華語學前發展性語言障礙兒童與典型發展兒童的執行功能與口語能力之關係

Relationship between executive functions and oral language among Mandarin-speaking preschoolers with and without developmental language disorder

本研究旨在比較華語學前發展性語言障礙兒童（DLD）和典型發展兒童（TD）4項核心的執行功能（持續性的選擇注意、抑制、更新工作記憶和認知轉換）之異同，並調查兩組兒童在控制了背景變項後（組別、非語文智商和社會地位指數），執行功能對其詞彙和語法表現的預測力。研究對象為22位4至5歲11個月的DLD兒童，以及44位與其性別和年齡配對的TD兒童。所有的受試兒童均接受一系列的電腦化執行功能作業，以及標準化的詞彙和語法測驗。每項執行功能皆由一個涉及語言處理程度較高的「語言為主的作業」，和一個涉及語言處理程度較低的「視覺空間為主的作業」測量，藉由比較兩組兒童在這兩個牽涉不同語言處理程度的作業表現，以釐清其執行功能缺陷是侷限於語言層面，或是廣泛性的障礙。研究結果顯示，華語學前DLD兒童除了在「持續的選擇注意力」表現與TD同儕相似，在其餘的執行功能表現上均顯著較TD兒童低落。另外，在控制了個案的背景變項之後，「持續性的選擇注意力」能預測學前兒童的詞彙理解與表達，「抑制」能預測兒童的詞彙和語法表達，而「更新工作記憶」可預測語法理解。整體而言，華語學前DLD兒童顯現廣泛性執行功能缺陷，且這些執行功能對於學前兒童的口語表現各有不同的預測力，據此推論DLD兒童的執行功能缺陷可能對其語言發展不利。建議臨床的語言治療師和老師需重視DLD兒童的執行功能表現，必要時應將其列入療育時的考量，以期能提升DLD兒童治療與教學成效。

Rationale and Purpose: A growing body of evidence suggests that children with developmental language disorder (DLD) experience difficulties in various aspects of executive functions (EFs). EFs are a set of processes responsible for goal-directed behaviors, such as inhibiting distractions, updating information held in working memory, and shifting between mental sets. EFs develop rapidly during the preschool years, and they have been demonstrated to be crucial for language development and for school readiness in areas such as early literacy and numeracy. However, most studies on EF deficits in children with DLD have focused on English-speaking school-age children and linguistic EF tasks that require children to apply their linguistic knowledge to formulate a correct response. Thus, whether the poor performance of children in EF tasks is due to language impairment, EF deficits, or both remains unclear. A few studies have used visual tasks to reduce language loading; however, their results are also inconclusive. To further explore the characteristics and behaviors of children with DLD, the present study first determined whether Mandarin-speaking preschoolers with DLD performed differently from their peers with typical development (TD) in terms of sustained selective attention, inhibition, updating working memory, and shifting tasks that required linguistical or visual processing. Through comparisons of the children’s performance on linguistic and visuospatial EF tasks, we clarified whether their EF deficits were limited to the language domain or were domain-general deficits. Furthermore, this study investigated the role of EFs in the performance of Mandarin-speaking preschoolers on receptive and expressive vocabulary and grammar assessments after various background variables were controlled for (i.e., group membership, nonverbal intelligence quotient [IQ], and the index-of-social-position [ISP] score of the primary caregiver). Methods: Twenty-two children with DLD who were aged 4 or 5 years old (14 boys and 8 girls) and 44 children with TD (28 boys and 16 girls) were matched by age and gender. All participants were recruited from public and private preschools in Taipei and had to meet the following criteria: (1) normal hearing; (2) native Mandarin speakers without any history or diagnosis of neurological impairment, psychological/emotional disability, motor disorder, or attention deficit hyperactivity disorder, as reported through a parent questionnaire; and (3) a nonverbal IQ score (measured using the Test of Nonverbal Intelligence-Fourth Edition) of >85. In addition, information on the education levels and occupations of the participants’primary caregivers was collected through questionnaires and used to generate the primary caregivers’ISP scores. The DLD group comprised participants who scored at least 1.25 standard deviations below the norm-referenced mean on three standardized oral language assessments (i.e., the Preschool Language Disorder Scale-Revised, Receptive and Expressive Vocabulary Test-Second Edition, and Diagnostic Test of Syntactic Ability for Preschoolers and Grade 1-2 Children in Elementary School). The non-DLD group comprised preschoolers with TD who had no history of language intervention and scored above the norm-referenced mean on all three standardized oral language assessments. All participants performed a series of computerized EF tasks that assessed their sustained selective attention, inhibition, updating of working memory, and shifting. Each EF was measured using two methods, namely a linguistic task with a higher language demand and a visuospatial task with a lower language demand. Four two-way analyses of covariance, in which group (DLD vs. TD) and task type (linguistic vs. visuospatial) were independent variables, were conducted to compare the mean EF scores of the two groups. Pearson’s correlation analysis was conducted to examine the relationship among EFs, receptive and expressive vocabulary, and receptive and expressive grammar. Finally, a series of hierarchical regressions were performed to determine the contribution of EFs to the participants’performance for receptive and expressive vocabulary and grammar. Notably, only visuospatial-based EF tasks were included for correlation and hierarchical regression analyses to eliminate the potential confounding effect due to language demand. Results/Findings: The results indicated that the participants with DLD performed worse than their peers with TD in updating working memory and performing shifting tasks, regardless of whether the tasks were linguistic or visuospatial. The DLD group also performed poorly on linguistic and visuospatial inhibition tasks in terms of response time but not accuracy. However, the two groups performed similarly on both linguistic and visuospatial sustained selective attention tasks. All participants performed more favorably on linguistic tasks than on visual tasks. This result may suggest that children apply theirlanguage ability to label stimuli and regulate their thoughts after completing EF tasks; it also indicates that visual tasks are inherently more difficult to complete than are linguistic tasks. A correlational analysis revealed significant correlations among visuospatial EF tasks, receptive and expressive vocabulary, receptive and expressive grammar. Hierarchical regression analyses revealed the following results. First, sustained selective attention accounted for 4% of the significant variance in receptive vocabulary after group and nonverbal IQ were controlled for. Second, after group and nonverbal IQ were controlled for, sustained selective attention explained 7% of the variance in expressive vocabulary, and inhibition (accuracy in the Simon task) explained 5% of this variance. Third, updating working memory explained 3% of the variance in receptive grammar after group and background variables were controlled for. Fourth, after group and nonverbal IQ were controlled for, inhibition (response time in the Eriksen flanker task) explained 15% of the variance in expressive grammar, and inhibition (accuracy in the Simon task) explained 4% of this variance. Conclusions/Implications: The overall results indicate that Mandarin-speaking preschoolers with DLD exhibit domain-general EF deficits, including challenges in updating working memory, shifting, and inhibition. These results align with those of studies demonstrating that English-speaking children with DLD exhibit domain-general EF deficits. However, the participants with DLD performed comparably to their peers with TD on sustained selective attention tasks. Moreover, each EF contributed differently to different aspects of the participants’oral language performance even after various background factors were controlled for. The participants’ability to maintain their attention on a given target was crucial for their receptive and expressive vocabulary performance. Their ability to suppress irrelevant information was crucial for their expressive language performance, including their performance on vocabulary and grammar. Finally, their ability to retain received information in their working memory and simultaneously update and manipulate this information with incoming information was key to their grammar comprehension performance. These findings suggest that the EF deficits of children with DLD can impede their language development. Thus, speech and language therapists and teachers should pay attention to the EF performance of children with DLD and practice caution when administering language assessments with a high EF demand to these children. Studies have increasingly been demonstrating that EF training can not only improve the EFs of children but also enhance their language performance. Thus, EF evaluation and training should be provided for children with DLD to enhance their language and EFs effectively.