Abstract

This study examined the executive function (EF) profile of Chinese boys with attention-deficit hyperactivity disorder (ADHD) using a large sample. Executive function performance within the ADHD subtypes and the effects of comorbidity were also investigated. Five hundred Chinese boys (375 with ADHD and 125 controls) aged 6–15 completed a battery of EF tests. Boys with all types of ADHD performed worse in all of the EF tests than age- and intelligence quotient-matched healthy controls. The boys with the inattention ADHD subtype and the combined subtype showed similar impairments across different EF tasks, whereas the boys with the hyperactive-impulsive ADHD subtype primarily displayed deficits in theory of mind and visual memory. Comorbid oppositional defiant disorder/conduct disorder had no additional influence on the EF characteristics of the boys with ADHD only, whereas comorbid learning disorder increased the severity of inhibition and shifting impairments.

The concept of “executive function” (EF), although variously defined, is generally agreed to be a product of the coordinated operation of various processes undertaken to accomplish a particular goal in a flexible manner (Funahashi, 2001). Executive function is a complex construct that involves intricate connections between various brain regions (Weyandt, 2005) and encompasses the five main domains of inhibition, working memory, set shifting, planning, and fluency (Pennington & Ozonoff, 1996; Sergeant, Geurts & Oosterlaan, 2002). These types of EF have been collectively termed “cold” EF, because their corresponding cognitive processes are relatively mechanistic or logically based. In contrast, types of EF that involves emotions, beliefs, or desires are regarded as “hot” EF (Chan, Shum, Toulopoulou, & Chend, 2008).

Executive function deficits are suggested to be an important component of attention-deficit hyperactivity disorder (ADHD) (Shallice et al., 2002; Willcutt, Doyle, Nigg, Faraone, & Pennington, 2005). Attention-deficit hyperactivity disorder is a prevalent disorder with recent estimates from studies in the United States and around the world, indicating that at least 3.0–7.8% of the general population meets the criteria for the disorder (Joseph, 2005). Attention-deficit hyperactivity disorder causes significant impairment in the vast majority of major life domains, such as education, family, and peer functioning (Barkley, Fischer, Smallish, & Fletcher, 2006). A growing body of literature suggests that ADHD is associated with EF deficits at different developmental stages, including childhood (Pasini, Paloscia, Alessandrelli, Porfirio, & Curatolo, 2007; Seidman et al., 2005; Sergeant et al., 2002; Willcutt et al., 2005), adolescence (Clark, Prior, & Kinsella, 2000; Martel, Nikolas, & Nigg, 2007; Seidman et al., 2005), and adulthood (Biederman et al., 2006; Boonstra, Oosterlaan, Sergeant, & Buitelaar, 2005; King, Colla, Brass, Heuser, & von Cramon, 2007). However, it is also known that EF deficits are common to many psychiatric disorders and are thus not specific to ADHD (Weyandt, 2005). Executive function weakness is significantly associated with ADHD, but is not the single necessary and sufficient cause of the disorder (Seidman, 2006; Willcutt et al., 2005).

The Diagnostic and Statistical Manual of Mental Disorder-IV (DSM-IV) (American Psychiatric Association [APA], 1994) describes three diagnostic subtypes of ADHD based on symptoms: inattentive (ADHD-I), hyperactive-impulsive (ADHD-HI), and combined (ADHD-C). The EF differences among these subtypes, however, still require further documentation. Barkley (1997) suggests that deficits in EF are related to ADHD-C but not ADHD-I, and that pervasive EF deficits in ADHD are due to a primary deficit in inhibitory control. This is supported by several other studies (Houghton et al., 1999; Klorman et al., 1999; Lockwood, Marcotte, & Stern, 2001; Nigg, Blaskey, Huang-Pollock, & Rappley, 2002; O'Driscoll et al., 2005; Schwenck et al., 2009). The results show that ADHD-C is accompanied by more serious EF impairment than ADHD-I. However, other researchers have failed to find reliable differences between the two subtypes (Chhabildas, Pennington, & Willcutt, 2001; Geurts, Verte, Oosterlaan, Roeyers, & Sergeant, 2005; Martel et al., 2007; Milich, Balentine, & Lynam, 2001; Murphy, Barkley, & Bush, 2002), and suggest that EF deficits in ADHD can be accounted for by symptoms of inattention (Chhabildas et al., 2001). Some studies have been carried out to investigate the EF profile of ADHD-HI. The results show that ADHD-HI does not exhibit EF deficits (Bedard et al., 2003; Chhabildas et al., 2001; Schmitz et al., 2002), giving rise to the conclusion that neuropsychological impairment seems to occur only in those ADHD subtypes in which inattention is clinically significant (Schmitz et al., 2002). However, another study reveals no differences between ADHD-HI and ADHD-C for a series of tasks of attention (Tucha et al., 2006). Accordingly, it is not clear whether the ADHD subtypes differ in EF as manifested in task performance.

Attention-deficit hyperactivity disorder is strongly comorbid with other disorders. Some studies found that ADHD patients without comorbidity may still demonstrate impairment in EF tests (Clark et al., 2000; Klorman et al., 1999; Seidman et al., 2005; Seidman, Biederman, Monuteaux, Doyle, & Faraone, 2001; Willcutt et al., 2001). These results suggest that the EF deficits displayed in ADHD are not accounted for by other psychiatric factors (Weyandt, 2005). Some studies show EF deficits are associated with such comorbidities as oppositional defiant disorder (ODD)/conduct disorder (CD) (Toupin, Dery, Pauze, Mercier, & Fortin, 2000), learning disorder (LD) (Wu, Anderson, & Castiello, 2002), and language problems (Jonsdottir, Bouma, Sergeant, & Scherder, 2006). Thus, investigating the performance of children who have ADHD without comorbidity would be useful for determining the EF characteristics of ADHD. Seidman (2006) also suggests that further study in the near future needs to address whether particular subgroups of children with ADHD comorbid with other psychiatric disorders are especially or distinctively impaired when compared with other subgroups of children with pure ADHD diagnosis.

There is also some disagreement on the relationship between intelligence quotient (IQ) and EF. However, regardless of the nature of this relationship, previous studies have found that IQ is associated with several aspects of EF (Arffa, 2007; Arffa, Lovell, Podell, & Goldberg, 1998; Harrier & Deornellas, 2005; Mahone et al., 2002; Sonuga-Barke, Dalen, Daley, & Remington, 2002). The results of studies that use IQ as a covariate are mixed (Martel et al., 2007). Some found that differences between ADHD and healthy control (HC) groups become non-significant when IQ is controlled (Martel et al., 2007; Riccio, Homack, Jarratt, & Wolfe, 2006; Scheres et al., 2004), whereas others support the conclusion that EF impairment in ADHD goes beyond IQ (Crinella & Yu, 2000; Seidman et al., 2005, 2006). Sergeant and colleagues (2002) indicate that a stronger case can be made for EF differences when IQ is taken into account.

What is needed to resolve some of these issues is a well-powered and detailed study to investigate the EF profile of treatment-naive people diagnosed with the different subtypes of ADHD with and without comorbidity. Accordingly, in this study, we test the hypothesis that EF is significantly impaired in Chinese boys with ADHD compared with age- and IQ-matched HCs. We also investigate whether different patterns of EF deficit are displayed for the three subtypes of ADHD, and how comorbidities including ODD/CD and LD affect EF characteristics in ADHD.

Materials and Methods

Participants

ADHD group

The study included 375 Chinese boys between the ages of 6 and 15 who had been diagnosed with ADHD. All of the boys were diagnosed using structured interviews with their parents following the criteria of the DSM-IV (APA, 1994) at clinics at the Mental Health Institute of Peking University in Beijing, China. The structured interviews assessed the lifetime history of the participants’ psychopathology. Children with symptoms of ADHD were included in the ADHD group. The parents were interviewed once each by two psychiatrists, including one senior psychiatrist. Following this, a consensus diagnosis was assigned to all participants.

The ADHD group was divided into three subtypes: ADHD-I (n = 200), ADHD-HI (n = 25), and ADHD-C (n = 150). Each group was matched by age (within 6 months) and IQ (within 15-scaled score points). The matching process aimed to decrease the error variance and prevent matching variables from becoming competing causal factors for any effects (Kirk, 1995).

In terms of comorbidity, 127 boys were categorized as having ADHD only, 55 as ADHD comorbid with ODD or CD, 39 as ADHD comorbid with LD, 20 as ADHD comorbid with tic disorder, and 16 as ADHD comorbid with mood disorder. The remaining 118 boys were categorized as having ADHD with as least two comorbidities.

We excluded children with major sensory-motor handicaps (e.g., paralysis, deafness, and blindness), a history of brain damage, epilepsy, or an estimated full-scale IQ of <80. None of the participants had received any medical treatment for their ADHD symptoms before administration of the tests.

HC group

The children in the HC group (n = 125) were recruited from two primary and two middle schools in Beijing. They were matched with the ADHD group by age and IQ. The recruitment process involved four stages. In Stage 1, parents of the children in the schools were given information about the research and a consent form. In Stage 2, the parents who agreed to participate underwent the same interviews by the same psychiatrists as for the ADHD group. In Stage 3, the healthy children completed the EF tests and IQ measures (Wechsler Intelligence Scale for Chinese Children-Revised). The assessments were administered and scored by examiners who were blind to the diagnostic status of the participants. In Stage 4, the healthy children were matched with the ADHD group by age and IQ to form the HC group.

The study was approved by the ethics committee of the Institute of Mental Health, and informed consent was obtained from all of the children and their parents. All of the participants were native Chinese speakers.

Instruments

The Stroop Color and Word Test

The Stroop Color and Word Test (Stroop, 1935) was used to capture the inhibition component of EF. The Stroop test consisted of four parts, represented by three cards (21 × 29.7 cm). The participants were required to name 30 stimuli in a 10 × 3 matrix as quickly and correctly as possible. Part 1 was a word card containing four differently colored words (red, green, yellow, and blue) that were printed in black ink and presented in random order. Part 2 involved a color card that contained blocks printed in red, green, yellow, and blue. Part 3 involved a color-word card. The participants were required to name the words of the color-content that did not match the color words. In Part 4, the same color-word cards were used, but the participants were required to name the colors. The distracter was the color meaning of the word. The time the children took to complete all 30 items and the number of errors that they made was recorded for each part. The time taken to complete Part 3 was subtracted from that for Part 1 to indicate color interference, and the time taken to complete Part 4 was subtracted from that for Part 2 to indicate word interference.

The Rey-Osterrieth Complex Figure Test

The Rey-Osterrieth Complex Figure Test (RCFT) was used to evaluate visuospatial construction ability, visual working memory, and organizational skills (Lezak, 1995). In the test, the participants were required to observe a complex geometric figure for 30 s and then reproduce it from memory immediately and after a brief delay (∼20 min) without prompting. This test allowed us to observe the participants’ short- and long-term memory performance and oblivion situation. Two traditional methods were used to assess structural and detailed memory. The structural score system divided each Rey geometric figure into five configural elements: a large rectangle, a diagonal cross, the vertical midline, the horizontal midline, and the vertex of the triangle on the right. The participants received points for constructing each element as an unfragmented unit. The large rectangle was assigned two points to reflect its importance to the fundamental organization of the figure. All of the other elements were each assigned one point, which resulted in a range of scores from 0 to 6 (Savage et al., 1999). The detailed score system broke each figure down into 18 storable elements (Lezak, 1995). Two points were awarded if an element was correct and properly placed, and one if it was correct but poorly placed or distorted but correctly placed (Loring, Martin, Meador, & Lee, 1990). The participants’ performance in the RCFT was scored by both systems, and the immediate scores were subtracted from the delayed scores to generate “forgotten” scores that indicated the information that was lost during the 20 min interval.

The Trail Making Test

The Trail Making Test (TMT) was used to assess visual scanning, graphomotor speed, and cognitive flexibility (Reitan & Wolfson, 1985). In Part A, the participants were instructed to connect 25 circles with numbers (1–25) randomly distributed over a sheet of paper (21 × 29.7 cm). This provided a baseline indication of visual search speed and visuo-motor functioning. Part B required the participants to connect 25 circles that contained numbers (1–13) or letters (A–L) and to alternate sequentially between the numbers and letters (that is, 1-A-2-B-3-C, etc.). This allowed the incorporation of the additional component of shift flexibility. The participants were instructed to connect the circles as rapidly as possible and received feedback when they connected them in the wrong order (Anderson, 2001). The time taken to complete the task and the errors made in each part were recorded. The time for Part A was subtracted from the time for Part B to indicate the shift time.

The Tower of Hanoi

The Tower of Hanoi (ToH) task (Simon, 1975) was used to assess the planning component of EF (Kopecky, Chang, Klorman, Thatcher, & Borgstedt, 2005). A model board (26 cm long × 8.5 cm wide) was fitted with three pegs (13 cm high). On the left peg, four graduated disks (diameter = 4.0, 4.5, 5.0, and 6.0 cm, respectively) were arranged as a tower, with the smaller disks stacked on top of the larger ones. The participants were required to move the four disks from the left peg to the right peg following three rules: (i) only one disk at a time could be moved; (ii) larger disks could not be placed on top of smaller disks; and (iii) the disks had to be on a peg or in the participant's hand at all times (Klorman et al., 1999). We reminded the children of the rules if they broke them and asked them to repeat the move when an error was made. The latency time from the examiner's signal to the children's initiation of the first move (initiation time), the total time to reach a solution (accomplish time), the number of moves (accomplish steps), and the number of rule violations (error steps) were recorded. Although the participants were not given a time limit for completing the task, if they found it overwhelming they were allowed to give up, which was recorded as a failure to complete the task (Murphy, 2002).

The Verbal Fluency Test

The Verbal Fluency (VF) Test was used to assess the VF component of EF. This was divided into phonemic and semantic tasks (Tucha et al., 2005). Due to the distinct features of the Chinese language, only the semantic VF task was examined. The participants were required to name as many animals as possible in 2min. The examiners recorded all correct words, and counted whether they were named in the first or second minute. Repetitions and words that were not identifiable as animal names (error responses) were also recorded.

The Deceptive Container Task

The Deceptive Container Task is a classic false-belief tasks used to assess hot EF (Carlson & Moses, 2001). In this task, the children were shown a typical container (a candy box) and asked about its contents (representation question). A practical answer would be “sugar.” The children were then shown that the candy box actually contained pencils, the box was closed, and they were asked about their former false belief: “When you first saw this box, before it was opened, what did you think was inside?” (own false belief question). They were told that another person had never looked inside the box and were asked: “What does he or she think is inside?” (other's false belief question). Their answers were recorded as right or wrong.

The False-belief Task

The False-belief Task was also used to assess the children's hot EF by determining their understanding of a protagonist's false belief (Perner & Lang, 1999). The children were presented with the following scenario. “A boy named Xiao Ming put his chocolate in a green cupboard in the kitchen. He then went out to play. In the meantime, his mother came into the kitchen to clean up. She transferred the chocolate from the green cupboard to a blue one and then went out to the garden to hang out the laundry. Xiao Ming grew tired and went back inside for his chocolate.” At this point, the children were asked the false-belief question: “Where will Xiao Ming look for his chocolate?” The right (green) and wrong (blue) answers were recorded.

The Chinese versions of the tests mentioned previously have been used by our group for Chinese samples, and they are found to discriminate well between ADHD and HC groups (Shuai & Wang, 2007).

Statistical analysis

Statistical analysis of the data was performed using SPSS for Windows (version 11.0). Before analysis, the data were entered twice. Differences were deemed significant when α < .05.

We first assessed the differences between the ADHD and HC groups using multivariate analysis of covariance (MANCOVA). The covariates were age and IQ. Second, we observed the differences among the ADHD subtypes and the HC groups using MANCOVA. The least significant difference (LSD) method was used to perform pairwise comparisons between any two groups. Third, we investigated the EF characteristics of the children with ADHD only using MANCOVA. Fourth, to deal with the comorbidity issue, we analyzed the differences among the ADHD + ODD/CD group, the ADHD + LD group, the ADHD-only group, and the HC group, using MANCOVA and LSD. After matching age, IQ, and subtype, there were 38 participants in the ADHD + ODD/CD group, 38 in the ADHD + LD group, 76 in the ADHD-only group, and 76 in the HC group.

Results

ADHD and HC

There were no significant differences in age (9.98 ± 2.07 vs. 9.99 ± 2.08) or IQ (108.68 ± 11.54 vs. 110.87 ± 10.67) between the ADHD and HC groups (p > .05). The ADHD group performed significantly worse than the HC group in all of the EF tests (see Table 1). In the Stroop test, the ADHD group took a longer time [Part 1: F(1, 496) = 11.91, p < .01, forumla; Part 2: F(1, 496) = 27.48, p < .01, forumla; Part 3: F(1, 496) = 20.47, p < .01, forumla; Part 4: F(1, 496) = 31.16, p < .01, forumla] and made more errors [Part 1: F(1, 496) = 7.72, p < .05, forumla; Part 2: F(1, 496) = 5.21, p < .05, forumla; Part 3: F(1, 496) = 9.91, p < .01, forumla; Part 4: F(1, 496) = 31.37, p < 0.01, forumla] in all four parts than the HC group. The ADHD group also displayed significantly greater color interference [F(1, 496) = 8.60, p < .01, forumla] and word interference [F(1, 496) = 12.90, p < .01, forumla] than the HC group. In the immediate memory part of the RCFT, the ADHD group obtained lower structural [F(1, 496) = 9.78, p < .01, forumla] and detailed scores [F(1, 496) = 21.67, p < .01, forumla], and displayed a significantly more impaired delayed recall [structural: F(1, 496) = 16.38, p < .01, forumla; detailed: F(1, 496) = 31.02, p < .01, forumla] than the HC group. There were no significant differences between the two groups in the structural forgotten scores [F(1, 496) = 3.45, p = .06], but the ADHD had larger detailed forgotten scores than the HC [F(1, 496) = 4.40, p < .05, forumla]. The ADHD group took longer than the HC to finish both Part A [F(1, 496) = 15.85, p < .01, forumla] and Part B [F(1, 496) = 28.06, p < .01, forumla] of the TMT. The shift time of the ADHD group was also longer [F(1, 496) = 21.78, p < .01, forumla] than that of the HC group. The number of mistakes made by the two groups in Part A did not differ [F(1, 496) = 3.24, p = .07], but differed significantly in Part B [F(1, 496) = 15.74, p < .01, forumla]. The percentage of the ADHD group completing the ToH task successfully was lower (p < .01) than that of the HC group. The ADHD group had a shorter initiation time [F(1, 496) = 9.64, p < .01, forumla], required more steps to complete the task [F(1, 363) = 4.16, p < .05, forumla], and committed more rule violations [F(1, 496) = 24.35, p < .01, forumla] than the HC group. In the VF test, the two groups produced the same total number of correct words in 2 min [F(1, 496) = 0.95, ns] and in the first minute [F(1, 496) = 1.20, ns], but the ADHD group produced fewer correct words than the HC group in the second minute [F(1, 496) = 11.53, p < .01, forumla] and gave more repeat responses [F(1, 496) = 10.69, p < .01, forumla]. In the deceptive container test, the answers of the two groups regarding representation did not differ (p = .07), but there was a significant difference (p < .01) between their answers regarding their own false beliefs and those of others. The ADHD group also gave more wrong answers (p < .01) in the false-belief task.

Table 1.

Executive function of three subtypes of ADHD boys and health controls

EF tests ADHD subtypes
 
ADHD total (n=375) HCd (n=125) Comparison (p<.05) 
ADHD-Ia(n =200) ADHD-HIb (n =25) ADHD-Cc (n =150) 
M ± SD M ± SD M ± SD M ± SD M ± SD 
Stroop 
 Part 1 time (s) 20.17 ± 6.78 19.12 ± 7.92 20.32 ± 6.44 20.16 ± 6.72* 17.92 ± 5.55 a, c > d 
 Part 2 time (s) 28.12 ± 9.84 24.96 ± 8.60 28.03 ± 9.62 27.87 ± 9.68* 23.30 ± 6.89 a, c > d 
 Part 3 time (s) 26.34 ± 10.73 25.00 ± 11.15 25.81 ± 9.03 26.04 ± 10.09* 21.76 ± 7.67 a, c > d 
 Part 4 time (s) 56.76 ± 19.11 50.08 ± 19.30 57.62 ± 19.72 56.66 ± 19.40* 47.10 ± 14.38 a, c > b, d 
 Part 1 error 0.30 ± 0.60 0.12 ± 0.33 0.29 ± 0.86 0.28 ± 0.70* 0.09 ± 0.36 a, c > d 
 Part 2 error 0.32 ± 0.72 0.48 ± 0.96 0.31 ± 0.68 0.33 ± 0.72* 0.15 ± 0.42 b > d 
 Part 3 error 0.56 ± 1.27 0.56 ± 0.87 0.51 ± 0.96 0.54 ± 1.13* 0.20 ± 0.52 a, c > d 
 Part 4 error 2.21 ± 3.09 1.64 ± 1.57 2.20 ± 2.69 2.17 ± 2.86* 0.66 ± 1.09 a, c > d 
 Color interference 6.17 ± 7.70 5.88 ± 8.41 5.48 ± 5.95 5.88 ± 7.10* 3.84 ± 5.23 a, c > d 
 Word interference 28.64 ± 14.15 25.12 ± 12.93 29.59 ± 15.12 28.78 ± 14.47* 23.79 ± 10.55 a, c > d 
RCFT 
 Structure immediate 2.93 ± 2.00 2.68 ± 2.05 2.45 ± 2.03 2.72 ± 2.02* 3.37 ± 2.02 c < a, d 
 Structure delay 2.79 ± 2.04 2.80 ± 2.04 2.41 ± 2.12 2.64 ± 2.07* 3.47 ± 2.01 c < a < d 
 Structure forgotten 0.14 ± 0.98 −0.12 ± 0.78 0.04 ± 0.93 0.08 ± 0.95 −0.10 ± 0.84 a > d 
 Detail immediate 9.80 ± 6.30 9.04 ± 6.34 8.27 ± 5.93 9.14 ± 6.18* 12.01 ± 6.92 a, b, c < d; c < a 
 Detail delay 9.26 ± 6.31 8.60 ± 5.78 7.85 ± 5.94 8.65 ± 6.15* 12.02 ± 7.26 a, b, c < d; c < a 
 Detail forgotten 0.55 ± 2.40 0.44 ± 2.12 0.42 ± 2.37 0.49 ± 2.37* −0.02 ± 2.18 a > d 
Trail making test 
 Part 1 time (s) 64.03 ± 28.54 58.36 ± 31.40 61.71 ± 24.31 62.73 ± 27.10* 52.62 ± 18.55 a, c > d 
 Part 2 time (s) 228.42 ± 157.76 184.44 ± 116.65 210.21 ± 118.24 218.20 ± 140.87* 150.42 ± 76.26 a, c > d 
 Shift time 164.38 ± 145.94 126.08 ± 113.87 148.50 ± 110.41 155.47 ± 130.92* 97.80 ± 66.45 a, c > d 
 Part 1 error 0.27 ± 0.58 0.20 ± 0.40 0.29 ± 0.70 0.27 ± 0.62 0.15 ± 0.36 ns 
 Part 2 error 1.64 ± 2.09 1.64 ± 1.72 1.48 ± 1.86 1.57 ± 1.98* 0.78 ± 1.14 a, b, c > d 
Tower of Hanoi 
 Complete/fail 147/53 19/6 98/52 264/111* 103/22 c < d 
 Initiation time (s) 0.90 ± 2.72 0.24 ± 1.01 1.05 ± 3.32 0.91 ± 2.90* 4.20 ± 20.61 a < d 
 Accomplish time (s) 187.14 ± 151.85 160.00 ± 111.97 191.46 ± 142.63 186.79 ± 145.68 184.44 ± 139.93 ns 
 Accomplish steps 34.44 ± 18.44 31.42 ± 17.95 42.58 ± 44.36 37.25 ± 30.89* 30.89 ± 15.24 c > a, d 
 Error steps 1.88 ± 2.54 1.60 ± 1.91 2.05 ± 2.52 1.93 ± 2.49* 0.76 ± 1.15 a, c > d 
VF test 
 Correct first 1 min 15.24 ± 4.31 16.60 ± 6.01 15.21 ± 4.65 15.32 ± 4.58 15.06 ± 4.84 ns 
 Correct last 1 min 5.57 ± 3.14 5.88 ± 3.24 5.43 ± 2.63 5.53 ± 2.95* 6.59 ± 3.07 a, c < d 
 Total correct 20.81 ± 6.08 22.52 ± 8.13 20.63 ± 5.58 20.85 ± 6.04 21.67 ± 6.76 ns 
 Repeat responses 0.86 ± 1.24 1.20 ± 1.60 1.15 ± 1.72 1.00 ± 1.47* 0.52 ± 0.71 a, b, c > d, c > a 
 Error responses 0.08 ± 0.38 0.12 ± 0.44 0.11 ± 0.35 0.09 ± 0.37 0.06 ± 0.23 ns 
Deceptive container task 
 Representation (C/W) 183/17 23/2 134/16 340/35 119/6 ns 
 Own false belief (C/W) 169/31 20/5 122/28 311/64* 123/2 a, b, c < d 
 Other's false belief (C/W) 169/31 20/5 121/29 310/65* 121/4 a, b, c < d 
False-belief task (C/W) 157/43 18/7 103/47 278/97* 112/13 a, b, c < d 
EF tests ADHD subtypes
 
ADHD total (n=375) HCd (n=125) Comparison (p<.05) 
ADHD-Ia(n =200) ADHD-HIb (n =25) ADHD-Cc (n =150) 
M ± SD M ± SD M ± SD M ± SD M ± SD 
Stroop 
 Part 1 time (s) 20.17 ± 6.78 19.12 ± 7.92 20.32 ± 6.44 20.16 ± 6.72* 17.92 ± 5.55 a, c > d 
 Part 2 time (s) 28.12 ± 9.84 24.96 ± 8.60 28.03 ± 9.62 27.87 ± 9.68* 23.30 ± 6.89 a, c > d 
 Part 3 time (s) 26.34 ± 10.73 25.00 ± 11.15 25.81 ± 9.03 26.04 ± 10.09* 21.76 ± 7.67 a, c > d 
 Part 4 time (s) 56.76 ± 19.11 50.08 ± 19.30 57.62 ± 19.72 56.66 ± 19.40* 47.10 ± 14.38 a, c > b, d 
 Part 1 error 0.30 ± 0.60 0.12 ± 0.33 0.29 ± 0.86 0.28 ± 0.70* 0.09 ± 0.36 a, c > d 
 Part 2 error 0.32 ± 0.72 0.48 ± 0.96 0.31 ± 0.68 0.33 ± 0.72* 0.15 ± 0.42 b > d 
 Part 3 error 0.56 ± 1.27 0.56 ± 0.87 0.51 ± 0.96 0.54 ± 1.13* 0.20 ± 0.52 a, c > d 
 Part 4 error 2.21 ± 3.09 1.64 ± 1.57 2.20 ± 2.69 2.17 ± 2.86* 0.66 ± 1.09 a, c > d 
 Color interference 6.17 ± 7.70 5.88 ± 8.41 5.48 ± 5.95 5.88 ± 7.10* 3.84 ± 5.23 a, c > d 
 Word interference 28.64 ± 14.15 25.12 ± 12.93 29.59 ± 15.12 28.78 ± 14.47* 23.79 ± 10.55 a, c > d 
RCFT 
 Structure immediate 2.93 ± 2.00 2.68 ± 2.05 2.45 ± 2.03 2.72 ± 2.02* 3.37 ± 2.02 c < a, d 
 Structure delay 2.79 ± 2.04 2.80 ± 2.04 2.41 ± 2.12 2.64 ± 2.07* 3.47 ± 2.01 c < a < d 
 Structure forgotten 0.14 ± 0.98 −0.12 ± 0.78 0.04 ± 0.93 0.08 ± 0.95 −0.10 ± 0.84 a > d 
 Detail immediate 9.80 ± 6.30 9.04 ± 6.34 8.27 ± 5.93 9.14 ± 6.18* 12.01 ± 6.92 a, b, c < d; c < a 
 Detail delay 9.26 ± 6.31 8.60 ± 5.78 7.85 ± 5.94 8.65 ± 6.15* 12.02 ± 7.26 a, b, c < d; c < a 
 Detail forgotten 0.55 ± 2.40 0.44 ± 2.12 0.42 ± 2.37 0.49 ± 2.37* −0.02 ± 2.18 a > d 
Trail making test 
 Part 1 time (s) 64.03 ± 28.54 58.36 ± 31.40 61.71 ± 24.31 62.73 ± 27.10* 52.62 ± 18.55 a, c > d 
 Part 2 time (s) 228.42 ± 157.76 184.44 ± 116.65 210.21 ± 118.24 218.20 ± 140.87* 150.42 ± 76.26 a, c > d 
 Shift time 164.38 ± 145.94 126.08 ± 113.87 148.50 ± 110.41 155.47 ± 130.92* 97.80 ± 66.45 a, c > d 
 Part 1 error 0.27 ± 0.58 0.20 ± 0.40 0.29 ± 0.70 0.27 ± 0.62 0.15 ± 0.36 ns 
 Part 2 error 1.64 ± 2.09 1.64 ± 1.72 1.48 ± 1.86 1.57 ± 1.98* 0.78 ± 1.14 a, b, c > d 
Tower of Hanoi 
 Complete/fail 147/53 19/6 98/52 264/111* 103/22 c < d 
 Initiation time (s) 0.90 ± 2.72 0.24 ± 1.01 1.05 ± 3.32 0.91 ± 2.90* 4.20 ± 20.61 a < d 
 Accomplish time (s) 187.14 ± 151.85 160.00 ± 111.97 191.46 ± 142.63 186.79 ± 145.68 184.44 ± 139.93 ns 
 Accomplish steps 34.44 ± 18.44 31.42 ± 17.95 42.58 ± 44.36 37.25 ± 30.89* 30.89 ± 15.24 c > a, d 
 Error steps 1.88 ± 2.54 1.60 ± 1.91 2.05 ± 2.52 1.93 ± 2.49* 0.76 ± 1.15 a, c > d 
VF test 
 Correct first 1 min 15.24 ± 4.31 16.60 ± 6.01 15.21 ± 4.65 15.32 ± 4.58 15.06 ± 4.84 ns 
 Correct last 1 min 5.57 ± 3.14 5.88 ± 3.24 5.43 ± 2.63 5.53 ± 2.95* 6.59 ± 3.07 a, c < d 
 Total correct 20.81 ± 6.08 22.52 ± 8.13 20.63 ± 5.58 20.85 ± 6.04 21.67 ± 6.76 ns 
 Repeat responses 0.86 ± 1.24 1.20 ± 1.60 1.15 ± 1.72 1.00 ± 1.47* 0.52 ± 0.71 a, b, c > d, c > a 
 Error responses 0.08 ± 0.38 0.12 ± 0.44 0.11 ± 0.35 0.09 ± 0.37 0.06 ± 0.23 ns 
Deceptive container task 
 Representation (C/W) 183/17 23/2 134/16 340/35 119/6 ns 
 Own false belief (C/W) 169/31 20/5 122/28 311/64* 123/2 a, b, c < d 
 Other's false belief (C/W) 169/31 20/5 121/29 310/65* 121/4 a, b, c < d 
False-belief task (C/W) 157/43 18/7 103/47 278/97* 112/13 a, b, c < d 

Note: ADHD total = ADHD-I + ADHD-HI + ADHD-C; M =mean; SD =standard deviation; RCFT = Rey-Osterreith Complex Figure Test; C/W = correct answer/wrong answer; ns = group contrast is not applicable as p > .05.

aADHD predominantly inattentive subtype.

bADHD predominantly hyperactivity-impulsive subtype.

cADHD combined type.

dHealth control.

*p < .05, comparison between ADHD total and HC group.

ADHD subtypes

The demographic information of the ADHD-I, ADHD-HI, ADHD-C, and HC groups showed that there were no significant differences (p > .05) among them in age (9.95 ± 2.08, 10.05 ± 2.06, 10.00 ± 2.07, 9.99 ± 2.08, respectively) or IQ (108.90 ± 11.39, 108.20 ± 11.59, 118.47 ± 11.80, 110.87 ± 10.67, respectively). The EF performances of the groups are summarized in Table 1. The ADHD-I and ADHD-C groups exhibited the same degree of poor performance in the Stroop test, the TMT, the VF, the deceptive container test and the false-belief task. In the structural aspect of the RCFT, the ADHD-C group obtained lower scores than either the HC or ADHD-I groups for immediate memory [F(3, 494) = 5.43, p < .01, forumla] and delayed recall [F(3,494) = 6.88, p < .01, forumla]. The ADHD-I group also obtained lower scores than the HC group for delayed recall (p < .01). In the detailed aspect of the RCFT, the ADHD-C group obtained significantly lower scores than either the HC or ADHD-I groups for immediate memory [F(3, 494) = 9.89, p < .01,forumla] and delayed recall [F(3, 494) = 12.59, p < .01, forumla], and the ADHD-I group also had lower scores than the HC group (p < .01). The ADHD-I group forgot more structural and detailed components than the HC group (p < .05). Fewer of the ADHD-C group than the HC group were able to successfully complete the ToH task (p < .01), and the ADHD-C group required more steps than both the HC and the ADHD-I group to complete it [F(3, 361) = 3.64, p < .05, forumla]. Both the ADHD-I and ADHD-C groups committed more rule violations than the HC group [F(3, 494) = 8.49, p < .01, forumla]. Finally, compared with the HC group, the ADHD-HI group made more errors in Part 2 of the Stroop test, had lower detailed scores in both the immediate and delayed parts of the RCFT, made more errors in Part 2 of the TMT, gave more repeat responses in the VF test, and gave more wrong answers in the false-belief task (p < .05). The ADHD-HI group also had a tendency to make more errors in Part 4 of the Stroop test than the HC group (p = .09).

ADHD only and HC

There were no significant differences in age (10.24 ± 2.40 vs. 10.21 ± 2.30) or IQ (107.22 ± 12.27 vs. 109.49 ± 11.45) between the ADHD-only and HC groups (p > .05). The ADHD-only group performed significantly worse in the EF tests (see Table 2). Compared with the HC group, the children with ADHD only took longer time to complete Part 2 [F(1, 148) = 21.63, p < .01, forumla], Part 3 [F(1, 148) = 9.73, p < .01, forumla], and Part 4 [F(1, 148) = 10.15, p < .01, forumla]; made more errors in Part 3 [F(1, 148) = 5.09, p < .05, forumla] and Part 4 [F(1, 148) = 14.24, p < .01, forumla]; and displayed greater color interference [F(1, 148) = 10.02, p < .01, forumla ] in the Stroop test. They also took longer in both parts of the TMT [Part A: F(1, 148) = 6.47, p < .05, forumla; Part B: F(1, 148) = 10.54, p < .01, forumla], had a longer shift time [F(1, 148) = 8.24, p < .01, forumla], and made more errors in Part B [F(1, 148) = 6.00, p < .05, forumla]. They made more error steps in the ToH [F(1, 148) = 14.65, p < .01, forumla]; and produced fewer correct words in the second minute [F(1, 148) = 5.50, p < .05, forumla] and gave more repeat [F(1, 148) = 9.16, p < .01, forumla] in the VF test. Finally, they performed worse in both the deceptive container test and the false-belief task (p < .05).

Table 2.

Executive function of ADHD only, ADHD comorbid ODD/CD, ADHD comorbid LD, and health control groups

EF tests ADHD comorbidities
 
ADHD onlyc (n =76) HCd (n =76) Comparison (p < .05) 
ADHD + ODD/CDa (n =38) ADHD + LDb (n =38) 
M ± SD M ± SD M ± SD M ± SD 
Stroop 
 Part 1 time (s) 19.26 ± 6.78 21.21 ± 9.77 19.18 ± 5.68 18.32 ± 6.51 b > c, d 
 Part 2 time (s) 26.82 ± 9.07 29.21 ± 13.68 28.08 ± 10.21 22.78 ± 7.21 a, b, c > d 
 Part 3 time (s) 25.21 ± 7.82 26.26 ± 9.94 25.04 ± 10.47 21.13 ± 7.92 a, b, c > d 
 Part 4 time (s) 51.82 ± 15.23 65.61 ± 28.02 53.61 ± 18.55 46.51 ± 15.74 b, c > d; b > a, c 
 Part 1 error 0.32 ± 0.70 0.24 ± 0.43 0.21 ± 0.55 0.13 ± 0.44 ns 
 Part 2 error 0.42 ± 1.08 0.42 ± 0.86 0.29 ± 0.65 0.16 ± 0.46 ns 
 Part 3 error 0.50 ± 0.83 0.71 ± 1.16 0.63 ± 1.38 0.22 ± 0.58 b, c > d 
 Part 4 error 2.11 ± 3.04 2.95 ± 3.40 2.50 ± 3.60 0.86 ± 1.38 a, b, c > d 
 Color interference 5.95 ± 6.69 5.05 ± 6.47 5.86 ± 7.37 2.82 ± 4.71 a, c > d 
 Word interference 25.00 ± 12.32 36.39 ± 18.52 25.53 ± 13.89 23.74 ± 10.96 b > a, c, d 
RCFT 
 Structure immediate 2.89 ± 2.19 3.13 ± 2.08 3.22 ± 2.11 3.32 ± 2.02 ns 
 Structure delay 2.95 ± 2.18 2.89 ± 2.25 3.08 ± 2.34 3.37 ± 2.10 ns 
 Structure forgotten −0.05 ± 0.93 0.24 ± 0.79 0.14 ± 0.67 −0.05 ± 0.83 b > d 
 Detail immediate 9.87 ± 7.41 9.50 ± 6.74 10.24 ± 7.43 11.74 ± 6.78 b < d 
 Detail delay 9.76 ± 7.55 8.61 ± 6.80 9.95 ± 7.52 11.53 ± 7.32 b < d 
 Detail forgotten 0.11 ± 2.40 0.89 ± 2.31 0.29 ± 1.60 0.21 ± 2.30 ns 
Trail making test 
 Part 1 time (s) 62.79 ± 32.16 67.05 ± 31.08 60.62 ± 31.22 51.32 ± 19.34 a, b, c > d 
 Part 2 time (s) 202.82 ± 138.03 256.95 ± 159.17 214.86 ± 150.96 153.93 ± 91.71 b > a, c > d 
 Shift time 140.03 ± 113.39 189.89 ± 137.21 154.24 ± 135.96 102.62 ± 81.59 b, c > d; b > a 
 Part 1 error 0.18 ± 0.56 0.34 ± 0.58 0.28 ± 0.70 0.21 ± 0.41 ns 
 Part 2 error 1.32 ± 1.71 1.79 ± 2.53 1.49 ± 1.98 0.80 ± 1.20 b, c > d 
Tower of Hanoi 
 Complete/fail 29/9 26/12 59/17 60/16 ns 
 Initiation time (s) 0.42 ± 1.13 0.37 ± 0.88 0.83 ± 3.23 4.54 ± 22.01 ns 
 Accomplish time (s) 224.03 ± 171.28 172.35 ± 122.64 179.56 ± 122.50 171.15 ± 151.28 ns 
 Accomplish steps 40.34 ± 21.27 33.15 ± 13.84 34.32 ± 15.64 28.80 ± 14.23 ns 
 Error steps 2.29 ± 2.75 2.00 ± 2.30 2.09 ± 2.67 0.82 ± 1.30 a, b, c > d 
VF test 
 Correct first 1 min 15.76 ± 4.51 15.74 ± 6.35 15.64 ± 4.56 15.42 ± 5.23 ns 
 Correct last 1 min 5.58 ± 3.13 5.11 ± 2.52 5.78 ± 3.38 7.00 ± 3.31 a, b, c < d 
 Total correct 21.37 ± 6.39 20.84 ± 7.94 21.39 ± 6.88 22.42 ± 7.56 ns 
 Repeat responses 0.79 ± 1.44 1.29 ± 1.94 1.18 ± 1.36 0.62 ± 0.78 b, c > d 
 Error responses 0.08 ± 0.27 0.08 ± 0.36 0.08 ± 0.32 0.07 ± 0.25 ns 
Deceptive container task 
 Representation (C/W) 36/2 36/2 70/6 73/3 ns 
 Own false belief (C/W) 28/10 30/8 64/12 73/3 a, b, c < d 
 Other's false belief (C/W) 29/9 30/8 65/11 72/4 a, b, c < d 
False-belief task (C/W) 29/9 25/13 57/19 70/6 a, b, c < d 
EF tests ADHD comorbidities
 
ADHD onlyc (n =76) HCd (n =76) Comparison (p < .05) 
ADHD + ODD/CDa (n =38) ADHD + LDb (n =38) 
M ± SD M ± SD M ± SD M ± SD 
Stroop 
 Part 1 time (s) 19.26 ± 6.78 21.21 ± 9.77 19.18 ± 5.68 18.32 ± 6.51 b > c, d 
 Part 2 time (s) 26.82 ± 9.07 29.21 ± 13.68 28.08 ± 10.21 22.78 ± 7.21 a, b, c > d 
 Part 3 time (s) 25.21 ± 7.82 26.26 ± 9.94 25.04 ± 10.47 21.13 ± 7.92 a, b, c > d 
 Part 4 time (s) 51.82 ± 15.23 65.61 ± 28.02 53.61 ± 18.55 46.51 ± 15.74 b, c > d; b > a, c 
 Part 1 error 0.32 ± 0.70 0.24 ± 0.43 0.21 ± 0.55 0.13 ± 0.44 ns 
 Part 2 error 0.42 ± 1.08 0.42 ± 0.86 0.29 ± 0.65 0.16 ± 0.46 ns 
 Part 3 error 0.50 ± 0.83 0.71 ± 1.16 0.63 ± 1.38 0.22 ± 0.58 b, c > d 
 Part 4 error 2.11 ± 3.04 2.95 ± 3.40 2.50 ± 3.60 0.86 ± 1.38 a, b, c > d 
 Color interference 5.95 ± 6.69 5.05 ± 6.47 5.86 ± 7.37 2.82 ± 4.71 a, c > d 
 Word interference 25.00 ± 12.32 36.39 ± 18.52 25.53 ± 13.89 23.74 ± 10.96 b > a, c, d 
RCFT 
 Structure immediate 2.89 ± 2.19 3.13 ± 2.08 3.22 ± 2.11 3.32 ± 2.02 ns 
 Structure delay 2.95 ± 2.18 2.89 ± 2.25 3.08 ± 2.34 3.37 ± 2.10 ns 
 Structure forgotten −0.05 ± 0.93 0.24 ± 0.79 0.14 ± 0.67 −0.05 ± 0.83 b > d 
 Detail immediate 9.87 ± 7.41 9.50 ± 6.74 10.24 ± 7.43 11.74 ± 6.78 b < d 
 Detail delay 9.76 ± 7.55 8.61 ± 6.80 9.95 ± 7.52 11.53 ± 7.32 b < d 
 Detail forgotten 0.11 ± 2.40 0.89 ± 2.31 0.29 ± 1.60 0.21 ± 2.30 ns 
Trail making test 
 Part 1 time (s) 62.79 ± 32.16 67.05 ± 31.08 60.62 ± 31.22 51.32 ± 19.34 a, b, c > d 
 Part 2 time (s) 202.82 ± 138.03 256.95 ± 159.17 214.86 ± 150.96 153.93 ± 91.71 b > a, c > d 
 Shift time 140.03 ± 113.39 189.89 ± 137.21 154.24 ± 135.96 102.62 ± 81.59 b, c > d; b > a 
 Part 1 error 0.18 ± 0.56 0.34 ± 0.58 0.28 ± 0.70 0.21 ± 0.41 ns 
 Part 2 error 1.32 ± 1.71 1.79 ± 2.53 1.49 ± 1.98 0.80 ± 1.20 b, c > d 
Tower of Hanoi 
 Complete/fail 29/9 26/12 59/17 60/16 ns 
 Initiation time (s) 0.42 ± 1.13 0.37 ± 0.88 0.83 ± 3.23 4.54 ± 22.01 ns 
 Accomplish time (s) 224.03 ± 171.28 172.35 ± 122.64 179.56 ± 122.50 171.15 ± 151.28 ns 
 Accomplish steps 40.34 ± 21.27 33.15 ± 13.84 34.32 ± 15.64 28.80 ± 14.23 ns 
 Error steps 2.29 ± 2.75 2.00 ± 2.30 2.09 ± 2.67 0.82 ± 1.30 a, b, c > d 
VF test 
 Correct first 1 min 15.76 ± 4.51 15.74 ± 6.35 15.64 ± 4.56 15.42 ± 5.23 ns 
 Correct last 1 min 5.58 ± 3.13 5.11 ± 2.52 5.78 ± 3.38 7.00 ± 3.31 a, b, c < d 
 Total correct 21.37 ± 6.39 20.84 ± 7.94 21.39 ± 6.88 22.42 ± 7.56 ns 
 Repeat responses 0.79 ± 1.44 1.29 ± 1.94 1.18 ± 1.36 0.62 ± 0.78 b, c > d 
 Error responses 0.08 ± 0.27 0.08 ± 0.36 0.08 ± 0.32 0.07 ± 0.25 ns 
Deceptive container task 
 Representation (C/W) 36/2 36/2 70/6 73/3 ns 
 Own false belief (C/W) 28/10 30/8 64/12 73/3 a, b, c < d 
 Other's false belief (C/W) 29/9 30/8 65/11 72/4 a, b, c < d 
False-belief task (C/W) 29/9 25/13 57/19 70/6 a, b, c < d 

Note: M =mean; SD =standard deviation; RCFT = Rey-Osterreith Complex Figure Test; C/W = correct answer/wrong answer; ns = group contrast is not applicable as p > .05.

aADHD comorbid ODD or CD.

bADHD comorbid LD.

cADHD only group.

dHealth control.

*p < .05, comparison between 108 boys with ADHD only and 108 HC boys which are mathched by age and IQ.

ODD/CD and LD Comorbids

There were no significant differences (p > .05) in age (10.34 ± 2.53, 10.38 ± 2.56, 10.24 ± 2.40, 10.21 ± 2.30, respectively) or IQ (107.97 ± 11.94, 106.92 ± 12.18, 107.22 ± 12.27, 109.49 ± 11.45, respectively) among the ADHD + ODD/CD, ADHD + LD, ADHD-only, and HC groups. The EF performances of the four groups were summarized in Table 2. Compared with the HC group, the ADHD + ODD/CD group performed significantly worse in all of the EF tests except RCFT, whereas the ADHD + LD group performed significantly worse in all of the EF tests. In addition, the ADHD + LD group took longer to finish Part 4 of the Stroop [F(3, 222) = 14.42, p < .01, forumla] and Part B of the TMT [F(3, 222) = 8.00, p < .01, forumla], and had greater word interference [F(3, 222) = 10.25, p < .01, forumla] than the ADHD + ODD/CD and ADHD-only group. The ADHD + LD group also had a longer shift time in the TMT than the ADHD + ODD/CD group (p < .05), and took longer to finish Part 1 of the Stroop than the ADHD-only group (p < .05).

Discussion

This study investigated the five domains of EF in a large sample of Chinese boys with ADHD, and considered the features of the different ADHD subtypes and comorbidities. We controlled age and IQ strictly to eliminate the influence of these factors on EF. The major findings can be summarized as follows. (i) EF was much more impaired in boys with ADHD compared with age- and IQ-matched HC. (ii) After controlling for comorbidity, the ADHD-only group still demonstrated EF impairments. (iii) The ADHD-I and ADHD-C groups displayed similar degrees of impairment in the EF tests for inhibition, shifting, VF, and theory of mind. Further, the ADHD-C group demonstrated more serious deficits in the tests for planning and working memory than the ADHD-I group. The ADHD-HI group primarily displayed deficits in the tests for working memory and theory of mind. (iv) Comorbid LD increased the inhibition and shifting deficits of the ADHD-only group, but comorbid ODD/CD caused no decrement in EF.

ADHD versus HC

Compared with the HC, the boys with ADHD exhibited poorer performance across all of the EF tests. The results suggest that EF deficits are an integral component of ADHD and support the findings of previous research (Barkley, 1997; Pennington & Ozonoff, 1996; Seidman et al., 2005; Sergeant et al., 2002; Shallice et al., 2002; Willcutt et al., 2005).

The results are also consistent with the findings of previous studies that have used the Stroop test to assess inhibition (Barkley, 1999; Houghton et al., 1999; Kerns, McInerney, & Wilde, 2001; Klorman et al., 1999; Seidman et al., 2006; Wodka et al., 2007; Wu, Anderson, & Castiello, 2006); the RCFT to measure visual working memory (Mahone et al., 2002; Seidman et al., 2005) or other tests to measure nonverbal memory (Cornoldi et al., 2001; Goldberg et al., 2005); the TMT to evaluate shifting (Murphy, 2002); and the ToH or other tower task to assess planning (Klorman et al., 1999; Papadopoulos, Panayiotou, Spanoudis, & Natsopoulos, 2005).

The participants with ADHD exhibited poorer performance across all four conditions in the Stroop test and both parts of the TMT. The results indicate that the specific deficits related to ADHD include not only basic processing speed (Wu et al., 2006), but also the higher level types of cognitive function involved in these tasks, such as interference inhibition (Doyle, Biederman, Seidman, Reske-Nielsen, & Faraone, 2005; Homack & Riccio, 2004) and efficient shifting (Murphy, 2002).

In the RCFT, the participants with ADHD had difficulty not only in grasping the structural concepts of complex stimuli such as RCF (Shin, Kim, Cho, & Kim, 2003), but also in memorizing detailed segments, which requires the activation of the short-term perceptual working memory for the retention of information (Sami, Carte, Hinshaw, & Zupan, 2003). Although the ADHD group performed worse in both the immediate and delayed trail tests, there was no additional loss of information after a 20 min delay, which is in line with the findings of earlier studies (Wood, Ebert, & Kinsboume, 1982). Barnett, Maruff, and Vance (2005) reported that this delay-independent memory impairment in ADHD indicates a dysfunction in encoding, rather than in the retrieval phase, of visuospatial memory.

The boys with ADHD showed an inability to generate and execute their own plans efficiently when engaged in cognitively challenging tasks such as the ToH (Culbertson & Zillmer, 1998; Murphy, 2002). However, although the ADHD group required more moves to solve the tasks, they did not take any longer. This is similar to the findings of Murphy and colleagues (2002), who reported that children with ADHD were unable to visualize correct moves, but rather kept moving the discs until they stumbled upon the solution. As in other studies using tower tasks (Culbertson & Zillmer, 1998; Oosterlaan, Scheres, & Sergeant, 2005; Sarkis, Sarkis, Marshall, & Archer, 2005), the participants with ADHD in this study committed more rule violations than the HC. Rule-breaking may represent disinhibition or memory lapse.

This study did not identify any significant difference in total correct number of words in the semantic VF test between the ADHD and HC groups. Previous studies suggest that VF tests are not very sensitive to ADHD (Pennington & Ozonoff, 1996; Sergeant et al., 2002). With the aim of measuring performance over time, we recorded correct word production in two periods. At first, a ready pool of frequently used words appeared to be available for production. As time passed, however, this pool became exhausted and the search for new words became effortful (Hurks et al., 2004). We found that in the second minute, the ADHD group produced significantly fewer words than the HC, indicating that the boys with ADHD were unable to systematically search for words from their memory store as efficiently as the HC. This result is inconsistent with that of a previous study (Hurks et al., 2004) that found participants with ADHD had more problems finding words in the first 15 s of “automatic-processing” than during the “effort-processing” period. This inconsistency may be due to the different ways that the two studies split the time period. Repetition and error responses also provide important information (Tucha et al., 2005). In our study, the ADHD group made more repetitions than the HC group, which is slightly different from the results of earlier studies (Hurks et al., 2004; Loge, Station, & Beatty, 1990), although this contradiction may simply be due to the use of different language versions of the tests.

In the deceptive container and false-belief tasks, the boys with ADHD were able to extract the object's representation as well as the HC, but had obvious difficulties in understanding others’ false beliefs or in recalling their own false beliefs. Although studies that investigate hot EF are scarce and their conclusions inconsistent (Perner, Kain, & Barchfeld, 2002), this study clearly shows that Chinese boys with ADHD had deficits in understanding the mental states of others.

After controlling for comorbidity, the HC group still outperformed the age- and IQ-matched ADHD-only group in the Stroop test, TMT, VF test, theory of mind task, and rule violation in the ToH, indicating that EF deficits are part and parcel of ADHD (Oosterlaan et al., 2005; Seidman et al., 2006). These results support the proposition that the impairments displayed in the tests of inhibition (Bitsakou, Psychogiou, Thompson, & Sonuga-Barke, 2008; Seidman et al., 2005, 2006; Sonuga-Barke et al., 2002), shifting, VF, theory of mind, and adhering to rule constraints (Culbertson & Zillmer, 1998), in children with ADHD go beyond the IQ and comorbidity (Weyandt, 2005).

Subtypes

The age- and IQ-matched ADHD-I and ADHD-C groups displayed similar degrees of impairment in the EF tests for inhibition, shifting, VF, and theory of mind. The ADHD-C group demonstrated more serious deficits in the tests for planning and working memory than the ADHD-I group. The ADHD-HI group primarily displayed deficits in the tests for visual memory and theory of mind.

Earlier studies have similarly failed to find reliable differences between ADHD-I and ADHD-C groups on measures of inhibition and cognitive flexibility (Geurts et al., 2005; Riccio et al., 2006). A study of response inhibition carried out by Chhabildas and colleagues (2001) showed that the symptoms of inattention, but not of hyperactivity/impulsivity, accounted for the response inhibition deficit in children with ADHD. This result conflicts with Barkley's model (1997), which shows a deficit in EF related to ADHD-C but not ADHD-I.

This study found that the ADHD-C group performed worse than the ADHD-I group in the RCFT and ToH, which is in line with previous studies investigating visual memory (Lockwood et al., 2001) and planning (Klorman et al., 1999; Kopecky et al., 2005; Nigg et al., 2002; Riccio, Wolfe, Romine, Davis, & Sullivan, 2004). The ADHD-C group's poor performance in the RCFT indicates that they had difficulty in retaining and producing complex visual information, which involves self-generated planning and organization (Lockwood et al., 2001). The ADHD-C group performed worse in the ToH than the HC, but the ADHD-I group did not. This indicates that the boys with ADHD-C had obvious deficits in planning an appropriate solution for the task, but that their ADHD-I counterparts did not. This result is similar to that observed by Nigg and colleagues (2002), who found that ADHD-C groups, but not ADHD-I groups, had lower Tower of London scores than healthy children.

The other ADHD subtype, ADHD-HI, was linked with deficits in fewer EF areas than the ADHD-I and ADHD-C subtypes, primarily in the tests for visual memory and theory of mind. There have been few studies of EF in ADHD-HI. Schmitz and colleagues (2002) carried out a study of adolescents with ADHD and found that participants with ADHD-HI did not differ from the controls in the measures of working memory (Digit Span) or set shifting (Wisconsin Card Sorting Test), but that participants with ADHD-I and ADHD-C did exhibit deficits. The authors thus claimed that neuropsychological impairment occurs only in the ADHD subtypes in which inattention is clinically significant (Schmitz et al., 2002). Other studies have also failed to find inhibition or shifting deficits in children with ADHD-HI (Bedard et al., 2003; Chhabildas et al., 2001). Although this study used different methods to assess EF, the ADHD-HI group showed less impairment in the cool EF tests for inhibition, working memory, shifting, and planning than the other two subtype groups. However, as was the case with the ADHD-I and ADHD-C groups, the ADHD-HI group gave more wrong answers in the hot EF tests, which indicates that children with ADHD-HI cannot understand the mental activity of others as well as HC.

In the RCFT, the ADHD-HI group achieved lower detailed scores than the HC, but comparable structural scores. This suggests that their impulsivity or hyperactivity might have made them too impatient to remember the detailed components correctly, but that they were able to grasp the structural segments successfully. Although the ADHD-HI group made more errors than the HC in Part B of the TMT, there was no difference in the time spent completing the task. The situation for Part 4 of the Stroop test was almost the same. The ADHD-HI group did not take any longer to complete the task, but had a tendency to make more errors than the HC. We assume that in the effortful, executive-control demand part of the test, the children with ADHD-HI were likely to have made a speed-accuracy trade-off (Wu et al., 2006); that is, they were in a hurry to finish the test regardless of correctness.

Comorbidity

The ADHD + ODD/CD group showed no distinctive performance pattern in the EF tests compared with the ADHD-only group. This is in line with the results of a meta-analysis in which an ADHD-only group could not be differentiated from an ADHD + ODD/CD group in tasks that required inhibition, working memory, flexibility, or fluency (Geurts, Verte, Oosterlaan, Roeyers, & Sergeant, 2004). This may either be because the EF deficits in ADHD are more marked than those in ODD/CD (Clark et al., 2000; Kooijmans, Scheres, & Oosterlaan, 2000), or because the children with ODD/CD do not have EF dysfunction (Barkley, 1997; van Goozen et al., 2004). In our view, ODD/CD is not associated with worse EF deficits than the ADHD-only condition.

The ADHD + LD group exhibited a worse performance in the tests for inhibition and shifting than either the ADHD-only group or the ADHD + ODD/CD group. Seidman et al. also found ADHD with LD performed worse in the Stroop tests than ADHD without LD (Seidman et al., 2001, 2006). Other results have also indicated that children with ADHD + LD display more severe deficits in tests of EF than the ADHD-only group (Seidman et al., 2001; Willcutt et al., 2001). It is clear from our data that comorbid LD has an effect on EF, and especially inhibition and shifting, in children with ADHD.

Limitations

A few limitations of this study should be noted. Although we used seven neuropsychological tests to assess hot and cool EF, only the RCFT was used to assess visual memory in the working memory part. Visuo-spatial working memory is also very important in ADHD. Future studies could cover the aspects of EF more comprehensively. Meanwhile, the assessment time horizon needs to be appropriate for children's range of tolerance. All of the EF tests were converted to a Chinese version. Thus, in analyzing the results, cultural bias must be taken into account. Further, the neuropsychological tests used cannot completely represent an individual's entire EF. In future research, it may be appropriate to use questionnaires to collect information on the children's EF in real-life scenarios from their parents and teachers.

Another limitation is that when we compared the performance of boys with different ADHD subtypes in the EF tests, the ADHD-HI sample was much smaller than the ADHD-I and ADHD-C groups. Clearly, we need to continue collecting data on children with ADHD-HI to confirm the EF characteristics of this subtype.

Further, we observed the EF impairment of ADHD with comorbid LD, but did not further refine LD into, for example, reading disorders or arithmetic disorders. This means that we were unable to explore the influence of specific types of LD on EF in ADHD. We were also unable to include an ODD/CD or LD clinical group as a comparison. If these clinical groups would be included in the future research, the conclusions drawn about the EF characteristics of ADHD, ODD/CD, and LD would be clearer and more definite.

Finally, we did not include a female sample in the study. Whether girls with ADHD have EF impairment or different patterns of EF deficits from boys with ADHD remains unclear. We intend to investigate the EF characteristic of Chinese children with ADHD of both genders in our next study.

Conclusions

Taken together, the results of this study clearly indicate that EF weaknesses, including inhibition, working memory, set shifting, planning, VF, and theory of mind, are significantly associated with ADHD in children. After controlling for comorbidity, the ADHD-only group still performed worse than the age- and IQ-matched HC group, thus demonstrating that EF impairment is clearly associated with ADHD. Our findings support Barkley's model, which shows ADHD to be linked to EF deficits that are independent of the presence or absence of comorbidity, although we are unable to find support for his model of ADHD subtypes (Barkley, 1997). Compared with HC group, both the ADHD-I and ADHD-C groups showed EF impairments, with the ADHD-C group exhibiting worse performance in planning and visual memory tasks than the ADHD-I group. The ADHD-HI group primarily displayed deficits in theory of mind and visual detailed memory. ODD/CD comorbidity is not found to be affecting EF impairment in the ADHD-only group, but comorbid LD seems to increase the severity of inhibition and shifting in ADHD-only cases.

Funding

This study was supported in part by grants from the National Foundation of the Ministry of Science and Technology, China (Grant number: 2007BAI17B03) and the Common Sciences Foundation of the Ministry of Health, China (Grant number: 200802073).

Conflict of Interest

None declared.

Acknowledgements

We would like to thank Qiaoxin Du and Rui Yang for their contributions to the study and the parents and children who participated in it.

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