The present study comprehensively investigated the characteristics of executive function in CWS. The results indicated that as initially hypothesized, the CWS exhibited lower performance in verbal and visual working memories. In the following, we will discuss the association between executive function and the occurrence of stuttering symptoms based on the results of the working memory task, in which the CWS had significantly lower performance than the CWNS, and cognitive flexibility and inhibition, in which both groups showed no significant differences in terms of performance.
First, the low performance of the CWS in the working memory task will be discussed in terms of Baddeley’s working memory component model31. In this model, under the central executive function, which is responsible for the control of attention, there are three components: the phonological loop, which stores verbal information; the visuospatial sketchpad, which stores visual information; and the episodic buffer, which retrieves information from long-term memory. The central executive function and phonological loop are classified as verbal working memory and the visuospatial sketchpad together as visual working memory. In the present study, the CWS had significantly lower performance in the verbal and visual working memory tasks (P = .04, partial η2 = .11). Most of the studies that investigated the executive function of young CWS have examined verbal working memory through a number-singing or nonword-recitation task. As a result, many studies reported that the performance of the CWS was significantly lower than that of the CWNS. However, young CWS have also been found to have delayed phonological development. Delayed phonological development may lead to difficulties in performing linguistic tasks and, consequently, to smaller verbal short-term memory capacity. In other words, the low verbal working memory capacity of CWS may be a confounded result of slow phonological development6. In the present study, visual working memory, which is less affected by phonological development, was also measured, suggesting that the performance in the verbal and visual working memory tasks was low. This indicates that such a low performance is not due to problems with subcomponents, such as phonological development and working memory capacity, but rather to the central executive system, a factor essential for driving working memory and to the reduced attentional capacity to drive it smoothly The results of this study also indicate the possibility that the central execution system and the reduced attentional capacity to drive it smoothly may be affected by the working memory. The low working memory and attention allocation capacity to drive it, as reported in the present study, have also been demonstrated in a previous study of adults who stutter32. Kaganovich, Wray, and Weber-Fox (2010)9 showed a similar trend in young children, but they examined only electrophysiological measures, not behavioral ones. The present study showed that working memory and the ability to allocate attention may also be low in CWS based on behavioral measures. These results indicate that the low performance on working memory tasks shown in children and persons who stutter may be an inherent characteristic of those who stutter rather than an acquired characteristic caused by long-term stuttering. In light of this, it has been reported that a high percentage of CWS have learning disabilities (LD)33, that both verbal and visual working memories are associated with LD34, 35, and that working memory deficits may have influenced these associations.
No significant differences in inhibition and cognitive flexibility were observed between the CWS and CWNS. Regarding the results of inhibition, it is possible that the bimodal distribution of the black-and-white task performance of the stuttering group had influenced the results, and thus, caution should be taken in the interpretation of these results. The bimodal distribution was observed only in CWS, suggesting that there may be a group of CWS who are extremely poor at inhibition, even among CWS who are typically developing. Furthermore, it has been suggested that the task of inhibition may be compensated for by the use of self-talk36 in late infancy due to the development of internalization of languages. The subjects in this study were also in late infancy and may have been affected by language. Anderson and Wagovich (2017)37 found no significant difference in performance between stuttering and nonstuttering infants in the explicit task of suppressing explicit verbal stimuli (grass-snow) and the implicit task of suppressing implicit verbal stimuli (baa-meow). They also reported that the CWS had lower response accuracy on implicit verbal tasks. These results indicated that the performance was better with explicit verbal stimuli, which are more directly compensatory linguistically. The black-and-white task used in this study was included in the explicit verbal stimuli, and although there was no significant difference in the number of correct responses to the task compared between CWS and CWNS by Anderson and Wagovich (2017)37, the possibility that the task was compensated for by other aspects, such as language, cannot be denied.
Hypotheses regarding the association between executive function and the occurrence of stuttering based on the results of this study will be discussed. DCM25 and MDPT26 are representative hypotheses regarding the occurrence of stuttering symptoms in early childhood. These hypotheses postulate that stuttering symptoms occur when the child’s capacities are unable to meet internal and external demands during the rapid development of speech-related factors in early childhood. Speech-related factors include speech motor, language, social–emotional, and cognitive abilities. Social–emotional abilities are assumed to correspond to executive functions. Among these factors are language (e.g., vocabulary38, speech length39) and executive function40 are a period of rapid development. In this context, there is no significant difference in language development between CWS and CWNS5. The present study suggests that subcomponent performance may be significantly lower with respect to executive functioning, which is highly associated with social–emotional competence. Thus, it is possible that CWS have an imbalance between language development and executive function development in early childhood, which may be a factor in the occurrence of stuttering. Further discussion of the interaction of these factors suggests that speech motor development, which is considered to be necessary in DCM and MDPT, is slower in 4- and 5-year-old toddlers who stutter than in CWNS, indicating that they have difficulty with consistent speech motor development41, 42. In addition, speech movements are influenced by linguistic and emotional factors. Regarding linguistic factors, speech movements are more unstable and are influenced by increased utterance length and word novelty41. Emotional factors also influence speech motor control43, and the degree of variability is greater in young children with persistent stuttering44. Based on these findings, it is possible that linguistic and emotional factors each influence speech motor control or are related to each other, which results in the development of stuttering symptoms. Moreover, executive function is the basis for the development of language and emotional control, which may indirectly affect these factors.
Finally, we discuss some issues to be addressed in this study.
The first is the method employed to measure executive function. The executive function tasks used in this study include those that require responses in speech. CWS may have felt anxious or stressed when responding with speech, which may have affected their performance on the task. In particular, the tasks used to measure verbal and visual working memories, which showed significant differences in this study, required responses in speech. In the future, it will be necessary to examine the performance in tasks that do not involve verbal responses. Furthermore, in this study, only the performance in the task was the dependent variable, and we were not able to assess the anxiety and effort required to perform the task, as described above. Even if the task performance does not change, an increase in the physiological indices of sympathetic activity that reflect anxiety and effortfulness may be observed. It would be desirable to examine executive function from the perspective of effortfulness in tasks without speech stress.
The second is the executive function model used in this study. In this study, we attempted to measure the executive function of CWS using Miyake’s model. There are other executive function models, such as that of Zelazo, Carter, Reznick, and Frye (1997)45, which focuses on selective attention and planning. Miyake’s model16 was based on a model of children with normal development. Therefore, it is necessary to determine whether the model used in this study is suitable as a framework of executive function for children with neurodevelopmental disorders, such as stuttering and communicative disorders. Indeed, Cirino, Ahmed, Miciak, Taylor, Gerst, and Barnes (2018)46 reported that Miyake’s model was not suitable for elementary school students with reading disabilities. Cirino et al. (2018)46 conducted a factor analysis of the components of executive function and extracted five specific executive function factors: working memory span, manipulation and planning, working memory updating, generative fluency, self-regulated learning, and metacognition. The components of executive function may differ among CWS and those who have communication difficulties due to speech impediments. In this study, we have elucidated the characteristics of executive function in CWS compared with normal children. However, to examine the executive functions required in the lives of young CWS, we believe that it is necessary to reexamine the subcomponents necessary for “goal-directed control of thoughts, actions, and impulses” in young CWS in the future.