Acquisition and extension of syntactic repertoires by severely mentally retarded youth

Research !n Developmenrel Dtsabrhrres, Vol. 8. pp. 549.574. Primed I” the USA. All right\ reserved.

Acquisition

1987 Copynpht

0891.4222187 $3.00 + 00 c 1987 Pcrpamon Journals In‘

and Extension of Syntactic

Repertoires by Severely Mentally Retarded Youth Howard

Goldstein,

Dianne Angelo, and Lori Mousetis UnIverslty

oi PWburgh

This study investigated the conditions that contribute IO generalized language learning in severely menrally retarded children. Matrix-training strategies were used IO reach rhree menrally retarded children syntactic rules for combining known words into two- and three-word utterances. The children applied these rules subsequently to learn unknown words. Generalized learning of responses not taughr direcrl_vwas shown IO be under experimenral conrrol using a multiple baseline design across submatrices. Training only a limired number of responses was sufficient fo promote recombinarive generalization in the [rained modality and transfer 10 untrained responses in rhe opposite modality. Teaching receptive and expressive language responses while simultaneously promoling unrrained responding through matrix training provides an economical and efficienr training approach for menially retarded individuals.

The difficult task of teaching functional communication skills to mentally retarded individuals has prompted investigations of conditions that promote generative language learning. Generative language refers to the comprehension and production of novel or untrained utterances (Goldstein, 1983a). It involves an ability to induce and apply linguistic rules in order to generate receptive and expressive language responses not taught directly. Delineating the conditions that efficiently contribute to generative language learning in mentally retarded individuals is the focus of this investigation. Miniature linguistic systems have been used to explore the relationship

This research was supported by Grant No. HD-17850 from the National Institutes of Child Health and Human Development to the University of Pittsburgh. Requests for reprints should be addressed to Howard Goldstein, Department of Communication, 1117 Cathedral of Learning, University of Pittsburgh, Pittsburgh, PA 15260. 549

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H. Goldstein, D. Angelo, and L. Mousetis

between environmental conditions and language learning (e.g., Esper, 1925; Foss, 1968; Morgan & Newport, 1981; Wolfle, 1933). Miniature linguistic systems have typically consisted of a made-up language or a subset of English, in which word classes are combined according to specified syntactic rules. A simple miniature linguistic system is a stimulus-response matrix in which responses (e.g., producing words) controlled by one stimulus class are taught to occur in sequential order with responses controlled by another stimulus class(es). For example, a child might be taught to refer to actionobject events with utterances such as “push car.” One could extend this example by including four actions and four objects, which would permit 16 possible action-object combinations. This process of stimulus control has been referred to as recombinative generalization (Goldstein, 1983b) and defined as the “differential responding to novel combinations of stimulus components that have been included previously in other stimulus contexts” (p. 281). Recombinative generalization occurs when the language learner discriminates the relations between words and referents and induces a word order rule allowing novel combinations of words from two or more word classes. Recombinative generalization enables individuals to comprehend and express untrained utterances. Esper (1925) first used miniature linguistic systems to study language learning experimentally. Esper taught two-word labels in response to 14 colored shapes and found that adults were able to label two untrained colored shapes. Since then, other investigators have used miniature linguistic systems to investigate the relationship between various experimental conditions and language learning (see Wetherby, 1978). The matrix training paradigm has been extended to mentally retarded populations, where it has become increasingly useful for conceptualizing the conditions that promote generative language learning among language delayed individuals (Goldstein, 1983b; Karlan, Brenn-White, Lentz, Hodur, Egger, & Frankoff, 1982; Wetherby & Striefel, 1978). Several investigations have focused on the acquisition of syntactic repertoires by mentally retarded individuals (Guess, Sailor, Rutherford, & Baer, 1968; Lutzker & Sherman, 1974; Schumaker & Sherman, 1970; Striefel, Wetherby, & Karlan, 1976, 1978). The matrix training paradigm can be used conceptually to explicate the conditions used in these studies (sometimes inadvertently) to establish generalized language learning (see Wetherby, 1978). The results of these studies suggest that the use of this systematic

teaching strategy was responsible for rule induction resulting in recombinative generalization. An important variable in matrix training studies is the selection of training items (Goldstein, Angelo, & Wetherby, 1987). Selection of training responses that include overlap of constituents (words) typically has been re-

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quired to facilitate recombinative generalization. Overlap training refers to the same word(s) appearing in more than one response being trained. Overlap among training stimuli requires subjects to discriminate and respond differentially to each constituent as it appears in various word combinations (Foss, 1968; Goldstein et al., 1985). However, Goldstein (1983b) has argued that overlap may not be necessary when the language learner knows the words comprising the miniature linguistic system. One must question whether matrix training procedures have been implemented in the most efficient way possible in previous studies. The emphasis of previous studies has been on the learning of a word order rule in matrices incorporating new or unknown words (Romski & Ruder, 1984; Striefel, Wetherby, & Karlan, 1976, 1978). This approach requires the language learner to learn both the referents for words and the word order rule simultaneously. Thus, mentally retarded subjects may have been required to learn under conditions more demanding than those normally encountered in the natural environment. An alternative approach is to use words already in the language learner’s repertoire thereby reducing the difficulty of the task and focusing only on the rule induction process. A recent matrix training study has explored ways to promote recombinative generalization more effectively (Goldstein, 1983a). Goldstein found that knowledge of one class of words in a two-dimensional matrix enhanced recombinative generalization results. To extend this approach further, one could begin training a matrix comprised of all known words. Thus, the word order rule could be learned still more efficiently. Subsequently, the child may apply the rule to learning new or unknown words. That is, based on a word order rule the subject may infer, for example, that words in the first position refer to objects and that words in the final position refer to locations in an object-location matrix. Initiating training with a submatrix comprised of all known words is hypothesized to represent an economical approach to generative language learning. Another generalization process is crossmodal transfer. Crossmodal transfer refers to receptive responding following expressive training or expressive responding following receptive training. Ruder, Hermann, and Schiefelbusch (1977) examined isolated comprehension and production learning and found that crossmodal transfer did not occur until a comprehension component was added to production training. In a related study, Lee (1981) pretrained the production of words imitatively and found that crossmodal transfer to production was observed after receptive learning was accomplished. This contradicts the findings of Ruder et al. (1977). Goldstein (1985) posited that crossmodal transfer may be related to the criterion used to terminate training. He suggested that crossmodal transfer involving multiword utterances may not occur after the learning of individual re-

552

H. Goldstein, D. Angelo, and L. Mousetis

sponses, but is more likely to occur after recombinative generalization in the trained modality. Clearly, more research is needed examining the effects of matrix training on receptive and expressive language learning and on transfer to the untrained modalities. Another factor that may determine the extent of crossmodal transfer is the degree to which responses probed in the opposite modality are reinforced. Striefel, Wetherby, and Karlan (1976) argued that consistent reinforcement of crossmodal probe items enhances the extent of crossmodal transfer. Otherwise, crossmodal responding may extinguish quickly. Identifying procedures that reliably promote extensive crossmodal transfer would be an important addition to the language intervention process. The purpose of this study was to identify the conditions that promote acquisition and extension of syntactic and lexical repertoires by mentally retarded individuals. Four questions were addressed: 1. Does matrix training with only known words facilitate recombinative generalization? 2. Is the learning of a word order rule applied to the learning of unknown words when training items are selected from the diagonal of the matrix (i.e., without overlap among the stimuli)? 3. To what extent are recombinative generalization and crossmodal transfer demonstrated subsequent to receptive and expressive matrix training with known word combinations and unknown word combinations? 4. Do receptive and expressive responding generalize to language use with nontrainers in other settings? METHOD

Subjects

Three severely mentally retarded individuals, aged 18;11, 7;5, and 9;3 years, enrolled in a self-contained public school for mentally retarded students participated. All subjects were identified as having language handicaps and were enrolled in the school’s speech/language therapy program prior to and during their participation in the study. The subjects were nominated by their teachers and speech pathologists and were selected on the basis of their limited expressive language ability. Specifically, the subjects demonstrated restricted use of two-word (Subjects 1 and 2) or three-word (Subject 3) combinations during spontaneous language sampling. Subjects 1 and 2 did not produce any object-location utterances (e.g., “hat chair”) during language sampling at the outset of the study. Subject 3 produced

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prepositional phrases, but did not produce object-preposition-location utterances (e.g., “juice in cup”). Subject 1 had an IQ of 23 and severely delayed language development. His language consisted primarily of single word utterances that were responses to direct questions. Few spontaneous utterances were observed. He demonstrated frequent echolalic and perseverative responses and a limited expressive vocabulary. He was not spontaneously interactive but responded in structured activities when tangible reinforcers were available. Although generally compliant, he often engaged in stereotypic behavior. At the beginning of the study, Subject 1 demonstrated a receptive language age of 20 months and an expressive language age of 24 months according to the Sequenced Inventory of Communication Development (SICD). Based on a spontaneous language sample, his mean length of response (MLR) was 1.07 words. Subject 1 was generally intelligible if the listener was aided by contextual cues. Subject 2 was a Down Syndrome female with an IQ of 43. This subject was highly inconsistent in the use of her expressive language skills. She was reluctant to communicate on numerous occasions and was often noncompliant. At the beginning of the study, Subject 2 demonstrated receptive and expressive language levels of 28 months each according to the SICD. Her MLR was 2.07 words. Subject 3 was a male with Down Syndrome with an IQ of 41. He had the most developed expressive language of the subjects. He consistently produced two-word combinations in spontaneous conversation and was generally intelligible. On the SICD, Subject 3 performed receptively at a 40 month level and 36 months expressively. His mean length of response was 2.23 words. Subjects 1, 2, and 3 were involved in the study for nine, six, and seven months, respectively. Setting

and Stimuli

Experimental sessions were conducted in a quiet room in the school. Each student was trained individually four days each week. The length of each session was 20 to 30 minutes. The students were seated beside the experimenter at a table facing a doll house containing miniature plastic furniture. The locations of the items were rotated daily to prevent inappropriate response strategies based on position. Six to eight small objects, including known and unknown objects, were placed in front of the doll house. The actual objects and locations varied among subjects (see Matrix Organization below).

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H. Goldstein, D. Angelo, and L. Mousetis

Pretraining Assessment A pretraining assessment was conducted with each subject to identify object, location, and preposition words that were then used to construct an individual matrix for each subject. The assessment included both receptive and expressive identification of potentially known and unknown words.

Selecting known words. In order to identify object and location words that already existed in the subjects’ receptive and expressive repertoires the following procedure was used. Initially, a group of five objects was presented and the subject was asked to identify an object on request by pointing. Five trials per item were presented. If the subject correctly identified an object four out of five times, it was assumed that the item was a known receptive vocabulary item. The same procedure and criterion was used to identify location words known receptively, using furniture items instead of objects. Expressive identification of known object and location words was accomplished by requiring subjects to label each item presented by the experimenter. Because the probability of providing a correct response by chance was so low, three trials per item were presented. If the subject correctly labeled the item on three out of three trials, the item was assumed to be a known object or location word. In addition to this procedure, Subject 3 was assessed on preposition words since he was more advanced linguistically. The experimenter placed a miniature table in front of the subject and instructed him to manipulate a small object (chicken) following a command containing a preposition (e.g., “put it on the table”). Following receptive assessment, the subject then expressively identified the location (preposition) of the object when placed by the experimenter in response to “where is the chicken?” The same criteria were used to affirm that a preposition was known receptively and expressively.

Selecting unknown

words. The assessment procedures also were used to identify object, location, and preposition words that were unknown for each subject. Unfamiliar objects (e.g., wrench and paddle) and locations (e.g., wardrobe and hutch) were used. The criteria for inclusion of the unknown words were set at three out of three incorrect responses receptively, and one of one incorrect response expressively. This criterion reflects the assumption that if the child never responded correctly on three receptive trials (with 20% chance probability for objects and locations), we could be reasonably confident that he/she was not likely to respond correctly on repeated expressive trials.

555

Acquisition and Extension of Syntactic Repertoires

Matrix

Organization

Based on the preassessment, miniature linguistic systems were developed with either two or three referential dimensions. As shown in Figure 1, object words were placed along the vertical axis while location words bordered the horizontal axis to constitute a two-dimensional matrix. An 8 x 8 objectlocation matrix was developed for Subject 1 (see Figure 1) and a 6 x 6 objectlocation matrix for Subject 2 (who started her participation in the experiment later in the year). To extend the matrix in a third dimension for Subject 3 (who was more linguistically sophisticated), preposition words were placed along another axis. Thus, a 5 x 5 x 5 object-preposition-location matrix was developed for Subject 3 (see Figure 2). The common features of the three matrices was the inclusion of an entire submatrix of only known words and several submatrices that included unknown words. Each unknown word submatrix reflected the possible generalization predicted experimentally given the introduction of a specific training item. For example, in Figure 1 the introduction of training item 2 (T2)

LOCATIONS UNKNOWN

KNOWN I

\

”

BALLOON

-6

-7

-8

COMB

-44

15 -

-16

KEY

-22

23 -

24 -32

MONEY

Y 2 0

I

PADDLE

38 T,

PAPER CLIP

% 5

-48

ifi

SCALE

56

WRENCH

TS 64

-

61

62 -

63 -

FIGURE 1. The object-location matrix including both known and unknown words taught to Subject 1. The five training items designated with a “T” show the most efficient progression through the matrix. The five submatrices are bordered by bold lines.

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H, Goldstein, D. Angelo, and L. Mousetis

FIGURE 2. The object-preposition-location matrix including both known and unknown taught to Subject 3. The five training items are designated with a “T.”

words

includes a new object word paddle and a new location hutch. Generalization under experimental control was predictable for novel combinations of paddle with previously learned locations (cells 33, 34, 35, 36) and hutch with previously learned objects (cells 5, 13, 21, 29).

Experimental Design and Conditions A multiple baseline design across responses was employed. A multiple probe technique was employed to monitor baseline and generalization performances. Baseline measures evaluated whether unexpected learning of words was accomplished before training was initiated. This design illustrated how the training of specific responses resulted in experimentally-predictable generalization within submatrices. If generalized responding occurred within submatrices only as predicted, one could be confident that the training conditions were responsible for experimental effects. When generalization across modalities was not demonstrated, an item from one submatrix at a time was taught in the second modality. The progression of training for each subject is presented in Table 1. Probes of untrained responses were interspersed among training trials. One response was taught at a time. The number of remaining untrained cells in each submatrix as training progressed is indicated in Table 1. For each subject, Submatrix 1 consisted of all known words. The goal of the first training phase was to establish recombinative generalization of known words after training a single response. Subjects 1 and 2 received receptive training and Subject 3 received expressive training on Tl (see Figures 1 and 2). Because Subjects 1 and 2 demonstrated noncompliant behavior during the preassessment, receptive responding was selected so respond-

Acquisition and Extension of Syntactic Repertoires

557

ing could be prompted physically, if necessary. Otherwise, beginning with expressive training may have been preferable, because of the enhanced crossmodal transfer to comprehension found in previous research (Cuvo & Riva, 1980; Keller & Bucher, 1979; Miller, Cuvo, & Borakove, 1977). When the mastery criteria were met for Tl and recombinative generalization in Submatrix 1, training proceeded to the unknown submatrices. Training of the unknown word submatrices required the subjects to understand or produce two- or three-word combinations that included unknown words. The learning of the word order rule in Submatrix 1 was thought to provide a basis for subjects to determine the relationship between new words and their referents, thus enabling subjects to extend their lexical and syntactic repertoires. For example, the subjects could learn new words by noting that the first word in the utterance refers to the object and the last word refers to the location. The selection of the new training item (T2) from the diagonal of the matrix made generalization to untrained items Submatrix 2 possible. For example, Cell 37 in Figure 1 was the second training item (T2-paddle on hutch) for Subject 1. Based upon the application of the syntactic rule learned

TABLE 1. Summary of Training Progression Used to Demonstrate I&combinative Generalization to Untrained Responses Within Submatrices According to a Multiple Baseline Design

Submatrix Subject 1:

1 2 3 A A

Subject 2:

1 2 A

Subject 3:

1 2 A (3)

Training Item

Training Modality

Number of Submatrix Cells Not Trained

Tl T2 T3 T4 T5 T6 Tl

balloon on bed paddle on hutch paperclip on speaker scale on globe paperclip on globe wrench on cabinet paddle on hutch

Ret Ret Ret Exp Exp Exp Exp

15 8 10 12 11 14 9

Tl T2 T3 T4

key on bed wrench on speaker scale on stool scale on speaker

Ret Exp Ret Exp

15 8 10 9

Tl T2 T3 T4 T5

money on desk paperclip right of wardrobe wrench left of cabinet key left of cabinet wrench right of cabinet

Exp Exp Ret Exp Exp and Ret

26 36 60 59 58

Note: Parentheses designate the selection of an overlapping training item within a submatrix.

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H. Goldstein, D. Angelo, and L. Mousetis

in Phase 1, recombinative generalization was expected to occur to Cells 5, 13, 21, 29, and 33-36. These cells represented novel word combinations containing one of the unknown words of the new training item (i.e., paddle or hutch). T2 was taught using the original training modality for Subject 1 (receptive) and Subject 3 (expressive), and the opposite modality (expressive) for Subject 2. Subject 2 had become resistive to receptive responding and prompting. The procedure and criteria for training Submatrix 2 were identical to those used to teach Submatrix 1. When the mastery criteria were achieved, training progressed to the next training item along the diagonal of the matrix. Overlap training was required if the subject failed to achieve the criteria for progressing to the next submatrix. The effects of training responses within submatrices that included unknown words were replicated across four submatrices (2-5) with Subject 1 and across two submatrices (2-3) with Subjects 2 and 3. Note in Table 1 that training was conducted in both the receptive and expressive modalities with each subject. This allowed a preliminary evaluation of the relative effectiveness of both training modalities in promoting recombinative generalization and crossmodal transfer. If discrimination problems were evident from systematic error patterns at the completion of training, brief reviews of training (described in the Results) were implemented to stabilize learning. A posttest of all-the responses in.each subject’s matrix then was conducted. Finally, transfer of training tests (described below) were conducted to assess generalization of learning to other settings and people. Training Procedures

Each time a new training item was introduced, massed practice trials were used to establish the response topography initially using modeling procedures and imitation. The subjects were required to respond correctly to the new training item on five consecutive trials before the training sequence was initiated. During receptive training trials, the experimenter presented an instruction such as “put the balloon on the bed.” The subject responded by placing an object with respect to a specific location. All correct responses were reinforced with social praise, edibles, and/or tokens. Following incorrect responses a correction procedure was instituted. The experimenter modeled the correct response and then repeated the trial. If the subject failed again to respond correctly, the experimenter physically assisted the subject to place the object after repeating the instruction. All corrected or prompted responses were followed by praise only. During expressive training trials, the experimenter placed one object with .

Acquisition and Extension of Syntactic Repertoires

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respect to a location and asked, “What did I do?” The subject was requested to describe the event with a complete and correct response (e.g., scale globe for a two-dimensional matrix and money on desk for a three-dimensional matrix). Tokens, edibles, and/or social praise were used to reinforce all correct responses. An incorrect response prompted a correction procedure in which the experimenter modeled the intended response. If the subject failed to imitate the complete response, he/she was encouraged to repeat each word individually. Corrected or prompted responses were reinforced socially (i.e., praise, pats, slapping five). Throughout training, all correct responses to any nontraining (probe) trials also were reinforced; no feedback was provided for incorrect responses. Probe trials. The selection of a training item from a submatrix determined whether other nontraining items remained in baseline or whether recombinative generalization or crossmodal transfer was possible. Thus, as the experiment progressed, more and more items that were originally baseline probes became recombinative generalization and crossmodal generalization probes, until no baseline probes remained. Nontraining items included the following: 1. Maintenance probes, which were previously trained items; 2. Recombinative generalization probes, which were untrained combinations of known words or words learned through exposure to training items; 3. Crossmodal generalization probes, which were training items and recombinative generalization items in the opposite or untrained modality; and 4. Baseline probes, which were combinations of untrained words. Recombinative and crossmodal generalization probes were used to assess the acquisition of a word order rule in the trained modality and to determine the extent of transfer to the untrained modality, respectively. Maintenance probes of previously trained items were included to monitor the stability of these responses over time. Baseline probes monitored any unexpected learning of untrained words. Training and probe trials were randomly ordered on computer-generated data sheets. The number of trials per data sheet ranged from 40 to 69. It typically took two sessions to complete each data sheet. Each data sheet included 12 training trials (17%-30% of the trials). Probes were increased over the course of the study from as few as 28 to as many as 57 (70%-83% of the trials). The likelihood of probes being responded to correctly increased over the course of the study. Nevertheless, in order to ensure that

560

H. Goldstein, D. Angelo, and L. Mousetis

subjects had ample opportunities to be reinforced, comprised more than 26% of the trials.

baseline probes never

Mastery criteria. Training blocks consisted of a minimum of four data sheets allowing ample opportunity to include probe trials sampled from the entire matrix. At the completion of each block, the performance of each subject was reviewed. First, mastery of the training item was set at 90% correct responding over two consecutive data sheets. If the subject failed to master the training item it was necessary to repeat the training block with the same item. Second, mastery reflecting recombinative generalization was set at 50% correct responding to untrained responses in the training modality. The 50% criterion reflects a weighting to compensate for the low likelihood of recombinative generalization as training began with a new training item. A higher criterion over fewer data sheets could have been selected, but a sampling of relatively few probes could have biased results. If mastery was demonstrated for the training item but recombinative generalization did not improve, then a second training item was selected from the same submatrix. In this way, overlap was instituted to remediate discrimination problems hindering recombinative generalization. If’both the mastery criteria for training and recombinative generalization were met, a diagonal training item from an unknown word submatrix was selected to expand the subject’s repertoire most efficiently. For example, because Subject 1 responded correctly to 100% of the T2 training trials (Cell 37 in Figure 1) on the third and fourth data sheets of block 2 and responded correctly to 92% of the 48 receptive recombinative generalization probes during block 2, the next item down the diagonal of the matrix (Cell 46 in Figure 1) was introduced into training. If his responding to Training item 2 fell below 90% correct on the third or fourth data sheets, we would have continued training that item. If his receptive recombinative generalization was below 50% during block 2, we would have introduced an overlapping item (either Cell 5 or 33 in Figure 1). Transfer Tests At the completion of training, two types of transfer tests were used to assessgeneralization across people and settings. In the first test, randomly selected matrix items were administered by a teacher or language clinician in different rooms in the school. The dollhouses were taken into the classrooms and the original stimuli were used. Only praise was provided for correct responses and no correction procedures were used. The second transfer test was conducted in the classroom or home environment by a different teacher or a parent. In consultation with the teacher or parent, familiar words were identified specific to the subject’s classroom

Acquisition and Extension of Syntactic Repertoires

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or home environment. Thus, the test consisted of combinations of known object and location words. These combinations were presented as in the first transfer test. This second transfer test assessed the extent of generalization to additional word combinations and reflected generalization of language use to a third setting. The testers were all familiar with the subjects. They were given practice administering and scoring receptive and expressive responses with the experimenter acting as the child. The experimenter generated the data sheets and provided the necessary stimuli, data sheets, and written instructions to the tester. The experimenter was not present during transfer testing and consequently, reliability measurements were not obtained. Recording

Responses

and Reliability

Each word of a training or probe response was recorded as correct ( + ) or incorrect (-) for each trial. In addition, all incorrect responses were documented by recording the receptive or expressive responses provided by the subject. Failure to respond and unintelligible responses also were recorded. A training item was considered correct only if all words of the response were scored ( + ). Reliability measures were recorded by an independent observer during training sessions approximately once a week for each subject. Reliability measures were obtained for 34% of the training sessions for all subjects. Reliability was calculated by comparing the trainer’s and the observer’s recordings of correct/incorrect responses on a trial-by-trial basis. Agreement was determined by dividing the number of agreements by the number of agreements plus disagreements, and multiplying this value by 100. Reliability ranged from 96% to loo%, with a mean of 99.5%. Reliability measures were not obtained for the transfer tests. RESULTS A summary of each subject’s performance on training trials, receptive and expressive recombinative generalization probes, and receptive and expressive baseline probes is presented in Table 2. Note that only one response was trained in the unknown word submatrix (1) with each subject. That is, Subjects 1 and 2 received training on one cell that included only two of the eight words in Submatrix 1 (see Figure 1) and Subject 3 received training on one cell that included three of the nine words in Submatrix 1 (see Figure 2). Each subject was able to extend their generalized repertoire to Submatrix 2 with the introduction of a diagonal item from the matrix. Recombinative generalization waned with the introduction of additional diagonal items from the matrix. Overlap training was instituted with TS with Subject 1 and

96 48 96 48 184 184

96% 94% 97% 81% 86% 75%

90% 77% 92% 62%

100% 96% 85% 72% 95% 49% 77%

86% 74% 66% 60% 78% 96%

II2 66 128 68 60 I25

60% 87% 73% 76% 93%

56% 57% 54% 75% 95%

I00 200 148 I52 128

60 100 180 42 72

89% 92% 70%

36 48 216

82% 86%

60 125

training

83% 65% 46”I’o 63%

44% 60% 73% 81% 92%

45% 38% 33% 28% 39% 60% 67% 85% 100%

104 63 128 60

68 96 188 42 72

40 48 216 96 192 80 88 128 64

Expressive Generalization Probes and Percent Correct

Note: Parentheses designate the selection of an overlapping included all items in the matrix including training items.

3: Exp Exp Ret Exp Exp Ret Posttest

48 48 96 26

Subject 2: TI Ret T2 Exp T3 Ret (T4) Exp Posttest

Subject TI T2 T3 (T4) (T5)

48 48 144 48 128 80 48

Subject 1: TI Ret T2 Ret T3 Ret T4 Exp (T9 Exp T6 Exp (T7) Exp Posttest

Training Item and Modality

No. Training Trials and Percent Correct

Receptive Generalization Probes and Percent Correct

- -

item within

_

_

48 30

_

IO 9

-

24 20 48 16 32 -

a submatrix.

13% 37%

20% 0%

13% 150/o 4% 19% 3107u

Receptive Baseline Probes and Percent Correct

TABLE 2. Summary of Trials and Performance Levels for Training Trials, Generalization Probes, and Baseline Probes During the Training of Each Response and a Review

0% 1707u

0%

ova

0% 0% 4% 0% 0%

The posttest

48 30

5 6

I2 8 24 8 I6 -

Expressive Baseline Probes Percent Correct

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563

T4 with Subjects 2 and 3 to alleviate apparent discrimination problems identified during probing, as described below. Final posttesting revealed nearly complete recombinative generalization for all subjects. Receptive responding ranged from 93%-96% correct and expressive responding from 86%-100% correct. &-rly 2%-6% of these responses could be attributed to direct training. In Table 2, data were aggregated for each block of trials. Consequently, correct responding percentages generally underestimate the subjects’ performance at the end of the block. For subsequent analyses, subjects needed to meet a 50% criterion to be credited with generalization in a particular cell of the matrix. Figures 3-5 present the percentage of cells in each submatrix for which generalization occurred. Subject

I

During receptive training of Tl (Cell 1) in the known word submatrix, 48 trials were presented and a performance level of 100% correct responding was obtained. Not only did Subject 1 achieve the mastery criterion of 90% correct responding over two consecutive data sheets, but he also generalized to 93% of the 15 cells receptively and 44% of the 16 cells expressively as shown in Figure 3. Baseline performance for unknown word Submatrices 2-5 remained low. Training was initiated with T2 (Cell 37) from Submatrix 2. Again, the 90% mastery criterion was met after only 48 trials. Subject’ 1 demonstrated receptive recombinative generalization for each of the 8 cells in Submatrix 2. Recombinative generalization in the untrained modality remained the same in Submatrix 1s but was nonexistent in Submatrix 2. Receptive learning of T3 (Cell 46, selected from the matrix diagonal) required extensive training, as Subject 1 had difficulty mastering the new training item. After 12 data sheets (144 trials), the mastery criterion of 90% correct responding over two consecutive days was met. As can be seen in Figure 3, receptive recombinative generalization was demonstrated for 8 of the 10 cells in Submatrix 3. During the training of the third receptive response, crossmodal transfer to production improved for Submatrix 1 and began to emerge for Submatrices 2 and 3. As expected, receptive and expressive baseline performance remained low for Submatrices 4 and 5. The next diagonal item T4 (Cell 55) was trained expressively. The training criterion was met after four data sheets. Despite training in the expressive modality, Subject l’s only recombinative generalization (4 of 12 cells) was demonstrated in the receptive modality. Because of a lack of expressive recombinative generalization, an overlapping training item T5 (Cell 47) was selected. Training trials for T4 and T5 were intermixed for eight data sheets. Expressive recombinative generalization was demonstrated to 7 of 11 cells in

564

H. Goldstein, D. Angelo, and L. Mousetis

L? p 100 2

60

g

60

g

40

3

20

= _i P

0

_ i I’ III

I I.

SUBMATRIX

2

SUBMATRIX

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I,

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I / 2 g

= t

I,,

I

L

100 60 60 t

SVBMATRIX 1

4

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60 40 SUBMATRIX

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5

2 3 4 5 6 7 REVIEW TRAINING ITEMS FOR SUBJECT 1

FIGURE 3. Percentage of generalired receptive and expressive responses for each submatrix taught to Subject I. The dashed vertical lines represent the introduction of each of the seven training items in the receptive modality (R) or the expressive modality (E). Closed circles denote receptive performance and open circles denote expressive performance.

Submatrix 4. Receptive responding (crossmodal transfer) was demonstrated for 9 of 13 cells in Submatrix 4. Some receptive recombinative generalization was evident in Submatrix 5 (see Figure 3). This partial loss of experimental control was due to the learning of the object word, wrench, apparently through an elimination strategy and differential reinforcement. Consequently, Subject 1 often responded correctly to probes that included wrench with a previously learned

Acquisition and Extension of Syntactic Repertoires

565

location. Expressive training of T6 (Cell 64) resulted in improved receptive and expressive generalization. As can be seen in Figure 3, crossmodal transfer to comprehension exceeded expressive recombinative generalization. Because receptive and expressive recombinative generalization remained limited in Submatrix 3, an overlapping response T7 (Cell 38) was trained expressively. Subject 1 demonstrated complete expressive recombinative generalization within Submatrix 3 and demonstrated crossmodal transfer to 8 of 10 cells in this submatrix. Before final posttesting of the entire matrix, a review of all objects and locations was conducted to determine whether lexical confusions persisted, attenuating generalization effects. Confusion was evident among the locations hutch, cabinet, and dresser. Although dresser was selected as a known location at the outset of the study and thus, never was included in a training stimulus, it apparently was not well established in the child’s repertoire. This problem was remediated through expressive training with the three locations. Subsequent posttesting of all cells in the matrix revealed complete receptive and expressive recombinative generalization.

TRAINING ITEMS FOR SUBJECT 2

FIGURE 4. Percentage of generalized receptive and expressive responses for each submatrix taught to Subject 2. The dashed vertical lines represent the introduction of each of the four training items in the receptive modality (R) or the expressive modality (E). Closed circles denote receptive performance and open circles denote expressive performance.

566

H. Goldstein, D. Angelo, and L. Mousetis

tsi=G-iI

_

A-”

I

! -

TRAINING

I

!

SUEMATRIX

2

SUBMATRIX

3

ITEMS FOR SUBJECT

3

FIGURE 5. Percenlage of generalized receptive and expressive responses for each submatrix taught IO Subject 3. The dashed vertical lines represent the introduction of each of the five lraining items in the receptive modality (R) or the expressive modality (EL Closed circles denote receptive performance and open circles denote expressive performance.

Subject

2

Subject 2 was trained receptively on Tl. The mastery criteria for training and receptive recombinative generalization were met in four data sheets. In Figure 4, one can see that crossmodal transfer to expressive responding was nearly complete. Also, receptive recombinative generalization was demonstrated during baseline to 50% of Submatrix 2 probes. Like Subject 1, this subject learned an object, wrench, apparently through the process of elimination and differential reinforcement. Learning of the location from Submatrix 2 and the object and location from Submatrix 3 was not evident during the baseline condition. Because the subject was resistive to receptive responding, T2 was trained expressively. After four data sheets, the mastery criteria for the training item and expressive recombinative generalization were met. Crossmodal transfer to receptive responding was nearly complete. The final diagonal training item T3 from the matrix was introduced receptively and eight data sheets were required to meet the criteria. Although receptive recombinative generalization and crossmodal transfer to expressive responding were demonstrated for 10 of 10 cells and 10 of 11 cells, respectively, performance was inconsistent. Subject 2 sometimes confused the two

Acquisition and Extension of Syntactic Repertoires

567

new objects, wrench and scale. Consequently, an overlapping training item, T4, was trained expressively. The consistency of generalized responding improved somewhat, but it was not until new reinforcers were introduced that Subject 2 began to respond with few errors. This may indicate that her errors stemmed from a motivation problem more than a discrimination problem. A final posttest of all cells in the matrix was conducted after two sessions of review of the four training items. Each cell was tested twice in each modality. Receptive responding averaged 93% correct and expressive responding averaged 91.5% correct. Subject

3

Subject 3 received expressive training on Tl selected from a three-dimensional language matrix (see Figure 2). The mastery criteria for training and recombinative generalization were actually met after four data sheets, but training was continued for an extra four data sheets in order to monitor a dramatic drop in expressive recombinative generalization demonstrated during the fourth data sheet that persisted on the fifth data sheet. As can be seen in Figure 5, Subject 3 generalized expressively and receptively to all the cells in Submatrix 1. Expressive training continued with T2 in unknown Submatrix 2. The criteria for training and expressive recombinative generalization were met in four data sheets, Note that for Submatrix 2 crossmodal transfer to receptive responding was slightly above expressive recombinative generalization (see Figure 5). The elevation in baseline responding for Submatrix 3 occurred because Subject 3 (like Subjects 1 and 2) appeared to learn the last unknown object through the process of elimination and differential reinforcement and, thus, responded correctly to stimuli that included wrench with known locations, such as “put the wrench on the desk.” T3, from the final unknown word submatrix, was trained receptively. After eight data sheets the training criterion was met, but expressive recombinative generalization remained below the 50% criterion. In addition, performance an receptive probes from Submatrix 1 was slightly reduced primarily because the newly introduced preposition, ieft of, was substituted for in front of. Also, recombinative generalization for Submatrices 2 and 3 was inconsistent due to confusion between the prepositions right of and left of. The training of two additional overlap items, T4 and T5, finally remediated these problems. Training item T4 was trained expressively and T5 was trained both receptively and expressively. A within-stimulus (loudness) prompt was used to help establish the discrimination between right of and left of and then faded. Only after receptive and expressive training of T5 did Subject 3’s generalized responding stabilize at a high level for Submatrices 2 and 3.

H. Goldstein, D. Angelo, and L. Mousetis

568

A review of training items from Submatrices 2 and 3 was conducted for six sessions to overtrain the discrimination between righr of and left of before the final posttest. Each of the 125 cells of the matrix was tested once in each modality during the posttest. Generalized expressive responding averaged 96.3% among the three submatrices and generalized receptive responding averaged 86.3%.

Transfer Tests Transfer tests were conducted to assess generalization across people and settings. The results are summarized in Table 3. Each subject’s teacher or speech pathologist administered a transfer test using the trained stimuli and the dollhouse in the child’s classroom. In addition, transfer tests were conducted in schoolrooms by teachers using familiar objects and locations with two of the children and a transfer test was administered by a parent in the home of one child. Transfer was consistently high for all subjects. Receptive responding ranged from 78% to 95% and expressive responding ranged from 75% to 96%.

Receptive vs. Expressive Learning and Crossmodal Transfer Additional analyses were conducted to examine the extent to which crossmodal transfer was demonstrated subsequent to receptive and expressive training. It was notable that recombinative generalization was complete in both modalities for all three subjects. Even on transfer of training probes reflecting generalization across settings and people, subjects maintained high accuracy rates, averaging 91% receptively and 89% expressively. Oppo-

Number

TABLE 3. of Trials and Percentage of Correct Performance During Transfer of Training Testing

Subject

Stimuli

Tester

Keceptive

I

Dollhouse School Home

Speech path. ADL teacher Mother

25

I6 I6

95% 94% 94%

2

Dollhouse

Teacher

25

3

Dollhouse Dollhouae School

Teacher Speech path. Tcachcr

25 25 20

Expressive 25 I6 I6

92% 94% 75%

95%

25

88%

84% 72% 90%

25 100% 25 92% 20 95%

Note: Known words used in the school and home tests t’or Subject I were: cup. plate, pot. spoon, dishwasher, ret’rigerator, stove, and table. Known word\ used in the whool test l’or Subject 3 were: book, ho\. crayon. paper. pen. basket, chair, desk, ~001, and table.

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site modality probes assessed performance in the receptive modality for the response undergoing expressive training and vice versa. On these probes, crossmodal transfer to comprehension during expressive training averaged 7 1% and transfer to production during receptive training averaged 40%. Responses for each cell of the matrix were analyzed to compare the modality in which responses first occurred (see Table 4). Untrained responses were demonstrated first more often in the receptive modality. On the average, 62% of the generalized responses first occurred in this modality. Nevertheless, all subjects first demonstrated some untrained responses in the expressive modality, averaging 17% of the responses. This expressive performance typically occurred while a training item was undergoing expressive training. DISCUSSION Previous matrix training studies typically taught miniature linguistic systems comprised exclusively of new or unknown words (e.g., Romski & Ruder, 1984; Striefel, Wetherby, & Karlan, 1976, 1978). Not only were the subjects required to learn these unknown words, but also the word order rule for combining them within the same learning task. Prior lexical learning was not considered in previous studies. Each of the words constituting the language matrix were included in the cells of the matrices taught to promote recombinative generalization (see Goldstein, 1983b). In contrast, Subjects 1 and 2 were taught just one response from 4 x 4 object-location submatrices and Subject 2 was taught just one response from a 3 x 3 x 3 object-preposition-location matrix. All subjects demonstrated extensive recombinative generalization. Subjects 2 and 3 also demonstrated extensive crossmodal transfer while Subject 1 demonstrated limited crossmodal transfer. When the subjects were presented with known words and taught to combine them during training, the learning demands focused on syntax. Without the competing demands of learning the referents for words, recombinative generalization was rapid and complete within the known submatrix for all subjects. This finding supported Goldstein’s (1983b) contention that minimal training is necessary when known words are recombined for the language learner. Whether syntactic learning was accomplished could only be evaluated later when unknown words were introduced. As expected, baseline performance remained low in submatrices that included unknown words. Some learning of object words was accomplished, however, apparently through the process of elimination and differential reinforcement. Such learning resulted in unexpected increments in baseline responding, which on one occasion was as high as 50% correct due to Subject 2 responding to instructions with the object wrench combined with known locations. One word from the object word class was learned in this manner

570

H. Goldstein, D. Angelo, and L. Mousetis TABLE 4. Summary of First Occurrences of Generalized and Expressive Responses

Receptive

1*

2

3

Mean

Occurrence of receptive response first Mean sessions before expressive response

53% (30) 7.0

66% (21) 2.6

66% (54) 1.4

62%

Occurrence of expressive response first Mean sessions before receptive response

21% (12) 1.6

3% (1) 1.0

26% (21) 1.4

17%

Occurrence of receptive and expressive response-same session

26% (15)

31% (10)

Subjects

8%

(7)

22%

*For Subject 1, three expressive responses were demonstrated for an item never demonstrated receptively. One receptive response was demonstrated for an item never demonstrated expressively. These items were not included when calculating mean session advantages.

by each subject. Nevertheless, this limited correct responding during the baseline conditions was followed by extensive correct responding to untrained responses subsequent to the introduction of training items from each submatrix. Thus, recombinative generalization occurred in a predictable fashion and was shown to be under experimental control. The introduction of a new training item from Submatrix 2 comprised of unknown words allowed us to evaluate whether syntactic learning had taken place during the acquisition of the known word submatrix. If subjects responded to known word combinations based simply upon semantic constraints, then one would not expect additional generalization to combinations of new words with previously known words. This was not the case. All subjects generalized extensively within Submatrix 2 following the learning of a single training item, irrespective of training modality. Thus, the subjects appeared to use syntactic learning to determine which words in the trained utterance corresponded to object, spatial preposition, and location referents. Despite this preliminary evidence of syntactic learning facilitating lexical learning, lexical learning did not progress subsequently without further confusions. Error analyses revealed lexical confusions for each of the subjects. Analyses of generalization probes allowed us to select training items to ameliorate these discrimination problems. For example, after receptive training of paperclip on the speaker (T3), Subject 1 confused the location speaker with hutch, the location trained in T2, and with dresser, a supposedly known location that was confused with a variety of locations through most of the study. The addition of an overlapping training item (T7: paddle on the speaker) contrasted speaker vs. hutch in combination with the object paddle as well as the objects paddle and paperclip in combination with speaker. Subject 2 exhibited inconsistent problems discriminating between

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571

scale and wrench, the two unknown objects. The selection of an overlapping training item T4, which contrasted wrench and scale in combination with the location speaker, remediated this confusion. Subject 3 confused the prepositions ieft of and right of. A stimulus fading procedure was finally

instituted in conjunction with overlap training to ameliorate this discrimination problem. The training of diagonal items has been hypothesized to be sufficient to extend the lexical repertoires of language matrix learners (Goldstein, 1983b). This condition was necessary, but not always sufficient to foster complete recombinative generalization for these mentally retarded subjects, however. It may be that confusions resulted because previously learned words were not adequately established in subjects’ repertoires. More stringent criteria for identifying words as “known” and for the amount of recombinative generalization required before introducing the next training item may have alleviated subsequent discrimination learning problems. Receptive vs. Expressive Learning and Crossmodal

Transfer

All subjects received both receptive and expressive training to contrast the effects of modality on recombinative generalization as well as crossmodal transfer. Although subject characteristics were considered in selecting the original training modality, neither modality seemed superior in ultimately achieving more extensive recombinative generalization. Crossmodal transfer to the untrained modality occurred following both receptive and expressive training. An apparent difference was evident, however, in the relative immediacy of effects. Expressive training seemed to promote more rapid crossmodal transfer to receptive responding than the effect of receptive training on expressive responding. Further analysis examined the learning of each cell of the matrix. During receptive training, untrained responses appeared in the receptive modality before the expressive modality almost exclusively. Untrained responses demonstrated in the expressive before the receptive modality almost always occurred during expressive training. But untrained responses often appeared first in the receptive modality during expressive training. Contrary to arguments made by Ingram (1974), it is notable that receptive responding did not always precede expressive responding. All subjects first demonstrated some untrained (i.e., generalized) responses in the expressive modality. The likelihood of these responses occurring by chance is remote given the nature of these two- and three-term responses. Chance performance inflating estimates of generalization is far more likely for receptive responses (cf. Baird, 1972; Fernald, 1972). In addition, rote responding could not account for these differences, as only untrained, generalized responses were analyzed. As Guess and Baer (1973) argued, it appears that receptive and expressive language repertoires

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H. Goldstein, D. Angelo, and L. Mousetis

can develop independently. Nevertheless, transfer after expressive training to receptive responding is less likely to require additional intervention. Yet the factors that facilitate transfer in either direction have yet to be delineated. One hypothesis is that the occurrence of recombinative generalization in one modality precedes crossmodal transfer (Goldstein, 1985). A contradiction was indicated, as generalized receptive responses sometimes occurred before generalized expressive responses were demonstrated within the submatrix being trained expressively. That is, the first generalized response was not always in the training modality. It seems that further resolution of this problem will require a more careful analysis of the component behaviors that contribute to receptive and expressive responding tasks. Implicalions

In all three cases, the extent of generalized learning was impressive for severely mentally retarded subjects. Subject 1 was directly taught just 3 out of 64 receptive responses and 4 of 64 expressive responses. Subject 2 was directly taught just 2 out of 36 receptive responses and 2 of 36 expressive responses. Subject 3 was directly taught just 2 out of 125 receptive responses and 4 of 125 expressive responses. Recombinative generalization and crossmodal transfer were responsible for 94%-98% of subjects’ learning. Although not measured extensively, generalization across people and settings was impressive. All three subjects perforined with the same stimuli in different rooms and with different teachers, as well as they did in the training setting. In addition, when the task was made more naturalistic by employing known objects and locations at school and at home, extensive receptive and expressive responding again was demonstrated. In fact, the mother who tested in the home was surprised that her child could follow such a large variety of object-location instructions not part of daily routines and she was more surprised to find the child could produce two-word utterances to describe such actions. Previous researchers have successfully taught generative language repertoires using matrix training strategies by focusing on lexical and syntactic learning simultaneously (see Goldstein, 1983a; Wetherby, 1978). The present study exemplifies the potential of using one language component to facilitate learning of another. The use of known lexical items to promote syntactic learning parallels natural language acquisition as young children combine their initial lexicon in syntactic forms. Syntactic learning also may be used to support lexical learning from utterances comprised of all unknown words. Using an established word order rule seems to accelerate the learning of the association between new words and environmental referents. When recombinative generalization is incomplete, an analysis of generalization probes may uncover the discrimination problems that account for errors.

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This error analysis can provide a firm basis for selecting an overlapping response or a lexical response for training to enhance generalization. Preliminary comparisons indicate that matrix training can be initiated either receptively or expressively. Neither training modality was prerequisite to the other. However, crossmodal transfer appeared more quickly and more extensively following expressive training and expressive recombinative generalization. The choice of training may depend on the subject characteristics and learning preferences. For example, receptive responding is far easier to prompt (physically) than expressive responding if a child evinces noncompliant behavior or a reticence to respond verbally, Expressive training may be appropriate with more linguistically-advanced subjects and with more verbally imitative subjects. REFERENCES Baird, R. (1972). On the role of chance in imitation-comprehension-production test results. Journal of Verbal Learning and Verbal Behavior, 11.474-477. Cuvo, A., & Riva, M. (1980). Generalization and transfer between comprehension and production: A comparison of retarded and nonretarded persons. Journal of Applied Behavior Analysis, 13, 3 15-33 1. Esper, E. (1925). A technique for the experimental investigation of associative interference in artificial linguistic material. Language Monographs, 1. Fernald, C. D. (1972). Control of grammar in imitation, comprehension, and production: Problems of replication. Journal of Verbal Learning and Verbal Behavior, 11, 606-613. Foss, D. (1968). Learning and discovery in the acquisition of structural material: Effects of number of items and their sequence. Journal of Experimental Psychology, 77, 341-344. Goldstein, H. (1983a). Training generative repertoires within agent-action-object miniature linguistic systems with children. Journal of Speech and Hearing Research, 26, 76-89. Goldstein, H. (1983b). Recombinative generalization: Relationship between environmental conditions and the linguistic repertoires of language learners. Analysis and Intervention in Developmental Disabilities, 3, 279-293. Goldstein, H. (1985). Enhancing language generalization using matrix and stimulus equivalence training. In S. E Warren and A. K. Rogers-Warren (Eds.), Teaching functional language (pp. 225-250). Baltimore: University Park Press. Goldstein, H., Angelo, D., & Wetherby, B. (1987). Effects of training item selection on adults’ acquisition of miniature linguistic systems. Psychological Record, 37, 89-107. Guess, D., & Baer, D. M. (1973). Some experimental analyses of linguistic development in institutionalized retarded children. In B. B. Lahey (Ed.), The modification of language behavior (pp. 3-60). Springfield, IL: Charles C Thomas. Guess, D., Sailor, W., Rutherford, G., & Baer, D. (1968). An experimental analysis of linguistic development: The productive use of the plural morpheme. Journal of Applied Behavior Analysis, 1, 297-306. Ingram, D. (1974). The relationship between comprehension and production. In R. L. Schiefelbusch & L. L. Lloyd (Eds.), Language perspectives: Acquisition, retardation, and intervention (pp. 3 13-334). Baltimore: University Park Press. Karlan, G., Brenn-White, B., Lentz, A., Hodur, P., Egger, D., & Frankoff, D. (1982). Establishing generalized, productive verb-noun phrase usage in a manual language system with moderately handicapped children. Journal of Speech and Hearing Disorders, 47, 31-42.

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10,735-736. Morgan, J. L., &Newport, E. L. (1981). The role of constituent structure in the induction of an artificial language. Journal of Verbal Learning and Verbal Behavior, 20, 67-85. Romski, M. A., & Ruder, K. F. (1984). Effects of speech and speech and sign instruction on oral language learning and generalization of action + object combinations by Down’s Syndrome children. Journul of Speech and Hearing Disorders, 49, 293-302. Ruder, K., Hermann, P., & Schiefelbusch, R. (1977). Effects of verbal imitation and comprehension training on verbal production. Journal of Psycholinguistic Research, 6, 59-72. Schumaker, J., & Sherman, J. (1970). Training generative verb usage by imitation and reinforcement procedures. Journal of Applied Behavior Analysis, 3, 273-287. Striefel, S., Wetherby, B., & Karlan, G. (1976). Establishing generalized verb-noun instructionfollowing skills in retarded children. Journal of Experimental Child Psychology, 22,

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Cl. (1978). Developing generalized instruction-following In C. Meyers (Ed.), Quality of life in severely and profoundly mentally retarded people: Research foundations for improvement (pp. 267-326). Washington, DC: American Association on Mental Deficiency. Wetherby, B. (1978). Miniature languages and the functional analysis of verbal behavior. In R. Schiefelbusch (Ed.), Bases of language intervention (pp. 397-448). Baltimore: University Park Press. Wetherby, B., & Striefel, S. (1978). Application of miniature linguistic system or matrix training procedures. In R. Schiefelbusch (Ed.), Language intervention strategies (pp. 318-356). Baltimore: University Park Press. Wolfle, D. L. (1933). The relative stability of first and second syllables in an artificial language.

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