A Review of Studies Examining the Nature of Selection-based and ...

The Analysis of Verbal Behavior 1997,14,85-104

A Review of Studies Examining the Nature ofSelection-based and Topography-based Verbal Behavior

Bill Potter and Deborah L. BrownCalifomia State University, Stanislaus

Selection-based (SB) verbal behavior, in most general terms, consists of selecting stimuli froman array, which presumably has some effect on a listener. Topography-based (TB) verbalbehavior consists of responses with unique topographies (e.g. speaking, signing, writing)which is also presumed to have some effect on a listener. This article reviews research examin-ing the nature of these two types of verbal behavior. Overall, TB verbal behavior appears to bemore easily acquired and may also function to mediate some SB verbal behavior.

Researchers are in the process of clarify-ing the differences between selection-basedand topography-based verbal behavior, asinitially specified by Michael (1985) andexpanded on by Cresson (1994) andStratton (1992). The purpose of this articleis to provide an overview of these differ-ences, then summarize the research whichhas been conducted in this area.Topography-based (TB) verbal behavior

consists of making a response with aunique form or topography (e.g., saying"What time is it?" or manually signing thesame). The resulting stimulus from such abehavior presumably affects the listener inan appropriate manner. Common exam-ples of topography-based verbal behaviorinclude speaking, signing (as with the signlanguage of the deaf), and writing.

Contrast this with selection-based (SB)verbal behavior, which consists of pointingto a stimulus, or series of stimuli, arrangedin an array. The listener, watching whichstimuli are pointed to, responds in an

This review was part of the first author's disserta-tion submitted in partial fulfillment of a Doctor ofPhilosophy degree at Western Michigan University.The authors would like to thank the reviewers (espe-cially Mark Sundberg and reviewer "B") for theirthoughtful comments on this review. In addition,thanks go to Jack Michael, Dick Malott, Lisa Baker,Phil Chase, Bruce Hesse and Shawn Huber for alsoproviding insightful comments. Correspondence maybe addressed to the first author at the Department ofPsychology, California State University, Stanislaus,801 W. Monte Vista, Turlock, CA 95382.

appropriate manner. For example, whenthe symbol for "bathroom" is pointed too,the listener may bring the student to thebathroom. In this case the form of theresponse, pointing, is always approxi-mately the same regardless of the stimulusselected. One example of SB verbal behav-ior is the use of communication boards, asused with developmentally disabled (DD)population (Shafer, 1993) and in ape lan-guage studies (Savage-Rumbaugh, 1984).Another example is the more recentlydeveloped Picture-Exchange Communi-cation System (PECS). This methodrequires students to select symbols or pic-tures from a stack, then hand them to a lis-tener. While still SB verbal behavior, thismethod facilitates more interactionbetween the speaker and listener (primar-ily used with children with developmentaldisabilities). This method also allows stu-dents to arrange pictures into "sentences"(see Bondy & Frost, 1993 for a more thor-ough explanation, and the application, ofPECS).

It is useful to contrast SB verbal behaviorwith TB verbal behavior to illustrate thedifferences pointed out by Michael (1985).Consider a situation in which a teacher istraining a student to use a communicationboard, consisting of a flat surface uponwhich a number of symbols are displayed.The student may point to a particular sym-bol on the device and the teacher will pro-

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86 BILL POTTER and DEBORAH L. BROWN

vide feedback about the correctness of theresponse. For example, the teacher maypresent a picture of a dog and the studentmay then point to the symbol for a dog inthe upper-left corner. If the selected stimu-lus is correct, the teacher will respond in apositive fashion. The teacher may thenhold up a picture of a cow. Again, anappropriate consequence follows depend-ing on the student's selection. Note thatexcept for minor positional differences, thetopography of the response (pointing) isvery similar whether the student is point-ing to the symbol for a dog or for a cow.

In the case of a teacher presenting a pic-ture of a cow to a deaf student learningsign language, the response evoked (ifaccurate) will be the particular sign forcow. The teacher judges the accuracy of theresponse based on the topography of theresponse. When the teacher presents a pic-ture of a dog, the sign emitted is differentfrom that of the sign for cow, or in otherwords, the topography of the signedresponse "cow" differs significantly fromthat of the signed "dog" response. This isnot the case with SB verbal behavior (i.e.,the pointing response is similar for all ver-bal responses). Other differences betweenthe two types of verbal behavior are pre-sented in Table 1 and are discussed later inthis article.There appear to be at least three general

reasons why research into the SB/TB dis-tinction is valuable. First, as someresearchers have recently noted, thereappears to be an increase in the use of SBtechniques (e.g., communication boards,PECS) with developmentally-delayed anddisabled populations (Bondy & Frost, 1993;Shafer, 1993; Shane & Bashir, 1980;Sundberg, 1993; Sundberg & Sundberg,1990). This increase has not been accompa-nied by much empirical research (Shafer,1993; Sundberg & Sundberg, 1990), andpublished studies are often in the form ofcase studies (Romski, Sevcik & Pate, 1988).It may be the case that the increasing use ofSB systems has resulted in a decrease of TBsystem use, without empirical justification.Understanding the relevant variables in

SB/TB verbal behavior would aid teachersin selecting and designing communicationssystems/programs for people with devel-opmental disabilities (see Shafer, 1993 andSundberg, 1993 for reviews of this area).Second, although there has been much

research using SB and TB systems withnonhumans, few if any, of these studieshave considered the distinction or madeany comparison between the two kinds ofverbal behavior. The aims of theseresearchers vary considerably, but each, atsome level, recognize the importance ofnonhuman research in understanding thenature of language (Epstein, Lanza &Skinner, 1980; Gardner & Gardner, 1969;Pepperberg 1988; Rumbaugh, 1977;Savage-Rumbaugh, 1990; Terrace, 1979). Itwould be valuable to determine if differ-ences exist between these types of verbalbehavior, in order to aid in interpreting theresults of such studies.

Third, research on stimulus equivalence,among other research areas, frequentlyuses SB procedures. Stimulus equivalenceis singled out here because it has strongimplications for the acquisition of verbalbehavior (see Sidman, 1994). Much of thisresearch involves the use of the "matching-to-sample" technique. This techniqueinvolves presenting some kind of ante-cedent stimulus (e.g., a picture of a car, orthe auditory stimulus "car") along withseveral choice stimuli from which the par-ticipant is required to select the correctrelation (e.g., pick the word "car" in thepresence of a picture of a car). It alsoshould be noted that research is conductedin which SB and TB responses are inter-mixed in a single equivalence experiment.For example, one of the earliest studies(Sidman, 1971) involved training a severelyDD participant to match pictures of a cat tothe printed word cat (SB response), as wellas to say the word "cat" upon seeing a pic-ture of a cat (TB response). In summary,given the differences which appear to existbetween these two types of verbal behav-ior, it would seem important to investigatethese differences to aid in designing andinterpreting future research.

A REVIEW OF SB AND TB VERBAL BEHAVIOR

Table 1

A comparison of selection-based and topography-based verbal behavior.

Item Compared Selection-based Topography-based

Stimulus Control * Conditional discrimination. * A direct relation. The antecedentThe pointing response is controlled stimulus condition evokes theby a stimulus or establishing oper- response (no second stimulus oration that alters the evocative stimulus array is involved).strength of a particular choicestimulus (from an array).* In a controlled/experimentalsetting, all choice stimuli arepresent allowing the student toreact to them.

Response * Nearly indistinguishable from * Clearly distinguishable fromother SB responses. other TB responses.* Requires an additional "scan- * Requires no "scanning"ning" repertoire to come into repertoire.contact with the choice stimulus * The topography of the responseeffective in evoking the pointing must be learned before it can beresponse. emitted.* It is likely that the pointingresponse already exists in thestudent's repertoire (no newtopography needs to be learned).

Response- * Nearly indistinguishable from * Clearly distinguishable fromproduced other SB response-produced other TB response-producedKinesthetic kinesthetic stimuli. kinesthetic stimuli.Stimulation

Correspondence * No point-to-point correspon- * Point-to-point correspondenceBetween dence between the response and exists between the response andResponse and the the response-produced stimulus the response-produced stimulusAntecedent functioning as an antecedent stim- functioning as an antecedent stim-Stimulation for ulus for the listener's behavior. ulus for the listener's behavior.the Listener

Environmental * Necessitates additional appara- * Generally, no additional appara-Arrangement tus (the stimulus array). tus required (writing is an excep-

* In most such communicative tion).activity, if it can be assumed that * There is no sense in which thethe correct symbol is on the board entire relevant repertoire can be(as with any training exercise), examined and incorrect responsesfinding it even if it is not well eliminated, other than by emittingknown is made easier by being the responses which are strong,able to eliminate the known sym- which may not include the relevantbols for other objects or events. one.* If the number of symbols is large * If the response is strong, thereenough to require a considerable is no time delay between thesearch time, there will be a loss of presentation of the controllingcontrol by the variable that initiated variable and the responsethe search, even if the relevant occurrence.symbol is well-known.

Notes Derived from Cresson, 1994; Michael, 1985; Stratton, 1992.

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Differences Between Selection-based andTopography-based Verbal Behavior

Table 1 (derived from Cresson, 1994;Michael, 1985; and Stratton, 1992) com-pares the differences between SB and TBtasks in five general categories: (1) thestimuli controlling the SB or TB response(stimulus control), (2) the response itself,(3) the nature of the response product(response-produced kinesthetic stimula-tion), (4) correspondence between theresponse and the response product whichcontrols the listener's response and, (5) thegeneral environmental arrangementrequired for each of the task types.The following paragraphs briefly sum-

marize some of the more critical parts ofTable 1. As noted in the part of Table 1addressing stimulus control, an SB task iscontrolled by a relation between two stim-uli, namely the sample stimulus (or anestablishing operation, although this isgenerally not referred to as a stimulus) anda choice stimulus. The sample stimulusaffects the participant in such a manner asto increase the evocative strength of one ofthe choice stimuli. This choice stimulusevokes the pointing response from the par-ticipant. Such a situation is often called aconditional discrimination, that is, itinvolves an extra degree of stimulus con-trol. Contrast this type of stimulus controlwith that of a TB task, in which no choicearray is present. In this case, the responseis often evoked directly by the antecedentstimulus.

Several differences also exist betweenthe types of responses characteristic of SBand TB verbal behavior. All final SBresponses are similar (e.g., pointing to achoice stimulus), while TB responses areall necessarily different as the form of theresponse (vs. what is pointed to for SB ver-bal behavior) is what has the desired effecton the listener (Michael, 1993). Prior to thefinal SB response, a scanning responsemust occur; that is, the speaker must comeinto contact with the appropriate compari-son stimulus before that stimulus canevoke the pointing response. This is not thecase with a TB response. When acquiringnew relations in an SB framework, no new

response needs to be added to the existingrepertoire (i.e., the pointing and scanningrepertoires having already been estab-lished). In a TB framework, however,acquiring a new relation generally involvesadding a new response to the speaker'sexisting repertoire such as learning a newsign or vocalization (Stratton, 1992).However, in both TB and SB types, theresponse must be brought under appropri-ate stimulus control. Most of studiesdescribed below (and in Table 2) describeresearch that has focused on examininggross differences between SB and TB rela-tions in terms of stimulus control andresponse type.There are clear differences between SB

and TB verbal behavior in regard toresponse-produced kinesthetic stimulation(versus the more ambiguous differences invisual and auditory feedback related toeach response form). The pointingresponse is nearly identical in all cases ofSB verbal behavior, producing little differ-ences in response-produced kinestheticstimulation. However, with TB verbalbehavior, the kinesthetic stimulation isdirectly related to the differences amongresponses, which must be distinct enoughto produce the desired effect in the listener(thus, also resulting in distinct response-produced kinesthetic stimulation).

Finally, certain logistical elements differbetween the two types of verbal behavior,which have some interesting practicalimplications as noted in Table 1 and aredescribed later in this review (Cresson,1994; Michael, 1993). For example, SB ver-bal behavior has been called an "aided"(Romski, et al., 1988; Sigafoos & Iacono,1993) communication system, in that itnecessitates the use of additional apparatussuch as a communication board or a com-puter. Other factors include the size of thestimulus array and the necessity to be closeto a listener.

OVERVIEW OF RELATIONSTRAINED AND TESTED

A number of studies have directly inves-tigated the nature of these two types ofverbal behavior. Many of these studies are

A REVIEW OF SB AND TB VERBAL BEHAVIOR 89

summarized below with additional infor-mation provided in Table 2. Nearly all thestudies conducted to date have focused onthe acquisition of tact and intraverbal rela-tions'. In addition, most of the studies haveincorporated some component of stimulusequivalence: reflexivity, symmetry, or tran-sitivity. A brief overview is provided ofeach of these types of relations prior toexamining the results of SB and TB studies.The tact relation as defined by Skinner

(1957) is "a verbal operant in which aresponse of given form is evoked (or atleast strengthened) by a particular object orevent or property of an event. We accountfor the strength by showing that in thepresence of the object or event a responseof that form is characteristically reinforcedin a given verbal community" (p. 82).Thus, in the tact relation, the controllingvariable is nonverbal - it is not the productof another's verbal behavior and theresponse is typically reinforced by othermembers of the verbal community. Anexample of this would be a child saying"red" in the presence of a red apple. Notethat the aspect of the environment control-ling the response is the "red" property ofthe apple, which of course might be sharedby other items (e.g., a barn, a sunset).Skinner classified this particular type oftact as abstract. An example in which anauditory stimulus controls the tactresponse would be someone saying "air-plane" in the presence of the sound of anairplane overhead. Notice that the defini-tion does not specify that a particular typeof reinforcer is required for this relation: itis not an important distinction (Peterson,1978). Some researchers have stressed thatgeneralized conditioned reinforcers proba-bly play a rather large role in the acquisi-tion of tact responses as well as theintraverbal (Michael, 1993; Skinner, 1957;Sundberg, 1990; Sundberg, Michael,Partington & Sundberg, 1996).

1 It should be noted that some of the intraverbalrelations reported in these studies used a nonsenseword (thus nonverbal) for the antecedent stimulus.According to Skinner's classification, this would makethose relations tacts, not intraverbals. The termintraverbal is used in this article to maintain consis-tency with previous reports.

The intraverbal relation (Skinner, 1957)is a verbal response controlled by a verbalstimulus, but little similarity existsbetween the controlling verbal stimulusand the product of the evoked verbalresponse. For example, saying "TheHerald" after hearing someone say "news-paper" would be considered an intraverbalrelation. The intraverbal relation is also notlimited to any sense modality. For exam-ple, signing "cat" after reading the lips ofsomeone saying "dog" would also be con-sidered an intraverbal response. As withother relations controlled by verbalantecedents, the intraverbal tends to beconsequated with generalized conditionedreinforcers (as noted above). Differencesfound between the acquisition of tact andintraverbal relations will be discussed laterin this article.As previously noted, a number of SB/TB

studies have incorporated some form ofstimulus equivalence. Stimulus equiva-lence training generally consists of partici-pants learning a series of relations. Forexample, after hearing the nonsense word"Ork," the participant is required to selecta picture of some object (e.g., a lump ofcoal). Next, the participant is required toselect the correct nonsense symbol (e.g.,"-"') from an array when presented withthe lump of coal. Once these relations aretrained, then equivalence testing can beconducted. The first test, called reflexivity,consists of presenting a particular stimulusboth as the sample and the correct compar-ison in an array of stimuli, to test if the par-ticipant has this untrained relation in his orher repertoire. Next, one could test to see ifreversing the sample and choice stimuliresults in accurate responding; for examplepresent "-" and require the participant toselect the lump of coal from an array ofstimuli (a symmetrical relation). The lastrelation, called transitivity, would consistof presenting a nonsense word (e.g.,"Ork") and having the symbol "-i" as thecorrect comparison stimulus in an array. Insymbolic terms, these relations can besummed as: A->B and B->C (training),then test A->A (and all other reflexiverelations), B->A, C->B (symmetrical


relations) and A->C (transitive relation).When all three emergent relations, transi-tivity, reflexivity, and symmetry have beendemonstrated, then stimulus equivalenceis said to exist for the set of stimuli used(Sidman & Tailby, 1982). While mostresearchers have focused on demonstratingreflexivity, symmetry and transitivity, oth-ers have explored various combinationsand arrangements of these relations. Forexample, Sidman and Tailby (1982) exam-ined the emergence of both a symmetricaland transitive relation. To do this theytrained the relations B->A, C->A, thentested B->C and C->B. Theseresearchers showed that equivalence couldbe demonstrated between two samplestimuli which had been paired only with acommon choice stimulus. For the purposeof this article, this type of emergent rela-tion will be called "equivalence" (Lazar,Davis-Lang & Sanchez, 1984) and wastested by Cresson (1994), Sundberg andSundberg, (1990), Wraikat, Sundberg, andMichael (1991), and Potter, Huber andMichael (1997).The relevance of these emergent rela-

tions to verbal behavior has been noted bymany researchers (Barnes, 1994; Fields,Verhave, & Fath, 1984; Hall & Chase, 1991;Sidman, 1986; Wulfert & Hayes, 1988)although some have raised questions as tothe applicability of stimulus equivalence toTB verbal behavior (Hall & Chase, 1991;Tan, Bredin, Polson, Grabavac, & Parsons,1995). Sidman (1994) generally believesthat the capacity to demonstrate stimulusequivalence is an innate feature of humans,one essential to the development of verbalbehavior. Hayes and Hayes (1989) alsobelieve stimulus equivalence to be essentialto verbal behavior, although they believethe development of this capability to be afunction of a specific learning history.Others have claimed that stimulus equiva-lence is a function of verbal behavior; thatis, without verbal behavior, stimulusequivalence would not emerge (Dugdale &Lowe, 1990; Mandell & Sheen, 1994). Todate, only one study has demonstrated thedevelopment of stimulus equivalence asdefined by Sidman and Tailby (1982) in

nonhumans (Schusterman & Kastak, 1993),although many others have tried andfailed. Horne and Lowe (1996) argue thatdistinctive aspects of the procedure usedby Schusterman and Kastak may haveallowed TB response forms to facilitate theperformance of the single sea lion whichwas used in this study. To date noresearcher has replicated these finding,which has been taken as evidence that lan-guage is a prerequisite for demonstrationsof stimulus equivalence (Horne & Lowe,1996). Although, as Hayes and Hayes(1989) have noted, this failure to demon-strate stimulus equivalence could also bedue to a lack of a particular training his-tory.

SUMMARIES OF SELECTEDSB/TB STUDIES

Below, ten studies that have examinedthe nature of SB and TB verbal behavior(most have examined difference in acquisi-tion and accuracy) are summarized. Aftersummarizing these studies, the implica-tions and ramifications of these studies willbe examined. Table 2 provides additionalinformation on each of these studies.

Bristow and Fristoe, 1984

Twenty nonhandicapped children (aver-age age 8.2 years old) participated in thisstudy. These researchers first trained allthe participants to select a particular pic-ture when presented with one of 12 audi-tory nonsense words. Half of the partici-pants then had six of the picturespresented as sample stimuli and wererequired to emit appropriate manual signsto each (TB condition). Later, these sameparticipants learned to select symbols froman array when presented with the remain-ing six pictures (SB condition). The remain-ing participants received similar trainingexcept that the pictures used in each condi-tion were counter-balanced. Correctresponses were simply acknowledged bythe researcher, while incorrect responseswere followed by the researcher notingthat it was incorrect and modeling the cor-rect response. A 30 s limited hold was ineffect that when timed out, was treated as

A REVIEW OF SB AND TB VERBAL BEHAVIOR

Table 2

Partial summary of research in SB/TB verbal behavior.

Participants Correction Training Antecedent Comparison TB ResponseStimuli Stimuli Form

Bristow & 20 children Incorrect- said incorrect or TB: pictures Symbols or Manual signsFristoe, (non DD) similar. Participant required to SB: pictures & pictures1984 av. age 8.2 imitate correct response. nonsense (12 or 6 used -

years old. words. ordering notreported)

Hodges & 52 retarded Fading and prompting were TB & SB: Symbols Manual signsSchwethelm, children Av. used, but no correction training objects (e.g., ordering not1984 age 12.2 was cited (e.g., remedial). candy, ball). reported

years (4 used)

Sundberg 4 adults Non answer = demonstration TB & SB: non- Symbols Manual signs& with DD with verbal prompt. sense objects Pseudo ran-Sundberg, Incorrect = told incorrect, then & nonsense dom order1990 model with verbal prompt, words each trial

correct. (auditory). (3 used)

Wraikat, 5 adults Non answer = demonstration TB & SB: non- Symbols Manual signs1990 with DD with verbal prompt. sense objects Pseudo ran-

Incorrect = told incorrect, then & nonsense dom ordermodel with verbal prompt, words each trialcorrect. (auditory). (3 used)

Wraikat, 7 adults Non answer = demonstration TB & SB: non- Symbols - 2 for Manual signsSundberg with DD with verbal prompt. sense objects 3 part. 3 for& Incorrect = told incorrect, then & nonsense other 4 part.Michael, model with verbal prompt, words Pseudo-ran-1991 correct. (auditory). dom order

each trial

Stratton, 28 college Non answer = correct answer TB & SB: Symbols Vocal1992 students and asked participant to do written (Kanji chars.) (Japanese 2

response. English word. random order syll. word).Incorrect = same as non answer. each blockIn both cases words "try again" (5 or 20 used)typed.

Wallender, 20 college Non answer = present correct SB only: Symbols None1993 students answer and ask participant to written (Kanji chars.)

do response. English or random orderIncorrect = same as non answer. Japanese each blockIn both cases tone heard & words. (20 used)"incorrect" shown.

Cresson, 16 college Incorrect = correction: repeat TB & SB: Symbols Write1994 students trial with correct answer dis- visual pattern (Katakana) Katakana

played (for both paradigms). & auditory pseudo- symbolnon. word. random

(8 used)

Tan et al., 8 college Incorrect answer resulted in a TB & SB: French or Typing1995 students low tone. Not reported if typed French English word

participant was required to or English (ordering notredo relation. word. reported)

(4 used)

Potter, 6 college Non answer = "incorrect, try SB & SB/TB: Dot patterns Mouse click onHuber & students again." Answer flashed and "flaglike" (10 to 14 used) each of 4 dotsMichael, participant emitted correct patterns & in a choice.1997 response. auditory non.

Incorrect = same as non answer. word.


an incorrect response. Participants werealso counterbalanced in the order of train-ing for SB and TB systems.

Finally, each participants was adminis-tered a transitivity test in which a nonsenseword was presented and the participantwas required to either emit the sign pairedwith the same picture that the samplestimulus had been paired with (after TBtraining), or to select a similarly pairedsymbol from an array (after SB training).Participants were also tested a day afterthe initial training to examine long-termretention.

Results indicated that acquisition wasslightly faster (no statistical significance)for the TB training. TB training alsoresulted in higher overall accuracy (alsonot statistically significant) in both trainingand for transitivity tests.

Hodges and Schwethelm, 1984

Using 52 profoundly retarded nonverbalchildren (average age 12.2 years old) theseresearchers examined the differencebetween acquiring signs (TB) or matching-to-sample (SB). In both cases participantswere shown common objects (e.g., candy,ball) as sample stimuli. In the TBparadigm, an object was held up while theresearcher modeled, then prompted(molded the child's hands) the participantsto emit the correct sign. In SB training theresearcher also modeled the correctresponse by tapping the sample object withthe correct symbol (used as choice stimuli).Correct responses were followed by vari-ous foods, objects and praise. Fading andprompting were used in training relations,but no remedial procedures were specifiedafter an incorrect response occurred.Mastery was defined as 80% correctresponding with 8-10 signs, or 80% on 9 SBrelations.Results indicated that participants

learned more signs, and also learned thosesigns faster than they learned the SB task.However, as noted later in this review, theresearchers actually trained mand relationsin the TB condition (vs. tact relations in theSB condition) which may account for theobtained results.

Sundberg and Sundberg, 1990

Using four adults with mild to moderatemental retardation, these researchersexamined the differences in acquisitionand accuracy between TB and SB verbalbehavior. In the SB paradigm participantswere trained to match nonsense objects(e.g., an oblong piece of wood) or nonsensewords (e.g., "Zug") to various symbolsarranged in an array of three. In the TBparadigm, the same type of sample stimuliwere used, but participants were requiredto emit a manual sign. In both cases, pre-training consisted of presenting each rela-tion five times while the experimentermodeled the correct choice.During the acquisition phase, correct

responses were followed by pennies andpraise, while incorrect responses wereacknowledged as incorrect and the correctresponse modeled. A 10 s limited hold wasin effect, that when timed out resulted inthe same conditions as an incorrectresponse. The acquisition phase continueduntil a participant obtained 9 out of 10 cor-rect responses for a particular training set(mastery). At the start of each session, par-ticipants received remedial training con-sisting of two demonstrations (as in pre-training) of each previously trainedrelation, but only within the paradigmbeing trained in that particular phase of theexperiment.Following mastery, an equivalence test

was administered. Participants were askedto select "Zug" (or some other nonsensename was provided) from an array of threenonsense objects. Thus, only the samplestimuli used in training were being use inthe equivalence test. The correct choice ineach trial was to match the name andobject which had been paired with thesame symbol (after SB training) or sign(after TB training) in the preceding trainingsessions.

Results indicated faster acquisition forall TB trained relations, as well as forequivalence tests following TB training. Inaddition, TB training resulted in higheraccuracy (the number of relations correct)across all conditions. The researchers alsoreported that several participants occasion-


ally emitted mediating TB responses priorto responding in an equivalence trial (pri-marily after TB training sessions).

Wraikat, 1990

Using five adults with mild/moderatedevelopmental-disabilities, Wraikat pre-sented participants with either an object(e.g., an "I" shaped plastic object) or a non-sense syllable (e.g., "ki"). Participants werethen required to either learn to select (SB)an appropriate symbol from an array ofthree or to emit the appropriate sign (TB).Correct responses were followed withpraises / stickers. Incorrect responsesresulted in the experimenter noting thatthe response was incorrect, then re-pre-senting the sample stimulus and modelingthe correct response (the participant wasnot required to emit the correct response ina remedial trial). A 20 s limited hold was ineffect, after which time the same conditionswere followed as when an incorrectresponse was emitted. Maintenance train-ing was provided for each relation (5 trialseach) at the start of a new session. Par-ticipants continued training until masterywas achieved, defined as 9 out of 10 correctresponses.

Results indicated that overall partici-pants acquired TB relations faster (in termsof trials to mastery). In addition, perfor-mance tended to be more accurate in theTB condition. Interestingly, Wraikatreported that participants appeared moreattentive and positive when performing inthe TB condition. Furthermore, some par-ticipants were observed to emit vocaliza-tions prior to emitting signs.

Wraikat, Sundberg and Michael, 1991

Seven participants, diagnosed as devel-opmentally disabled and exhibiting mod-erate to severe learning disabilities, wereused in this study. Similar to the Sundbergand Sundberg (1990) and Wraikat (1990)studies, these researchers presented eithernonsense objects or nonsense (auditory)words to a participant as samples, thenasked the participant to select an appropri-ate symbol from an array of either two orthree (SB), or to emit a manual sign (TB). In

addition, these researchers added anequivalence test in which the objects usedin training were placed on a table and theparticipants was asked "Select the 'Nack' "or some other nonsense word. Thus, theparticipant was required to select theobject that had been paired with the samesymbol that the spoken nonsense wordhad been paired with in SB training, and toselect the object that had been paired withthe same sign in TB training as was pairedwith the nonsense word. These researchersalso incorporated the use of interspersaltraining in their study. After the first rela-tions were trained, each subsequent ses-sion mixed new relations along withreview trials of previously trained relations(half of the trials in each session were dedi-cated to this interspersed training). Thus,in each subsequent session, 24 trials werealways dedicated to new relations, while24 trials were split equally among all rela-tions previously trained.Correct responses were followed by

praise and stickers, while incorrectresponses resulted in an acknowledgmentthat the response was incorrect, then thecorrect response was modeled by theresearcher as the trial was repeated. A 20 slimited hold was in effect and when timedout, resulted in the same conditions as anincorrect response. Participants continueduntil mastery was reached, defined as 11out of 12 consecutive trials correct whenusing two objects and 7 out of 8 correctconsecutive trials when using three objects(interspersed training was not counted asbeing part of mastery).

Results indicated faster acquisition andmore accurate responding when the partic-ipant was asked to manually sign (TB) ver-sus select a symbol from an array.Accuracy was also better in the equiva-lence test after TB training. Wraikat et al.noted that participants again appearedmore alert and engaged in the TB condi-tions (as Wraikat reported in 1990).

Stratton, 1992

Using 28 college students, Strattonexamined stimulus set size (5 vs. 20) andlanguage paradigm (SB or TB). In the SB


paradigm, participants were asked to selecta written Japanese Kanji character afterbeing presented with an English word(e.g., "clam"). They did this by clicking amouse over the appropriate Kanji characterpresented in an array of either five ortwenty (all stimuli were presented on acomputer). In the TB paradigm, partici-pants were also shown an English word onthe computer screen, but in this case wererequired to say the matching Japaneseword. In both paradigms participantsreceived one trial of pretraining indicatingthe correct matches.Correct responses were followed by

"Good" written on the computer screen inboth cases (the experimenter evaluatedwhether the TB response was correct andentered it into the computer). Incorrectresponses were followed by the writtenstatement "Try again" at which point par-ticipants were shown the correct choiceand required to emit the correct responsebefore continuing. A 20 s limited hold wasin place, which when timed out, resulted inthe same conditions as an incorrectresponse. Participants continued until mas-tery occurred which was defined as threeconsecutive blocks (one trial for each rela-tion trained) with no errors. Participantswere also tested for emergence of symmet-rical relations. Symmetry tests were notconducted on a computer. For the SB con-ditions, participants were required to select(circle on a paper) the correct English wordfrom an array, after being shown a writtenKanji character. In the TB condition, partic-ipants selected the correct English wordafter the experimenter said the matchingJapanese word. Participants only receivedone condition, thus there were seven par-ticipants in each of the four groups: SBwith five relations; SB with twenty rela-tions; TB with five relations and TB withtwenty relations.Results indicated little difference

between the SB and TB conditions whenthe five stimulus set was used all partici-pants easily mastered the relations. When20 relations were trained, performanceswere much better in the SB conditions vs.

the TB condition (on average, SB perfor-mances reached mastery six blocks earlierthen TB performances). This was the onlystudy demonstrating better performance inthe SB task. All participants performedwell in symmetry tests, regardless of whichparadigm they were trained under.Stratton reported that participants werelikely engaging in TB responding in the SBtask (covert vocalizations).

Wallender, 1993

Using 20 highly verbal college students,Wallender examined the effect of familiar-ity (very familiar and unfamiliar) with thesample stimulus on matching-to-sampleperformance. His research extended that ofStratton (1992) in examining some of theparameters which might affect SB perfor-mances. All experimental conditions werepresented, and responses recorded, using aMacintosh personal computer.Using 20 pairs of stimuli, Wallender

asked different groups of participants (10per group) to select the appropriateJapanese Kanji characters when presentedwith either an English animal name (famil-iar group) or a Japanese Katakana charac-ter (unfamiliar group). All comparisonstimuli were simultaneously presented inan array (randomly positioned for eachblock) while only one sample stimulus waspresented at a time. The SB response wasusing a computer mouse to select the cor-rect comparison stimulus. A response wasconsidered incorrect if the wrong compari-son was selected, or if a 20 s limited holdtimed out. Incorrect responses resulted inthe written feedback "Incorrect" (and adistinct tone) at which time the correctchoice was revealed and the participantwas required to click on the correct choice.Correct answers were followed by thewritten feedback "Good" and a distincttone. Dependent variables consisted of thenumber of blocks (each of the 20 relationspresented once) prior to two consecutiveblocks in which no errors occurred (mas-tery). In addition, reaction times wererecorded - that is the time from the onset of


the sample stimulus to the occurrence ofthe selection response.Results indicated that when English

words (familiar) were used as sample stim-uli, half as many blocks were needed priorto mastery, versus using JapaneseKatakana symbols as samples. No differ-ence in reaction times were found betweenthe two conditions. Wallender noted thatfamiliar stimuli "made the SB task easierfor the verbal adults because of otherresponses such stimuli might evoke inthese subjects" (p. 15).

Cresson, 1994

Cresson arranged for 16 college studentsto learn relationships between arbitrarypairs of stimuli. Either nonsense sounds orvisual patterns were used as sample stim-uli, while Japanese Katakana characterswere used as choices. In one condition (TB)the participants were asked to write theKatakana character which matched eitherthe nonsense sound or the visual pattern.In the second condition (SB) participantswere asked to select the correct Katakanacharacter from an array, in the presence ofeither the auditory nonsense syllable or thevisual pattern. Participants were randomlyassigned to the order of presentation (TBor SB first). The participants learned 8 rela-tions at a time.The dependent variable consisted of

tracking the number of errors (incorrectselections or incorrect written responseproducts) for each condition. Pretrainingconsisted of familiarizing the participantswith the choice stimuli (e.g., writing thecharacters or selecting them) and initiallyshowing the participants the correctmatches. Correct responses were followedby praise ("Correct") and $5 was given atthe end of each session. Incorrect responsesresulted in the participant being shown thecorrect selection/written response productand asked to emit the correct response. Thesame choice stimuli were used for theauditory sample stimuli as for the visualpatterns. This arrangement allowedCresson to conduct an equivalence test. Inthis test, participants heard the nonsensesounds and were required to select the cor-

rect choice from an array of the previouslyused visual patterns. The correct choicewas the visual pattern which had beenpaired with the same Katakana characteras the nonsense sound had been paired.No time limit (limited hold) was imposedon participants. Two trials of each previ-ously trained relation was provided whentraining within the TB or SB paradigmextended to more than one session (thesereviews occurred at the start of the nextsession).

Results indicated better TB performanceacross all conditions, including equiva-lence testing. In addition, participantsacquired the auditory discriminationsfaster.

Tan, Bredin, Polson, Grabavac and Parsons,1995

Using eight college students, theseresearchers arranged for a computerizedtask in which the participants saw Frenchwords as sample stimuli and eitherselected English words from an array (SB)or typed English words (TB2). Differenttones along with the written words"Right" or "Wrong" provided feedbackafter each response indicating whether theresponse was correct or not. Interspersedin this training were two types of symme-try relation testing. In both cases the sam-ple stimuli were the previous choice stim-uli (namely English words displayed onthe computer screen). Participants werethen required to either select comparisonFrench words from an array (SB symmetrytest) or to type in the appropriate Frenchword (TB symmetry test). For each trainingcondition (SB and TB), half of the trainedrelations were tested for symmetry usingthe TB response form and half were testedusing the SB response form.

Results indicated all training conditionswere learned equally well. Near perfectperformance was demonstrated when theresponse form was SB in symmetry tests,

2 Tan et al. used typing words as the TB responseform. It should be noted however, that even if theparticipants were touch typists, typing is not a pureTB response. Typing relies on a mechanism (key-board) and involves composing words from letters.Thus, such an arrangement is likely to involve SBcomponents (i.e., seeing the stimulus array).


whether training was SB or TB. Poorer per-formance was demonstrated when symme-try testing involved TB responses. Theauthors noted: "For the behavior analystthis should come as no surprise: seeing thestimulus word, whether during selection-training or topography-training, no matterhow often, does not guarantee that onewill later be able to produce it, especially ifthe word is unfamiliar" (p. 2).

Potter, Huber and Michael, 1997

Using six college students, Potter et al.arranged for a computerized matching-to-sample task. Participants were either pre-sented with a nonsense sound or a visual"flag-like" pattern. In both cases the partic-ipants were asked to learn the relationbetween each of the 12 or 14 sample stim-uli and a set of 12 or 14 dot-like patterns (asquare consisting of four dots in it)arranged in an array on the screen.

In one condition (point-to-point or PTP),participants were asked to select compari-son stimuli by clicking on each of the dotsin the selected comparison. This arrangedfor a SB task with an added TB component,namely unique response-produced kines-thetic stimulation. This was the case aseach dot pattern was significantly differentfrom all others. In the second condition(non point-to-point or NPTP), the partici-pants were required to select the appropri-ate comparison stimulus by clicking twicein the upper-left corner and twice in thelower-right corner of the square. Thus, thesame number of clicks were requiredacross conditions but only the PTP condi-tion had unique response-produced kines-thetic stimulation. Both visual and audi-tory sample stimuli (as describe above)were used in the PTP and NPTP condi-tions, with the same comparison stimuliused with the visual and auditory samplestimuli. This arrangement allowed forequivalence testing; namely presenting anonsense word and requesting the partici-pant to select the matching visual pattern.The correct choice was always the visualpattern which had been paired with thesame dot pattern as the nonsense wordhad been paired. During training, but not

during equivalence testing, a correctresponse was followed by the auditory"correct" via the computer, while incorrectresponses resulted in the auditory"Incorrect." A remedial trial followedincorrect selections, and consisted of theparticipant being shown the correct choiceand asked to complete that response. A 20s limited hold was in effect - which whentimed out resulted in the same conditionsfollowing an incorrect response.

Results showed no consistent differencebetween the PTP and NPTP conditions inacquisition (number of trials/blocks toacquisition) or in accuracy (the number ofcorrect responses in each conditions). Exitinterviews and protocol analyses (concur-rent vocalizations) showed consistentvocal-verbal behavior by the participants,indicating that this may have obscured anytrue PTP and NPTP differences, as well asimplicating the use of TB verbal behaviorin SB tasks, especially with highly vocal-verbal participants.

TOPOGRAPHY-BASED ANDSELECTION-BASED STUDIES

This section will examine some of thefindings and ramifications of the abovesummarized studies (with additional infor-mation provided in Table 2).

Acquisition

For the nine studies that investigatedease of acquisition across language type(Wallender, 1993, investigated SB parame-ters only), there was an advantage for TBverbal behavior demonstrated in six stud-ies.The typical arrangement for each of

these six studies was to either present anonsense sound (the intraverbal condition)or present some object (tact condition),then ask participants to either select thecorrect symbol from an array (SB), or toemit the correct manual sign, vocalization,or written response (TB). While eachresearcher equated all stimuli used (similarin both SB and TB conditions), it is nearlyimpossible to equate response types acrossconditions, for the reasons noted in Table1. Across these six studies, SB response


types were similar (pointing), but choicestimuli array sizes varied, a point dis-cussed below. Again, across these six stud-ies, TB response forms were primarilymanual signs (Bristow & Fristoe, 1984;Hodges & Schwethelm, 1984; Sundberg &Sundberg, 1990; Wraikat, 1990, Wraikat etal., 1991) with one study incorporating awriting response (Cresson, 1994).The types of participants used in these

studies also varied, but the majority ofthem used participants with poor verbalabilities, primarily adults and childrenwith developmental disabilities (Hodges &Schwethelm, 1984; Sundberg & Sundberg,1990; Wraikat, 1990, Wraikat et al., 1991).These studies demonstrated the greatestadvantage for TB verbal behavior. Bristowand Fristoe (1984) and Cresson (1994)using normal children and adults respec-tively, also demonstrated a TB advantage,but both reported the differences to be rel-atively small. Considering that SB systemsare seldom used with highly verbalhumans, it would appear odd to use suchparticipants in SB/TB studies. Cresson(1994) provided his rationale:

College students have a further advantage: theyhave extensive verbal histories. If a variable isfound that affects performance on TB and SBtasks in these subjects, then it would be likely tobe even more important with students forwhom the stimulus control is tenuous (p. 20).

In addition, as pointed out earlier in thisreview, SB and TB tasks are often inter-mixed in stimulus equivalence studies (aswell as other types of research), thus mak-ing research in this area useful for designingbetter studies. It would appear however,that TB advantages are more significant forless verbal participants. Why this might bethe case is discussed later in this review.Perhaps more interesting to analyze are

the three studies that did not demonstratea TB advantage. Potter et al. (1997) showedlittle difference between a SB task requir-ing a stereotypical response (pure SB) anda SB task requiring a unique topography(SB with a TB component). Theseresearchers conducted an additional exam-ination (protocol analysis) which revealedthat participants appeared to be engagingin vocal-verbal (TB) behavior under both

conditions. Tan et. al, (1995) found no dif-ference in acquisition between the two lan-guage types, but this may have been due tothe small number of relations trained ineach condition (four). These results werethe same as those obtained by Stratton(1992). In one part of the Stratton (1992)experiment, only five relations weretrained in the SB and TB conditions. Thisarrangement resulted in little differencebetween SB and TB performance. Strattonattributed these results to a ceiling effect.However, when Stratton increased thenumber of relations to be learned in boththe TB and SB conditions to twenty, the SBrelations were acquired on the average ofsix blocks (i.e., a single presentation of all20 relations) faster than the TB relations.This is the only study reviewed whichdemonstrated an advantage for the SBparadigm.

Stratton's study was also the first studyto test SB/TB differences with normaladults - most other researchers focused onchildren or adults with DD. Otherresearchers using normally functioninghumans included Bristow and Fristoe(1984) and Cresson (1994), using childrenand college undergraduates, respectively.Both of these researchers found only smalldifferences between the SB and TBparadigms which favored the TB paradigm(Cresson did not record the number ofblocks to acquisition; however, to theextent to which accuracy is related toacquisition, this holds true). As mentionedearlier, using normal adults, Potter et al.(1997) also showed little differencebetween a pure SB response and a SBresponse with an added TB component.Preliminary results indicate that clearerdifferences emerge between the twoparadigms when participants with poorinitial verbal skills are tested, and onlyslight differences appear when participantswith well-developed verbal skills aretested. However, more investigation isneeded to clarify this issue.One possible reason for the preceding

observations may be that the highly verbalparticipants are engaging in vocal-verbalbehavior that functions to mediate SB


responses. Several researchers haveattempted to explain the emergence ofstimulus equivalence, and acquisition ofconditional relations, in this manner(Dugdale & Lowe 1990; Horne & Lowe1996; Lowenkron, 1991, 1996 ). If one wereto adopt the hypothesis that SB verbalbehavior may be composed of TB compo-nents (Dugdale & Lowe, 1990; Lowenkron,1991), one would expect to see the resultsdescribed above. Given no verbal skillswith which to use in learning SB relations,it would be necessary for a participant tolearn a conditional discrimination to selectcorrectly, which has been demonstrated innonhuman research, but often takes a con-siderable amount of time (Cumming &Berryman, 1961; Shafer, 1993). An alterna-tive to this is that a participant might learna topographical response and then have itfunction as an aide in responding correctlyin the SB condition. This of course wouldrequire an extra learning step, one not pro-grammed by the researcher, and wouldalso be likely to require an extended train-ing period. However, with highly verbalparticipants, the training period could con-ceivably be relatively short due to theextensive verbal repertoires of such partici-pants.

This hypothesis is partially supported bya follow-up study to Stratton (1992) con-ducted by Wallender (1993). Wallendersuggested that Stratton's results were dueto the fact that English (very familiar)words were used as sample stimuli(Stratton used the English names of vari-ous animals). According to Wallender, thisfamiliarity probably allowed participantsto readily emit verbal responses to thesample stimuli and also to the choice stim-uli, introducing a TB response as a media-tor in the SB task (i.e., identifying commonfeatures, emitting the TB response whilescanning, etc.). As described previously,Wallender had two groups of participantsengage in a SB task similar to that used byStratton. With one group, however, unfa-miliar Japanese characters served as sam-ples and for the other group English ani-mal names served as sample stimuli. Inboth groups, unfamiliar Japanese charac-

ters functioned as choice stimuli. Thus the"familiar" group saw English animalnames then selected the appropriateJapanese character and the "unfamiliar"group saw Japanese characters thenselected the appropriate (but dissimilar)Japanese character. A clear difference wasdemonstrated between the two groups; thegroup receiving the English words as sam-ples learned the relations nearly twice asfast as the other group (Wallender, 1993).Other researchers have examined this samephenomenon, drawing similar conclusions.Mandell and Sheen (1994) examined theeffects of pronounceable (e.g., "FLODG")and nonpronounceable (e.g., "NSJBM" ori+]*A!/) sample stimuli on the acquisition

of conditional discriminations (SB tasks)and on the formation of equivalenceclasses. In both cases, acquisition was bet-ter with more pronounceable sample stim-uli, leading the researchers to note thatstimulus equivalence (and conditional dis-criminations) are likely to be mediated byverbal behavior. Some evidence exists forthe role of mediating TB responses in thedevelopment of stimulus equivalence andconditional discriminations (Dugdale &Lowe, 1990; Lowenkron & Colvin, 1995;Potter et al., 1997; Wulfert, Dougher &Greenway, 1991) but most of this researchis necessarily correlational. More investiga-tion is needed to clarify this issue.Of course, if one were to adopt the

hypothesis that TB verbal behavior medi-ates SB verbal responding, one is still leftwith the task of explaining Stratton's (1992)results in which SB performance was betterthen TB performance. Stratton noted:

The topography-based task is essentially learn-ing 20 new foreign words for 20 English words.There are clearly two aspects to this task: learn-ing to say the foreign words as units, and learn-ing to say the correct one when the Englishword is shown....whereas no new topographiesare required in the selection-based task (pp. 23-24).

Stratton also noted that the SB task offersthe highly verbal participant the opportu-nity to engage in verbal behavior whichmay eliminate some of the choice stimuli(e.g., "it's not the one with the grid pat-tern") as comparisons on any given trial.


Others have reported some evidence forthis (Cresson, 1994; McIlvane et al., 1987;Potter et al., 1997; Wallender, 1993).McIlvane et al. (1987) showed thatresponding (using normal adults) in amatching-to-sample task could be con-trolled by the relation between the samplestimulus and the incorrect choice stimulus.These same researchers found similarresults when three and four comparisonswere used, versus only two. McIlvane et al.note that previous research with childrenindicates that exclusion responses "...havebeen reported to emerge in the secondyear, coinciding with initial acquisition ofverbal behavior" (p. 206).

It is important to examine the results ofStratton's study in light of the differentprocedures adopted in each study. Threeseem particularly relevant: the number ofrelations trained (which is usually thenumber of choice stimuli used for thesestudies); the nature of the TB response(e.g., a sign, writing, or vocal response);and finally, the extent to which the TBresponse was pretrained, thus necessitat-ing acquisition of the response during reg-ular training or not, in addition to comingunder the appropriate stimulus control forthat response.

Stratton (1992) and Potter et al. (1997)were the only researchers to require morethan 12 relations be learned, arranging for20 relations and 14 relations, respectively.Cresson required 8 relations be learned inany given condition and Bristow andFristoe (1984) required 12 SB relations belearned, but only a maximum of 6 TB rela-tions. No other researcher required morethan four relations be learned in any condi-tion. Stratton was also the only researcherincorporating a vocal-verbal TB response.All others used sign (Bristow & Fristoe,1984; Hodges & Schwethelm, 1984;Sundberg & Sundberg, 1990; Wraikat,1990; Wraikat et al., 1991), typing (Tan et.al, 1995), or writing (Cresson, 1994).Wallender (1993) and Potter et al. (1997)did not directly incorporate a pure TBresponse in their studies. Stratton's verbalresponse consisted of two-syllableJapanese words. It is possible that some of

those words contained phonemes not inthe English language as Japanese containssome phonemes which English does notcontain. In comparison, the writing, typ-ing, and sign responses required of partici-pants in all other studies had no similarsubcomponent of the final response thatwas unfamiliar to them. For example, allwould have had experience with drawinglines, with characters in the English alpha-bet (used when typing French words in theTan et al., 1995 study), and it is likely allparticipants had previously emitted allsubparts of the sign responses included inthese studies.

Stratton (1992) also only provided mini-mal pretraining of the TB response, namelyone preexposure to the sample stimulusand the correct response, at which pointthe participant would echo the response. Incontrast, Cresson (1994) thoroughly trainedthe TB writing response used. He requiredthe participants to quickly go through aseven step process, starting from copyingthe Katakana symbols used, to generatinga series of them, in any nonrepeatingorder. Pretraining for the TB response var-ied across the remaining studies, but couldbe classified as being more extensive thanin Stratton's study, but less extensive thanin Cresson's study (e.g., Sundberg &Sundberg, 1990, demonstrated andrequired imitation for each relation fivetimes). Bristow and Fristoe (1984) andHodges and Schwethelm (1984) did notreport the extent of pretraining.

In summary, it is unclear whetherStratton's results, as compared to otherresearcher's results, are due to true differ-ences in SB and TB responding, proceduraldifferences, or to both. However, at thevery least, the preceding analysis doesillustrate the number of variables whichmight influence obtained results.

Accuracy

Of the seven studies reporting accuracydata (the number of correct responses ineach condition) between SB and TB tasks,six reported more accurate responding inthe TB condition (Bristow & Fristoe, 1984;Hodges and Schwethelm, 1984; Sundberg


& Sundberg, 1990; Wraikat, 1990; Wraikat,et al., 1991; Cresson, 1994) and onereported no difference between conditions(Potter et al., 1997). These performancesparalleled those cited above; relativelylarge differences were recorded in studiesusing participants with poor verbal skillsand relatively small differences weredemonstrated in studies using participantswith well-established verbal skills.Although Hodges and Schwethelm (1984)showed clear differences between TB andSB accuracy (and number of relationsacquired), it should be noted that theseresearchers used nonspecific reinforcerswhen training the SB task (praise and food)and specific reinforcers for the TB task(e.g., candy when "candy" was said).These researchers actually conducted train-ing which ultimately resulted in mandrelations. Skinner (1957) defined the mandas "a verbal operant in which the responseis reinforced by a characteristic conse-quence and is under the functional controlof relevant conditions of deprivation oraversive stimulation" (p. 36). For example,participants in Hodges and Schwethelm'sstudy were required to sign "Candy" whenthe researcher held up a candy bar. If theemitted sign was correct, the candy wasgiven to the child. Prompting (molding thechild's hands) and fading of these promptswere also incorporated in this training.Eventually, the child was only asked"What do you want?", at which point if thechild emitted an appropriate sign, therequested object was given to the child.Skinner (1957) and Sundberg (1990) sug-gest that the mand is probably the firsttype of verbal relation acquired byhumans. They also note that it is likely tobe an easily acquired response, given thepowerful nature of the reinforcers used. Itis possible that the results of Hodges andSchwethelm's study were actually due tothe type of relation trained and not a truedifference between SB and TB verbalbehavior. Overall, however, TB perfor-mance appeared to be more accurate thanSB performance across studies.

Stimulus Equivalence

For those studies that incorporated someaspect of stimulus equivalence (reflexivity,symmetry, transitivity, and equivalence),performances tended to be better after TBtraining versus after SB training. Again, themagnitude of the differences recordedseem related to the type of participantstested. For the two studies using partici-pants with DD, a relatively large differencewas demonstrated between the TB and SBconditions (Sundberg & Sundberg, 1990;Wraikat et al., 1991). With verbal partici-pants, TB performance was only slightlyhigher for two studies (Bristow & Fristoe;1992; Cresson, 1994) and showed no differ-ence in two studies (symmetrical relations- Stratton, 1992; equivalence relations -Potter et al., 1997). Tan et. al (1995) showednearly perfect symmetry performancewhen the response form was SB and poorsymmetry performance when the responseform was TB (however see footnote #2regarding this issue). Tan et al. examinedthe difference between symmetry perfor-mances under four conditions: (1) SB train-ing/SB Testing, (2) SB Training/TBTesting, (3) TB Training/SB Testing and (4)TB Training/TB Testing. These researchersused either French words (unfamiliar) assamples and English words as the choicestimuli (either typed out - TB, or selected -SB). In testing for symmetry, when theresponse was selection-based, regardless ofthe training, accuracy was generally high.However, when the response was topogra-phy-based in symmetry testing, accuracywas generally low. This was an expectedoutcome as the authors noted: "...seeingthe stimulus word, either during selection-training or topography training, no matterhow often, does not guarantee that onewill later be able to produce it, especially ifthe word is unfamiliar" (p. 2).

Tact vs. Intraverbal

In those studies that trained both tactand intraverbal relations in the SB and TBconditions, overall, the tact relationappears to have been acquired more read-ily than the intraverbal relation. Cresson(1994) demonstrated an exception to this,


with the intraverbal relation acquired morereadily, and Potter et al. (1997) showed lit-tle difference between the two conditionsoverall. Potter et al. (1997) examined thismore closely and showed that intraverbalperformances were better in earlier ses-sions and tact performances better in latersessions. These researchers believed thatthis showed support for the hypothesisthat vocal-verbal mediation plays a role inSB responding, as the same choice stimuliwere used in the intraverbal and tact con-ditions to allow for equivalence testing.The participants were apparently generat-ing and learning vocal-verbal responses inthe intraverbal condition and simply re-emitting the same responses in the tactcondition.

It is difficult to draw conclusions fromthese tact and intraverbal results for sev-eral reasons. First, different participantswere used in those studies (Cresson usednormal adults, others used adults and chil-dren with DD). Second, within each study,the stimuli used as samples are in differentmodalities (i.e., hearing a sound for theintraverbal and seeing a graphic for thetact), thus making it difficult to equate thetwo in terms of complexity. Since theresearchers in each study used differentchoice stimuli, it becomes even more diffi-cult to interpret these data.

Additional Results

Some studies incorporated reaction timemeasures (Potter et al., 1997; Stratton, 1992;Wallender, 1993). Stratton was the onlyresearcher to incorporate a reaction timemeasure and have participants engage inboth a SB and a TB task. As would beexpected, reaction times were higher forthe SB group in Stratton's study as partici-pants needed to scan the choice stimuli,whereas no such scanning response isrequired in a TB task. Wallender (1993) andPotter et al. (1997) trained and tested onlyin the SB paradigm, and both demon-strated little difference between conditionsin terms of reaction times.

Finally, many researchers reported anec-dotal observations of participants engagedin the SB or TB tasks. For example, Hodges

& Schwethelm (1984), Sundberg andSundberg (1990), Wraikat (1990), andWraikat et al. (1991) all reported that par-ticipants appeared more engaged in the TBvs. the SB task. Participants in the Potter etal. (1997) study reported in exit interviewsa preference for the SB task which incorpo-rated a TB component. Other researchersnoted that participants appeared to engagein vocal-verbal mediating behavior(Cresson, 1994; Potter et al., 1997; Stratton,1992; Sundberg & Sundberg, 1990;Wraikat, 1990). While most of these appar-ently mediating vocal-verbal responsesoccurred during SB tasks, Wraikat (1990)reported some participants emitted vocalresponses prior to performing a sign, andthose participants tended to perform bet-ter. Wallender (1993) proposed that usingmore familiar sample stimuli "made it eas-ier for the verbal adults because of otherresponses such stimuli might evoke inthese subjects." (p. 15). As discussed ear-lier, Wallender was examining whetherfamiliarity with the stimuli used in a con-ditional discrimination task would aidacquisition, which it did, both in his studyas well as in a similar study by Mandelland Sheen (1994).

Potter et al. (1997) attempted to studythis apparent vocal-verbal mediation byarranging for participants to engage in acomputer-based matching-to-sample taskwhile talking aloud. The "Talk Aloud"procedure is one of several types of proto-col analysis techniques offered by Ericssonand Simon (1993). Basically, this procedurerequires that the participant vocalize nor-mally covert verbal behavior, which is thentape-recorded. Participants are given sometraining in this to prevent additionalintraverbal behavior from creeping into thevocalizations. For example, participantsare discouraged from specifying what theyare doing at any moment, or from describ-ing mental imagery. Ericsson and Simon(1993) found little difference betweengroups solving problems, one using thetalk aloud procedure and the other solvingproblems silently. The objective in suchresearch is to simply clarify the role, if any,that mediating vocal-verbal behavior


might have in these tasks, without interfer-ing with the task itself. Potter et al. (1997)transcribed tape recordings of participants'vocal-verbal behavior, then classified theparticipants' statements according toSkinner's (1957) elementary verbal oper-ants (primarily tacts and intraverbals). Theparticipants aided in clarifying controllingvariables by listening to the tapes whilebeing exposed to the situation in effectwhen the vocalization was made. Veryconsistent types of responses were shownto precede correct responses for all partici-pants. The authors concluded that it islikely that conditional discrimination tasks,and emergent equivalence relations, aremediated by TB verbal responding, at leastwith verbal organisms.

CONCLUSIONS

Most of the research reviewed in thisarticle demonstrated better overall resultsin acquisition and accuracy when TB ver-bal behavior was trained vs. SB verbalbehavior.However, given the limited number of

studies conducted, and the data these stud-ies generated, these conclusions should beconsidered with caution. Special attentionshould be given to the type of participantsconsidered. Greater differences in SB andTB performances are obtained when per-sons with poor verbal skills are used asparticipants, with TB verbal behaviorresulting in better acquisition and accuracyperformances. This is an interesting find-ing as SB systems were developed primar-ily for persons with low, or nonexistentverbal abilities. These issues, and others,are addressed below.

First, it is not clear that a distinct separa-tion of SB and TB verbal behavior wasattained in the studies reviewed. In thestudies in which vocal-verbal participantswere used, there is a fair amount of evi-dence indicating that the SB task wasmediated via vocal-verbal behavior. Thiswas supported by the fact that SB verbalbehavior appears to be more difficult toacquire than TB verbal behavior - for bothverbal and nonverbal participants.However, the difference between the two

language types is greater when partici-pants with weaker verbal repertoires areused, indicating that TB verbal behaviorplays a role in acquiring SB verbal behav-ior (along with other supporting evidence).Interestingly, taken alone, this observationwould argue against the use of SB verbalbehavior for the very reasons it was ini-tially used, that is to aid nonverbal clients.However, other factors also need to be con-sidered such as the skills of potential lis-teners (i.e., do they know sign language),the physical capabilities of the client, andthe response forms to be trained (e.g.,mands, tacts, etc. - see Shafer, 1993 andSundberg, 1993 for a thorough discussionof these issues).

It would appear that TB responses aresomewhat easier, or preferred over SBresponding (as most participants of thestudies discussed in this article reported).If SB responding were easier or more effec-tive, it would seem that responding wouldsettle into a pure SB form (the vocal-verbalsupplements would drop out), which wasnot the case in some of these studiesreviewed. Perhaps with extensive trainingthis would occur. It is possible that existingTB repertoires "block" the acquisition ofpure conditional discriminations (that is"pure" SB responding), but again, intu-itively, if SB responding were easier, itwould seem likely that mediating TBresponses would eventually drop out of aconditional discrimination. While SBresponding appears on face value to beeasier then TB verbal behavior, it probablyis so for the teachers/caretakers only, inthat they would not be required to learn anew TB form of communication (versusreading words off a communicationboard). It is clear that more research isneeded in this area.Second, numerous factors, including

participant and methodological considera-tions must be considered when attemptingto determine acquisition rates and accu-racy for each type of verbal behavior. Theamount of pretraining, the extent of exist-ing repertoires, the number of relationstrained, the size of a typical response unit(i.e., sentences vs. single words), all would


seem to be relevant determinants ofobtained results.

Finally, some other distinctions betweenSB and TB verbal behavior may contributeto the observed differences in acquisition,accuracy and overall utility. SB verbalbehavior, being dependent on hardware, isless likely to function as verbal behaviorthat the speaker can respond tohimself/herself (i.e., talking to oneself).This has multiple implications when oneconsiders that much of verbal behaviormay be acquired and strengthened viaautomatic reinforcement (Sundberg,Michael, Partington & Sundberg, 1996). Inaddition, SB verbal behavior is likely torestrict observing/interacting with a verbalcommunity that models and directly rein-forces appropriate verbal behaviors(Sundberg, 1993).

Future Directions

More research is necessary to clarify thedifferences between SB and TB verbalbehavior. Many of the differences outlinedin Table 1 have yet to be investigated.Some research that might be conductedincludes: the relevance of scanning reper-toires; the effect of time delays imposedbetween the presentation of someantecedent stimulus and the responseemissions; the impact point-to-point corre-spondence might have on acquisition; theeffect of SB/TB training with verbal oper-ants other than the tact or intraverbal (e.g.,mand); and differential effects across vari-ous populations. With the advent of high-powered microcomputers and supportingsoftware, it is becoming easier to conductresearch in this area. For example, a TBresponse might be emitted via mousemovements (or even finger movementswith the appropriate hardware inputs) ona computer resulting in auditory emissionsfrom the computer that serve as antecedentstimuli for the listener. By manipulatingthe nature of the TB response one couldexamine the effect the point-to-point corre-spondence has on acquisition and accu-racy. Indeed, this very technique might beuseful in providing a TB response form toclients with DD who have vocalization dis-

abilities, but good manual dexterity. Thistechnique would not require staff to learn anew TB response form as the computercould be programmed to emit vocalresponses in the listener's native tongue.A thorough understanding of the factors

contributing to the differences betweenthese types of verbal behavior can onlyimprove our understanding of verbalbehavior. In addition, it will allow us todesign better programs for nonverbal per-sons, as well as design research that takesthese differences into consideration. It ishoped that this review has aided in thistask.

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