do executive function deficits differentiate between adolescents with adhd and oppositional...

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Journal of Abnormal Child Psychology Pl118-38 August 7, 2000 11:9 Style file version July 26, 1999

Journal of Abnormal Child Psychology, Vol. 28, No. 5, 2000, pp. 403–414

Do Executive Function Deficits Differentiate BetweenAdolescents with ADHD and Oppositional Defiant/ConductDisorder? A Neuropsychological Study Using the SixElements Test and Hayling Sentence Completion Test

Cheryl Clark, 1 Margot Prior, 2 and Glynda J. Kinsella1

Received March 8, 1999; revision received February 21, 2000; accepted February 22, 2000

Two neuropsychological measures of executive functions—Six Elements Tests (SET) and HaylingSentence Completion Test (HSCT)—were administered to 110 adolescents, aged 12–15 years. Par-ticipants comprised four groups: Attention Deficit Hyperactivity Disorder (ADHD) only (n = 35),ADHD and Oppositional Defiant Disorder/Conduct Disorder (ODD/CD) (n = 38), ODD/CD only(n = 11), and a normal community control group (n = 26). Results indicated that adolescents withADHD performed significantly worse on both the SET and HSCT than those without ADHD, whetheror not they also had ODD/CD. The adolescents with ADHD and with comorbid ADHD and ODD/CDwere significantly more impaired in their ability to generate strategies and to monitor their ongo-ing behavior compared with age-matched controls and adolescents with ODD/CD only. It is arguedthat among adolescents with clinically significant levels of externalizing behavior problems, execu-tive function deficits are specific to those with ADHD. The findings support the sensitivity of thesetwo relatively new tests of executive functions and their ecological validity in tapping into everydaysituations, which are potentially problematic for individuals with ADHD.

KEY WORDS: executive functions; attention deficit hyperactivity disorder; conduct disorder.

INTRODUCTION

Executive function (EF) deficits have been postulatedas important neuropsychological correlates of AttentionDeficit Hyperactivity Disorder (ADHD) (Pennington &Ozonoff, 1996). Executive functions involve control pro-cesses such as goal-oriented planning, flexible strategygeneration, sustaining set maintenance, self-monitoring,and inhibition (Luria, 1973; Shallice, 1988; Welsh &Pennington, 1988). These cognitive processes are fre-quently associated with the prefrontal cortex, which is con-sidered to be the prime neural substrate mediating adaptivebehavior, social conduct, flexibility of thought and action,and goal attainment (Dennis, 1991; Grattan & Eslinger,

1School of Psychological Science, La Trobe University, Victoria,Australia, 3083.

2Department of Psychology, Royal Children’s Hospital, Parkville,Australia, 3052.

1991; Luria, 1973; Shallice, 1988). Findings of advancedneuroimaging research that indicate subtle neurobiolog-ical abnormalities in children with ADHD (Castellanoset al., 1994; Tannock, 1998) support the involvement ofthe prefrontal cortex and its interconnections with subcor-tical regions.

Despite these findings, the nature of the neuropsy-chological deficits specifically associated with ADHD re-main elusive, most likely, for at least two important rea-sons. The first involves the longstanding issue of thedistinction between ADHD and Oppositional defiant dis-order (ODD) or conduct disorder (CD) or both, with highco-occurrence of these two disorders. Reported prevalencerates of comorbid ADHD and CD range approximatelybetween 40% and 90% (see review by Jensen, Martin, &Cantwell, 1997). Despite this, there is substantial evidenceto suggest that these behavioral syndromes can be inde-pendent disorders (Schachar, 1991) with differential cor-relates; ADHD being related more to cognitive deficits and

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404 Clark, Prior, and Kinsella

CD to psychosocial factors such as dysfunctional familysystems and low family income (Abikoff & Klein, 1992;Hinshaw, 1992; Schachar & Logan, 1990). Nevertheless,as Moffitt (1993) argues, the observed similarity betweenthe behavior of delinquents and criminals, on the one hand,and the disinhibited, antisocial behavior of patients withfrontal lobe injury, on the other, continues to provide arationale for research into neuropsychological correlatesof ODD/CD, in a similar way as do the parallels betweenthe behavioral symptoms of ADHD and those of patientswith frontal lobe injury.

The second reason is that EFs reflect an array of cor-related, but distinct cognitive processes, not all of whichmay be associated with ADHD. Several neuropsycholog-ical tests have been developed to measure EFs. However,despite the general consensus that EFs do not representa unitary construct, and hence, different tests most likelytap different aspects of executive functioning, the speci-ficity of the EF deficit in ADHD, as opposed to a moregeneralized deficit, has not been addressed.

The use of different sample populations (clinical orcommunity-based), different EF measures across studies,the comorbidity of ADHD and ODD/CD, and the likelyrange in severity of symptoms are possible explanationsfor mixed findings in the neuropsychological research intodisruptive behaviors. Giancola’s review (Giancola, 1995)concluded that the prefrontal cortex and EFs are asso-ciated with antisocial behavior, whether it be physicalaggression or disinhibited–nonaggressive behavior. Us-ing a clinically referred sample and a composite depen-dent variable derived from various measures, Giancola,Mezzich, and Tarter (1998) found that female adolescentswith CD exhibited poorer EF capacity than controls did,and this relationship was not attenuated after confound-ing factors, particularly ADHD, were controlled. In con-trast, among a group of referred children, aged 5–12 years,McBurnett et al. (1995) found EF deficits to be associatedwith ADHD symptoms and not aggressive–defiant behav-ior, using a single EF measure. A meta-analysis of eightstudies (Oosterlaan, Logan, & Sergeant, 1998) concludedthat EF deficits, specifically response inhibition, were notunique to ADHD when compared to CD (both performedpoorly compared to controls), although the findings ofpoor response inhibition among the CD groups from thevarious studies were inconsistent.

Methodological differences may also contribute tothe uncertainty surrounding the clinical identity and, speci-fically, the neuropsychological status of the comorbid con-dition, ADHD and ODD/CD. In the population-based NewZealand longitudinal study, significant neuropsychologi-cal deficits were found only among delinquent adoles-cents who also had ADHD (Moffitt & Henry, 1989). But,

as Moffitt (1993) points out, in a representative sampleof ODD/CD adolescents, where performance scores arepooled, group means may mask clinically significant dif-ferences. In another community sample, Leung andConnelly (1996) failed to find EF deficits between comor-bid ADHD and CD on a modified version of the StroopTest. Among clinically referred children, however, Nigg,Hinshaw, Carte, and Treuting (1998) found that the ADHDand comorbid ADHD plus CD groups exhibited similarEF deficits on some, but not all, EF measures used. Areview by Pennington and Ozonoff (1996) revealed thatEF deficits were consistently associated with ADHD, butnot CD, in both community and referred samples. Theseauthors speculate that although EF deficits in CD with-out ADHD are not empirically supported, the EF deficitsfound in comorbid ADHD/CD groups may play a crucialrole in the severity and persistence of antisocial behaviorin that group.

Traditional EF measures usually comprise a single,explicit problem, where the goal is provided by the ex-aminer and trial length is very short. In other words, self-regulation is not demanded of the examinee, and decisionsas to task goal, strategies to employ, and whether ongoingbehavior needs modification are not necessary. Shalliceand Burgess (1991) argued that a valid test of EFs must be aquantifiable analogue of the open-ended, problem-solvingsituations in everyday life where, although one’s generalintellectual abilities may be intact, the ability to integratethese skills to use them to organize, monitor, and regu-late behavior in carrying out real-life tasks is impaired.Deliberate attentional resources are required to generateexplicit intentions or goals and to ensure that ongoing be-havior complies with plans made at some earlier time andwith rules of behavior or social norms that are not cur-rently salient. In highly structured situations where rulesare explicit, and consequences are short-term and per-ceived by the child as directly related to inappropriate orrule-breaking behavior, hyperactive children can be calmand attentive (Draeger, Prior, & Sanson, 1986). In con-trast, an everyday situation in which the child with ADHDhas difficulties, for example, is when waiting his/her turn,whether during free play, conversation, or in loosely struc-tured games and classrooms. Where automatic behaviorneeds to be resisted and a new behavior sequence plannedand monitored, parents, clinicians, and researchers reportthat ADHD behavior is most likely to be seen (Campbell,Pierce, March, Ewing, & Szumowski, 1994; Hinshaw,Simmel, & Heller, 1995).

Situations in which children with ADHD appearto have difficulty and which require decision-making andself-monitoring are consistent with the conditions underwhich the Supervisory Attentional System (SAS) (Norman



Executive Functions in ADHD and CD 405

& Shallice, 1986; Shallice, 1982, 1988) is activated. Froman information-processing perspective, Norman andShallice (1986) have used the distinction between auto-matic and controlled processing to parallel the cognitiveprocessing required in routine, overlearned situations andin novel, problem-solving situations. The SAS is a theo-retical model of the cognitive processes purported to un-derlie goal-directed behavior necessary in the nonroutinesituation. It represents a set of cognitive processes involv-ing goal initiation, strategy generation in pursuit of thegoal, and evaluation of ongoing performance required toperform complex, nonroutine cognitive tasks. The SAS isactivated when thought and action schemas representingsubroutines capable of realizing a goal effectively can-not be selected through the automatic triggering by well-learned cues. The SAS plays a vital role in novel, problem-solving situations, where previously well-learned actionor thought sequences are inadequate or inappropriate, orwhere new behaviors need to be planned and monitoredfor satisfactory performance of the task.

Two tests have recently been developed that may havegreater promise of tapping the cognitive processes in thisarea. One is the Six Elements Task (SET) (Burgess et al.,1996), which is a simplified version of the original Shalliceand Burgess (1991) test, based on the theoretical model ofSAS. Shallice and Burgess (1991) demonstrated that theSET was sensitive to everyday problems associated withgoal-directed behavior. Their empirical work showed thatpatients who were unable to act effectively in everydaylife because they experienced impaired attentional con-trol, self-regulation of behavior, and utilization of feed-back in carrying out everyday tasks, but who could per-form satisfactorily on traditional tests considered sensitiveto frontal damage, performed poorly on the SET. The SEThas been found to reliably distinguish patients with frontallobe injury from normal controls, and significant correla-tions exist between frontal lobe patients’ SET scores andobservers’ objective ratings of their everyday executiveproblems (.40, p < .001) (Burgess et al., 1998). Becausenovelty is a critical feature of EF tests, test-retest relia-bility is often not high on these tests. The SET appearsa typical example as Burgess and his colleagues (1996)found moderate test-retest correlations for the SET, which,nevertheless, were similar to those of other EF measuresadministered at the same time. Unlike most tests of EFs,which are highly structured and in which the goals areprovided by the examiner, the SET transfers the tasks ofstructuring performance and decision-making to the re-spondent. It measures the individual’s ability to schedulehis or her performance of a number of simple tasks wherethe demands are open-ended and where there are a num-ber of simple rules, which apply across all subtasks. In

addition, Shallice and Burgess (1991) claim that, in usingthis task, the participant’s ability to plan, organize, andself-monitor can be studied in isolation from subordinateprocesses.

The other test in this assessment is the Hayling Sen-tence Completion Test (HSCT) (Burgess & Shallice, 1996,1997). The HSCT generates a measure of the ability togenerate a strategy in order to fulfil task requirements.Among 91 patients with frontal lesions, poor performanceson the HSCT were found among patients with anterior le-sions than among age- and IQ-matched patients with le-sions to other parts of the brain (Burgess & Shallice, 1996).Functional imaging evidence also indicates that activationof left prefrontal regions is associated with performanceon the HSCT (Nathaniel-James, Fletcher, & Frith, 1997).The HSCT also demands planning and strategy genera-tion. Burgess and Shallice (1996) argue that it is the fail-ure to generate appropriate strategies which underlies poorperformance on the HSCT and which leads to the produc-tion of errors.

In the present study, our goals were (a) to employtests that are designed to measure an individual’s ability toself-regulate, that is to plan and organize his/her responses,and to generate strategies which contribute to successfulperformance during ongoing goal-directed behavior and(b) to determine whether EF deficits are specific to ADHDwhen compared to conduct disorders.

If the difficulties of the child with ADHD involvepoor planning and poor self-monitoring abilities, it is im-portant that the research measures used to explicate theirdeficits demand such capacities, and for this, the Six El-ements Test (SET) and the Hayling Sentence CompletionTest (HSCT) were used in this study. We hypothesizedthat these two neuropsychological tasks would differen-tiate adolescents with an ADHD diagnosis from thosewithout ADHD. Thus, it was anticipated that participantswith ADHD only and those with co-occurring ADHDand ODD/CD would perform less well on both EF tasksthan the adolescents without ADHD, that is those withODD/CD only and a control group.

METHOD

Participants

Four groups of adolescents were used in this study:ADHD only group (n = 35), ODD/CD only group (n =11), group representing the co-occurrence of ADHD andODD/CD (n = 38), and a nonclinical comparison group(n = 26). ODD and CD were combined as the former dis-order is frequently found to be a developmental antecedent




to the latter, the two disorders are related to the samerisk factors, and ODD is generally considered a milderform of CD (American Psychiatric Association, 1994).Participants were drawn from two sources: the AustralianTemperament Project (ATP) (Sanson, Prior, & Oberklaid,1985), a longitudinal, community-based study of child de-velopment, and from additional schools in metropolitanMelbourne. In total, 110 adolescents between the ages of12–15 years participated in this study: 67 were from theATP and 43 from other secondary schools. The ATP isa large community study, which began in 1983 with aninitial sample of 2443, 4- to 8-month-old infants fromacross the state of Victoria, Australia. There have been10 waves of data on this sample: at ages 4–8 months,18–24 months, 32–36 months, 44–48 months, 5–6 years(preparatory grade), 7–8 years (Grade 2), 9–10 years(Grade 4), 11–12 years (Grade 6), 13–14 years, and 15–16 years. Sample retention over 15 years is approximately70%. Attrition within the sample has been caused by somewithdrawals and more frequently by loss of contact withina rather mobile population. Although there has been someselective loss of families with low SES and non-Australianbackgrounds, the sample still covers a wide range of so-ciodemographic characteristics, remains broadly compa-rable to the Victorian population, and is not significantlydifferent in SES from the original sample (see Prior,Sanson, Smart, & Oberklaid, 1999). Adolescents from theATP who were selected for participation in one of thethree clinical groups met the following criteria—ADHDgroup: 19 adolescents (14 boys, 5 girls), who scored morethan 1 SD above the mean on the Hyperactive–Distractiblefactor scale (Hyp) on at least two of the following threequestionnaires: parent-completed Child Behavior Ques-tionnaire Scale A (CBQ-A), teacher-completed ChildBehavior Questionnaire (CBQ-T) (Rutter, Tizard, &Whitmore, 1970), and child-completed Child BehaviorQuestionnaire Scale C (CBQ-C) (Prior et al., 1999), onat least two of the following three data collection points:7–8 years, 9–10 years, 11–12 years, as well as on theCBQ-A at the time of testing for this study (13–14 years).They also scored less than 1SD above the mean on theHostile–Aggressive scale (H–A) on each of these ques-tionnaires, at each of these times; ODD/CD group: 9 ado-lescents (6 boys, 3 girls), scoring over 1SDabove the meanon the H–A scale on at least two of the three question-naires, CBQ-A, CBQ-C, CBQ-T at 7–8 years, 9–10 years,11–12 years, as well as on the CBQ-A at the time oftesting, and less than 1SD above the mean on the Hypscale on each of these questionnaires, at each of thesetimes; ADHD+ODD/CD group: 25 adolescents (21 boys,4 girls), who scored over 1SD above the mean on bothscales on at least two of the three questionnaires, as well

as on the CBQ-A at the time of testing. These partici-pants thus met criteria across informants and across time.In addition, a control group consisting of 14 adolescents(10 boys, 4 girls) was randomly drawn from the remainderof the total ATP sample. None of these adolescents wasmore than 0.5SDabove the mean on either scale at any age.

In order to select participants from schools, adoles-cents with behavior problems were identified initially bytheir teachers according to a checklist based on DSM-IV(APA, 1994) criteria for ADHD, ODD, and CD. Parents ofstudents thus identified, together with a number of com-parison students of similar age and recruited initially bythe teachers, were contacted. If informed consent to par-ticipate in the study was received from both parent andstudent, participants were assigned to the four groups asfollows: ADHD group: 16 adolescents (16 boys, 0 girls),who scored more than 1SDabove the mean on the Hypscale on both the parent-completed and teacher-completedquestionnaires (CBQ-A and CBQ-T) at the time of testing,while scoring less than 1SDabove the mean on the H–Ascale; ODD/CD group: 2 adolescents (1 boy, 1 girl), scor-ing over 1SDabove the mean on the H–A scale of boththe CBQ-A and the CBQ-T at the time of testing and lessthan 1SD above the mean on the Hyp scale; ADHD+ODD/CD group: 13 adolescents (13 boys, 0 girls), whoscored over 1SDabove the mean on both scales on bothquestionnaires at the time of testing; control group: 12adolescents (10 boys, 2 girls) randomly selected by theirteachers from the remainder of the student group betweenthe ages of 12 and 15 years. None of these adolescentswas more than 0.5SDabove the mean on either scale atthe time of testing.

The dimensional score-based selection was checkedagainst DSM-IV listed criteria, using the checklist givento teachers as noted earlier (copy available from authors),but administered to parents at the conclusion of the test-ing session. Of the 73 adolescents with ADHD, 59% metDSM-IV criteria for the ‘combined’ type, 36% for the‘inattention’ type, and 5% for the ‘hyperactive-impulsive’type. Twenty-four percent of the adolescents with oppo-sitional defiant/conduct behavior problems met diagnos-tic criteria for CD and 76% met criteria for ODD at thetime of testing. The use of parent, child, and teacher re-ports added reliability and minimized informant bias, aswell as allowed the selection of adolescents who showedconsistent cross-situational behavior patterns. For the par-ticipants from the ATP, the use of various sources overtime also indicated consistency in behavioral symptoms.Adolescents with a history of neurological dysfunction orbrain injury, or with estimated FSIQ below 80 were ex-cluded from the study. The SES score for each participantwas derived from the means of both maternal and paternal




Table I. Descriptive Dataa and Classification Variables of the Groups

Variable ADHD (n = 35) ADHD+ ODD/CD (n = 38) ODD/CD (n = 11) Control (n = 26) F valuedf(3,106)

Ageb 14.2 (0.9) 13.9 (0.7) 13.8 (0.4) 14.0 (0.8) nsFSIQ (est) 92.7 (12.4) 96.2 (11.7) 103.2 (14.0) 110.4 (14.0) 10.3∗CBQ-A Hypc 1.12 (0.39) 1.23 (0.50) 0.22 (0.40) 0.02 (0.09) 64.46∗∗

CBQ-A H-Ad 0.40 (0.46) 1.08 (0.42) 0.92 (0.32) 0.02 (0.08) 47.37‡SES 3.9 (1.2) 4.5 (1.4) 4.5 (1.2) 3.5 (1.7) ns

aMean (standard deviation).bIn years.cCBQ-A Hyp.= parent-completed Child behavior Questionnaire Scale A, Hyperactive scale.dCBQ-A H-A = parent-completed Child behavior Questionnaire Scale A, Hostile–Aggressive scale.∗ p < .001, ADHD, ADHD+ ODD/CD< control.∗∗ p < .000, ADHD+ ODD/CD, ADHD> ODD/CD, controls.‡ p < .000, ADHD+ ODD/CD, ODD/CD> ADHD, controls.

occupation and education ratings (education ratings werefrom 1-Postgraduate to 8-Primary schooling only, and oc-cupation ratings were from 1-Professional to 6-Unskilled.No significant differences between participants from theATP and the schools were found with respect to age, esti-mated IQ, and behavior rating scores at the time of recruit-ment, except for SES where the ATP participants had ahigher average SES rating (4.4, SD1.3) than the non-ATPparticipants had (3.5, SD 1.4; p < .000). At the time oftesting, the three clinical groups differed from each otherand from the controls as expected on the parent-completedCBQ-A rating scales. In summary, of the 110 participants(91 boys, 19 girls) in this study, there were 73 with ADHDand 37 without ADHD. Demographic data and behavioralvariables are summarized in Table I.

Measures

Six Elements Task (SET) (Burgess et al., 1996)

This task is a simplified version of a subtest of theBehavioral Assessment of the Dysexecutive Syndrome(Wilson, Alderman, Burgess, Emslie, & Evans, 1996).The SET is a test of planning, task scheduling, and perfor-mance monitoring. Participants are required to carry outsix open-ended tasks in ten minutes, and in doing so, theyneed to comply with two rules. The six tasks are dividedinto two sets of three: telling a story, completing arith-metic problems, and writing down the names of picturesof objects printed on a series of cards. There are two sto-ries to be told: (1) a story describing the best holiday orthe best birthday one has had and (2) a story describingany memorable event in one’s life. The two sets of arith-metic problems are printed on two separate sheets marked“Set A” and “Set B,” and each contains 60 problems of

increasing difficulty. In Set A, for example, problem no. 4is “4+ 4= ,” no. 20 is “3× 3= ,” and no. 40 is “14×3= .” The two sets are equivalent in difficulty. The twosets of pictures each contain 60 brightly coloured cardswith a single line drawing of a common object (e.g., ham-mer, ship, jug) on each.

The participant is given two sheets of paper, oneheaded “Set A,” the other, “Set B” on which to write thenames of the objects on the cards. The participants are toldthat the examiner is not interested in either their handwrit-ing or spelling. The examiner first explains the six tasks,indicating the two sets of cards, which are in separatestands marked “A” and “B” and the corresponding writingsheets and the two sets of arithmetic. A brightly coloured,A4 size card with the six tasks listed on it is then placed infront of the participant, and it is explained that this list willbe left in front of him/her at all times to consult at will.The two rules are then explained as fully as necessary tothe participant. The first is that he/she has ten minutes totry and complete at least part of each of the six individualtasks. It is explained that it is not possible to complete allof the tasks in just ten minutes, and that the most impor-tant thing is to at least attempt part of all six tasks. Theparticipant is then given a digital stop watch and it is ex-plained that he/she can use it to check how much time hasalready passed since starting. The second rule is that theparticipant cannot move onto part B of any task immedi-ately after having attempted part A, instead he/she musttry something else. For example, if he/she does some ofSet A of the picture naming, he/she cannot immediatelythen go on to try Set B of picture naming, but insteadmust try one of the arithmetic or story telling tasks. Thetwo rules are then summarized for the participant, afterwhich he/she is asked to explain them to the examiner.The test begins when the participant can accurately ex-plain the tasks and rules to the examiner. The number of




tasks attempted within ten minutes minus the number ofrule breaks comprises the score.

Hayling Sentence Completion Test (HSCT)

This test was developed recently by Burgess andShallice (1996, 1997) and comprises two parts. Test-retestreliability values (Part A time, .62,p < .001; Part B time,.78, p < .001; errors, .52,p < .01) are comparable toother well-established neuropsychological measures of ex-ecutive functions (Burgess & Shallice, 1997). In the firstpart of the test, the participant is read a series of 15 sen-tences, which have the last word omitted and they are askedto provide the word which completes the sentence. In eachcase this last word is strongly cued by the sentence content.For example, “He posted the letter without a. . . . . . . . .” Itis easy to construct such sentences in English, and Bloomand Fishler (1980) found a probability of .99 that given thesentence in this example, a respondent would complete thesentence with the word “stamp.” Two practice sentencesare read to the participant and when he/she has satisfacto-rily given reasonable responses, each of the 15 trial sen-tences are read to the participant at a normal reading pace.Timing of response latency is by stop watch operated bythe examiner. In the second part of the test, which is con-ducted immediately after Part A and comprises another 15similar sentences, the participant is required to produce aword that makes no sense in the sentence context. Again,two practice sentences are given and after reasonable re-sponses, the 15 trial sentences are read, one at a time,and response latencies recorded. If the participant had notproduced a word within 60 s, that trial was terminatedand a response latency of 60 s was recorded. The HaylingTest yields two measures used in this study: (1) thinkingtime for strategy generation in Part B—total of responselatencies in Part B minus Part A total response latencies,which Burgess and Shallice (1996) suggest represents theadditional time required to produce a novel word ratherthan a straightforward sentence completion and (2) errorscore—a measure of inappropriate sentence completion,where response word is either the word most strongly cuedby sentence content or is in some way semantically relatedto the sentence (see Burgess & Shallice, 1996).

Weschler Intelligence Scale for Children (3rd ed.)(WISC-III; Weschler, 1991)

Vocabulary, Block Design, and Digit. Span subtestswere completed and the scores obtained were used for theestimated full scale IQ following the guidelines of Sattler(1992).

Wide Range Achievement Test-Revision 3 (WRAT3;Jastak, 1993)

Reading subtest was used as a measure of readingability. The subtest yields a standard score relative to same-age peers with a mean of 100 and a standard deviationof 15.

Behavioral Measures

Data on the ATP sample collected at 7–8 years, 9–10 years, and 11–12 years were analysed to investigate be-havioral disorder status. At the time of testing, behavioraldisorders were assessed using the Rutter Child BehaviorQuestionnaires and Scales A (for parents) and T (for teach-ers of the non-ATP participants). These widely used scalescontain subscales of Hostile-Aggressive and Hyperactive-Distractible (also Anxious-Fearful). They have good psy-chometric qualities and have been shown to have goodclinical and predictive validity (Mc Gee et al., 1985; Rutteret al., 1970). A Scale C for children has been devised,based on minor modifications in wording to the parentscale (Prior et al., 1999).

Procedure

All participants were tested individually and in theirown homes. ADHD adolescents currently on medication(n = 17) were tested after a 24-hour medication-freeperiod. Current behavior questionnaires were completedby mothers and adolescents during the home visit testingsession. A diagnosis was scored if DSM-IV criteria weremet on the basis the mother’s responses. Because DSM-IVdiagnoses were sometimes obtained prior to testing, theexaminer was not always blind to diagnosis. After permis-sion was received from participants’ parents during thehome visit, teachers’ behavior questionnaires were sentout to the respective schools and completed by the rele-vant teacher within a few weeks of testing. Administrationof the three WISC-III subtests was followed by the SETand HSCT and finally the WRAT3 reading test. All taskswere administered in the same sequence. As the depen-dent measures, we utilized a single score from the SET andtwo scores—B− A latencies and number of errors—onthe HSCT.

RESULTS

Adolescents with ADHD, ODD/CD, and both dis-orders were compared with normal controls on age, esti-mated full scale IQ, and socioeconomic status. The control




Table II. Correlations Among Executive Function Measuresand Behavior Ratings

Measure 1 2 3 4 5

SET — −.42∗ −.40∗ −.38∗ −.14HSCT, B− A — .65∗ .30∗∗ .04HSCT err — .33∗ .15CBQA, Hyp. — .48∗CBQ-A, H-A —

∗ p < .000.∗∗ p < .001.

and clinical groups did not differ with respect to age andSES, and clinical groups did not differ from each other onthese variables. However, there was a statistically signif-icant group difference for estimated IQ, with the controlgroup having a higher mean score than that for the ADHDonly and the comorbid group (Table I). Where ADHD waspresent (n = 73), participants had a significantly lowerestimated FSIQ (M = 94.5, SD= 12.6) than those with-out ADHD (n = 37; M = 108.3, SD= 14.2; p < .000).The presence or absence of ODD/CD was not associatedwith any significant differences in age, estimated FSIQ,or SES. There were no significant gender differences onage, estimated FSIQ, SES, or the current parent-completedbehavior rating scales.

Correlations between the three neuropsychologicaltest measures and the current parent-completed CBQ-AHyperactive–Distractible and Hostile–Aggressive scalesare shown in Table II. Each test score showed significant,moderate correlations with the Hyperactive–Distractiblescale (SET, r = −.38, p < .000; HSCT, B− A, r =.30, p < .001; HSCT errors, r = .33, p < .000), al-though there was little or no relationship between theHostile–Aggressive scale and the dependent measures(SET,r = −.14; HSCT, B− A, r = .04; HSCT errors,

Table III. Performance Scores on 3 Dependent Measures for 4 Groups

ADHD ADHD + ODD/CD ODD/CD Controls ANOVAMeasure (n = 35) (n = 38) (n = 11) (n = 26) F(3,106) Patterna

SETM 3.4 3.8 4.9 5.2 10.04∗∗∗ 1 2 3 4SD 1.3 1.6 1.4 1.1

HSCT, B− AM 31.3 34.5 9.9 10.1 5.52∗∗ 2 1 4 3SD 34.0 30.3 9.9 19.4

HSCT errorM 4.3 5.4 1.4 1.0 3.9∗ 2 1 3 4SD 6.6 6.7 3.3 2.1

aPattern of group performance scores from poorest to best.∗ p < .01, ADHD+ ODD/CD> control.∗∗ p < .001, ADHD+ ODD/CD> controls.∗∗∗ p < .000, ADHD, ADHD+ ODD/CD< ODD/CD, controls.

r = .15). Moderate, statistically significant correlationswere found between the SET and HSCT, B− A (−.42,p < .000), and SET and HSCT errors (−.40, p < .000).There was a strong correlation between HSCT, B−A andHSCT errors (.65, p < .000).

Between-group effects were evaluated with a set of1-way analyses of variance, and as Table III shows therewere significant differences between the four groups oneach of the three dependent measures. The ADHD-onlyand ADHD+ ODD/CD groups performed more poorlyon the SET than the control and ODD/CD-only groupsdid. Similarly, the ADHD-only and ADHD+ ODD/CDgroups performed less well than the controls did on theHSCT, B− A measure. There were significantly fewererrors made by the control group than by the ADHD+ODD/CD group on the HSCT. Multivariate effect size(partial eta-squared) was .112 (observed power= .86)after covarying estimated IQ, and ANOVA effect sizes(partial eta-squared) ranged between .22 and .10. Withineach group,t tests were used to examine performance ofboys vs. girls on the three dependent measures. No signif-icant gender differences were found.

To directly evaluate the hypothesis that EF deficits arespecific to ADHD, a 2-way ANCOVA for each dependentvariable was performed, the results of which are shownin Table IV. The two independent variables were ADHDand ODD/CD, both with two levels, namely the presenceand absence of the disorder. Preliminary multiple linearregression analyses of the three dependent variables in-dicated that estimated IQ, but not age, was a significantpredictor of performance on the three measures (SET:t ra-tio 5.59,p < .000; HSCT, B−A: t ratio−3.05, p< .003;HSCT error score:t ratio−2.95, p< .004). The analysesof covariance performed therefore used estimated IQ as acovariate. Where significant main effects were obtained,t tests were used in the post hoc analyses. All analyses




Table IV. Performance Scores for ADHD Present, ADHD Absent, ODD/CD Present and ODD/CD Absent withEstimated IQ Covaried

ADHD ODD/CD ANCOVA F(4,105)

Measure Present (n = 73) Absent (n = 37) Present (n = 49) Absent (n = 61) ADHD ODD/CD

SETM 3.6 5.1 4.0 4.2 11.18∗∗∗ nsSD 1.4 1.2 1.6 1.5

HSCT, B− AM 39.9 10.0 29.0 22.2 8.38∗∗ nsSD 31.9 16.9 28.8 30.4

HSCT errorM 4.9 1.1 4.5 2.9 4.49∗ nsSD 6.6 2.5 6.3 5.4

SET noa

M 4.1 5.4 4.5 4.6 11.08∗∗∗ nsSD 1.5 1.0 1.6 1.4

SET rbksb

M 0.5 0.3 0.5 0.4 ns nsSD 1.0 0.8 1.0 0.7

aSET number of tasks completed.bSET number of rule breaks.∗ p < .03.∗∗ p < .005.∗∗∗ p < .001.

of the dependent variables were two-tailed and appliedBonferroni adjustments to guard against Type I errors inmultiple comparisons.

For the SET, the 2-way ANCOVA revealed signif-icant main effects for ADHD,F(4,105)= 11.18, p <.001, but not for ODD/CD nor the interaction. Similarly,the results of 2-way ANCOVA of HSCT B− A per-formance scores (time) indicated a significant main ef-fect for ADHD, F(4,105)= 8.38, p < .005, but not forODD/CD nor for the interaction. The 2-way ANCOVA ofHSCT error scores also indicated a significant main ef-fect for ADHD, F(4,105)= 4.49, p < .03, but not forODD/CD nor the interaction. Results of post hoct testsshowed that significantly poorer performances on SET(p < .000) and HSCT, in terms of both B− A latencies(p < .000) and errors (p < .001), were associated withthe presence of ADHD.

Additional analyses were also conducted on the com-ponent scores obtained from the SET performances (seeTable III). A 2-way ANCOVA of thenumber of tasks at-temptedin the SET indicated a significant main effectfor ADHD, F(4,105)= 11.08, p < .001, but no maineffect for ODD/CD nor the interaction. Post hoct testshowed that participants with ADHD attempted signif-icantly fewer tasks than those without ADHD. Analy-sis of the number of rule breaks in the SET by a 2-wayANCOVA revealed no significant main effects for eitherADHD, ODD/CD, or the interaction.

Because reading difficulties (RD) are also often as-sociated with ADHD and ODD/CD (Hinshaw, 1992), wereran analyses controlling the effects of any reading dis-ability. Two-way ANCOVAs were again performed wherethe two independent variables were ADHD and ODD/CD,both with two levels, namely the presence and absence ofthe disorder, and WRAT3 reading standard score was thecovariate. Significant ADHD effects on EF measures re-mained robust to this covariance analysis (SET:F(4,105)= 17.07, p < .000; HSCT, B−A: F(4,105)=8.36, p < .01; HSCT errors:F(4,105)= 4.35, p < .04).There were no significant effects for ODD/CD or theinteraction.

DISCUSSION

The results of this study provide support for the hypo-thesis that among adolescents from a community sample,with clinically significant levels of externalizing behav-ior problems, EF deficits, as assessed by the SET andthe HSCT, are specific to the group with ADHD. The re-sults were robust to the effects of IQ and reading ability,as shown in the covaried analyses. Our findings confirmprevious evidence that showed that children and adoles-cents with ADHD have underlying neuropsychologicaldeficits (Barkley et al., 1992; Grodzinsky & Diamond,1992; Seidman, Beiderman, Faraone, Weber, & Ouellette,




1997) and, more specifically, difficulty with self-regulation (Shue & Douglas, 1992) and an inability to planand generate strategies to manage their problem-solvingperformance (Sonuga-Barke, Williams, Hall, & Saxton,1996). Furthermore, the results replicate past findings inboth clinically referred and population-based studies,which indicate that similar deficits are not primarily asso-ciated with ODD/CD (McBurnett et al., 1995; Moffitt &Henry, 1989; Nigg et al., 1998).

On the SET, adolescents with ADHD attempted fewertasks, but did not commit more rule breaks, than thosewithout ADHD. This indicated that those with ADHD, orwith both ADHD and ODD/CD, were significantly moreimpaired in their ability to strategically plan and organizeinformation and to monitor their ongoing performancewhen compared with age-matched controls and adoles-cents with ODD/CD only. Despite their lack of planning,the adolescents with ADHD were not dissimilar to thosewithout ADHD in preventing themselves from makingthe error of moving from Part A to Part B of a task duringcompletion of the SET. On the HSCT, adolescents withADHD required more additional time to produce an un-related word rather than a straightforward sentence com-pletion and made more errors when they did respond, inthat they gave a word that was semantically related to thestimulus sentence, despite instructions not to do so.

The Supervisory Attentional System (SAS) is a cog-nitive model, which represents an attempt to integratethese processes and conceptualize the functional systemrequired when controlled or nonautomatic cognition is re-quired. In terms of the SAS model, successful completionof the SET requires generation of a strategy, implementa-tion of the strategy involving the activation of appropriateschema and the triggering of markers to represent the rulesof the test, and, finally, evaluation of one’s ongoing behav-ior, which may lead to the modification of one’s plan and,in turn, one’s actions accordingly. We can infer from theperformance of those with ADHD in our study, that theydid trigger markers that represented the rules of the taskwherever necessary. However, they appeared to have diffi-culty generating and implementing appropriate strategiesand monitoring performance effectively.

The results on the HSCT support these claims. Theadolescents with ADHD took longer to respond than thosewithout ADHD when required to provide a response thatwas not the well-learned or prepotent response. Althoughtheir responses reflected errors when they finally did re-spond, these words were not necessarily the word moststrongly cued by sentence content. An example of suchan error was provided in response to the sentence, “all theguests had a very good. . . . . . .,” when the word “food”was given. Although the participants with ADHD often

responded inappropriately, they were able to activate andtrigger the markers relating to the highly cued words, butwere poor in generating a plan or strategy to perform thistask efficiently. In contrast, the non-ADHD participantswere quicker to respond and when they did, the wordsprovided were quite unrelated semantically to the sen-tence. These participants were often observed to be look-ing around the room as the sentence was read to them, re-sponding quickly with the name of an object in the room.When asked, they were able to explain the strategy theyused, and it was not uncommon for some of them to re-spond (correctly) before the examiner had finished readingthe sentence.

Executive functions have often been used as an um-brella term referring to an array of higher-order cogni-tive processes such as planning and organization, strat-egy generation, set maintenance, inhibition or impulsecontrol, and working memory. These processes under-lie successful performance on both the SET and HSCT,and, as Fuster (1991) points out, also apply to behavioralsequences that involve mutually contingent and tempo-rally separated events. They are the key features of thetemporal organization of behavior, which Fuster (1989,1991) argues reflect the “time-bridging function” of theprefrontal cortex. In this study, adolescents with ADHDappeared impaired in their ability to effectively processthe temporal components of the tasks confronting them.According to Barkley’s model of ADHD (Barkley, 1997),a temporal window of opportunity is created when a re-sponse to a stimulus or event is delayed, and it is the in-ability to delay that constitutes the primary deficit asso-ciated with ADHD. The interpretation of findings offeredhere, based on the theoretical model of the SAS, providesan alternative explanation of ADHD symptoms. Ratherthan EF deficits arising secondary to behavioral disinhibi-tion, as Barkley proposes, they are given primary status.In other words, it may be, as Douglas (1983) suggests,the capacity to use the delay period created between stim-ulus and response in a strategic way that is impaired inADHD.

There are a number of other implications of our find-ings. Our results indicate that EF deficits are differentiallyassociated with ADHD and not with CD and ODD, con-sistent with Hinshaw (1987), Leung et al. (1996), Sanson,Smart, Prior, and Oberklaid (1993), and Schachar (1991)who argue that, despite the high incidence of comorbiditybetween ADHD and ODD/CD, they are likely to be twodifferent disorders with different aetiologies. This claimalso gains support from a recent study in which a factoranalysis of a large number of measures of impulsivity in asample of more than 400 boys revealed a behavioral anda cognitive impulsivity factor, with the behavioral factor




having greater predictive association with CD than thecognitive factor had (White et al., 1994). The similarlyimpaired performance of the adolescents with ADHD andthose with co-occurring ADHD and ODD/CD suggestthat the EF deficit associated with ADHD may be theunderlying neuropsychological feature of the comorbidcondition.

Neuropsychological performance measures are im-portant validating criteria for ADHD because they do notshare method variance with currently used diagnostic in-struments (Seidman et al., 1997). The diagnosis of ADHDis limited currently to behavioral rating scales, which arevulnerable to a range of confounding factors such as recallbiases and halo effects, and which assess behavior but notspecific domains of attention (Halperin, McKay, Matier,& Sharma, 1994). The impaired performance of adoles-cents with ADHD on measures of EFs may be one usefulindicator for ADHD diagnosis.

Ecological validity of a neuropsychological test en-compasses both a theoretical relation between the test andeveryday behavior, and a conceptual similarity betweentask demands and the demands of everyday life (Franzen& Wilhelm, 1996). At a theoretical level, the SET hasbeen designed to measure the cognitive processes involvedin behavioral self-regulation as explained by the Super-visory Attentional System. Within the same theoreticalframework, the HSCT is considered to reflect an individ-ual’s ability to generate strategies that assist goal-directedbehavior. In order to complete the SET, respondents needto make decisions as to how to structure their performanceand to monitor ongoing performance to ensure that theyremain on track. This parallels the requirements of every-day situations where children and adolescents with ADHDhave difficulty, i.e., where distractions need to be ignoredand temptations resisted. The sample used in this studyhas also been used in a larger investigation of EFs, usingmore traditional measures and in which a statistically sig-nificant correlation was found between the SET and Rey-Osterreith Complex Figure, organisation score (ROCF)(r = .44, p < .000), and weak, but nevertheless signif-icant, correlations between the SET and Controlled OralWord Association Test (COWAT) (r = .21, p < .05), theHSCT, B− A and COWAT (r = −.22, p < .05), HSCT,B − A and ROCF (r = −.20, p < .05), and HSCT errorand ROCF (r = −.21, p < .05). Relationships of vary-ing strengths among EF measures have been noted byother researchers (Anderson, Lajoie, & Bell, 1995; Kafer& Hunter, 1997) and provide supportive evidence for theconcept of EFs as an array of separable subprocesses.

This study also indicates that successful performanceon these two tests is not necessarily associated with IQ

(Grattan & Eslinger, 1991; Welsh, Pennington, & Groisser,1991). It seems reasonable that, as Seidman et al. (1997)suggests, it is unlikely that IQ would remain unaffected bya developmental disorder, such as ADHD, which begins inearly childhood. As in this study, it is not uncommon to findthat, at a group level, IQ scores for ADHD children are sig-nificantly lower than those without ADHD (Carte, Nigg,& Hinshaw, 1996; Mariani & Barkley, 1997; Melnick &Hinshaw, 1996; Schachar & Logan, 1990). Clarifying thecomplex relationship between EFs, IQ, and learned be-havior, as it evolves throughout development, remains animportant issue in ADHD research.

Developmental hypotheses that attribute neuropsy-chological impairment during childhood to a delay in brainmaturation, which is overcome by adolescence, are notsupported by the findings of outcome studies, which con-sistently find that, compared with controls, there is a higherrate of attention deficits among young adults who have re-ceived diagnoses of hyperactivity during childhood(Biederman et al., 1993; Downey, Stelson, Pomerleau,& Giordani, 1997; Mannuzza et al., 1991). The resultsof this study suggest that there is no attenuation in EFdeficits among adolescents with ADHD between 12 and15 years, a finding which is consistent with previous re-search (Macleod & Prior, 1996; Seidman et al., 1997).Nevertheless, our study used a cross-sectional design, andtherefore, the results cannot be viewed as definitive. Lon-gitudinal studies, such as the ATP, are necessary to deter-mine if the impaired neuropsychological processes asso-ciated with ADHD continue into adulthood.

In conclusion, our findings have important impli-cations for the understanding of the neuropsychologicalcharacteristics of ADHD. An important limitation to thepresent study was the nonblinded assessment of partici-pants. This occurred in some 10% of instances when thechecklist based on DSM-IV criteria was administered toa parent prior to the testing, which was carried out by thesame experimenter. Inferences drawn from the impairedperformance of children with ADHD revealed in this studycan contribute to our theoretical understanding of the na-ture of EFs. The ability to plan and generate a strategy, toimplement the strategy effectively, and to monitor one’songoing performance involves the processing of temporalcontingencies, and it may be in this area of higher-ordercognitive processing that children and adolescents withADHD have a primary deficit. The two experimental EFtests used in this study can be valuable additions to theexisting array of neuropsychological instruments avail-able. Their clinical and predictive utility in the assessmentand treatment of ADHD needs to be confirmed by futureresearch.




ACKNOWLEDGMENTS

The authors wish to acknowledge the helpful com-ments of Dr. Rob McGee on an earlier draft of the manu-script. The study was supported by an Australian ResearchCouncil scholarship to the first author to complete her doc-toral thesis at La Trobe University.

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