Reinforcer Strength in Transfer of Stimulus Control From Tacts to Mands

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1 EUROPEAN JOURNAL OF BEHAVIOR ANALYSIS 2011, 12, NUMBER 1 (SUMMER 2011) 289 Reinforcer Strength in Transfer of Stimulus Control From Tacts to Mands Aurelia Power 1,2 and J. Carl Hughes 1 Bangor University 1, Stepping Stones Aba School For Children With Autism 2 The purpose of the present study was to investigate the role of reinforcer strength in transfer of stimulus control from tacts to mands conditions. Three children diagnosed with autism were initially taught to tact most and least preferred abstract stimulus properties of preferred items that had been identified in two preference assessments. Subsequently, we ran probe trials to determine whether the participants would exhibit the same topographical responses under mand conditions. Previous mand responses that did not include the abstract stimulus properties were placed on extinction. Our results show that tact responses may emerge as mands for those abstract stimulus properties identified as most preferred, but not for those that were identified as least preferred. Results are discusses in terms of the role of motivating operations, and potentially, extinction, as variables that facilitate transfer of stimulus control from tacts and mands. Key words: verbal behavior, autism, tacts, mands, transfer, motivating operations Skinner (1957) described our verbal repertoire in terms of a number of separate verbal operants that are defined by the particular controlling variables under which they occur, and not by their form or topography. Two of these verbal operants are the mand and the tact. A mand was defined by Michael (1988) as a type of verbal operant in which a particular response form is reinforced by a characteristic consequence and is therefore under the control of an establishing operation relevant to that consequence. In the mand response the response form has no specified relation to a prior stimulus discriminative (p.7). In contrast, a tact was defined by Skinner (1957) as a verbal operant in which a response of a given form is evoked or at least strengthened by a particular object or event or property of an object or event (p.81-82), and further, is reinforced in the presence of one stimulus with many Correspondence to Dr. J. Carl Hughes, Wales Centre for Behaviour Analysis, Brigantia Building, School of Psychology, Bangor University, Gwynedd, Wales, LL57 2AS. c.hughes@bangor.ac.uk different reinforcers or with a generalized reinforcer (p.83). In other words, in the mand relation the response is evoked by a motivating operation and is reinforced by a specific consequence relevant to that motivating operation, whereas in the tact relation the response is discriminated by a non-verbal stimulus signalling the availability of a nonspecific generalized reinforcer. Therefore, as Skinner suggested (1957), a mand response and a tact response, even though formally or topographically indistinguishable, are responses emitted under the control of different antecedent and consequent events they are different operant relations and from a functional perspective must be accounted for separately. The functional independence of mands and tacts has been investigated under a number of different contexts. Lammare and Holland (1985) evaluated the collateral effects of either mand or tact training of two prepositional phrases ( on the left and on the right ) on the spontaneous emergence of 289

2 290 Aurelia Power and J. Carl Hughes mands after tact training, and of tacts after mand training. Hall and Sundberg (1987) investigated the effects of tact training on the spontaneous emergence of mands for items that were necessary to complete a task that had a number of components in a response chain. Sigafoos, Doss, and Reichle (1989) initially taught three adults with developmental disabilities to tact three sets of food items and the utensils required to consume those food items by using graphic symbols, and subsequently tested to determine whether they occur as mands. Finally, Twyman (1996) investigated the functional independence of mands and tacts at the level of abstract stimulus properties. The results of all these studies indicated that participants showed no transfer or very little transfer of stimulus control from mands to tacts or vice versa, and the authors suggested that they were independent at the time of acquisition, providing empirical support for the functional independence of mands and tacts. However, despite the evidence that mands and tacts are separate verbal operants, there are instances of spontaneous transfer of stimulus control from one to the other. Skinner (1957) maintains that this phenomenon may be explained in several ways. First, it may explained through the fact that there is more than one variable at work at the time of the acquisition of one verbal operant, and such multiple control provides the opportunity for the same topographical response to come under the control of various contingencies. Second, the presence of a reinforcing stimulus may signal the opportunity for receiving reinforcement (i.e., the stimulus may have discriminative functions); thus a mand for that reinforcing stimulus could be evoked, but the conditions may resemble both mand and tact because the mere presence of the reinforcement may function as a discriminative stimulus that makes it partially a tact; in these cases it is often described as an impure mand (Skinner, 1957). Other conditions under which transfer may occur include the similarity between stimulus conditions when a person mands and tacts: as Skinner (1957) notes, the milk which the child gets with the mand Milk! resembles the milk which controls the tact milk in response to the question What is that? This may facilitate the acquisition of whichever operant is acquired second. (p.189), and the presence of an established mand frame, such as I want, within which acquired labels of objects or events may be used spontaneously to access those objects or events in the form of a mand (Skinner, 1957). Such transfer has been noted in some of the previously mentioned studies. For example, in the Hall and Sundberg study (1987) even though direct mand training was initially needed in order to establish manding for missing items, after acquiring four mands, both participants showed the ability to generalise manding to any item that they could tact. Similarly, Sigafoos et al. (1989) observed that, after the acquisition of a few symbol responses as tacts or mands, the participants showed a tendency to generalise transfer of stimulus control to untrained stimuli. Other studies have shown that transfer from tact to mand conditions can readily occur without direct training, once participants had an already established mand repertoire. For example, Sigafoos, Reichle, and Doss (1990) demonstrated that following tact intervention, manding spontaneously occurred for two out of three utensils needed to consume food items in the two participants adults with severe mental retardation who had in their verbal repertoires a generalised request in the form of pointing to a want symbol prior to the beginning of the study. Similarly, Sundberg, San Juan, Dawdy, and Arguelles (1990) reported transfer from tact and intraverbal conditions to mand conditions for three sets of common items in two subjects with traumatic brain injury with an established manding repertoire. In contrast with previous research, Wallace, Iwata, and Hanley (2006) found that the establishment of mands following tact training may be facilitated without the

3 Transfer From Tacts to Mands 291 requirement of any prior established mand repertoire. The authors argued that previous research did not take in consideration the reinforcing capability of the stimulus trained during tact condition and thus overlooked the effects that it may have on the transfer of stimulus control. As a result, they employed a preference assessment before the tact training was implemented for three adults with mental retardation who did not engage in verbal behaviour in order to obtain desired items. The results of the study showed that transfer of stimulus control from tacts to mands occurred for highly preferred items and initially even for less preferred items. However, manding for less preferred items rapidly decreased while manding for highly preferred items increased and were maintained at that level. Thus in terms of reinforcer strength, mands may be established following tact training once the stimuli used in tact training function as reinforcers, underlying the importance of taking advantage of naturally occurring establishing operations [motivating operations; Laraway, Snycerski, Michael, & Poling, 2003] as the antecedent events that evoke a mand response. The present study is intended to extend the scope and the generality of Wallace et al. (2006) findings to abstract stimulus properties that may be responsible for the items being reinforcing, such as size, colour etc. Thus the question we addressed concerns the possibility of transfer of stimulus control to mands following tact training on abstract stimulus properties that participants selected as highly preferred. Method Participants and Settings We recruited two boys, Solomon and Martin, and one girl, Sue, aged 5 years, 6 years, and 5 years, respectively. All three had a diagnosis of autism. The participants were selected on the basis that they could reliably mand for desired items, but without specifying abstract properties of those items. The selection process also involved adhering to the ethical governance procedures of the School of Psychology, Bangor University, Wales. Consent to take part in the study was received from the parents or guardians and the school. All three children had attended the same school for at least one and a half years and all received 30 hours of intensive behavioural intervention each week. Solomon was vocal and could spontaneously mand for desired items on an average of 300 mands per day, could follow simple and 2-step directions, and imitate vocally phrases up to 5 words long. At the time of the study he had a tact repertoire of at least 100 words for common items and at least 100 words for pictures of common items, and could receptively identify at least 100 objects in his immediate environment (data available from the school). Martin and Sue were using sign language as the main means of communicating. Although they both could vocalise, their vocalisations were weak and unintelligible, especially to listeners unfamiliar with them. Martin had 26 mastered signs, and Sue had 22 mastered signs which they used to request desired items, on average of 350, and 280 requests per day respectively. They both could follow simple and 2-step instructions, and both could imitate 1- and 2-step actions. They also could receptively identify 100 objects in their environment (data also available from the school). The first author conducted all sessions in the school, and these sessions took place in a portion of the classroom divided by a partition to minimise distractions. The experimenter and the participant sat at the table and the only items present were those included in the study. Response Definitions and Data collection During the prerequisite skills assessment we defined mand as emitting a response that specified (either by using sign language, or vocally) the item presented, which in turn produced access to that item. We collected

4 292 Aurelia Power and J. Carl Hughes data on the number of trials in which the participant correctly requested the items presented, and reported this as mands per minute. We used rate measures for mand responses because we could not directly control antecedent motivational conditions. However, during the experimental sequence we further elaborated the definition of the mand to include abstract stimulus properties. Thus, we defined the mand as emitting a response that specified both the relevant property and the actual item, when two items that differed only on the relevant abstract stimulus property were held in front of the participant (for example, butter/dry cracker). We reinforced correct responses by providing access to the specified item. We considered as incorrect those mand responses that specified the actual item only without specifying an abstract property, thus mand responses that would have gained access to the reinforcer in the past no longer did so. We collected data on the number of trials in which the participant correctly requested for the items presented using an abstract stimulus property, but also on the number of trials that were incorrect when no abstract stimulus property was specified. We defined the tact response as a correct response if it included both specifying the relevant property and the actual item (for example, butter/dry cracker), to the question what is this? when the experimenter pointed to one of the two items present. The items differed only on one relevant characteristic or property. All correct tact responses were followed by verbal social praise (verbal praise was an established form of reinforcement for these children in their setting). We coded as incorrect any tact responses that specified the actual item only, as well as any tact responses that specified an item other than the one that the experimenter pointed to; no response within 5secs were also coded as incorrect. We collected data on the number of trials in which the participant tacted the items presented, and reported this data as the number of correct responses since the number of trials for both items was equal during a tact training session, and across all tact sessions. Inter-observer Agreement The first author collected all primary data. A second observer who was trained in the specific method of data collection and had over one year of experience working in ABA settings served to assess inter-observer agreement. We calculated inter-observer agreement by dividing the total number of trials with agreements by the total number of trials (agreements and disagreements), and multiplying by 100%. In Martin s case, inter-observer agreement observations were conducted for 44.43% of the sessions, with an overall percentage of agreement of 99.4, ranging from 95% to 100%. In Sue s case, reliability checks were conducted for 42.37% of the sessions, with an overall percentage of agreement of 96.04, ranging from 91.6% to 100%, and in Solomon s case 47.37% of the sessions were assessed for reliability, with an overall percentage of agreement of 99.61, ranging from 95% to 100%. Preference Assessments During this phase, using the paired-stimulus procedure (Fisher, Piazza, Bowman, & Hagopian,1992), we conducted two preference assessments for each participant in order to identify highly preferred food items and subsequently the abstract stimulus properties responsible for the food items being highly preferred. We conducted the first preference assessment for four food items which were reported by the staff as being preferred, and to which the participants had access consistently, on a daily basis. The experimenter presented the items in pairs until each item was paired with all the others twice in each session. Each pair was presented for five seconds and the experimenter instructed the participant to pick one item. The participant was then allowed to consume the item. Reaching for more than one item was blocked, and the experimenter reinstructed the participant to

5 Transfer From Tacts to Mands 293 pick one item only. We selected the most preferred item to be presented in different exemplars differing on only one dimension during the second preference assessment and subsequent phases of the study. However the pairs did not represent all possible combinations of exemplars, but only those naturally available to the participants (for example, dry cracker and butter cracker were the only variations of cracker available to Martin, white chocolate and brown chocolate were the only chocolate variations available to Solomon, and big and small orange were the only variations available to Sue). Employing the same procedure as in the first preference assessment, the experimenter presented the two items in pairs six times each session. We selected these items to be taught as tacts and then probed for transfer from tacts to mands. Prerequisite Skills Assessment We designed this phase to determine that all three participants could reliably mand for the most preferred item identified in the first preference assessment, and to provide a level of the participants responding where extinction was not applied. The experimenter held the item in front of the participant, but did not deliver any instructions or prompts. If the participant correctly signed for the item (in Sue and Martin cases) or said the word corresponding to the item (in Solomon s case), the experimenter gave the item to the participant and allowed him/her to consume it. No corresponding words or signs for abstract stimulus properties were required in order to access the specified reinforcer. Experimental Design We used a multiple baseline design across subjects. Procedure For each participant we conducted mand probes for those items that were identified during the second preference assessment as most and least preferred before tact training was initiated in order to provide a baseline; we then initiated tact training for the least and most preferred abstract stimulus properties; once each participant had demonstrated reliable tact responses, we re-introduced mand probes in order to test for influences that tact training might have on the emergence of mands. Mand Probes In order to enhance as much as possible motivational control, we conducted mand sessions just before snack time; in addition we limited the duration of sessions to 5 minutes each to avoid satiation, and cut the items into small pieces of equal size (the only exception being in Sue s case, where the orange was cut into smaller and larger pieces to denote the two abstract stimulus properties: small and big). During the mand probes, the experimenter began the session by stating phrases to which the participants were exposed daily when snack time was scheduled (e.g., It is time for snack or Snack time ) in order to signal that food items were available for consumption. The experimenter simultaneously held in front of the participant the two items identified in the second preference assessment as the most and the least preferred (i.e., dry cracker and butter cracker). No prompts or instructions were delivered during this phase. If the participant signed correctly or said the correct words corresponding to the item presented specifying one of the relevant abstract stimulus properties, the experimenter delivered that item and allowed the participant to consume it. The experimenter then initiated a new trial as soon as the participant consumed the item delivered. Mand responses that specified only the item and did not incorporate one of the relevant abstract stimulus properties were considered incorrect and no longer accessed the item requested, thus being placed on extinction. Tact Training In an attempt to reduce the probability that tact responses would function as mand responses, we conducted tact training ses-

6 294 Aurelia Power and J. Carl Hughes sions just after the snack time. Furthermore, the two items were placed in front of the participant just before the commencement of the session with no further instructions; if the participant approached any of the items and consumed it, indicating that potentially the motivating operation for those stimuli was active, the tact training session was not conducted. During this phase, the experimenter simultaneously placed both items on the table and pointed to one of them, asking the question What is it?. If the participant responded correctly using the corresponding words or signs for the relevant abstract stimulus property and the actual item, the experimenter delivered verbal social praise, in the form of statements such as That s right! or Well done!. If the participant responded incorrectly, or made no response within 5 seconds, the experimenter modelled the correct response (either signing, or vocalising), and immediately represented the question What is it?. We determined that tact responses were acquired when the participant could correctly tact both items on at least 90% of the trials over three consecutive sessions. During tact training, 10 trials were presented for each item within each session in random fashion, and the sessions lasted approximately 5 to 10 min., depending on the number of errors. Over the period of time while tact training was conducted, all participants could access in their natural settings (for example at snack time) the food items employed in the present study by using the previous form of manding where specifying a stimulus property was not required. Pure Mand Probes We also conducted Pure Mand Probes to further reduce the probability that target responses were emitted as tacts during mand probe phases, given the possibility that the presence of the items could evoke tact responses (Skinner, 1957). The conditions were similar to those during mand probes, apart from the fact that the items were not in the participant s visual field, but instead were kept in a small closed box hidden under the table. Results Figure 1 (left hand panel) shows the outcomes of the first preference assessment. Martin s most preferred food item was cracker, chosen on 80% of the trials, Sue s was orange, chosen on 96.6% of the trials, and Solomon s was chocolate, chosen on 73.3% of the trials. These items were presented in the subsequent phases of the study. Figure 1 (right hand panel) shows the outcomes of the second preference assessment in which the most preferred item identified during the first preference assessment was presented in different exemplars differing on only one relevant property in order to identify the stimulus property responsible for the item being highly preferred. The exemplars represented only those items that the participants would naturally have access to. Thus the most and least preferred items identified during the second preference assessment for Martin were butter cracker, chosen on 100% of the trials, and dry cracker, chosen on 0% of the trials; for Sue, the most and least preferred items were big orange, chosen on 100% of the trials, and small orange, chosen on 0% of the trials; and for Solomon, the most and least preferred items were brown chocolate, chosen on 100% of the trials, and white chocolate, chosen on 0% of the trials. Words or signs corresponding to these items, and their relevant stimulus properties were taught during tact training and tested to determine if they emerged as mands, without direct training. Figure 2 shows the results of prerequisite skills assessment, mand and pure mand probes, and tact training. All participants emitted correct mand responses during the prerequisite skills assessment for the preferred item, which increased over sessions. During the first phase of mand probes which served

7 Transfer From Tacts to Mands 295 as baseline, none of the participants emitted abstract mand responses. However they all continued to mand for those items, but without using words or signs corresponding to the relevant abstract properties. These mand responses decreased considerably over sessions since they were placed on extinction, and Solomon did not emit any mand responses during the last 6 sessions of the first phase of mand probes. During tact training, Martin acquired the signs for butter cracker and dry cracker within 9 sessions, Sue acquired the signs for big orange and small orange within 12 sessions, and Solomon acquired the words for brown and white chocolate within 6 sessions. When the mand probes were conducted again, all participants emitted mand responses for the preferred items using signs or words corresponding to the abstract properties. Additionally, the rate of these responses increased across sessions, reaching 4.4 mands per minute for Martin, 4.8 mands per minute for Sue, and 10.4 mands per minute for Solomon. All participants also emitted at the beginning mands that did not include abstract stimulus properties, but at a very low rate that decreased to 0 in subsequent sessions. Martin emitted these responses, during the first 2 sessions at a rate of 0.2 mands per minute, Sue emitted such responses during 4 sessions, at a rate ranging from 0.2 to 1.4 mands per minute, and Solomon emitted such responses only during the first session, at a rate of 0.2 mands per minute. Solomon also manded during the first session for the item with the least preferred stimulus property on only one trial (0.2 mands per minute), whereas Martin and Sue never emitted such responses. During pure mand probes, all participants continued to mand for the most preferred item using the signs or words corresponding to the preferred abstract stimulus property, and with the exception of Martin, who manded for the item with the least preferred property only on one trial (0.2 mands per minute), they never manded for the item with the least preferred property. However the rate of mand responses for the item with the most preferred property decreased in comparison to the previous phase in the Preference Assessment 1 Preference Assessment Martin Sue Solomon Martin Sue Solomon percentage of trials selected crisp cracker apple yoghurt crisp orange popcorn yoghurt apple chocolate bread orange Figure 1. Percentage of trials in which each item was chosen during preference assessments 1 and 2. butter cracker dry cracker big orange small orange brown chocolate white chocolate

8 296 Aurelia Power and J. Carl Hughes Figure 2. Mands per minute (left scale-unshaded) during the prerequisite skill assessment, mand probes, and pure mand probes for the most preffered item without using abstract stimulus properties ( ), and using the most preferred abstract stimulus property ( ) or least preferred abstract stimulus property( ), and number of correct tact responses (right scale-shaded) during tact training of most and least preferred abstract stimulus properties of most preferred item.

9 Transfer From Tacts to Mands 297 case of both Martin and Sue, but remained approximately the same for Solomon. No participant emitted mand responses using only the name of the item. Discussion The results of the present study show that none of the participants exhibited target responses during the first mand probes phase (baseline), prior to tact training. After acquiring words or signs corresponding to most and least preferred abstract stimulus properties as tacts, all participants showed transfer to mand conditions. However such transfer was achieved only in the case of most preferred properties, suggesting that reinforcer strength is a key variable responsible for the emergence of tacts into mands. As research has shown (Fisher et al., 1992; Roscoe, Iwata, & Kahng, 1999), in a free operant concurrent arrangement, when a preferred and a non-preferred item are simultaneously available, participants allocate their responding towards the preferred item, or as Baum (1974) describes it with respect to the generalised matching law towards the higher-quality item. This finding is supported by data which shows that mand responses for the item with the most preferred abstract stimulus property occurred at high rates, and increased over sessions during mand probes, whereas mand responses for the item with the least preferred abstract stimulus property never occurred in the case of both Martin and Sue, and in Solomon s case occurred on only one trial (0.2 mands per minute). Such pattern of responding was predicted by the second preference assessment, which showed that choice responses were always allocated towards the item with the most preferred property. It is possible that during the mand probes conducted after tact training, initially responding for most preferred abstract stimulus property might have served a tact function. However this is very unlikely since differentiation in responding for the item with the most preferred property and least preferred property was very clear for all participants. As Wallace et al. (2006) pointed out, it is extremely unlikely, however that all of the participants would continue to tact one item (HP) consistently but not the other (LP) (p.23). On the other hand, the lack of opportunities to provide consistently differential consequences for most and least preferred properties during mand probes seems to indicate that prior tact training facilitated the emergence of mands, and the procedures described in the present study did not function to train manding directly. Additional evidence to support the fact that mand responses were evoked by motivating operations comes from the data collected during pure mand probes. Even though the rate of responding during pure mand probes was lower than during mand probes in Martin s and Sue s cases (for Solomon the rate of mand responding remained approximately the same during pure mand probes), all participants continued to mand correctly for the item with the most preferred abstract stimulus property, but not for the item with the least preferred property. The fact that during mand probes the rate of responding for the item with the most preferred abstract stimulus property was higher than during pure mand probes in Martin s and Sue s cases, suggests that the presence of items in the participant s visual field during mand probes may have acted as a prompt or cue for mand responses, signalling the availability of reinforcement (Skinner, 1957). In the present study we attempted to ensure that discriminative control is achieved during tact training sessions, and motivational control during mand and pure mand probes in four ways. First, by conducting tact sessions just after snack time, and mand sessions just before snack time; second, by testing the motivating operation for both items before commencing tact training sessions; third, by keeping the sessions short in order to reduce the probability of participants becoming satiated during mand probes, and

10 298 Aurelia Power and J. Carl Hughes deprived during tact training sessions; and fourth, by providing different consequences for tact responses (generalised reinforcement in the form of praise), and mand responses (the participant was allowed to consume the item specified with the specified property). This is further supported by data which shows that during tact training correct responding for the item with the most preferred abstract stimulus property was never higher than the responding for the item with the least preferred property, and for all participants correct tact responses for the item with the most preferred property were lower at times than those for the item with the least preferred property, suggesting that during tact training discriminative control was achieved. In other words, it seems that responding during tact training did not significantly favour any of the items, whereas during mand probes conducted after tact training there was clear differentiation in responding between least and most preferred items. One other variable that may have influenced the transfer from tacts to mands, and for which the present study did not control, was the fact that all participants, prior to the beginning of the study, had an established manding repertoire. As Skinner (1957) suggested, an established mand frame may facilitate the spontaneous use of acquired tacts of objects and events as mands, in order to access those objects or events. Sigafoos et al., (1990) demonstrated that, in adults with severe mental retardation who already had in their verbal repertoire a generalised request in the form of pointing to a want symbol, spontaneous transfer from tact to mand conditions had successfully occurred. Hall & Sundberg (1987) and Sigafoos et al. (1989) have also noted that once a minimal mand repertoire is established, subsequent mands for other items were more likely to occur. In contrast, other studies (Lammare & Holland, 1985; Twyman, 1996) have shown that, despite having an established mand repertoire, participants did not show transfer of stimulus control from tact to mand when the use of more sophisticated linguistic variables, not already in the participants repertoires, were required such as prepositions, or abstract stimulus properties suggesting that a second order mand frame that includes such variables may be necessary to establish transfer from tacts into mands. However, in the studies mentioned above the reinforcing capability of the stimuli employed was not assessed, and thus it was unclear if they would have evoked mands by naturally occurring motivating operations. In the present study, preferred items and their abstract stimulus properties responsible for the items being preferred were identified before proceeding with the experimental sequence. All participants showed transfer from tacts to mands for the item with the most preferred property, but not for the least preferred property, even though they did not have in their repertoire an already established mand frame that included the use of abstract stimulus properties, indicating that reinforcer strength was one of the main variables responsible for such transfer. Although we used the term abstract stimulus property to refer to properties of the items employed during the present study, as with the Twyman (1996) study, they were abstract only from the experimenter s point of view since they were tested only for one item for each participant. As Skinner (1957) pointed out, abstraction refers to a verbal process in which any property of a stimulus present when a verbal response is reinforced acquires some degree of control over that response, and this control continues to be exerted when the property appears in other combinations (p.107). From this perspective, to be truly abstract, a property has to be used in conjunction with items other than those employed in the present experiment. The results for the items with the least preferred abstract stimulus properties are consistent with previous research (Hall & Sundberg, 1987; Lammare & Holland, 1985; Sigafoos et al., 1989; Twyman, 1996) in which tact to mand transfer was not

11 Transfer From Tacts to Mands 299 achieved. However, the aspect regarding their reinforcing nature must be further elaborated. Thus because they were not nonpreferred properties, but only less preferred in relation to the most preferred properties identified in the present study, transfer might have occurred if they were the only ones available, or if other less preferred properties were simultaneously available suggestions also made by Wallace et al. (2006). Further research might focus explicitly on addressing this aspect. Another potential variable that may have facilitated transfer of stimulus control from tacts to mands is extinction. Extinction may have contributed to transfer of stimulus control from tacts to mands in the sense that the tact as a verbal operant in one s repertoire may occur as a mand during extinction burst, since it has been shown that novel behaviours behaviours that do not typically appear in a given situation may occur (Lalli, Zanolli, & Wohn, 1994), along with increased intensity, duration or rate (Lerman & Iwata, 1995) during extinction burst. The data from the present study seem to reflect the occurrence of such novel behaviours, since all participants continued during the second phase of mand probes to briefly mand for the most preferred item without using words or signs corresponding to any of the stimulus properties presented. These mand responses quickly decreased to zero levels, since they no longer accessed the items requested, and they were replaced by the new responses that included words or signs corresponding to the most preferred stimulus property, even though these new responses were never trained under mand conditions. In addition, it is possible that extinction may have served to maintain the new form of responding as the new responses might have required greater effort. Based on the predictions of the matching law (see, Baum, 1982), it would be very unlikely that the participants would mand using the new form of responding, when previous form of responding could still access the reinforcing items, requiring less effort. However, the present study did not allow for an experimental evaluation of the effects of extinction, and more empirical support is needed to clarify this relationship. Further research might investigate extinction effects by incorporating phases with extinction and no extinction. It would be also interesting to investigate transfer from tact to mand conditions where the new form of response requires less effort, for instance instead of reaching for an item that is normally up high (which may also require a chain of behaviours to be performed, such as bringing a chair to stand on, getting up on the chair, etc.), a word corresponding to that item might be taught as a tact, and then tested to see if it readily occurs as a mand. The results of the present study, taken together with the results of the study conducted by Wallace et al. (2006), may have several implications for teaching communication skills to people with severe and moderate communication difficulties. Though mand training maybe the first option to consider when beginning teaching verbal behaviour to the population mentioned above (Sundberg & Michael, 2001), the view that transfer of stimulus control from tact to mand conditions may be achieved provides an alternative tool for expanding verbal repertoires when teaching mands directly proves to be difficult: first when contriving or capturing the relevant motivating operations cannot be achieved, as teaching mands directly involves identifying particular states of deprivation; second, when teaching mands for items that are said to be harmful to long-term health (BACB Guidelines for Responsible Conduct for Behaviour Analysts, 2004); and third, when procedures for teaching mands involving states of marked deprivation may pose ethical issues (BACB Guidelines for Responsible Conduct for Behaviour Analysts, 2004). The procedures described in the present study may help avoid such issues, because they only take advantage of naturally occurring motivating operations.

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