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Teaching equivalence relations to individuals with minimal verbal repertoires: are visual and auditory-visual discriminations predictive of stimulus equivalence?

A controversy exists as to whether or not individuals with minimal verbal repertoires can learn stimulus equivalence. Studies that have addressed this question have typically relied on global measures of language assessment (e.g., Carr, Wilkinson, Blackman, & McIlvane, 2000). Perhaps research on this topic can be advanced by measures of more precise language discriminations of which participants are capable, such as visual and auditory discriminations assessed by the Assessment of Basic Learning Abilities (ABLA) test (Kerr, Meyerson, & Flora, 1977), and/or by a test of echoics, tacts, and mands. The present research is a step in that direction.

Research Findings on the ABLA Test

An abundance of research has supported use of the ABLA test for selecting and sequencing training tasks to enable optimal learning for individuals with intellectual disabilities (Martin & Yu, 2000). The ABLA test assesses an individual's ability to perform a simple imitation task and five two-choice motor, visual, and auditory discriminations. A description of the ABLA levels and the types of discriminations required to perform each level are provided in Table 1.

Hierarchical ordering. The ABLA levels are hierarchically ordered in level of difficulty (e.g., Kerr et al., 1977; Martin, Yu, Quinn, & Patterson, 1983). Each level exceeds the previous level in terms of the types of discriminations required (see Table 1).

Predictive validity of the ABLA. The ABLA test is highly predictive of the ease or difficulty with which an individual is able to perform educational, prevocational, and vocational tasks (Stubbings & Martin, 1995, 1998; Wacker, Kerr, & Carroll, 1983; Wacker, Steil, & Greenbaum, 1983). For example, Wacker, Kerr, et al. examined whether the ABLA test predicted participants' abilities to perform two-choice and four-choice vocational analogue tasks. Results on the ABLA test predicted performance on the analogue tasks for 11 of 12 participants.

Failed levels are difficult to teach. Failed ABLA levels are difficult to teach using standard reinforcement and prompting procedures (e.g., Conyers, Martin, Yu, & Vause, 2000; Meyerson, 1977; Witt & Wacker, 1981; Yu & Martin, 1986). For example, Meyerson (1977) attempted to train participants on their first failed ABLA level, and reported that participants needed 100 to 900 training trials before a higher level of discrimination was attained.

ABLA and a test of verbal operants. The relationship between the ABLA test and performance on a test of echoics, tacts, and mands was examined by Marion et al. (2003). In this study, 38 individuals were first tested on levels of the ABLA test. Following testing, individuals were directly assessed on a test of vocal imitation, tacting, and manding (Marion et al.). To assess vocal imitation, 11 words were presented to each individual, and he or she was given the vocal prompt, "Say (word)." To assess tacting, the participant was presented with each of 11 items (based on words used in the vocal imitation assessment) and given the vocal prompt, "What's this?" The dependent variable for both assessments was the percentage of correct responses. Correct responses were defined as the pronunciation of all vowels and consonants of a word.

To assess manding, five activities were chosen and were repeated, in the same order, three times each. The activities were as follows: manding for (a) juice in the presence of a cup, (b) pudding in the presence of a spoon, (c) a piece of foam in the presence of ABLA test materials (i.e., box and can), (d) a puzzle piece in the presence of a partially assembled puzzle, and (e) a piece of paper in the presence of a pen. To assess an individual's ability to mand for juice, for example, the participant was first given a sip of juice accompanied by the vocal prompt, "Have some." After repeating this twice (and observing the participant consuming the juice), the mand assessment for juice began. Step 1 involved providing the prompt "Have some," with the cup present on the table, and the juice hidden underneath the table and out of sight of the participant. If the participant said the word correctly or approximated the word (vocalized specific segments of the word), the participant was given the juice and that trial was terminated. If the participant said the word incorrectly, or did not say anything within 10 seconds, Step 2 was implemented. Step 2 involved hiding the item underneath the table and giving the participant the vocal prompt, "What do you want?" The dependent variable was the percentage of correct or approximated responses emitted in Step 1 or Step 2.

An 80% correct criterion was used as a pass for vocal imitation, tacting, and manding. Results indicated that (a) individuals who were only able to pass visual discriminations, Levels 3 and 4 on the ABLA test, passed only 2% of the verbal assessments; and (b) individuals who passed ABLA Level 6 (but failed an auditory matching task) passed 36% of the verbal assessments. Additionally, results on the test of the three verbal operants showed high test-retest reliability after 1 month.

A prototype visual nonidentity matching task. Recently, Sakko, Martin, Vause, Martin, and Yu (2004) examined a prototype visual nonidentity matching (VVNM) task as a potential addition to the ABLA test. The task consists of an individual matching a silver-colored piece of wood that is shaped into the word "BOX," in capital letters, to a red box, and a purple-colored piece of wood that is shaped into the word "Can," in upper and lower case letters, to the yellow can. Results of 23 participants suggested that the VVNM prototype task fits between ABLA Levels 4 and 6.

The present study examined how stimulus equivalence relates to performance on the ABLA test, including the VVNM prototype task.

Research on Stimulus Equivalence

Stimulus equivalence is an efficient and powerful approach for establishing stimulus classes (e.g., Sidman, 1971; Sidman, Kirk, & Willson-Morris, 1985; Sidman & Tailby, 1982; Sidman, Willson-Morris, & Kirk, 1986). Abstract symbols and drawings have been widely used in teaching equivalence relations to persons with intellectual disabilities (e.g., Barnes, McCullagh, & Keenan, 1990; Carr et al., 2000; Devany, Hayes, & Nelson, 1986; Dixon & Spradlin, 1976; Eikeseth & Smith, 1992; Saunders, Wachter, & Spradlin, 1988; Vyse & Rapport, 1989).

A small but growing number of studies have used stimulus equivalence procedures to teach practical repertoires to individuals with developmental disabilities. Over the years, several studies have used a stimulus equivalence paradigm to teach simple reading skills (Sidman, Cresson, & Willson-Morris, 1974) as well as a variety of other practical skills to individuals with disabilities including manual signing (Osborne & Gatch, 1989; VanBiervliet, 1977); pre-arithmetic skills (Gast, Vanbiervliet, & Spradlin, 1979); spelling (Stromer & Mackay, 1992, 1993; Mackay, 1985); name-face matching (Cowley, Green, & Braunling-McMorrow, 1992); shopping skills (Taylor & O'Reilly, 2000); monetary skills (McDonagh, McIlvane, & Stoddard, 1984); relations among objects, spoken words, and lexigrams (Brady & McLean, 2000); and relations among consonants, spoken words, and pictures (Carr et al., 2000).

Stimulus equivalence and language. Controversy exists concerning the relationship between language and stimulus equivalence. For example, some researchers (e.g., Dugdale & Lowe, 1990; Horne & Lowe, 1996) have argued that the emergence of stimulus equivalence is accounted for by an individual's language or "naming" skills. Simply stated, individuals who do not possess naming skills will not be able to demonstrate equivalence relations. The notion that stimulus equivalence is related to an individual's language ability was also supported by Hayes (1991).

In an attempt to empirically support this notion, Horne and Lowe (1996) cited a number of studies (e.g., Barnes et al., 1990; Devany et al., 1986; Dugdale & Lowe, 1990; Eikeseth & Smith, 1992). For example, Devany et al. (1986) studied 12 children, with mental ages ranging from 14 to 36 months. Results suggested that children who had some language skills were able to demonstrate equivalence classes. However, individuals with mental retardation and no language skills required more trials to learn conditional discriminations and failed to form equivalence classes. Further, in accordance with their argument that a linkage exists between language and stimulus equivalence, Horne and Lowe (1996) cited a series of studies conducted with pigeons, monkeys, baboons, and chimpanzees, whereby stimulus equivalence was not established (e.g., D'Amato, Salmon, Loukas, & Tomie, 1985; Hogan & Zentall, 1977; Holmes, 1979; Kendall, 1983; Lipkens, Kop, & Matthijs, 1988; Rodewald, 1974; Sidman et al., 1982). Lastly, Horne and Lowe (1996) offered several criticisms concerning one recent study (Schusterman & Kastak, 1993) in which stimulus equivalence was claimed to be demonstrated with a sea lion.

In response to Horne and Lowe (1996), several researchers (e.g., McIlvane & Dube, 1996; Saunders & Green, 1996) offered points of criticism. Among their points, they asserted that previously cited studies (e.g., Devany et al., 1986) presented a number of confounds. For example, Saunders and Green suggested that, rather than an inadequate naming repertoire, "an initial history of simple discriminations, fragile baselines, and confounding effects related to instructional control" (p. 313) may have accounted for the inability of some individuals to establish equivalence relations. Further, researchers (e.g., Fields, 1996; McIlvane & Dube; Saunders & Green) commented that criticisms regarding the Schusterman and Kastak (1993) study are unfounded. Sidman (2000) cited additional studies conducted with animals that demonstrated equivalence (e.g., Reichmuth, 1997; Schusterman & Kastak, 1998).

On a related note, some studies (e.g., Dugdale & Lowe, 1990; Eikeseth & Smith, 1992) have reported that teaching individuals to name stimuli can facilitate the learning of equivalence relations. However, a recent study conducted by Carr and Blackman (2001) provided data that does not support this statement. In Sidman's (1996) commentary, he argued, in response to the Dugdale and Lowe study, that it is unclear whether naming or some other variable is responsible for the emergence of equivalence relations.

To date, there are few studies that have examined the relationship between naming and stimulus equivalence (O'Donnell & Saunders, 2003). Recently, Brady and McLean (2000) examined whether 4 individuals with severe developmental disabilities and limited verbal repertoires were able to demonstrate equivalence relations with objects, spoken words, and lexigrams. Participants scored between 2 years 8 months and 4 years 2 months on the Peabody Picture Vocabulary Test-Revised (Dunn & Dunn, 1981); they were able to demonstrate rudimentary tacting skills by using gestures, but could not communicate vocally (i.e., spoken words or augmentative communication). Three-choice match-to-sample training was used, with the implementation of blocked trials for 2 of the 4 participants. Three participants showed symmetry, with 2 of these participants also demonstrating transitivity. Similarly, Carr et al. (2000) looked at whether adolescents and adults with limited verbal repertoires and severe mental retardation were able to form equivalence relations with visual and auditory stimuli. In Experiment 1 involving 3 individuals with mental retardation, participants scored slightly above or below an age-equivalent score of 2 years on global assessment measures including the Peabody Picture Vocabulary Test-Revised (PPVT-R) (Dunn & Dunn, 1981) and the Gardner Expressive One-Word Picture Vocabulary Test-Revised (Gardner, 1990); none of the participants possessed oral naming skills. Three-choice match-to-sample training, with incorporation of within-stimulus prompting, was used. Posttests indicated that participants successfully demonstrated stimulus equivalence, including all symmetric and transitive relations.

Two adolescent males with mental retardation and autism participated in Experiment 2 of Carr et al. (2000). The study reported that 1 participant had a verbal repertoire similar to participants in Experiment 1, and the other had higher oral skills, including an extensive echoic repertoire. The authors indicated that the former participant was assessed on the Derbyshire Language Scheme Assessment (Knowles & Masidlover, 1982) placing him at 2 years of age, and the latter was assessed on the Reynell Developmental Language Scales (Reynell & Huntley, 1985) with a score of 3 years 2 months. The study employed two-choice match-to-sample training with abstract stimuli. Results indicated that only the former participant demonstrated one symmetric relation and gradual emergence of the transitive relation and the symmetry of the transitive relation.

The present study further examined whether individuals with minimal verbal repertoires are able to learn stimulus equivalence. To provide a more comprehensive quantification of verbal repertoires, we expanded the language assessments to include both standardized measures and the assessment of vocal imitation, tacts, and mands. It was hypothesized that individuals with minimal verbal repertoires would be able to demonstrate equivalence relations. Second, it was hypothesized that individuals who were able to perform ABLA Level 6 (including the VVNM prototype task) would learn AB and BC relations quicker and demonstrate a greater emergence of equivalence relations than individuals who failed ABLA Level 6 and the VVNM prototype task. Participants were matched, as closely as possible, on language assessments.

Method

Setting and Participants

Assessment and training sessions were conducted in the testing room of the Psychology Department of the St. Amant Centre, a residential and community resource facility for persons with developmental disabilities. During sessions, the tester was seated directly across from the participant.

Five male participants with intellectual disabilities participated in the study. They were selected based on their scores on the ABLA test, and because they had minimal language skills (described later). Ages of Participants 1 through 5 were 36, 44, 28, 33, and 27 years, at the beginning of the study. The Wechsler Adult Intelligence Scale (WISC-III) indicated that all participants had moderate mental retardation (American Psychiatric Association, 1994). The 5 participants were assessed on the Scales of Independent Behavior-Short Form (Buininks, Woodcock, Weatherman, & Hill, 1984), which measures adaptive behavior. All participants scored in the severe range. According to agency records, Participant 3 had a diagnosis of autism.

Materials

The ABLA tasks. The materials for the ABLA test consisted of a red box with black stripes, a yellow can, a small red block with black stripes, a small yellow cylinder, and a small piece of irregularly shaped beige foam.

The VVNM prototype task. Materials for the VVNM prototype task consisted of a red box and a yellow can, a silver-colored piece of wood shaped into the word, "BOX," in capital letters, and a purple-colored piece of wood shaped into the word "Can," in upper and lower case letters.

Verbal operant assessment materials. Materials for the tact assessment included 11 tangible items: a red box, a yellow can, a piece of beige foam, a blue pen, orange or apple juice in a plastic container, a Styrofoam cup, a container of chocolate or vanilla pudding, a metal spoon, a round brown bowl, a puzzle in the shape of a bear that was embedded on a 25- X 38-cm platform, and a sheet of 22- X 28-cm white bond paper. Materials for the tact assessment were also used to assess manding, with the exception of the brown bowl.

Standardized assessment measures. The communication portion of the Interview Edition, Expanded Form of the Vineland Adaptive Behavior Scales (VABS) (Sparrow, Balla, & Cicchetti, 1984) was administered to a care-worker of each participant. The PPVT-R (Dunn & Dunn, 1981) was directly administered to each participant.

Stimuli required for pretesting and posttesting relations. The same materials were used for pretesting, training, and posttesting of match-to-sample relations. The materials included nine 10- X 15-cm white cards that each contained a picture, a printed word, or a symbol that varied in color, shape, and size. See Figure 1 for a visual display of the stimuli. Class 1 included the word DISK, with letters filled with blue and white polka dots, located in the center of the card; a picture of a black computer disk in the center of the card; and the symbol IBM, in black stripes, positioned at the bottom of the card. Class 2 consisted of the word plant, alternating lowercase letters in light blue and pink, in the center of the card, with the "l" and "n" positioned lower than the other letters; a picture of a green cactus with a black outline displayed on the left side of the card; and the symbol H2O, in yellow, running diagonally from left to right. Class 3 consisted of the word CLOCK, with respective letters positioned down the card; a picture of a clock consisting of a red outline and black numbers and hands displayed at the bottom of the card; and the symbol 10:00 PM written, in brown, near the top of the card.

[FIGURE 1 OMITTED]

Pretraining Assessments and Assessment Results

ABLA assessment, VVNM assessment, and tests of auditory matching. According to procedures specified by Kerr et al. (1977), each participant was assessed on the ABLA tasks and the VVNM prototype task. Throughout testing, each correct response was immediately followed by verbal praise (e.g., "Good job!"). An incorrect response was followed by a correction procedure that consisted of a demonstration, a guided trial, and a practice trial. Testing continued for each task until either eight consecutive correct responses (passing criterion) or eight cumulative errors (failing criterion) occurred.

Prior to testing, in accordance with the ABLA procedure, a demonstration, a guided trial, and a practice trial were provided. Individuals did not advance to test trials until they were able to perform one independent correct response with each sample stimulus. Participants 1 through 3 passed ABLA Level 4, but failed Levels 5 and 6 and VVNM. Participants 4 and 5 passed all six ABLA levels and the VVNM prototype task.

Assessment of vocal imitation, tacts, and mands. Subsequent to ABLA assessments, participants were assessed on vocal imitation, tacting, and manding according to procedures specified by Marion et al. (2003) and as summarized in the introduction. None of the participants reached the passing criterion of 80%. Specific scores of participants on the vocal imitation, tact, and mand assessments are presented in Table 2.

Standardized language assessment measures. At the beginning of the study, participants were assessed on the VABS (Sparrow et al., 1984) and the PPVT-R (Dunn & Dunn, 1981). Participants who were tested for stimulus equivalence were posttested on the VABS (Sparrow et al.) and the PPVT-R (Dunn & Dunn).

Pretesting stimulus equivalence relations. Nine relations (AA, BB, CC, AB, BA, AC, CA, BC, CB) were tested, with 18 trials per relation, for a total of 162 test trials. A three-choice match-to-sample format was used. During pretesting, differential reinforcement was not provided. All responses were followed by the tester saying, "Thank-you." However, considering the reinforcement history of participants for correct responding on tabletop tasks (e.g., the ABLA test), an edible was presented immediately before each trial began. It was assumed that this would maintain participants' responding during pretesting, without influencing the results. For the 5 participants, pretest results for the nonidentity relations were at chance level (i.e., 33%) or slightly above.

Training Procedure to Teach the Match-to-Sample Relations, AB and BC, Prior to Testing For Stimulus Equivalence

Participants were first taught the AB relation, followed by the BC relation. Throughout training, multiple components were used. If minimal or no progress was observed after a number of trials with certain components, they were deleted and additional components were added. The final training procedure, used for all participants, that was successful in training AB and BC for Participants 4 and 5 consisted of components such as (a) a reinforcer preference assessment; (b) the establishment of an attending response prior to the presentation of the sample stimulus, (c) edible and tangible reinforcement for correct responses, and (d) a correction procedure following errors. Size fading of one comparison stimulus, in a two-choice match-to-sample task, that, across trials, served as both the correct and incorrect comparison, was used. The sample stimuli and the other comparison stimulus remained at their target size. The comparison stimulus was initially presented at a very small size, and then was gradually increased to its target size across steps. When the criterion was achieved with size fading of one comparison stimulus, in a two-choice match-to-sample task, size fading occurred with a third comparison stimulus, during which the third stimulus was presented at a very small size, and then gradually increased to its target size across steps. For Participants 4 and 5, who showed consistent errors with a particular stimulus, position and color fading procedures were also used. Mastery criterion for each relation was two consecutive sets of at least 15 out of 18 correct responses, with no more than one error per stimulus class. Detailed descriptions of the training procedures are available from the first author.

Bridge Between Training and Testing of Equivalence Relations with Respect to Reinforcement Delivery

Before advancing to posttest, Participants 4 and 5 were required to meet the same criterion, set for mastery of AB and BC, on one session of 18 trials (i.e., at least 15 out of 18 with no more than one error per stimulus class) with no differential reinforcement. This was conducted to ensure that performance was maintained, on taught relations, in the absence of differential reinforcement.

Testing for Equivalence

Similar to pretesting, all test relations were conducted in the absence of differential reinforcement. In an attempt to maintain responding, immediately before each trial began, the participant was presented with a preferred consumable item. Due to performance on posttesting for both participants (see Results section), additional training was implemented, and repeated posttests were conducted.

Retention

Approximately 1 month after the final posttest, retention of performance was tested.

Reliability Assessments

For interobserver agreement (IOA) checks, an observer and a tester independently recorded the responses of each participant. Agreement scores were calculated, on a trial-by-trial basis, for each participant by dividing the number of agreements by the number of agreements plus disagreements and then multiplying by 100%. IOA checks for trial outcome were conducted on 86% of ABLA testing and auditory matching tests, 81% of verbal operant tests, 71% of pretest sessions, 47% of training sessions, and 79% of posttest sessions. Across participants, agreement scores for ABLA and auditory matching tests ranged from 93% to 100% (mean = 100%). On verbal operant tests, agreements ranged from 67% to 100% (mean = 82%). For pretesting, training, and posttesting sessions, agreements ranged from 99% to 100% (mean = 100%), 93% to 100% (mean = 100%), and 94% to 100% (mean = 100%), respectively. All agreement scores are rounded to the nearest whole number, which is why means were frequently 100%, even though occasional scores were lower.

Procedural integrity (PI) checks were conducted to ensure that key treatment components (e.g., proper set-up of materials, appropriate consequences for correct and incorrect responses) were implemented by the tester. During PI checks, an observer recorded whether steps were carried out correctly by the experimenter on each trial according to procedural checklists. In addition, procedural observer reliability checks (POR) were conducted by a second observer for the PI checks. An agreement was scored if both observers recorded that a procedural component was implemented on a given trial; otherwise, it was a disagreement. Reliability of PI checks was calculated in the same manner as IOA assessments.

PI checks for trial outcome were conducted on 61% of ABLA testing and auditory matching tests, 86% of verbal operant tests, 53% of pretest sessions, 36% of training sessions, and 77% of posttesting. Across participants, PI scores for ABLA and auditory matching tests ranged from 93% to 100% (mean = 100%). For verbal operant tests, PIs ranged from 86% to 100% (mean = 98%). For pretesting, training, and posttesting sessions, PIs ranged from 95% to 100% (mean = 100%), 90% to 100% (mean = 100%), and 90% to 100% (mean = 94%), respectively. All PI scores were rounded to the nearest whole number.

POR checks were conducted on 61% of ABLA testing and auditory matching tests, 67% of verbal operant tests, 51% of pretest sessions, 34% of training sessions, and 77% of posttest sessions. Reliability of POR scores for ABLA and auditory matching tests ranged from 70% to 100% (mean = 96%). For verbal operant tests, PORs ranged from 93% to 100% (mean = 99%). For pretesting, training, and posttesting sessions, PORs ranged from 95% to 100% (mean = 100%), 85% to 100% (mean = 99%), and 95% to 100% (mean = 100%), respectively. All POR scores were rounded to the nearest whole number.

Results

Despite extensive training, Participants 1 through 3 were not able to meet mastery criterion for relation AB, even after comparison stimuli were reduced from 3 to 2. Combining all procedures for Participants 1 through 3, the total number of training trials for AB were 1808, 2158, and 1995, respectively.

Participant 4

Combining all procedures for Participant 4, who passed up to ABLA Level 6, the total number of training trials to reach mastery criterion for the AB and BC relations were 1375 and 756, respectively. Results of pretesting and posttesting for Participant 4 are presented in Figure 2.

[FIGURE 2 OMITTED]

Posttest 1. After mastery on the AB and BC relations had been demonstrated, Posttest 1 scores for reflexive relations (AA, BB, CC) remained high (see Figure 2). The taught relations, AB and BC, were not tested, but their symmetric relations, BA and CB, rose to 56% and 50%, respectively. The transitive relation, AC, rose to 56%, and the symmetry of the transitive relation, CA, rose to 44%.

Posttest 2 with AB and BC interspersed. On Posttest 2, subsequent to retraining AB and BC relations to mastery criterion, the reflexive relations remained high. AB and BC, the taught relations that were interspersed with the test relations, remained intact at 100% and 94%, respectively (not shown in Figure 2). Their symmetric relations, BA and CB, rose to 72% and 61%, respectively. The transitive relation, AC, rose to 89%, and the symmetry of the transitive relations, CA, rose to 67%.

One-month retention. One month following Posttest 2, the taught relations, AB and BC, were 100% and 67%, respectively. Retention scores of the symmetric and transitive relations all showed a decrease, reverting to Posttest 1 levels except for BA, which remained at 72% (see Figure 2).

Participant 5

Combining all procedures for Participant 5, who passed up to ABLA Level 6, the total number of training trials to reach mastery criterion for the taught relations, AB and BC, were 1382 and 2243, respectively. Results of pretests and Posttests 1, 2, and 3 for Participant 5 are presented in Figure 3.

Posttest 1 with AB and BC interspersed. After mastery on AB and BC relations had been demonstrated, Posttest 1 scores for reflexive relations (AA, BB, CC) remained high (see Figure 2). The taught relations AB and BC averaged 96% and 74% (not shown in Figure 3). Symmetric relations, BA and CB, rose to 61% and 72%, respectively. The transitive relation, AC, rose to 72%, and the symmetry of the transitive relation, CA, rose to 50%.

Posttest 2 with AB and BC interspersed. On Posttest 2, following retraining on the BC relation, all reflexive relations remained high. AB and BC, the taught relations averaged 89% and 74%, respectively (not shown in Figure 3), and their symmetric relations, BA and CB, dropped to 44% and 28%. The transitive relation, AC, dropped to 44%, and the symmetry of the transitive relation, CA, dropped to 17%.

Posttest 3 with AB and BC interspersed and reinforced. On Posttest 3, reflexive relations were not tested. Subsequent to retraining on AB and BC, during Posttest 3, the taught relations averaged 96% and 82%, respectively (not shown in Figure 3), and their symmetric relations, BA and CB, rose to 56% and 61%, respectively. The transitive relation, AC, remained at 44%, and the symmetry of the transitive, CA, was 50%, which was similar to that of Posttest 1.

One-month retention. One-month retention scores (see Figure 3) for reflexive relations were 100%. The taught relations, AB and BC averaged 94% and 65%, respectively. Symmetric relation, BA, remained at 56%, and CB dropped to 28%. The transitive relation, AC, was 50%, which was similar to that of Posttests 2 and 3, and the symmetry of the transitive dropped to 22%.

[FIGURE 3 OMITTED]

Posttest 4 (2-choice) with AB and BC interspersed and reinforced. Due to a large number of errors that occurred when the clock stimuli served as samples and comparisons, they were excluded in Posttest 4, which changed the task from three-choice match-to-sample to two-choice match-to-sample (see Figure 4). For trained relations, AB and BC, scores averaged 97% and 94%, respectively (not shown in Figure 4). Their symmetric relations, BA and CB, were 58% and 83%. The transitive relation, AC, was 92%, and the symmetry of the transitive relation was 80%.

[FIGURE 4 OMITTED]

One-month retention (2-choice). Reflexive relations were not tested. One-month retention scores for the taught relations, AB and BC averaged 89% and 92%, respectively (not shown in Figure 4). Symmetric relation, BA, dropped to 50%, and CB remained at 83% (see Figure 4). The transitive relation and the symmetry of the transitive relation both dropped to 67%.

Discussion

The results of Participants 4 and 5 provided support for the hypothesis that, contrary to Horne and Lowe (1996), individuals with minimal verbal repertoires were capable of showing positive outcomes on equivalence tests. Equivalence results that were considered "positive" were comparable to those reported in previous studies (Brady & McLean, 2000; Study 2 of Carr et al., 2000; Devany et al., 1986). However, it is important to acknowledge that both participants in the present study did not demonstrate stimulus equivalence as it is defined by Sidman and Tailby (1982), in that positive results did not occur on all tests of reflexivity, symmetry, and transitivity.

In a review article by O'Donnell and Saunders (2003), related to measurement of communication skills, they state "it would be helpful if part of that documentation came from standard assessment tools that involve direct measures of specific behavior (i.e., not based on observer ratings), so that comparable information would be provided across studies" (p. 135). The present study extended the results of past studies (e.g., Barnes et al., 1990; Carr et al., 2000; Devany et al., 1986) and, similar to Brady and McLean (2000), went one step further than suggested by O'Donnell and Saunders by reporting language skills using a test of basic verbal operants, rather than relying solely on informal observation of language skills or global measures of language assessment.

As can be seen from Table 2, according to a global measure of language assessment (the communication domain of the VABS), Participants 1, 2, 4, and 5 were all very similar in language ability, with Participant 3 scoring slightly higher than the other participants. As indicated in Table 2, all participants also failed a test of echoics, tacts, and mands, with a failure set at 80%. Nevertheless, there were some differences in performance in terms of their actual scores, and these differences suggest directions for future research. For example, Participants 3, 4, and 5 were all able to perform at least some tacts, and yet only Participants 4 and 5 demonstrated some stimulus equivalence relations. Because Participant 3 had at least a minimal "naming" repertoire, and yet did not learn stimulus equivalence, perhaps, contrary to Horne and Lowe (1996), the emergence of stimulus equivalence cannot be accounted for by an individual's "naming" skills. In contrast, only Participants 4 and 5 demonstrated a few mands, and they were the only two to demonstrate stimulus equivalence. Perhaps a manding repertoire is prerequisite to learning stimulus equivalence. Future research is needed to assess this possibility. At the very least, the current results suggest that future research in this area might benefit from an assessment of the echoic, tacting, and manding repertoire of participants, along with global measures of language.

The study supports the hypothesis that individuals who are able to perform arbitrary relations, as measured by performance on ABLA Level 6 and the VVNM prototype task, are better able to learn the relations necessary to test for stimulus equivalence than are individuals who are not able to perform ABLA Level 6 and the VVNM prototype task. This suggests ABLA level and VVNM performance as possible predictors of an individual's ability to perform the visual nonidentify relations necessary to test for stimulus equivalence. This seems logical given that similar discriminations are required to perform ABLA Level 6, VVNM, and the trained relations AB and BC. As shown in Table 1, all tasks require an individual to make a conditional arbitrary visual discrimination with color, shape, and size as relevant visual cues.

Finally, in contrast to past studies (e.g., Barnes et al., 1990; Brady & McIean, 2000; Carr et al., 2000; Devany et al., 1986), the present study collected 1-month retention data on test relations. As indicated in the Results section, with a few exceptions, performance on tests of 1-month retention for Participants 4 and 5 was lower or the same as the last posttest score. Also, there were instances where faulty matches made during training were "retained" during posttest. For example, Participant 5's difficulty with the trained BC relation during training carried over to posttests. Considering the lack of retention data collected in past studies regarding individuals with minimal verbal repertoires, it is not possible to make comparisons across studies.

Several alternative explanations need to be considered regarding the demonstration of equivalence relations with Participant 4. Results for Participant 4 indicate performance above chance level (i.e., 33%) on symmetric relations, BA and CB, with an improvement in performance from Posttest 1 to Posttest 2. Similar to results of Carr et al., these data may support the gradual emergence phenomenon (Sidman, 1994) that is commonly reported in equivalence studies. It is also possible that an increase in performance may have been caused by changes in test trial order (Green & Saunders, 1998). Specifically, Posttest 1 began with test trials for the transitive relation and symmetry of the transitive relation, AC and CA, followed by test trials for symmetric relations, BA and CB. In contrast, Posttest 2 began with symmetric relations, followed by blocks of test trials for the transitive relation and symmetry of the transitive relation. Additionally, in Posttest 2, blocks of symmetric test trials were preceded by blocks of corresponding training trials (e.g., AB training trials followed by BA test trials). It is therefore possible that changes in test trial order, combined with incorporation of taught relations, may have led to improvements in equivalence test results. However, it is also possible that repeated testing functioned as training.

Alternative explanations of performance for Participant 5 also need to be considered. Posttest 1 results for Participant 5 indicated high scores on reflexive relations, and performance above chance level on all symmetric and transitive relations. Following retraining on the taught relation, BC, a marked drop in performance in symmetric and transitive relations was evident in Posttest 2, followed by an increase in performance in Posttest 3, that was similar to results of Posttest 1. The taught BC relation ranged between 74% and 82% on the three posttests. One possible explanation for lower performance on the BC relation and a failure to demonstrate equivalence relations, despite the meeting of mastery criterion for the taught relations, AB and BC, prior to each posttest, was the change in reinforcement density between training and testing. It has been suggested by several researchers (e.g., Carr et al., 2000; McIlvane & Stoddard, 1985; Sidman, 1986) that individuals with learning difficulties may be especially sensitive to changes in reinforcement parameters, and it may be necessary to adjust the reinforcement schedule in order to maintain an individual's motivation to respond. Specifically, considering that test trials are novel to the participant, he or she may learn to quickly discriminate test trials from training trials, and responses may come under the control of extraneous variables (e.g., stimulus preference) during test trials. In an attempt to prevent this problem, researchers (e.g., Carr et al.; McIlvane & Stoddard) designed posttests such that training trials were interspersed with test trials, and reinforcement was provided during a certain percentage of training trials and test trials. Considering that the present study included no differential reinforcement on Posttests 1 and 2, and differential reinforcement for only taught relations in Posttest 3, a sudden change in the density of reinforcement between training and testing is a possible contributor to the findings.

However, as mentioned in the Results section, extraneous stimulus control for Participant 5 was indicated by the large number of erroneous matches that occurred when the clock stimuli were presented as both samples and comparisons. These errors may be only partially explained by a change in the reinforcement schedule during posttests. When the clock stimuli were removed, and posttests were conducted with only two comparisons, the taught relations remained intact, the symmetric relations, BA and CB, and the symmetry of the transitive, CA, were above chance level, and the transitive relation, AC, was demonstrated. It is not possible, however, to rule out the possibility that repeated testing may have functioned as training for Participant 5 (Saunders & Green, 1992).

The present study has several limitations. First, it is possible that the stimuli chosen for the present study may have led to the development of faulty stimulus control. In choosing the stimuli, the researchers did not take into account the possibility of participants matching by similar component features among stimuli in different classes (e.g., the roundness of the letter "O" and the picture of the clock).

Several limitations regarding the tests of echoics, tacts, and mands need to be acknowledged. One is the limited number of words that were used. The echoic and tact assessment each consisted of 11 words, and the mand assessment consisted of 5 words. Future studies should attempt to expand the test by including a larger number of words from the list of beginning words suggested by Sundberg and Partington (1998). A second limitation is the focus on only spoken words as acceptable responses. Unlike Brady and McLean (2000), the test does not take into account other modes of communication such as picture-based communication, signing, or gesturing. Expansion of the test to include other forms of communication is needed. Lastly, in the mand assessment, prior to each trial, the participant was presented with the item and asked to consume it, in order to provide him or her with a brief history of reinforcement. It was assumed that subsequent presentation of the task with the missing component would create an establishing operation (Michael, 1982) for the item to be manded.

In summary, the present study yielded two main findings. First, it provided evidence against Horne and Lowe's hypothesis that "naming" is necessary for the formation of equivalence relations. In the present study, Participants 4 and 5 demonstrated positive outcomes on some equivalence tests, even though both participants failed a test of echoics, tacts, and mands. Second, in contrast to Participants 4 and 5, who passed ABLA Level 6 and the prototype VVNM prototype task, Participants 1, 2, and 3, who only passed up to ABLA Level 4, were not able to learn the first taught relation, AB, even after approximately 2000 training trials. This suggests that the ability to perform Level 6 and a VVNM prototype task may facilitate the learning of relations necessary to demonstrate positive results on a test for stimulus equivalence.
Table 1 A Description of the ABLA Levels and the Types of
Discriminations Required

ABLA Levels Types of Discriminations

Level 1, Imitation: A simple imitation
A tester puts an object into a container and
asks the client to do likewise.
Level 2, Position Discrimination: A simultaneous visual
When a red box and a yellow can are discrimination with
presented in a fixed position, a client is position, color, shape,
required to consistently place a piece of and size as relevant
foam in the container on the left when the visual cues
tester says, "Put it in."
Level 3, Visual Discrimination: A simultaneous visual
When a red box and a yellow can are discrimination with color,
presented in randomly alternating positions, shape, and size as
a client is required to consistently place a relevant visual cues
piece of foam in the can when the tester
says, "Put it in."
Level 4, Match-to-Sample: A conditional visual-
A client demonstrates Level 4 if, when visual quasi-identity
given a yellow can and a red box in discrimination with color,
randomly alternated left-right positions, shape, and size as
and is presented randomly with a yellow relevant visual cues
cylinder and a red cube, he/she consistently
places a yellow cylinder in the yellow can
and a red cube in the red box.
Level 5, Auditory Discrimination: A conditional auditory-
When presented with a yellow can and a visual discrimination with
red box (in fixed positions), a client is pitch, pronunciation, and
required to consistently place a piece of duration as relevant
foam in the appropriate container when auditory cues and
the tester randomly says, 'red box' or position, color, shape,
'yellow can.' and size as relevant
 visual cues
Level 6, Auditory-visual Combined A conditional auditory-
Discrimination: visual discrimination with
Same as Level 5, except the left-right the same auditory cues as
positioning of the containers is randomly Level 5, and with only
alternated. color, shape, and size as
 relevant visual cues

Table 2 Results on Tests of Language for the Five Participants

Participant Highest Vocal Tacts Mands
 ABLA Imitation (pretest; (pretest;
 Level (pretest) posttest) posttest)
 Passed

1 4 0% 0% 0%
2 4 0% 0% 0%
3 4 73% 18% 0%
4 6 39% 30% 67%
5 6 33% 3% 6%

Participant Age-Equivalent on Age-Equivalent on
 the Communication the PPVT-R
 Domain of the (pretest)
 VABS
 (pretest; posttest)

1 1 year, 3 months 1 year, 10 months (untestable)
2 1 year, 5 months 2 years, 0 months
3 2 years, 2 months 2 years, 0 months
4 1 year, 6 months 2 years, 7 months
5 1 year, 8 months 2 years, 3 months

Note. ABLA tests were administered as specified by Kerr et al., 1977;
tests of Vocal Imitation, Tacts, and Mands were administered as
described by Marion et al. (2003); the VABS was developed by Sparrow et
al. (1984); and the PPVT-R by Dunn and Dunn (1981).


This research was supported by Grant MOP6353 from the Canadian Institutes of Health Research, and grants from the St. Amant Foundation, St. Boniface General Hospital Research Centre, and the Faculty of Nursing, University of Manitoba. Grateful appreciation is expressed to the participants and direct-care staff at St. Amant Centre, and to Drs. Carl Stephens and Angela Cornick for their support throughout the study. We also acknowledge Deanna Betteridge, Rene Hiebert, Shayla Harapiak, and Todd Martin for their help with reliability assessments. Address correspondence to Tricia Vause, Department of Child and Youth Studies, Brock University, 455 South Block, St. Catharines, Ontario, Canada, L25-3A1. (E-mail: tvause@brocku.ca).

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TRICIA VAUSE, GARRY L. MARTIN, C. T. YU, CAROLE MARION, and GINA SAKKO

University of Manitoba and St. Amant Centre
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