Development of Executive Functions in 5- to 12- Years old Iranian Children with and without ADHD

The aim of this research was to investigate the development of executive functions in children with and without ADHD of preschools and primary schools. The population included the male and female children with and without ADHD that were 5 to 12 years old in Iran. Therefore among each age group, 60 subjects (15 girls without ADHD, 15 boys without ADHD, 15 girls with ADHD and 15 boys with ADHD) were selected. Children without ADHD were selected by multistage cluster random sampling and ADHD children were selected by clinicians and Conner’s parental questionnaire. The instruments that have been used are Conner’s parental questionnaire and NEPSY tower test. The results showed a significant differences in executive functions scores between children with and without ADHD at different age groups, But gender differences was not significant. The results reveal that the development of executive functions in children with and without ADHD is rising, but this development has a significant difference in children with and without ADHD which is the noticeable lower performance of ADHD children. Nevertheless, there is no significant difference between male and female groups with and without ADHD.


Introduction
Across development, children become increasingly more able to control their thoughts and actions (for a review see : Diamond, 2002).This change has been associated with the development of executive functions, which refers to a broad set of cognitive control processes that enable individuals to manage and direct their attention, thinking and actions to meet adaptive goals (Best & Miller, 2010;Blair & Raver, 2012;Diamond & Lee, 2011).Today executive functions are described as multidimensional construct consisting of several sub-functions.Executive functions, as studied in the cognitive development literature, has come to refer to specific interrelated information processing abilities that enable the resolution of conflicting information; namely, working memory which is defined as the holding in mind and updating of information while performing some operation on it; inhibitory control, defined as the inhibition of prepotent or automatized responding when engaged in task completion; and mental flexibility, the ability to shift attentional or cognitive set among distinct but related dimensions or aspects ofa given task (Davidson, Arnso, Anderson, & Diamond, 2006;Diamond, Kirkham, & Arnso, 2002;Garon , Bryson, & Smith, 2008;Zelazo & Muller, 2002).Neuropsychologically, executive functions have been linked to the prefrontal cortex of the brain and supporting subcortical loops (Stuss & Alexander, 2000).
Regarding development, it was historically thought that executive functions only emerged in late adolescence to adulthood, and therefore played no significant role in development during infancy and early childhood brain development (Anderson, 2002;welsh, Pennington & Groisser, 1991).Specifically, executive functions appears to emerge in three distinct growth periods, birth to 2 years, 6 to 9 years, and adolescence to early 20s (Anderson, 1998, Anderson, Levin & Jacobs, 2002, romine & Reynolds, 2005).During these growth periods executive functions develop rapidly and are particularly vulnerable to disruption (Ewing-Cobbss, Prasad, landry, Kramer, & Deleon, 2004).
The impairment of executive function is evidenced by poor attention, distractibility, reduced impulse control, difficulties with planning or organization, a reduction in goal oriented behaviors, reduced insight, a tendency to blame others, or perseveration on thoughts or actions (Anderson, 2002;Bradshow, 2001;Busch, McBride, Curtiss & Vanderploeg, 2005).Furthermore, problems with executive functioning development may impact several childhood disorders, such as attention deficit hyperactivity disorder (ADHD), learning disabilities, autism spectrum disorders (Semrud-Clikeman, Goldenring Fine, & Bledsoe, 2014).
Attention Deficit Hyperactivity Disorder (ADHD) is defined as a developmental disorder characterized by difficulties with sustained attention, distractibility, hyperactivity, and impulse control (Barkley, 1998).A review of eighteen studies by Pennington and Ozonoff (1996) concluded that executive functions deficits were consistently associated with ADHD, in both community and referred samples of children and adolescents with this disorder.A meta-analysis of 83 studies of executive functions in ADHD (Willcutt et al., 2005) found that children and adolescents with ADHD exhibited significant deficits compared to those without ADHD in neuropsychological measures of executive functions (inhibitory control, vigilance, planning, verbal and spatial working memory, and cognitive flexibility), with effect sizes ranging from 0.46 to 0.69 (Soriano-Ferrer, Félix-Mateo, & Begeny, 2014).
Neuropsychological deficits, particularly those related to executive function, are common in individuals with ADHD (Willcutt, Doyle, Nigg, Faraone, & Pennington, 2005).Seidman (2001) and Doyle and colleagues (2001) used neuropsychological measures of executive functions to assess impairments of executive function in children and adolescents with ADHD.They found that about 30% of children and adolescents in their ADHD sample were significantly impaired on traditional neuropsychological tests of executive functions.They concluded that psychometrically defined impairments of executive functions should be considered a comorbid problem that is present in about one third of individuals with ADHD, a comorbidity that compounds the already compromised school functioning of children and adolescents with ADHD (Brown, Reichel, & Quinlan, 2011).
Considering the fact that ADHD costs millions a year in treatment and has lasting effects into adulthood, it is imperative that it should be studied more thoroughly (Sarkis, Sarkis, Marshall, & Archer, 2005).Although ADHD is one of the most commonly diagnosed mental disorders (Conners & Jett, 1999).If a relationship can be determined between ADHD and the executive functions, new interventions may be developed, and more effective medication can be prescribed (Sarkis et al, 2005).
With regard to the results of these studies, the aim of the present study was to obtain more insight into the developmental timetable of executive functions in preschool and school-age without ADHD and ADHD children and to compare development of these functions in Iranian children as well.

Participants
The samples consisted of 420 Iranian children (210 children without ADHD, 210 children with ADHD, 50% boys and 50% girls) aged 5 years to 12 years.Participants were divided into seven groups (5-6 years, 6-7 years, 7-8years, 8-9 years, 9-10 years, 10-11 years, and 11-12 years) and each age group included30 children without ADHD and 30 ADHD children, also multistage random cluster sampling method was used for selecting children.In order to select subjects, from 6 educational areas, 3 areas were randomly selected.Subsequently from these 3 areas 20 preschools and primary schools were randomly selected.Finally from these 20 preschools and schools, 210 children with ADHD and 210 children without ADHD were randomly selected.Also, the following criteria were considered for samples in order to enter the research: 1) No intellectual disability exist 2) Having no specific and clear disorder except ADHD (for ADHD samples).
3) Parents' written consent for their children's participation

NEPSY Neuropsychology Test (Tower subtest)
This subtest is designed to assess the executive functions of planning, monitoring,self-regulation, and problem solving.This multidimensional problem-solving subtest is an adaptation of Shallice's (1982,1988) Tower of London test (TOL).The child moves three colored balls to target positions on three pegs in a prescribed number of moves.There is a time limit (30-60 sec) and also rules to which the child must adhere: (a) Only one ball can be moved at a time; (b) a ball cannot be placed on the table or in the lap or held in the hand while moving another ball, and (c) only a certain number of balls can be put on each of the different pegs.The difficulty of the 20 tasks varies according to the complexity and the number of moves (from 1 to 7).The score is the number of correctly achieved target positions (maximum 20 points) (Klenberg, Korkman & Lahti-Nuuttila, 2001).

Conner's Rating Scale
In this research, Conner's rating scale was filled by parents in order to rate their children's ADHD symptoms.This scale was designed by Keith Conner (2001) and consists of 27 items.The scale is designed to measure the intensity of ADHD symptoms.Conner's scale is one of the most well-known instruments for assessing of ADHD which is used in various researches.

Research Method
For selecting ADHD children, parents of children were asked to fill Conner's parental questionnaire (2001).Then, 210childrenwho had been identified as children with ADHD by parents and professional and 210children without ADHD were selected.All children were examined individually in a quiet room at their respective school.
Squirrel item was used for children aged 5 to 6 years old and bullet item was used for children aged 7 to 12 years old.The means and standard deviations of executive functions for the four groups are presented in Table 1.As itis shown, development of executive functions is ascending.

Results
To assess the impact of age and gender on executive function, a two-way analysis of variance was performed.As it is presented in Table2, with the exception of10-11years old group; there is a significant difference between the means cores of groups with and without ADHD among other age groups.There is no gender effect between two groups with and without ADHD, also there is not significant interaction between group and gender.Tables 3 and 4 shows results of the Scheffe test between the means of executive functions of groups with and without ADHD.

Discussion
This study investigated the development of executive functions among preschool and primary schools children with and without ADHD aged 5 to 12 years in Iran.
The findings of research show that development of executive functions is rising in children with and without ADHD.There are also significant differences between executive functions means in two groups (with and without ADHD).Thus, executive functions scores in ADHD children are significantly lower than children without ADHD.Interaction of gender and group is not significant and there are no significant differences between executive functions means in girls and boys in both groups.
Findings of researches Penington & Ozonof (1996), Barkley (1997), andTehranidoostet al (2003) and … indicate that preschool and school children with ADHD have lower performance in executive functions in comparison with children without ADHD.Levin et al (1991) found significant increases in sensitivity to feedback, problem solving, concept formation, and impulse control between children without ADHD 7-8 years old and 9-12 years old.
The concept of executive functions usually refers to cognitive abilities responsible for controlling and coordinating performance in complex cognitive tasks.The subfunctions or components of goal selection, working memory, planning, inhibition of irrelevant impulses and responses, monitoring and regulation of activity and evaluation ofthe results of activity are most often included in executive functions (Baddeley, 1990(Baddeley, , 1996;;Lezak, 1993;Stuss & Benson, 1986;Vilkki, 1995).
The empirical evidence available to evaluate Barkley's theoretical model, suggesting inhibition as being the fundamental core in effective executive functioning.Children with ADHD are assumed to be developmentally delayed with regard to inhibitory function (Barkley, 1997).The model of Barkley (1997) would predict that, at each age level studied children with ADHD would perform like younger children without ADHD.In congruence with Barkley's model, Pennington and Ozonoff (1996) concluded that children with ADHD demonstrate deficits in motor inhibition.This conclusion was based on a literature review of five executive function domains (inhibition, planning, set-shifting, working memory, and fluency) in different developmental disorders.Also, according to Douglas (2008) deficient self-regulation is responsible for impaired performance of children with ADHD on cognitive, information-processing and neuropsychological tasks.Regulatory deficit models offer an alternative conceptual framework, integrating cognitive and motivational processes across levels of information processing to understand behavioral regulation.Since self-monitoring and adaptive control processes work together to produce goal-directed behavior, deficits in either or both of these regulatory processes may result in maladaptive or suboptimal behavior.Given heterogeneity of ADHD and the lack of a common core neuropsychological deficit among children with this disorder, regulatory models provide a plausible alternative to core-cognitive/motivational-deficit models of ADHD (Douglas, 2008).Douglas' model (2008) suggests that complex effortful control processes (or adaptive) contribute to efficient attention and inhibition.Effortful control is conceptually related to self-regulation and executive functions (Martel & Nigg, 2006;Rothbart, Ellis, Rueda, & Posner, 2003) and is thought to modulate other cognitive processes implicated in ADHD, including working memory, self-monitoring, and planning (Douglas, 2008).
Executive functions were hypothesized to predict achievement, consistent with evidence linking executive functions to school achievement and readiness (Blair & Raver, 2012;Raver et al., 2011;Willoughby et al., 2011).
There is also a considerable evidence linking executive functions to peer acceptance and rejection (Hughes & Ensor, 2011;Riggs, Jahromi, Razza, Dillworth-Bart, & Mueller, 2006;Willoughby et al., 2010).There is less evidence linking executive functions directly to prosocial behavior; however, the thoughtful and empathic behaviors that are central to ratings of prosocial behavior in school-age children have theoretical ties to theory-of-mind abilities in children(understanding the perspectives of other people), which have strong empirical, as well as conceptual, links to executive functions (e.g., Riggset al., 2006).In contrast, executive functions task performance has been directly linked by theory and empirical findings to symptoms of in attention and impulsivity (Hughes & Ensor, 2011;Willoughby et al., 2010).Thus, we hypothesized that executive functions would be inversely predictive of these symptoms reported by teachers.Children who had better executive functions, did better in kindergarten or first grade with respect to school adjustment criteria, with better academic achievement, peer acceptance and prosocial behavior and fewer problems of impulsivity, inattention, aggression, or noncompliance (Matson et al, 2012).

Table 1 .
Mean and standard deviation scores of executive functions in girls and boys aged 5-12 years with and without ADHD

Table 2 .
Summary of Analysis of Variance in children with and without ADHD

Table 3 .
The results of scheffe post hoc test in executive functions of children without ADHD

Table 4 .
The results of scheffe post hoc test in executive functions of ADHD children