Emergence of Mands and Tacts of Novel Objects among Preschool Children

According to Skinner''s (1957) analysis of verbal behavior, the mand and the tact are functionally independent verbal operants, each of which is acquired through a unique history of reinforcement. The present study attempted to replicate the findings of Lamarre and Holland (1985), who empirically demonstrated functional independence of mands and tacts in typically developing preschool children. Five children participated. All were initially trained to complete two 4-piece assembly tasks. Four children were then trained to tact the four pieces that comprised one of the assembly tasks, and to mand for the four pieces that comprised the other task, using arbitrary vocal response forms. The remaining child received tact training only, and only on one task. The effects of training on the untrained operant were evaluated in a multiple-probe design across tasks. Following mand training, 4 out of 4 children reliably emitted tacts under testing conditions, while the effects of tact training differed across participants. The results differ from those of Lamarre and Holland, but are not necessarily surprising from the point of view of either Skinner''s analysis or more recent behavioral accounts of language. Future research should attempt to identify variables that affect transfer between mand and tact relations.     

Constructs and Events in Verbal Behavior

Skinner''s (1957) analysis of verbal behavior has been the subject of much controversy in recent years. While criticism has historically come from outside the field of behavior analysis, there are now well-articulated arguments against Skinner''s analysis of verbal behavior from within the field as well. Recently, advocates of Skinner''s analysis have attempted to respond to the critiques, particularly to those regarding Skinner''s definition of verbal behavior articulated by proponents of relational frame theory. Specifically, it has been suggested that talk about definitions equates to making the essentialist error. This paper provides an overview of these issues in the context of understanding the role of constructs in science more generally. It will be argued that definitions are central to scientific progress, and are not only relevant to a functional analysis, but a central prerequisite to the pursuit of such an analysis.     

Empirical Applications of Skinner’s Analysis of Verbal Behavior with Humans

In Verbal Behavior, Skinner (1957) provided a conceptual framework and taxonomy for the controlling variables of language that defined independent verbal operants by their functional relations to antecedents and consequences (rather than by topography or meaning). Although professional interest in this area has recently increased within the behavior analytic community, Skinner''s conceptual framework may not yet have fully impacted the experimental literature. This quantitative review of the literature examined the studies on verbal behavior that were empirical in nature, concerned with human verbal behavior, and addressed at least one verbal operant (e.g., mand, tact, echoic, autoclitic, and/or intraverbal behavior) within the experiment. The results of this review suggest that a growing body of research exists to support many of the tenets of Skinner''s conceptualization and taxonomy but many areas of verbal behavior research have yet to be addressed. Continued research in this area is crucial for the development and implementation of effective verbal behavior interventions for people with disabilities.     

Evaluation of Verbal Behavior in Older Adults

Approximately 5% of older adults have a dementia diagnosis, and language deterioration is commonly associated with this disorder (Kempler, 2005). Several instruments have been developed to diagnose dementia and assess language capabilities of elderly adults. However, none of these instruments take a functional approach to language assessment as described by Skinner (1957). The purpose of this study was to develop and evaluate a function-based assessment for language deficits of older adults. Thirty-one participants were categorized into a control group (n  =  15) and a dementia group (n  =  16) based on their score on the Dementia Rating Scale-2. Individuals with dementia performed significantly worse on the tact assessment than those without dementia. Participants from both groups performed better on measures of tacts than intraverbals or mands, even though topographically identical responses were required in these assessments. The data provide support for Skinner''s conceptualization of functionally independent verbal operants.     

Transferring Control of the Mand to the Motivating Operation in Children with Autism

Few studies have made use of B. F. Skinner''s (1957) behavioral analysis of language and precise taxonomy of verbal behavior when describing the controlling variables for the mand relation. Consequently, the motivating operation (MO) has not typically been identified as an independent variable and the nature of a spontaneous mand has been imprecisely described. The purpose of this study was to develop procedures to bring the mand response under the control of the relevant MO and therefore free it from the multiple controls that are more easily identified by practitioner''s who rely on Skinner''s analysis and taxonomy. Using a rolling time delay and prompt fade procedure both participants'' mand repertoires were successfully transferred to the relevant MO and a listener and described within the context of a behavioral analysis of language.     

A Review of Training Intraverbal Repertoires: Can Precision Teaching Help?

Intraverbal behavior is common in conversation and academic and professional settings. Many individuals with disabilities fail to acquire intraverbal repertoires. Some individuals who do acquire intraverbal behavior fail to acquire responses that are functional and complete. Research has examined procedures to establish or increase intraverbal repertoires of individuals with impaired verbal repertoires. Several of these studies integrate a precision teaching methodology with Skinner''s (1957) conceptual framework. This paper reviews the literature on the most commonly examined methods of establishing or increasing intraverbal repertoires. Articles that incorporate precision teaching are emphasized. The review highlights the strengths (including the effectiveness) and limitations of each procedure. The review concludes with a discussion of the implications of using precision teaching to establish or increase an intraverbal repertoire and with suggestions for future research.     

Jack Michael’s Motivation

Among many of Jack Michael''s contributions to the field of behavior analysis is his behavioral account of motivation. This paper focuses on the concept of motivating operation (MO) by outlining its development from Skinner''s (1938) notion of drive. Conceptually, Michael''s term helped us change our focus on how to study motivation by shifting its origins from the organism to the environment. Michael''s account also served to stimulate applied research and to better understand behavioral function in clinical practice.     

An Underdiscussed Aspect of Chomsky (1959)

Chomsky''s (1959) review of Skinner''s (1957) Verbal Behavior has been influential and attributed with a role in the cognitive revolution. However, while counter reviews from within behavior analysis have noted that Chomsky misunderstood the subject matter, certain aspects of his scholarship have been underdiscussed. This includes several instances where Chomsky misquotes Skinner or takes his quotes out of context. Similar to the findings of Sokal (1996a, 1996b), it is speculated that the problems with Chomsky were overlooked by cognitive psychologists because his general outlook was accepted. Implications for the editorial review process are discussed.

Transgressing disciplinary boundaries . . . [is] a subversive undertaking since it is likely to violate the sanctuaries of accepted ways of perceiving. Among the most fortified boundaries have been those between the natural sciences and the humanities.—Greenberg (1990, p. 1), as quoted in Sokal (1996a)Every man has a right to his opinion, but no man has a right to be wrong in his facts.—attributed to Bernhard Mannes Baruch (Montapert, 1964, p. 145)

Areas of study are sometimes profoundly affected by people outside them. For example, the linguist Noam Chomsky is familiar to many psychologists for his influence in the cognitive revolution, the major shift in the orientation of American psychology from behavioral to cognitive in the 1960s and 1970s (e.g., Auyang, 2000, p. 105; Erneling, 1997; Hunt, 1982, p. 217; Kosslyn & Rosenberg, 2001, p. 238). Some have even suggested that Chomsky''s (1959) review of B. F. Skinner''s (1957) Verbal Behavior, the latter''s theoretical account of how language can be explained in behavioral terms, was responsible for initiating the cognitive revolution (e.g., Bialystock, 1997; Harnish, 2002, p. 44).When Chomsky set out to write his review, he faced a daunting task. While apparently well read in certain areas such as political theory, there is little to indicate he had any previous exposure to psychology except for psychoanalysis (Barsky, 1997) and through linguists who have drawn upon psychology, such as Bloomfield (1933). Psychology in general in the early and mid-20th century was broad and diverse (e.g., Heidbreder, 1933; Hilgard & Bower, 1966), and so too was behavioral psychology. At that time “behaviorism” was used to refer a variety of behavior-oriented work, such as the strict methodological behaviorism of John B. Watson (e.g., Watson, 1913, 1924), the mathematical modeling with intervening variables of Clark L. Hull''s school (e.g., Hull, 1943, 1951, 1952), and the eclectic, molar, purposive behaviorism of Edward C. Tolman (e.g., Tolman, 1922, 1932, 1948), each with its own assumptions and terminologies. Skinner''s approach, radical behaviorism, differed from all of them in that he recognized the fundamental respondent-operant distinction (Skinner, 1935, 1937, 1938, p. 61ff., 1953, pp. 45–90), accepted the reality of private events, and rejected purely hypothetical mental constructs (Skinner, 1945). Verbal Behavior was very specifically based on Skinner''s approach, and any reviewer would have to understand this to do the job properly.Behavior analysts’ responses to Chomsky''s (1959) review of Verbal Behavior have not been positive (e.g., MacCorquodale, 1970; McLeish & Martin, 1975; Richelle, 1976); their counter reviews generally have noted that Chomsky fundamentally misunderstood the subject matter and emphasized major points where he differed from Skinner. The existing counter reviews, however, do not exhaust the problems with Chomsky. An especially important yet underdiscussed problem of Chomsky is the poor quality of his scholarship, which, to this author''s best knowledge, has not been previously addressed. The purpose of this paper is to address this issue and discuss its implications for future literature reviews.

Chomsky''s Review of Skinner

As noted above, Chomsky''s apparent understanding of radical behaviorism was poor, and at numerous points he makes claims not supported by the existing literature. Some parts of his review, such as citing a supporting claim that stimuli cannot be manipulated (Chomsky, 1959, p. 31, footnote 5) demonstrate severe problems with his understanding of the subject matter, and his handling of the most basic terms was so poor that Richelle (1976) said his “misunderstanding . . . would prompt most examination graders to read no further.”Many serious problems in this area have already been discussed by the above-mentioned counter reviewers; these include instances where Chomsky misattributed views to Skinner that clearly belong to others. For example, Chomsky (1959, p. 28) in describing response strength claimed: “The strength of an operant is defined by Skinner in terms of the rate of response during extinction. . . .” Skinner never claimed this, but rather defined it as the probability a response will occur under certain circumstances (Skinner, 1957, p. 22). MacCorquodale (1970) noted that rate of response during extinction was actually Hull''s definition of probability (Hull, 1943, pp. 260–262), and claiming that Skinner accepted this definition given his criticism of Hull''s work (Skinner, 1944) would be difficult to justify. Several other of Chomsky''s errors may have resulted from his failing to distinguish Skinner''s views from those of others. For example, Chomsky claimed that Skinner said “that children can learn language only through ‘meticulous care’ on the part of adults who shape their verbal repertoire through careful differential reinforcement” (Chomsky, p. 42) and that it is part of “the doctrine of Skinner and others that slow and careful shaping of verbal behavior through differential reinforcement is an absolute necessity” (op. cit., p. 42). Aside from the fact that other people''s doctrines are irrelevant here, nowhere does Skinner make this claim. Chomsky may have actually been reacting to Miller and Dollard (1941, p. 82), who in a discussion on language acquisition wrote: “The child is given meticulous training in connecting words to objects and connecting acts to words.” Skinner (1957), however, says almost nothing about language acquisition and certainly did not describe it as arising from a laborious and intentional shaping procedure. Chomsky (pp. 35, 52), perhaps supposing all responses would act like respondents, also wrongly indicated that a “strong” (operant) verbal response should, according to Skinner, necessarily be a high-pitched scream. However, as Skinner (1950, p. 196) noted,

In the sort of behavior adapted to the Pavlovian experiment (respondent behavior) there may be a progressive increase in the magnitude of response during learning. But we do not shout our responses louder and louder as we learn verbal material, nor does a rat press a lever harder and harder as conditioning proceeds. In operant behavior the energy or magnitude of response changes significantly only when some arbitrary value is differentially reinforced—when such a change is what is learned.

Not all of Chomsky''s errors involved confounding views or pure misunderstandings. There are several instances in Chomsky (1959) where Skinner is quoted out of context or the quote is incorrect such that Skinner''s views are not accurately portrayed. A few examples follow.

1. The above-mentioned false definition of response strength was again made when Chomsky (1959, p. 34) claimed that “Skinner has argued that this [rate of emission during extinction] is ‘the only datum that varies significantly and in the expected direction under conditions which are relevant to the “learning process.” The actual quote from Skinner (1950, p. 198) began with “Rate of responding appears to be the only datum which [sic] varies significantly. . . .” Extinction was not mentioned at all. Furthermore, this occurred in a discussion of learning theories, far removed from anything Chomsky discussed.
2. Chomsky (1959, p. 34) claimed that Skinner said that “the frequency of the response may be ‘primarily attributable to the frequency of occurrence of controlling variables’ . . . .” The actual sentence in Skinner (1957, p. 27) was “Nor can we be sure that frequency is not primarily attributable to the frequency of occurrence of controlling variables.” This occurred in a discussion of factors affecting frequency, during which Skinner emphasized that, contrary to the practice of “word counts” used in formal analyses of language, “it is also important to know the prevailing conditions,” that is, the circumstances under which they occur. Skinner was expressing a possibility or an uncertainty, which Chomsky failed to acknowledge.
3. Chomsky (1959, p. 34, footnote 9) quoted from Skinner (1950, p. 199) regarding allegedly using “the notion of probability in analyzing or formulating instances of even types of behavior which are not susceptible to this analysis.” Chomsky used this quote in a discussion in which he attacked Skinner''s usage of probability (and failed; see MacCorquodale, 1970). However, the passage from Skinner occurred at the end of a paragraph discussing complex activities that, as a whole, may never be repeated by the individual. In the same paragraph, Skinner (p. 199) said,

They are not simple unitary events lending themselves to prediction as such. If we are to predict marriage, success, accident, and so on, in anything more than statistical terms, we must deal with the smaller units of behavior which lead to and compose these unitary episodes. If the units appear in repeatable form, the present analysis may be applied.

Chomsky, however, attempted to apply this analysis to language behaviors, which typically involve a large number of units that may occur very frequently; thus the limitations that may apply to predicting marriage, success, accident, and so on as wholes are irrelevant to Skinner''s program. Chomsky acknowledged none of this.4. Chomsky (1959, p. 35, footnote 10), in discussing pitch, quoted Skinner as saying “Fortunately, ‘In English this presents no great difficulty’ since, for example, ‘relative pitch levels . . . are not . . . important,’” the “this” apparently referring to that pitch, among things, has linguistic functions (pp. 34–35). He further indicated (op. cit.): “No reference is made to the numerous studies of the function of relative pitch levels and other intonational features in English.” The implication is that Skinner ignored the importance of pitch and intonation in actual speech. But what Skinner (1957, p. 25) actually wrote was:

In English, this [energy level, speed of response, and even repetition entering into the construction of different forms of response] presents no great difficulty. Absolute levels of pitch and intensity are not “distinctive,” nor are relative pitch levels important. Changes in pitch, however, distinguish different types of utterance.

Skinner then discussed the functions of pitch in English. While Skinner and Chomsky may have disagreed about their exact functions, it is clear that Skinner did not contend that intonation or pitch in English were completely “unimportant” as Chomsky implied.5. Chomsky (1959, p. 52, footnote 42) quipped, “Similarly, ‘the universality of a literary work refers to the number of potential readers inclined to say the same thing’ (. . . i.e., the most ‘universal’ work is a dictionary of clichés and greetings).” However, the paragraph in Skinner (1957, pp. 274–275) this quote was taken from begins:

A reader seeks out other works of a given writer or other literature of a given type because of the reinforcement he has received. The reinforcement depends upon his own verbal behavior. A thematic correspondence between a reader and a literary work is likely to involve a matching of variables in the fields of motivation and emotion. The universality of a literary work refers to the number of potential readers inclined to say the same thing, at least in some measure. The success of a book is some indication of the number of people who possess a given kind of verbal behavior in strength.

In other words, a work has “universality” when readers would be likely to say something similar in content to the work, not specific utterances.6. Chomsky (1959, p. 53) claimed that “Skinner considers a sentence to be a set of key responses (nouns, verbs, adjectives) on a skeletal frame” and then proceeded to criticize Skinner as if this was his position. However, the actual quote was: “Others [other sentences] are nearly complete skeletal ‘frames’ upon which an exceptional response or two may be hung” (Skinner, 1957, p. 346), and this was not presented as the sole possibility. Rather, it was part of a much longer chapter (pp. 344–367) in which Skinner discussed how verbal responses are combined. Much of this involves a class of responses called autoclitics (p. 311ff.), which are verbal operants whose occurrence depends in part on the occurrence of other verbal operants; in Skinner''s hypothesis, grammar results from an interaction of autoclitics with other verbal operants. For example, when one is confronted by the appropriate stimulating conditions (seeing a hungry man) one would not only emit such responses as hungry and man, but also autoclitics related to the specificity of the situation (the) and occurrence (is) to produce a longer, structured utterance (The man is hungry; Skinner, p. 345). Not all sentences are necessarily created this way, and in the quote Chomsky cites, Skinner does indicate some may be partially or completely preformed; however, he certainly did not claim all were.     

Extending the Assessment of Functions of Vocalizations in Children with Limited Verbal Repertoires

The current study examined the effectiveness of an experimental functional analysis for assessing the functions of emergent vocal-verbal behavior in children with developmental disabilities. Experiment 1 consisted of a systematic replication of Lerman et al. (2005). Participants were 3 children with developmental disabilities, between the ages of 2 and 5 years, with limited vocal-verbal repertoires, who could emit at least two clear vocalizations. Results indicated that for all participants, targeted vocalizations functioned as mands, tacts, or both. The purpose of Experiment 2 was to address some of the limitations of Experiment 1 while using a different experimental design. Results of Experiment 2 suggested that participants'' targeted vocalizations served as tacts, echoics, or both. Results are discussed in terms of the utility of this methodology for the selection and development of effective language interventions as well as implications for our current knowledge of verbal behavior.     

A Behavioral Conceptualization of Aphasia

Aphasia is an acquired language impairment that affects over 1 million individuals, the majority of whom are over age 65 (Groher, 1989). This disorder has typically been conceptualized within a cognitive neuroscience framework, but a behavioral interpretation of aphasia is also possible. Skinner''s (1957) analysis of verbal behavior proposes a framework of verbal operants that can be integrated with the work of Sidman (1971) and Haughton (1980) to describe the language difficulties individuals with aphasia experience. Using this synthesis of models, we propose a new taxonomy of aphasia based on the observed deficit relations. Assessment and treatment implications are also discussed.     

Generalized Verbal Behavior Increases Following a Speaker Immersion Intervention

A goal of behavior-analytic interventions is to produce behavior that is maintained under naturalistic conditions. In this experiment, we studied the effects of a speaker immersion protocol (SIP) on the number of speaker responses (tacts and mands) emitted by 3 preschool students under naturalistic, not directly targeted, conditions. During the SIP, the researchers provided 100 daily opportunities for the participants to emit mands using the target mand form by contriving establishing operations (EOs) throughout the school day. The effects of the intervention were evaluated using a multiple-probe design by measuring target mands during EO probe sessions and the number of mands and tacts emitted during noninstructional-setting probe sessions. The researchers found that the SIP produced increases in both targeted and generalized verbal behavior.Electronic supplementary materialThe online version of this article (10.1007/s40616-020-00133-2) contains supplementary material, which is available to authorized users.     

A Comparison of Topography-based and Selection-based Verbal Behavior in Typically Developed Children and Developmentally Disabled Persons with Autism

This study is a replication of Sundberg and Sundberg (1990) that compared topography-based verbal behavior with selection-based verbal behavior in terms of acquisition, accuracy, and testing for the emergence of a new verbal relation. Participants were three typical children and three developmentally disabled persons with autism. The study sought to determine which paradigm (topography-based or selection-based) resulted in more rapid acquisition of tacts and intraverbals, which was associated with the fewest errors, and which paradigm resulted in the emergence of the highest number of new verbal relations. The results of the study showed that the six participants performed quite differently from one another. Most importantly, the results from the person with autism contradicted previous findings favoring selection-based verbal behavior over topography-based approaches for teaching verbal behavior to low-functioning individuals.     

The effects of tact training on stereotypic vocalizations in children with autism

This study was a systematic extension of Karmali, Greer, Nuzzulo-Gomez, Ross, and Rivera-Valdes (2005) and Ahearn, Clark, MacDonald, and Chung (2007). We investigated the effects of a tact correction procedure on stereotypic vocalizations in 4 children diagnosed with autism who ranged in age from 6 to 16 years. Participants had limited vocal verbal repertoires and were primarily dependent on prompts for the emission of appropriate vocalizations. A multiple-baseline design across participants was used. Data were collected on instances of stereotypic vocalizations and independent tacts during baseline conditions and on instances of stereotypic vocalizations, independent tacts, and echoic-tacts during intervention. Procedural integrity and social validity data were also obtained. The results indicated a decrease in stereotypic vocalizations for 3 of the 4 participants and a slight increase in appropriate vocal verbal behavior (i.e., tacting) for all participants. The study provides support for the use of tact correction procedures to decrease stereotypic vocalizations and increase appropriate vocalizations in children with autism.     

Meaning and Verbal Behavior in Skinner's Work from 1934 to 1957

This paper explores the historical development of Skinner''s treatment of meaning from 1930 to 1957. Twelve papers published between 1934 and 1957, and parts of The Behavior of Organisms and Science and Human Behavior related to verbal behavior, were analyzed. Before 1945 meaning was taken as a property of the verbal response, and from 1945 on, meaning was supposed to be found among the determiners of a verbal response. We argue that these different conceptions of meaning were related to distinct aspects of Skinner''s explanatory system. Finally, the notion of meaning presented by Skinner in 1945 is related to the theoretical breakthrough represented by Skinner''s assertion of the three-term contingency. This idea permitted verbal behavior to be analyzed in terms of its functions.     

Teaching mands by manipulating conditioned establishing operations

Skinner’s (1957) analysis of verbal behavior suggests the functional independence of the verbal operants. However, only a few empirical studies have directly examined the nature of these operants, and their independence. The present study evaluated whether teaching topographies as tacts would lead to their emission as mands. The results indicated that manding only occurred reliably after direct mand training, which consisted of the use of imitative and tact prompts, and fading those prompts, to transfer stimulus control from nonverbal stimuli to conditioned establishing operations. The results contribute to the existing data on the functional independence of mands and tacts, as well as demonstrate the value of manipulating conditioned establishing operations for mand training.

     

The Hefferline Notes: B. F. Skinner’s First Public Exposition of His Analysis of Verbal Behavior

B. F. Skinner''s first public exposition of his analysis of verbal behavior was the Hefferline Notes (1947a), a written summary of a course Skinner taught at Columbia University during the summer of 1947 just prior to his presentation of the William James Lectures at Harvard University in the fall. The Notes are significant because they display Skinner''s analysis as it made the transition from spoken to written form; moreover, they are an effective supplemental source of examples and early approximations for comprehending Skinner''s functional verbal operants.     

Thirty Points About Motivation From Skinner’s Book Verbal Behavior

Skinner discussed the topic of motivation in every chapter of the book Verbal Behavior (1957), usually with his preferred terminology of “deprivation, satiation, and aversive stimulation.” In the current paper, direct quotations are used to systematically take the reader through 30 separate points made by Skinner in Verbal Behavior that collectively provide a comprehensive analysis of his position regarding the role of motivation in behavior analysis. In addition, various refinements and extensions of Skinner''s analysis by Jack Michael and colleagues (Laraway, Snycerski, Michael, & Poling, 2003; Michael, 1982, 1988, 1993, 2000, 2004, 2007) are incorporated, along with suggestions for research and applications for several of the points.     

Assessing Transfer of Stimulus Control Procedures Across Learners With Autism

The purpose of this study was to evaluate the effectiveness of 2 transfer of stimulus control procedures to teach tacting to individuals with autism. Five participants with differing verbal skills were assessed by a subset of the ABLLS prior to intervention, then were taught 36 previously unknown tacts using the receptive-echoic-tact (r-e-t) and echoic-tact (e-t) transfer procedures. Each transfer method was used separately to establish different tacts, in a multiple baseline design across tacts for 3 sets of stimuli. The results showed that 4 out of 5 participants (who demonstrated mands, tacts, echoics, and sometimes intraverbals prior to the study) acquired all targeted tacts when either r-e-t or e-t training was presented. One participant (who emitted no verbal operants at the onset of the study) did not acquire any tacts. While some participants appeared to learn more quickly with one transfer method, neither method emerged as more efficient with learners with fewer or more extensive verbal skills. The results indicate that both transfer methods promoted the acquisition of tacts for learners with autism with at least minimal verbal skills.     

Recollections of Jack Michael and the Application of Skinner’s Analysis of Verbal Behavior

The Analysis of Verbal Behavior - Jack Michael offered a course on verbal behavior almost every year throughout his teaching career. Jack was also interested in the application of Skinner's...     

Acquisition of Mands,Tacts, and Intraverbals Through Sign Exposure in an Individual With Autism

Many children with autism communicate through the use of alternative communication systems, such as sign language. Limited research has been conducted on the situations under which sign language will be acquired across verbal operants without direct teaching. The purpose of the current study was to evaluate exposure to sign language on the acquisition of signed mands, tacts, and intraverbals in a male child with autism. Results indicated fast acquisition of mands, tacts, and intraverbals without direct teaching. Results are discussed in the context of future research investigating exposure without direct teaching in individuals who communicate with alternative communication systems.