لکنت زبان در دوران کودکی و جداشدگی در سراسر حوزه زبانی

The purpose of this investigation was to evaluate the possible presence of dissociations in the speech and language skills of young children who do (CWS) and do not stutter (CWNS) using a correlation-based statistical procedure [Bates, E., Appelbaum, M., Salcedo, J., Saygin, A. P., & Pizzamiglio, L. (2003). Quantifying dissociations in neuropsychological research. Journal of Clinical and Experimental Neuropsychology, 25, 1128–1153]. Participants were 45 preschool CWS and 45 CWNS between the ages of 3;0 and 5;11 (years;months), with the two groups matched by age, gender, race, and parental socioeconomic status. Children participated in a parent–child interaction for the purpose of disfluency analysis and responded to four standardized speech-language tests for subsequent analyses as main dependent variables. Findings indicated that CWS were over three times more likely than CWNS to exhibit dissociations across speech-language domains, with 44 cases of dissociation for CWS and 14 for CWNS across 10 possible comparisons. Results suggest that there may be a subgroup of CWS who exhibit dissociations across speech-language domains, which may result in a greater susceptibility to breakdowns in speech fluency.

The issue of whether children who stutter (CWS) differ from children who do not stutter (CWNS) in terms of linguistic abilities has been a topic of much interest and controversy (see Ratner, 1997 for review). However, findings from descriptive studies of the speech and language abilities of CWS have been less than consistent. On the one hand, some literature reviews and empirical studies have suggested that CWS may have less developed phonology, vocabulary, or overall language abilities than their normally-fluent peers (Anderson and Conture, 2000 and Anderson and Conture, 2004; Byrd & Cooper, 1989; Louko, Conture, & Edwards, 1999; Paden, Yairi, & Ambrose, 1999; Pellowski, Conture, Anderson, & Ohde, 2001; Ryan, 1992 and Silverman and Ratner, 2002). On the other hand, some empirical studies have found no evidence to suggest that the speech or language abilities of CWS are less robust than those of CWNS (e.g., see Nippold, 2002 for review). For example, Howell, Davis, and Au-Yeung (2003) reported that CWS and CWNS (aged 2–10 years) performed similarly on the Reception of Syntax Test, a measure of syntactic development. To further challenge any clear-cut interpretation of this area of empirical investigation, some studies have reported that CWS may have above average expressive language abilities relative to their developmental expectations (Watkins & Yairi, 1997; Watkins, Yairi, & Ambrose, 1999). Nevertheless, despite apparent differences in findings among descriptive studies of the speech and language abilities of CWS, most would appear to agree that the linguistic characteristics associated with instances of stuttering are relatively consistent in their distribution and loci. That is, instances of stuttering exhibited by CWS tend to occur on (a) low frequency words (Anderson, 2005 and Soderberg, 1966; Palen & Peterson, 1982), (b) first three words of an utterance (Bernstein, 1981; Howell & Au-Yeung, 1995; Wall, Starkweather, & Cairns, 1981), (c) function words (Bernstein, 1981; Bloodstein & Grossman, 1981; Graham, Conture, & Camarata, 2005; Howell, Au-Yeung, & Sackin, 1999; Natke, Sandreiser, van Ark, Pietrowsky, & Kalveram, 2004), and (d) longer or more syntactically complex utterances (Ratner & Sih, 1987; Howell & Au-Yeung, 1995; Kadi-Hanifi & Howell, 1992; Logan and Conture, 1995 and Logan and Conture, 1997; Melnick & Conture, 2000; Yaruss, 1999). These linguistic factors have also been shown to influence the fluency with which words are produced in adolescents and adults who stutter (e.g., Bergmann, 1986 and Brown, 1945; Danzger & Halpern, 1973; Hubbard & Prins, 1994; Klouda & Cooper, 1988; Natke, Grosser, Sandrieser, & Kalveram, 2002; Prins, Hubbard, & Krause, 1991; Ronson, 1976 and Wingate, 1984). However, unlike young CWS, older children and adults tend to stutter more on content words than function words (e.g., Brown, 1938a and Brown, 1938b; Dayalu, Kalinowski, Stuart, Holbert, & Rastatter, 2002; Howell et al., 1999). Taken together, the relatively consistent association observed between certain utterance characteristics and the loci of stuttering seems to suggest that there may be an interaction between linguistic processing and instances of stuttering. Findings that the linguistic characteristics of stuttering events are fairly predictable, along with evidence from some studies suggesting that CWS may have less developed linguistic skills than CWNS has prompted some theorists to speculate that CWS may have dissociations or asynchrony within or between subcomponents of their linguistic formulation processes. For example, speculation associated with the Neuropsycholinguistic Theory of Stuttering (Perkins, Kent, & Curlee, 1991) suggests that people who stutter experience asynchrony between linguistic and paralinguistic processing components as a result of linguistic uncertainty or inefficient neural resources. This asynchrony induces disfluencies that are transformed into stuttering events under conditions of time pressure; the speaker is required to initiate and accelerate the disrupted utterance and experiences loss of control in the process. More generally, this theory suggests that stuttering occurs as a function of reduced efficiency in one or more processing systems (linguistic, paralinguistic, integrative, and segmental), resulting in an imbalance in the production of language “as different components arrive at a central language integrator at different times and thus have a mistimed impact on the motor production of speech” (Tetnowski, 1998; p. 243). The notion that CWS may have temporal asynchrony or disequilibrium between components of their speech-language processing systems is also indirectly reflected in the Covert Repair Hypothesis (Postma & Kolk, 1993). The basic premise of this theory is that CWS have slower-than-normal phonological encoding systems, which increases the probability that their phonetic plans will include more phoneme errors. If these errors are detected by the internal speech monitoring system, then there will be more error correction opportunities prior to overt execution of speech. It is this “covert repair reaction” to errors in the phonetic plan that is thought to disrupt the forward flow of speech production, resulting in hesitations, repetitions, and prolongations. This theory generally implicates speech-language production planning processes as a potential source of difficulty for CWS. If these linguistic planning processes were, indeed, problematic for CWS, then there would seem to be potential for the presence of dissociations among components of their speech-language systems. In this way, the phonological processing systems of CWS would be considerably less efficient or more susceptible to disruptions than other linguistic formulation processes, leading to dissociations in performance. Such dissociations in the speech-language systems of young children, who may already be vulnerable to errors and/or delays in processing as they work to develop more abstract, adult-like linguistic representations, could potentially contribute to the onset and development of stuttering (see Savage & Lieven, 2004; cf. Anderson & Musolino, 2004; Demuth, 2004). The two aforementioned psycholinguistic theories are based on the notion that the loci of dissociation is relatively static for all CWS—that is, dissociations exist between linguistic and paralinguistic components (Neuropsycholinguistic Theory) or, more indirectly, between phonological encoding and other processing domains (Covert Repair Hypothesis). However, it may be, as suggested by Smith and Kelly (1997) that no single factor can be conclusively identified in the etiology of stuttering. In other words, different factors may be responsible for the development of stuttering in different CWS (see e.g., Schwartz & Conture, 1988, for similar discussion and data pertaining to behavioral subgroups among young children who stutter). According to this line of thinking, CWS may experience a range of potential dissociations across speech-language domains. If this is the case, then perhaps models that focus more generally on the interaction among speech-language domains may be more germane to speculation concerning the role of linguistic dissociations in developmental stuttering. For example, linguistic trade-off models (e.g., Crystal's (1987) “bucket” theory) commonly presume that increased demands or complexity requirements in one speech-language domain are associated with decreases in complexity or accuracy in another domain (e.g., speech fluency). Thus, the mere presence of a dissociation (regardless of its nature) in the speech-language systems of CWS could, at least theoretically, lead to a greater expenditure of resources being directed towards linguistic processes. With more resources being allocated towards managing these dissociations, fewer resources would be available for the production of fluent speech, with the net effect being an increase in speech disfluencies. Although there has been a considerable amount of speculation regarding the potential role that linguistic dissociations may play in developmental stuttering, few research studies have examined whether CWS do, in fact, exhibit more dissociations across speech and language domains than CWNS. However, the concept of dissociations has played a significant role in neuropsychological research, particularly with respect to examining the relationship between specific brain regions and their behavioral functions (Bates, Appelbaum, Salcedo, Saygin, & Pizzamiglio, 2003). For example, a dissociation in performance can be said to exist when Clinical Population X exhibits low performance on one behavioral task (Task A) and high performance on another behavioral task (Task B) when compared to Clinical Population Y who exhibits high performance on Task A and low performance on Task B (Bates et al., 2003). More specific to the fields of speech-language pathology and neuropsychology, dissociations between object (noun) and action (verb) naming have been reported in the aphasia literature, such that nonfluent aphasics tend to exhibit high performance on noun naming and low performance on verb naming, whereas fluent aphasics tend to have the opposite pattern of performance (verb > noun; Caramazza & Hillis, 1991; Damasio, Grabowski, Tranel, Hichwa, & Damasio, 1996; Zingeser & Berndt, 1990). Alternatively, the probability of a proposed dissociation in a single clinical population can be evaluated by comparing the clinical population with a normal control population on several behavioral measures (Bates et al., 2003). For example, although children with language disorders have been found to perform similarly to children with typical language development on prosodic tasks, they score lower on measures of segmental phonology, suggesting a dissociation between lexical and prosodic phonology (Snow, 2001). While dissociations in brain-damaged individuals are an important source of evidence in the study of the neural bases of behavioral functions, they have, as previously suggested, less often been studied in other clinical conditions, such as developmental stuttering. However, one study attempted to quantify the presence of language dissociations in childhood stuttering by examining differences in performance between speech and language measures in CWS and CWNS. In particular, Anderson and Conture (2000) found that CWS, when compared to CWNS, exhibit a significantly greater difference between standardized measures of receptive/expressive language and receptive vocabulary, with receptive/expressive language being better developed than receptive vocabulary. On average, CWS scored almost 30 percentile points higher on the receptive/expressive language measure than on the receptive vocabulary measure. Likewise, CWNS exhibited the same relative trend of lexical development lagging that of syntactic development, but there was only an average of a 13 percentile point difference between the two measures. Anderson and Conture took these findings to suggest that preschool CWS may have an imbalance among components of their speech-language systems. Although Anderson and Conture (2000) appear to be among the first to directly examine the notion of dissociations in the speech and language skills of CWS, language dissociations have been previously observed for children with language disorders who are “highly disfluent” (Hall, Yamashita, & Aram, 1993). In specific, Hall et al. investigated the relationship between fluency and language in 60 children with language disorders and found that “highly disfluent” children (n = 10; 8.04% or higher total disfluencies) had significantly greater semantic than morphosyntactic capacities. The authors concluded that for these children, “the automaticity with which they are able to produce the more rule based components of spoken language, such as morphology and syntax is not as efficient as their capacity to manage semantic parameters or vocabulary” (p. 577). In a follow-up study of these children, Hall (1996) found that while the overall frequency of speech disfluencies tended to decrease over time, most of these children continued to exhibit higher-than-average frequencies of total disfluencies. Further, Hall reported that the overall decrease in speech disfluencies over time tended to be associated with improvements in overall language skills (i.e., the dissociation between semantic and syntactic capabilities diminished). Although the frequency and type of speech disfluencies exhibited by these “highly disfluent” children with language disorders may not be quantitatively or qualitatively the same as those exhibited by children with developmental stuttering, these findings, along with those of Anderson and Conture (2000), provide some preliminary evidence to suggest that childhood stuttering could be associated with linguistic dissociations, at least for some children who stutter. In this way, whether the dissociation is, for example, between linguistic and paralinguistic inputs or between lexical and morphosyntactic abilities, such dissociations may disrupt the forward flow of speech-language planning and production, resulting in hesitations, repetitions, and prolongations of sounds, syllables, or words. Any proposed dissociations, however, need not reflect a significant delay or disorder in one component of the system. Indeed, it is quite possible that dissociations could exist among components of the system even though the system is, overall, well within or even above normal limits. In other words, an individual could be classified “…as a dissociated case, where Y is abnormally low for that patient's value of X, even though this individual is performing close to the group mean on both measures” (Bates et al., 2003; p. 1144). In essence, the present authors neither explicitly nor implicitly imply that dissociations cannot be a part of normal speech-language development; rather the issue these authors seek to explore is whether CWS differ from CWNS in the quantity and/or quality of any potential dissociation(s). One problem with using high versus low performance profiles (see Anderson & Conture, 2000) to identify dissociations in one or more clinical populations, according to Bates et al. (2003), is that the probability of finding a proposed dissociation by chance is typically not taken into consideration. Further, when using this “high versus low performance” approach in conjunction with inferential statistics, some researchers may make faulty assumptions about the independence and equivalence of variances and means between measures, thereby increasing the risk of false positives and/or false negatives. In other words, this approach is based on the assumption that the measures are independent of one another, meaning that they are not correlated. Measures that are weakly correlated more closely approximate the assumption of measurement independence, a necessary prerequisite for statistical testing. However, when this assumption is violated because the measures are highly correlated, it has a significant effect on the level of significance (results in incorrectly small p-values) and statistical power, leading to an increased risk of false negatives and false positives. And, the higher the correlation between measures, the greater the effect it will have on the results. As an alternative to techniques that assume measurement independence (i.e., examining high versus low performance profiles), Bates et al. (2003) developed a statistical procedure to determine the probability of dissociations that takes the means and standard deviations of the population into account, along with the correlation between behavioral measures. As Bates and her colleagues suggest, if the correlation between two measures is low, then there will be little difference in outcomes between this correlation technique and those that assume measurement independence. On the other hand, if the correlation between two measures is high, as is often the case with speech-language measures, then this correlation-based technique will increase the probability of finding dissociations that may be of interest theoretically. In addition to protecting against false positives and false negatives, this correlation-based procedure has the advantage of enabling identification of individual differences in performance profiles. In summary, given the aforementioned observations, further empirical study of possible dissociations between linguistic variables in CWS and CWNS is warranted. If such dissociations exist, it will be important to examine how they relate to childhood stuttering. Therefore, the purpose of this investigation was to use the correlation-based procedure developed by Bates et al. (2003) to evaluate whether CWS and CWNS have dissociations between or within components of their speech-language systems. In specific, the primary goal was to assess whether CWS and CWNS differ in terms of differences between standardized measures of (a) one-word receptive and expressive vocabulary; (b) overall receptive and expressive language; and (c) speech sound articulation. Although standardized measures may not be the most precise or direct means of examining children's performance across linguistic domains (see Hakim & Ratner, 2004 for commentary), the use of these measures along with the Bates et al. (2003) correlation-based procedure represents, at the very least, a first step towards further examining the empirical reality of theoretical notions of potential dissociations in CWS. A secondary goal of this study was to determine whether children who exhibit dissociations differ from children who do not exhibit dissociations in speech disfluency and speech-language measures.