their relations to the comparison stimuli (e.g., "Squiggly lines and tree go with the top hat"); and (b) descriptions of the relations between sample and comparison stimuli which explicitly omitted reference to the "X" elements (e.g., "I'm going to ignore that one and just keep matching the squiggly lines with the top hat"), or continued to describe only the "A" elements and their relations to the other stimuli even though the "X" elements were also presented (e.g., "the squiggly lines go with the top hat").

Reliability of the categorizing of subjects' verbal behavior was calculated by dividing the number of category agreements by the number of category agreements plus disagreements on 50% of the subjects' transcripts. Overall agreement between observers (interobserver reliability) was 99%. Table 1 shows the interobserver reliability for each category, for Subjects 7 and 8.

[TABULAR DATA FOR TABLE 1 OMITTED]

Assessment of Percentage Reinforcement Estimates

At the conclusion of the test phase, subjects were given the following instructions:

You have now successfully completed the experiment! Good work. Now, there are a few questions we would like for you to answer about your experiences during the experiment. Please do your best to answer honestly. Look at the arrangement of images below and make an estimate of the percentage of times that you received a point for choosing the image on the bottom when presented with the image on the top (it does not matter in which of the three positions the image on the bottom was presented when you chose it). Your estimate could range from 0% if you think that you never earned a point for that response, up to 100% if you think that you always earned a point for that response. Please enter your response in the box below and then click the mouse on the 'Go On' box. (instructions adopted from Pilgrim & Galizio, 1996).

A series of 18 stimulus displays were presented, in which one stimulus was presented in the sample stimulus position, and a second stimulus was presented directly below it in the middle comparison stimulus position. Below each display was a box in which subjects were to enter their numerical response. After entering a response, a mouse-click in a box labeled "Go On" produced a new display. The following stimulus arrangements were presented, with the first stimulus always appearing in the sample stimulus position: B1-A1, B2-A2, B3-A3, B1-X1, B2-X2, B3-X3, A1-C1, A2-C2, A3-C3, X1-C1, X2-C2, X3-C3, C1-A1, C2-A2, C3A3, C1-X1, C2-X2, C3-X3.

Results

All subjects demonstrated criterion performance on A-B, AX-B, B-C, and mixed AX-B and B-C training sets. Shown in Table 2 is the number of trial blocks required for each subject to attain criterion during each set.
Table 2

Trial Blocks to Criterion for Each Training Set per Subject

 Training Set
Subject A-B AX-B B-C MIX

1 2 1 2 2
2 4 1 1 1
3 2 1 1 2
4 4 1 2 1
5 2 1 2 1
6 2 1 2 2
7 4 1 2 1
8 5 1 2 1
9 6 1 2 2
10 3 1 1 1


Equivalence Class Formation

Shown in Table 3 is the proportion of trials which were responded to accurately by each subject, for each relation tested. The table shows that Subjects 1, 2, 4, 7, and 9 performed with high accuracy on test trials for the B-A, C-B, A-C, and C-A relations, thus demonstrating the establishment of [TABULAR DATA FOR TABLE 3 OMITTED] three three-member equivalence classes (A1B1C1, A2B2C2, and A3B3C3). Subjects 3, 5, 6, 8, and 10 did not perform with high accuracy on test trials for the B-A, C-B, A-C, and C-A relations, thus demonstrating a failure to form those same three-member classes.

Blocking

Blocking was evaluated by visually comparing the proportion of accurate responses on test trials for symmetry, transitivity, and combined symmetry and transitivity relations in which elements A1, A2, or A3 were presented as sample or comparison stimuli, to the proportion of correct responses on test trials for symmetry, transitivity, and combined symmetry and transitivity relations in which elements X1, X2, or X3 were presented as sample or comparison stimuli. If the proportion of accurate responses appeared to be substantially higher on test trials assessing the emergence of relations between the "A" elements and the "B" and "C" stimuli than on test trials assessing the emergence of relations between the "X" elements and the "B" and "C" stimuli, the "A" elements were held to have blocked the entry of the "X" elements into equivalence classes.

Figure 3 presents the proportion of accurate test trials for symmetry, transitivity, and combined symmetry and transitivity relations for the 6 subjects who did not have the talk-aloud requirement, shown separately for trials in which the "A" elements were presented and for trials in which the "X" elements were presented. Figure 4 presents the same results for the 4 subjects who did have the talk-aloud requirement. An examination of Table 3 and Figures 3 and 4 reveals that the "X" elements appear to have been blocked from entering into equivalence classes with the "B" and "C" stimuli for Subjects 1, 2, 4, 7, and 9. Each subject responded with substantially higher accuracy on all test trials in which the "A" elements were presented, and substantially lower accuracy on all test trials in which the "X" elements were presented. A weak blocking effect can be observed for Subjects 3 and 10, while Subjects 5, 6, and 8 did not demonstrate the effect.

Within-Compound Relations

Figure 5 displays the proportion of test trials assessing the establishment of within-compound relations which were responded to accurately by each subject. It can be seen that the subjects for whom a strong blocking effect was observed (Subjects 1, 2, 4, 7, and 9) also performed with low accuracy on test trials for the A-X and X-A relations. It can also be seen that the subjects for whom a weak blocking effect or no effect whatsoever was observed (Subjects 3, 5, 6, 8, and 10) performed with high accuracy on test trials for the A-X and X-A relations, with the exception of Subject 10.

Concurrent "Talk-Alouds"

Shown in Table 4 is the proportion of descriptions of matching sample and comparison stimulus relations emitted during training and testing by the 4 subjects who were required to talk aloud. All of the subjects described the relations between sample and comparison stimuli during training, and this proportion increased substantially during the equivalence test for all subjects. A relationship can be observed between the proportion of descriptions emitted during the test and subjects' test performances. Of the verbal behavior emitted by Subjects 8 and 10 during the equivalence test, 98% was descriptions of matching sample and comparison stimulus relations. Both subjects performed with moderate overall accuracy on test trials for the B-A, C-B, A-C, and C-A relations, and demonstrated either a weak blocking effect or none at all. Of the verbal behavior emitted by Subjects 7 and 9 during the equivalence test, 53% of Subject 7's and 47% of Subject 9's were descriptions of matching sample and comparison stimulus relations. Both subjects performed with high accuracy on test trials for the B-A, CB, A-C, and C-A relations and demonstrated a strong blocking effect. Thus, the degree to which three-member classes and the blocking of the "X" elements was observed appears to be conversely related to the proportion of descriptions of sample and comparison stimulus relations emitted during testing.
Table 4

Proportion of Descriptions of Matching Sample and Comparison
Stimulus Relations Emitted during Training and Testing

Subject Training Testing

7 .45 .53
8 .56 .98
9 .30 .47
10 .17 .98


Of the descriptions of sample and comparison stimulus relations emitted by subjects during training trials on which the "AX" compounds were presented, all subjects described both elements of each compound more frequently than they described the "A" elements only. This was particularly the case for Subjects 7 and 9, who described both elements of each compound and their relation to comparison stimuli more frequently than did Subjects 8 and 10. Hence, the blocking of the "X" elements appears to be positively correlated with the proportion of descriptions of both elements of each sample stimulus compound and their relation to comparison stimuli.

Percentage Estimated Reinforcement

Figures 6 and 7 present the percentage of reinforcement estimated by each subject for symmetry, transitivity, and combined symmetry and transitivity relations, shown separately for test trials on which the "A" elements were presented and test trials on which the "X" elements were presented. The results are shown in Figure 6 for the 6 subjects who did not have the talk-aloud requirement, and Figure 7 presents the same results for the 4 subjects who were required to talk aloud. It can be seen that, for those subjects for whom a strong blocking effect was observed (Subjects 1, 2, and 7), reinforcement estimates were consistently lower for the tested relations involving the "X" elements than for tested relations involving the "A" elements. The same is true for Subjects 3 and 10, for whom a weak blocking effect was observed. (Subjects 4 and 9 failed to complete this portion of the experiment). It can also be seen that for those subjects for whom a blocking effect was not observed (Subjects 5, 6, and 8), estimates for all of the relations were either high or varied.

Discussion

The present results indicate that a prior history of symbolically relating one element of a sample stimulus compound to other stimuli will block a second, redundant element of that compound from becoming a member of an equivalence class. Three subjects who did not talk aloud during the experiment and two subjects who did talk aloud during the experiment demonstrated a strong blocking effect, as well as successfully forming three three-member equivalence classes. Likewise, three subjects who did not talk aloud during the experiment and two subjects who did talk aloud during the experiment demonstrated a weak blocking effect or none at all, and they failed to demonstrate the formation of three-member equivalence classes. In addition, the relations between the elements of each compound were consistently shown to be strongest for those subjects who demonstrated a strong blocking effect, and weakest for those subjects who did not demonstrate the effect. These findings suggest that temporal contiguity may not always be a sufficient condition for the acquisition of functional control by elements of stimulus compounds (see Williams, 1975), thus contributing to our knowledge of conditions in which, despite their involvement in equivalence class training, not all stimuli become class members.

Percentage of reinforcement estimates were also found to be congruent with observations of the blocking effect. Subjects for whom a blocking effect was observed consistently estimated a lower percentage of reinforcement for tested relations involving the "X" elements than for tested relations involving the "A" elements, whereas the estimates of the other subjects were either high for all of the tested relations or more varied. Thus, the degree of untrained stimulus control exerted by the "A" and "X" elements during the test for equivalence appears to be reflected in this measure: The more effective the control, the higher the estimates for relations involving those elements, and the less effective the control, the lower the estimates for tested relations involving those elements. Percentage of reinforcement estimates might thus be viewed as additional measures of the strength of emergent relations (see Pilgrim & Galizio, 1996).

Although the strength of the within-compound relations and percentage of reinforcement estimates were shown to be consistent with the degree to which a blocking effect and class formation was observed, the verbal behavior of the four subjects who talked aloud during the experiment was not. The two subjects who demonstrated a blocking effect and the formation of three-member classes emitted fewer descriptions of matching sample and comparison stimulus relations during the equivalence test than did the two subjects who did not demonstrate a strong blocking effect or class formation. In addition, all four subjects described both elements of each stimulus compound more frequently than they described the "A" elements only. Finally, the two subjects who demonstrated a strong blocking effect described both elements of each compound and their relation to comparison stimuli more frequently than did the other two subjects. Because the performance of these four subjects as a group did not differ significantly from that of the six subjects who did not talk aloud, it can be assumed that this requirement did not differentially affect subjects' performance. Unlike results from previous studies (e.g., Dickins, Bentall, & Smith, 1993; Dixon & Hayes, 1998; Wulfert et al., 1991), these findings suggest that verbal behavior and nonverbal behavior may not necessarily correlate at all times, possibly reflecting a limitation of descriptive analyses of verbal behavior (see Hayes, 1986, p. 361).

The subjects who did not demonstrate the blocking effect also failed to demonstrate the formation of three-member equivalence classes, indicating weak contingency control. It is likely that the "X" elements were not simply redundant for these subjects during training, but rather competed with the "A" elements for conditional control (see Chase & Heinemann, 1972). This may have been attenuated by the fact that the left and right positions of the elements varied randomly throughout training. Had the positions been fixed, the "A" elements might have acquired greater control than they did for these subjects, and the establishment of three-member classes and the blocking of the "X" elements might have been observed.

In order to unequivocally infer that a blocking effect was demonstrated in this experiment, two features of the methodology employed might be adjusted in follow-up research: First, more trials were presented during the A-B training set than were presented during the AX-B training set, possibly enhancing control by the "A" elements relative to that by the "X" elements. Although some research suggests that increased training with one element may not necessarily influence the blocking effect (e.g., Blumenthal, 1980), and other reports of the blocking effect have included such extended training (e.g., Marchant & Moore, 1973), a clearer demonstration of the effect might have been obtained had the number of trials been held constant between the two training sets. Second, in order to ensure that the redundant element of a stimulus compound would have acquired functional control under normal circumstances, experiments often employ control subjects who do not receive a prior training history with one element (e.g., Kamin, 1969). If both elements are then shown to exert functional control over the responding of control subjects but a discrepancy is observed for the subjects who received a prior training history with one element, an apparent blocking effect can be assumed. These two clarifications might be addressed in subsequent research, in continued efforts to learn more about the conditions under which stimuli become and fail to become members of equivalence classes.

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Author:Rehfeldt, Ruth Anne; Dixon, Mark R.; Hayes, Linda J.; Steele, Amy
Publication:The Psychological Record
Date:Sep 22, 1998
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