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. Author manuscript; available in PMC: 2024 Mar 14.
Published in final edited form as: J Psychopathol Behav Assess. 2017 Sep 23;40(1):130–138. doi: 10.1007/s10862-017-9625-7

Verbal Working Memory as a Longitudinal Mechanism of Vocabulary Problems in Preschoolers with ADHD

Monica L Gremillion 1, Tess E Smith 1, Michelle M Martel 2
PMCID: PMC10939059  NIHMSID: NIHMS1927791  PMID: 38486942

Abstract

Elucidation of early potential risk factors of Attention-Deficit/Hyperactivity Disorder (ADHD) is important to allow for early identification of ADHD and targeted early intervention for children with ADHD. Delayed language skills, particularly poor vocabulary, is an early-developing potential risk factor that is thought to be involved in developmental pathways to ADHD; however, mechanisms explaining the relationship between poor vocabulary skills and ADHD symptoms are unclear and warrant investigation. The present study examines the relationship between poor vocabulary skills and ADHD symptoms by testing cognitive mechanisms, namely verbal working memory (WM), that might account for this link. Participants were 109 young children between the ages of three and six and their primary caregivers. Diagnostic information on ADHD symptoms was available from parents and teachers/daycare providers via standardized rating forms. Vocabulary skills and WM were measured through child performance on laboratory tasks. Mediation analyses found poor verbal working memory significantly partially explained the vocabulary-ADHD association for both parent and teacher-rated ADHD symptoms. Further, effects of verbal WM on the association between poor vocabulary and increased ADHD symptoms largely held at one-year follow-up. Development of early interventions targeting verbal WM may be a promising new direction for early ADHD intervention work.

Keywords: ADHD, Verbal working memory, Vocabulary skills, Developmental pathways, Longitudinal mechanisms

Attention-Deficit/Hyperactivity Disorder (ADHD) is a childhood neurodevelopmental disorder characterized by inattention, hyperactivity, and impulsivity (American Psychiatric Association [APA], 2013) that affects approximately 7% of preschool-aged children (Froehlich et al. 2004). Symptoms of ADHD typically emerge during early childhood, or by age 4 (American Psychiatric Association 2013; Keenan and Wakschlag 2002). ADHD exhibits a chronic course from preschool throughout childhood and into adolescence and young adulthood (Faraone and Biederman 2005). Further, ADHD is often associated with poor academic functioning, negative peer relationships, deficits in cognitive processing, and language impairment (Hamilton and Armando 2008; Speltz et al. 1999). Thus, elucidation of early potential risk factors of ADHD is important to allow for early identification of ADHD and targeted early intervention for children with ADHD. Delayed language skills, particularly poor vocabulary, are one such early-developing potential risk factor that is thought to be involved in developmental pathways to ADHD (Gremillion and Martel 2012; Keenan and Shaw 2003); however, mechanisms explaining the relationship between vocabulary skills and ADHD symptoms are unclear and warrant investigation. Shared etiological factors, such as verbal working memory (WM), are one potential explanation for the association between poor vocabulary skills and ADHD symptoms.

Executive functioning is a broad construct which includes attention control, behavioral inhibition, and WM (Welsh, Pennington, & Grossier, 1991). Within the literature, executive functioning deficits for individuals with ADHD have been well-documented (Barkley 1997). Additionally, research indicates executive functioning is highly correlated with language skills (Carlson et al. 2005; Gooch et al. 2014). Therefore, different models explaining the relationship between deficits in executive function and poor language skills have been proposed (Bishop et al. 2014). One possible model suggests language skills are causally related to the development of executive function (Bishop et al. 2014). This model is in line with research suggesting verbal mediation is utilized during some executive function tasks (Bishop and Norbury 2005). Additionally, in line with this model, Kuhn et al. (2014) found children’s language skills predicted later executive functioning skills.

Research indicates WM is a key component of executive function (Barkley 1997). WM refers to the ability to maintain and manipulate information over brief periods of time without reliance on external aids or cues (Baddeley 2000). WM is important to daily functioning, as it allows individuals to retrieve relevant information while distractions are present (Kane and Engle 2002). Research indicates individuals with ADHD exhibit deficits in both spatial and verbal WM (Cockcroft 2011; Matinussen et al. 2005; Willcutt et al. 2005). Additionally, the neurophysiological substrates related to WM, particularly the prefrontal cortex, are also impaired in individuals with ADHD (Cockcroft 2011; Cook et al. 1995), further validating the role of WM in ADHD.

The development of vocabulary skills and verbal WM appear to be intertwined early during development. Developmental theory suggests that general language ability both relies on and facilitates cognitive processes, such as holding information in short-term memory and shaping attention (Gartstein et al. 2008; Marchman and Fernald 2008). In line with this idea, language facilitates the development of verbal WM, and verbal WM also appears critical to the development of language (Archibald and Gathercole 2006; Gathercole and Baddeley 1989; Marchman and Fernald 2008; Montgomery 2003). As children learn to talk, this helps them to focus their attention and memory on verbal concepts, which in turn develops their attention (Petersen et al. 2013). Consistent with this, the development of both language and verbal WM appears to be delayed in children with ADHD (Denckla 1996; Sowerby et al. 2011). Thus, it is possible that verbal WM deficits provide mechanistic explanations for the association between emerging language, specifically vocabulary skills, and ADHD symptoms. Petersen et al. (2015) found language ability predicted self-regulation, a component of WM. In the same study, the relationship between language ability and later inattentive-hyperactive behavior ratings was mediated by self-regulation (Petersen et al. 2015).

Examining the association among vocabulary, verbal WM, and ADHD symptoms in preschool is important because this is a key period during which these processes are developing. Impaired language and vocabulary development can be reliably and validly identified in children by age 3 using standardized performance measures (Paul 1996). During this time period, WM becomes distinguishable from short-term memory (Alloway et al. 2006). Further, by this age or slightly later, ADHD can be reliably diagnosed and exhibits substantial temporal stability (Keenan and Wakschlag 2002; Lahey et al. 1994). Language and cognitive deficits associated with ADHD may be most noticeable during this age range when these skills are actively developing, and children with ADHD are lagging behind. Previous work has found that preschoolers with ADHD, especially those with high hyperactivity/impulsivity, exhibit poor receptive and expressive vocabulary compared to same-aged peers (Gremillion and Martel 2012). Language deficits, specifically receptive vocabulary deficits, may be less pronounced by the time that children with ADHD begin school as they rapidly catch up to their peers (Cohen et al. 2000).

The present study will examine the relationship between vocabulary skills and ADHD symptoms, by testing cognitive mechanisms, namely verbal WM, that might account for this link. Previous research has suggested poor language skills predict later executive functioning (Kuhn et al. 2014), and further that components of executive functioning (i.e. self-regulation) mediate the relationship between poor language skills and inattentive-hyperactive behavior ratings (Petersen et al. 2015). This study adds to the literature by investigating specific early emerging components of executive function (verbal WM) and language skills (vocabulary) in a community-recruited sample of preschoolers aged 3 to 6 at risk for ADHD, using both parent and teacher report of ADHD symptoms. Prior work has suggested that ADHD symptoms manifest differentially in specific contexts (Bartels et al. 2004; Bird et al. 1992; Piacentini et al. 1992). Therefore the current study, sought to replicate previously found associations between poor language skills and deficits in executive functioning using both parent and teacher report. It is predicted that verbal WM will mediate the association between vocabulary skills and ADHD symptoms for both parent and teacher-reported symptoms. Further, this study will investigate the longitudinal mediation effects of WM on the association between vocabulary skills and ADHD symptoms at one-year follow-up. It is predicted that poorer vocabulary skills at the initial assessment will be significantly associated with ADHD symptoms at one-year follow-up and that WM will significantly mediate this association.

Methods

Participants

Overview

Participants included 109 young children between the ages of three and six (M = 4.77, SD = 1.10) and their primary caregivers (hereafter termed parents for simplicity; 69% mothers with the remaining 31% fathers and mothers together, fathers only, foster parents, or grandmothers with guardianship). As shown in Table 1, 59% (n = 64) of the sample was male, and 33% of the sample was ethnic minority (26% African American and 7% other including Latinos and mixed race children). Parental educational level ranged from unemployed to highly skilled professionals, with incomes ranging from below $20,000 to above $100,000 annually. Based on multistage and comprehensive diagnostic screening procedures (detailed below), children were recruited into two groups: children with clinically significant ADHD symptoms (n = 61; 6 met criteria for primarily inattentive subtype, 26 for primarily hyperactive-impulsive subtype, and 29 for combined type) and children without clinically significant ADHD symptoms (n = 48). The non-ADHD group included children with subthreshold symptoms (i.e., fewer than 6 ADHD symptoms) to provide a more continuous measure of ADHD symptoms, to be sensitive to the young age of the sample and to be consistent with research suggesting that ADHD may be better captured by continuous measures than categorical diagnosis (e.g., Haslam et al. 2006; Marcus and Barry 2011). No siblings were included.

Table 1.

Descriptive statistics for the sample

ADHD n = 61 non-ADHD n = 48
Age 4.57(1.17) 4.93(1.04)
Sex (n; % Male) 40(65.6) 24(50)
Ethnic Minority 27(44.2) 9(18.8)*
Income (mode; see below) 0 2,5
Parent-rated Inattention 14.62(7.17) 5.49(5.52)**
Parent-rated Hyper-Imp 18.08(6.8) 6.82(5.6)**
Teacher-rated Inattention 19.04(5.67) 4.05(4.04)**
Teacher-rated Hyper-Imp 17.48(6.15) 4.96(4.98)**
Vocabulary Skills −.21 (1.03)* .27 (.87)*
Verbal Working Memory 1.20(2.01) 1.93(2.40)
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*

p < .05.

**

p < .01 based on chi-square or ANOVA/MANOVA. Family income modes: 0 = annual income less than $20,000, 1 = between $20,000 and $40,000, 2 = between $40,000 and $60,000, 3 = between $60,000 and $80,000, 4 = between $80,000 and $100,000, and 5 = over $100,000 annually. Vocabulary skills are measured using a vocabulary composite of PPVT and EVT standard scores. Parent n = 109, teacher n = 50

Recruitment and Identification

Participants were recruited primarily through direct mailings to families with children between the ages of three and six from the Greater New Orleans area, including urban New Orleans, suburbs of the city, and surrounding rural areas. Advertisements in newspapers and on craigslist.com and flyers posted at doctors’ offices, community centers, daycares, and on campus bulletin boards were also used to recruit from the community. Two sets of advertisements were utilized: one set of advertisements targeted children between ages 3 and 6 with disruptive behavior problems and/or attention problems and a second set of advertisements targeted children between ages 3 and 6 without these types of problems. After recruitment, families passed through a multi-gated screening process. An initial telephone screening was conducted to rule out children prescribed long-acting psychotropic medication (e.g., antidepressants) or children with neurological impairments, intellectual disability, autism spectrum disorders, seizure history, head injury with loss of consciousness, or other major medical conditions. Only 10 families were screened out at this phase. All families screened into the study at this point completed written and verbal informed consent procedures consistent with the university Institutional Review Board, the National Institute of Mental Health, and APA guidelines.

During the second stage, parents and children attended a campus laboratory visit after which participating caregivers received $30 apiece and children received a small $5 prize as incentives for participation. Parents of children taking psycho-stimulant medication were asked to consult with a physician about discontinuing children’s medication for 24 to 48 h prior to the visit depending on their dosage and type of medication in order to ensure a more accurate measure of cognitive performance (less than 5% of children in the study were currently taking medication for attention problems and all discontinued at least 24 h before cognitive testing). Before and during the laboratory visit, diagnostic information was collected via parent and teacher ratings. Parents completed the Kiddie Disruptive Behavior Disorders Schedule (KDBDS: Leblanc et al. 2008), a semi-structured diagnostic interview modeled after the Schedule for Affective Disorders and Schizophrenia for School-Age Children (Orvaschel and Puig-Antich 1995) administered by a trained graduate student clinician. Questions about endorsed ADHD were followed by questions that determine symptom severity, duration, onset, and cross-situational pervasiveness. The K-DBDS demonstrates high test-retest reliability and high inter-rater reliability (Leblanc et al. 2008). Fidelity to interview procedure was determined via stringent check-out procedures, in which staff administering tests had to first successfully administer tests to a trained graduate student or the principle investigator before interview administration to participants. In addition, reliability of interviewer ratings was determined by blind ratings of interviews of each interviewer on 10% of families. Inter-rater reliability for ADHD symptoms was adequate (ICC = .969) on the KDBDS.

Families were mailed teacher/daycare provider questionnaires one week prior to the laboratory visit and instructed to provide the questionnaire to children’s teacher and/or daycare provider who then mailed the completed questionnaire back to the university. When available (i.e., available on 46% of participating families), teacher/daycare provider report on DBD symptoms was obtained via report on the Disruptive Behavior Rating Scale (DBRS; Barkley and Murphy 2006). In the current study, of the 46% of teacher/daycare provider report, approximately 67% of that was available from teachers, with most of the remaining questionnaires completed by daycare providers or babysitters (see Table 2). Response rate did not differ based on child diagnostic category (X2(3) = 0.59, p = .90). Ultimately, clinical diagnoses were determined by the Principal Investigator, a licensed clinical psychologist, after a review of parent ratings on the KDBDS and (when available) teacher/daycare provider ratings on the DBRS, consistent with current best practice guidelines for current diagnosis (Pelham et al. 2005). Specifically, if children had both parent and teacher/daycare provider ratings available, both reporters had to endorse 6 or more symptoms as present for ADHD diagnostic criteria to be achieved.

Table 2.

Measures

Measure Completed by Time point completed Data missing (%)
ADHD symptoms
 DBRS Parent Initial 4
1 year follow-up 27
Teacher/Daycare provider Initial 54
Vocabulary
 PPVT Child Initial 2
 EVT Child Initial 2
Working memory
 DSB Child Initial 5
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DBRS Disruptive Behavior Rating Scale, PPVT Peabody Picture Vocabulary Test-Fourth Edition, EVT Expressive Vocabulary-2, DSB Digit Span Backward subtest of WISC-IV

Measures

ADHD Symptoms

Parental and teacher/daycare provider reports on symptoms related to ADHD were available via the DBRS (Barkley and Murphy 2006), which assess symptoms of ADHD using a 0 to 3 scale for a more continuous dimension. The DBRS has high internal consistency ranging from .78 to .96 in the preschool age range (Pelletier et al. 2006. All scales for parent and teacher/daycare provider report on the DBRS had high internal reliability in the current sample (all alphas >.92). Primary analyses were conducted using continuous symptoms via parent-report on the DBRS, and secondary analyses were conducted using continuous symptoms via teacher-report of ADHD symptoms on the DBRS in order to examine possible context effects on ADHD symptoms. Parent report on ADHD symptoms was available via the K-DBDS described above one year after the initial appointment.

Vocabulary Skills

Vocabulary skills were measured using the Peabody Picture Vocabulary Test-Fourth Edition (PPVT-4; Dunn and Dunn 2007), which measures receptive vocabulary skills, and the Expressive Vocabulary-2 (EVT-2; Williams 2007), which measures expressive skills. The PPVT-4 and EVT-2 both exhibit high internal consistency (between .88 and .97) and high test-retest reliability (between .92 and .97). Raw scores were calculated by subtracting the number of errors made from the highest numbered item completed; standard scores ranged from 66 to 138 for the PPVT-4 and from 71 to 140 for the EVT-2. Because receptive and expressive vocabulary were highly correlated (r = .95, p < .001), a language composite using standard scores from the PPVT-4 and EVT-2 was calculated using Principal Axis Factoring in SPSS. The factor analysis yielded a one-factor solution explaining a total of 97.5% of the variance. Factor scores were retained for subsequent analyses.

Verbal WM

The Digit Span Backward (DSB) subtest of the WISC-IV (Wechsler 2003) was administered to assess verbal WM, the type of WM theoretically believed to be most related to the development of early academic skills which are highly dependent on verbal reasoning and memory (Sattler 2008). DSB tests verbal WM and auditory sequential processing, or the recall and manipulation of auditory information (Sattler 2008). Raw scores were used for primary analyses with lower scores denoting worse verbal WM. Scores for the current sample ranged from 0 to 7.

Data Analysis

Missingness was minimal in the current study, with the exception of teacher report of ADHD symptoms, which was available on 46% (n = 50) of the sample, and parent report of ADHD symptoms at one-year follow-up, available on approximately 73% (n = 80) of the sample. Despite this missing data, power was still adequate (.80) to detect a medium size effect (r = .3).

Data analysis proceeded in a stepwise fashion. Preliminary statistics were conducted in SPSS to examine mean differences between the ADHD and non-ADHD groups on demographic variables. Partial correlations were conducted to examine initial patterns of associations among vocabulary, verbal WM, and ADHD symptoms, controlling for ethnicity. Mediation analyses were then conducted in order to examine the idea that poor WM might explain the relationship between poor vocabulary and ADHD. A series of multiple linear regressions using a macro by Preacher and Hayes (2004) were conducted to test whether verbal WM mediated the association between vocabulary skills and ADHD symptoms.

Results

Preliminary evaluation of group differences on demographic variables indicated that the ADHD and non-ADHD groups did not differ on age (t[107] = −1.69, p = .095), gender, (χ2[1] = 2.69, p = .101), or family income (χ2[5] = 8.02, p = .155; see Table 1). However, preschoolers with ADHD did significantly differ from preschoolers without ADHD on ethnicity minority status (χ2[1] = 7.90, p = .005). More preschoolers with ADHD were ethnic minorities, compared to those without ADHD. In order to control for group differences in ethnicity, ethnicity was covaried in all subsequent regression analyses. In line with study hypotheses and as shown in Table 1, independent samples t-tests indicated that preschoolers with ADHD had significantly lower vocabulary skills than preschoolers without ADHD (t[103] = 2.51, p = .013). Preschoolers with ADHD did not significantly differ on verbal WM from preschoolers without ADHD (t[102] = 1.69, p = .094). Partial correlations covarying ethnicity were conducted to examine the relationship among vocabulary skills, verbal WM, and ADHD symptoms. In line with hypotheses, increased ADHD symptoms were significantly correlated with poorer vocabulary skills and poorer verbal WM (r range = −.26 to −.28, p < .05, Table 3).

Table 3.

Partial correlations among vocabulary skills, verbal WM, and ADHD symptoms covarying ethnicity

1 2 3 4 5 6 7
1 Vocabulary
2 Verbal WM .16
3 ADHD-P −.26* −.28*
4 Inatt-P −.23* −.25* 95**
5 HI-P −.27* −.29* .96** .83**
6 ADHD-1 −.12 −.29* .68** .62** .69**
7 Inatt-1 −.09 −.22 .58** .57** .55** 93**
8 HI-1 −.14 −.31** .70** .58** .74** .93** .73**
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WM working memory, ADHD attention-deficit/hyperactivity disorder, P parent report, Inatt inattention, HI hyperactivity/impulsivity, 1 = one year follow-up parent report

*

= p < .05;

**

= p < .01

Does Verbal WM Mediate the Association between Vocabulary Skills and ADHD Symptoms?

To test the hypothesis that verbal WM mediated the association between vocabulary skills and ADHD symptoms, a series of multiple regression analyses, covarying child ethnicity, were performed. For analyses predicting ADHD symptoms, all pathways were significant in the expected direction (Fig. 1). Poorer vocabulary skills were significantly associated with poorer verbal WM (β = .66, p < .001) and increased ADHD symptoms (β = −.23, p = .024). Poor verbal WM was significantly associated with increased ADHD symptoms (β = −.23, p = .022). Once verbal WM was entered into the model, the association between vocabulary skills and ADHD symptoms became nonsignificant (β = −.14, p = .286), suggesting that WM significantly mediated the association between vocabulary skills and ADHD symptoms (Δr2 = .07, p = .033; indirect effects: z = −2.21, p = .027).

Fig. 1.

Fig. 1

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Does verbal WM mediate the association between vocabulary skills and ADHD symptoms? NOTE: * = p < .05; ** = p < .01; ns = non-significant. Vocabulary skills measured by creating a composite variable using the PPVT and EVT raw scores. Verbal working memory measured by WISC-IV Digit Span Backward raw score

Do Results Generalize to Teacher-Reported Symptoms?

To evaluate informant effects on report of ADHD symptoms, primary analyses were checked utilizing teacher-reported (n = 50 [vs. parent-reported (n = 109)]) preschool ADHD symptoms. Poorer vocabulary skills and poorer verbal WM were significantly associated with increased teacher-reported ADHD symptoms (β = −.30, p = .050 for vocabulary; β = −.32, p = .041 for verbal WM). With verbal WM entered as an independent variable in the second step of the regression analysis, the association between vocabulary skills and ADHD symptoms became nonsignificant (β = −.17, p = .499), suggesting significant partial mediation and in line with generalization of effects across reporters (Δr2 = .11, p = .077; indirect effects: z = −2.21, p = .027).

Longitudinal Mediation Effects

To evaluate longitudinal mediation effects, primary analyses were conducted using ADHD symptoms measured one year after the initial laboratory visit. All pathways remained significant in the expected directions, except for the pathway from verbal WM to ADHD symptoms at one-year follow-up, which was marginally significant. Poorer vocabulary skills were associated with increased ADHD symptoms at Time 2 (β = −.38, p = .029), although poorer verbal WM was only marginally associated with increased ADHD symptoms at one year (β = −.31, p = .082). Once verbal WM was entered into the model, the association between vocabulary skills and ADHD symptoms measured at one year became nonsignificant (β = −.27, p = .226), suggesting that verbal WM significantly partially mediated the association between vocabulary skills and ADHD symptoms measured at Time 2 (Δr2 = .35, p = .006).

Secondary Checks

Secondary checks on the data analyses were conducted to test the effects of not covarying ethnicity. When ethnicity was not used as a covariate, results did not change their pattern of significance (i.e., previously significant findings remained significant, whereas non-significant findings remained non-significant). Additionally, primary analyses were conducted again with ADHD symptoms predicting vocabulary skills, with verbal working memory as the mediator. For both parent-reported and teacher-reported ADHD symptoms, neither model was significant (Δr2 = .064, p = .411 for parent-reported ADHD symptoms; Δr2 = .019, p = .101 for teacher-reported ADHD symptoms).

Discussion

The present study examined verbal working memory as a potential mechanism of the association between vocabulary skills and ADHD symptoms, as assessed by multiple raters, and over a one-year longitudinal period. In line with developmental theory suggesting that language, including vocabulary, and verbal working memory are intertwined, poor verbal working memory significantly partially explained the association between poor vocabulary and ADHD during preschool. Further, these results held whether using parent or teacher-rated ADHD symptoms and over a one-year longitudinal period.

Based on results of the present study, poorer vocabulary skills and poorer verbal WM was associated with increased ADHD symptoms in preschoolers. This is consistent with prior research suggesting that children with ADHD exhibit language and WM problems (Kim and Kaiser 2000; Purvis and Tannock 1997; Sowerby et al. 2011). Importantly, results of the current study extend prior work in this area by suggesting that verbal WM can explain the association between vocabulary skills and ADHD symptoms. In line with developmental models, delayed language development may hinder the development of higher order cognitive functions, such as verbal WM, which, in turn, increases risk for ADHD (Gartstein et al. 2008; Marchman and Fernald 2008; Montgomery 2003). These deficits in verbal WM likely hinder subsequent vocabulary development, potentially in a bidirectional manner, such that preschoolers with ADHD may develop vocabulary skills and verbal WM at a slower pace than typically developing peers (Denckla 1996; Sowerby et al. 2011). Thus, it is possible that deficits in verbal WM provide the mechanism by which poorer vocabulary skills increase risk for ADHD symptoms. This suggests language, specifically vocabulary, as an important early-emerging mechanism of ADHD that sheds neurodevelopmental origins of the disorder (Frick and Nigg 2012).

Further, this study investigated the effects of multiple informants on ADHD symptoms since prior work has suggested that ADHD symptoms manifest differentially in specific contexts (Bartels et al. 2004; Bird et al. 1992; Piacentini et al. 1992). Results suggested that deficits in vocabulary and verbal WM are associated with both parent- and teacher-rated ADHD symptoms. Regardless of reporter of ADHD symptoms, verbal WM appears to explain the relationship between poor vocabulary and ADHD in the current study. This finding suggests deficits in verbal WM might represent an overarching problem that spans across differential situational demands on the child, both at home and at school.

Importantly, results held across a one-year longitudinal follow-up period, and early vocabulary deficits were associated with ADHD symptoms one year later during the preschool period. Therefore, early language assessment and intervention may be helpful, particularly for those preschoolers showing early signs of ADHD. Development of early interventions targeting verbal WM may be a promising new direction for early ADHD intervention work. Previous work targeting WM in children has shown promising results (Klingberg et al. 2002, 2005). This line of work uses a training paradigm with intensive and adaptive training of WM tasks, and children who completed the training exhibited improved WM, as well as ADHD symptom reduction (Klingberg et al. 2005). Additionally, it is possible interventions targeting vocabulary skills could be beneficial for children with ADHD.

The present study provides a good starting point for investigating mechanisms by which vocabulary skills are related to ADHD symptoms; however, it is not without limitations. Though verbal WM was the focal point of this investigation, it is possible that other aspects of higher order cognitive functioning, such sustained attention and voluntary control, or lower-order cognitive functioning, such as short-term memory, or even other unknown variables may predispose children to both poor language development and also make them more susceptible to developing ADHD. Additionally, it is possible that attentional problems impair performance on verbal working memory tasks. The current study was a first step in examining a potential mechanism accounting for the association between poor vocabulary and ADHD; however it is possible that other factors, such as language spoken at home, IQ, school attendance, etc. have an impact on language development, and these factors were not included in the current study. Additionally, the current study was unable to examine whether this model is specific to ADHD versus other psychiatric disorders or learning disorders, which is important direction for future work. Further, general cognitive ability and environmental factors were not assessed and may potentially confound results. The current study conducted mediation analyses in a cross-sectional sample, and therefore it is not possible to directly test causal hypotheses and there are a number of possible alternate explanations for the present findings. Bidirectional associations between vocabulary and verbal WM were not assessed, but this is another important direction for future work. This study utilized a small community-recruited sample enriched for ADHD and teacher reports were only obtained for about 45% of the sample, which potentially threatens the generalizability of the current study; replication with larger general population samples and clinic-recruited samples would be beneficial. Finally, the current study did not have enough power to examine within-ADHD subgroups. However, as ADHD is a heterogeneous disorder, an important future direction will be to examine whether this model may differ for individuals with different ADHD presentations (inattentive, hyperactive-impulsive, or combined).

This study makes an important contribution to the existing literature by examining the role of verbal WM as a potential mechanism explaining the association between vocabulary skills and ADHD symptoms during preschool. Study results indicated that verbal WM partially explained the association between vocabulary problems and ADHD over a one-year period during preschool. This work suggests useful new directions in regard to the development of tailored interventions addressing deficits in vocabulary and verbal WM in preschoolers with ADHD.

Acknowledgments

We are indebted to the families who made this study possible. This research was supported by National Institute of Health and Human Development Grant 5R03 HD062599–02 to M. Martel.

Funding

This study was funded by the National Institute of Health and Human Development Grant 5R03 HD062599–02.

Footnotes

Conflict of Interest Monica L. Gremillion, Tess E. Smith, and Michelle M. Martel declare that they have no conflict of interest.

Ethical Approval All procedures performed in studies involving human participants were in accordance with the ethical standards of the institutional and/or national research committee and with the 1964 Helsinki declaration and its later amendments or comparable ethical standards.

Informed Consent Informed consent was obtained from all individual participants included in the study.

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