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European Journal ofDevelopmental PsychologyPublication details, including instructions for authorsand subscription information:http://www.tandfonline.com/loi/pedp20

CPT performance, motoractivity, and continuousrelations to ADHD symptomdomains: A developmental studyKarin C. Brocki a , Carin M. Tillman a & Gunilla Bohlin aa Uppsala University , Uppsala, SwedenPublished online: 11 Apr 2008.

To cite this article: Karin C. Brocki , Carin M. Tillman & Gunilla Bohlin (2010) CPTperformance, motor activity, and continuous relations to ADHD symptom domains: Adevelopmental study , European Journal of Developmental Psychology, 7:2, 178-197,DOI: 10.1080/17405620801937764

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CPT performance, motor activity, and continuous

relations to ADHD symptom domains:

A developmental study

Karin C. Brocki, Carin M. Tillman and Gunilla BohlinUppsala University, Uppsala, Sweden

Using a population-based sample consisting of 401 6- to 12-year-olds, this studyexamined normative age and sex distributions on motor activity as measured inan actigraphic-based motion tracking system (MTS) and on attention-relatedfunctions derived from a Continuous Performance Test (CPT). Specificobjectives were to present new knowledge on age-related change in motoractivity and to study age effects on changes in motor activity and CPTperformance as a function of time on task. Further, continuous relations betweenthe two ADHD symptom domains and CPT performance and motor activity,and importantly, age effects in these relations were examined. CPT performanceimproved, and level of motor activity decreased with age. Linear associationsbetween the two ADHD symptom domains and several of the CPT and MTSparameters support available research describing the nature of ADHD as acontinuous dimension with variable expression throughout the generalpopulation. Further, our study is one of the first to provide developmentaldata using a time on task design, particularly with regard to motor activity.Imperative for ADHD future research are our results showing that age matters inthe relation between ADHD behaviours and neuropsychological function.

Keywords: Development; Continuous Performance Test (CPT); Motoractivity; ADHD.

Research in developmental psychology suggests great improvements duringearly and middle childhood in children’s attention abilities. It is alsoaccepted that children’s levels of motor activity decreases with age.However, objective measurements of the typical developmental progress in

Correspondence should be addressed to Karin C. Brocki, Uppsala University, Department

of Psychology, PO Box 1225, SE-751 42 Uppsala, Sweden. E-mail: Karin.Brocki@psyk.uu.se

This study was supported by grants from The Bank of Sweden Tercentenary Foundation. We

thank QbTech AB for providing technical apparatus and support.

EUROPEAN JOURNAL OF DEVELOPMENTAL PSYCHOLOGY2010, 7 (2), 178–197

� 2008 Psychology Press, an imprint of the Taylor & Francis Group, an Informa business

http://www.psypress.com/edp DOI: 10.1080/17405620801937764

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the ability to sustain attention as measured by changes in performance overtime on task as well as in levels of motor activity are rare. Such knowledge isimportant from the perspective of developmental psychopathology asdeviations in these functions are considered symptomatic for a wide range ofdevelopmental disorders, particularly so for attention deficit hyperactivitydisorder (ADHD). In the current study we examined typical age-relatedchanges on objective measurements of motor activity using actigraphyand attention-related functions derived from a Continuous PerformanceTest (CPT). With reference to sustained attention, defined as the abilityto maintain alertness over time (cf. Nigg, 2006; van der Meere & Sergeant,1988), we also used time-on-task measures to investigate developmentalchanges in CPT performance as well as in motor activity. Importantly,effects of time on task have not been studied on motor activity at all inprevious research and very rarely so with regard to CPT performance.Further, assuming the dimensional and developmental perspectives ofADHD suggesting that the disorder represents a developmental delay intraits that are present throughout the general population (e.g., Levy, Hay,McStephen, & Wood, 1997), we studied continuous relations betweenbehavioural ratings of the two ADHD symptoms domains (i.e., hyper-activity/impulsivity and inattention) and the objective measurements ofADHD-relevant behavioural and cognitive variables. Finally, we wished tofill a gap in extant research by also examining age effects in these relations.

Normal development of motor activity and CPT performance

The normal development of motor activity as measured by actigraphy hasnot been extensively explored. In fact, to our knowledge, the paper byEaton, McKeen, and Campell (2001), showing an inverted U-shapeddevelopmental movement pattern between 0 and 16 years of age, is the onlyone that includes developmental data on motor activity using limbactometers in middle childhood. As for CPT performance, however,developmental effects have been widely demonstrated (e.g., Betts, McKay,Maruff, & Anderson, 2006; Conners, Epstein, Angold, & Klaric, 2003;Greenberg & Waldman, 1993; Levy, 1980; Lin, Hsiao, & Chen, 1999;Seidel & Joschko, 1990). For example, using 341 children 6 to 15 years ofage, Lin et al. (1999) reported strong age effects on CPT performance asreflected in increased hit-rate, decreased number of false alarms, and anincreased sensitivity, with particularly active periods of developmentoccurring between the ages of 6 to 12. Further, Conners et al. (2003) foundrobust age effects for RT and variability in RT as measured by CPT in alarge sample of 9- to 17-year-old children.

Findings from CPT studies have often been interpreted in terms ofsustained attention and thus developmental effects on CPT parameters have

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been understood as development of sustained attention (cf. Lin et al., 1999).However, if sustained attention is defined as ‘‘the decline in humanperformance over time’’ (van der Meere & Sergeant, 1988, p. 643),statements about this aspect of attention, whether it be developmental orgroup differences, should only be made with respect to the amount of declinein performance over time on task. To our knowledge, the only study that hasexamined effects of age on decrement in CPT performance over time wasconducted by McKay, Halperin, Schwartz, and Vanshdeep (1994), whofound no appreciable development in the ability to sustain attention between7 and 11 years of age, but significant development in this ability between age11 and adulthood. Thus, further studies on the effect of age on performanceover time on task are clearly needed.

Relations between CPT and MTS measures and ratingsof hyperactivity/impulsivity and inattention: Age effectsand specificity

Findings from studies using actigraphy in ADHD, indicate that childrenwith ADHD have 25% to 30% higher levels of activity (e.g., Porrino et al.,1983; Teicher, Ito, Glod, & Barber,1996) as well as a less complex (morelinear) movement pattern (Teicher et al., 1996) compared to controls. Thismethod has been used in both naturalistic settings (e.g., Porrino et al., 1983)and structured academic settings (e.g., Halperin, Matier, Bedi, Sharma, &Newcorn, 1992), with the greatest difference in activity level being foundduring structured school activities (Porrino et al., 1983).

To increase the ecological validity in measurement of activity level inlaboratory settings, children’s motor activities have been recorded whilethey are engaged in a CPT. The use of actigraphy together with the CPT hasshown high discriminative validity in distinguishing children with ADHDfrom controls, good test–retest reliability, as well as significant relationsbetween activity measures and both parent and teacher ratings of ADHDsymptoms (Reichenbach, Halperin, Sharma, & Newcorn, 1992; Teicheret al., 1996). A relation between CPT-derived variables and ADHDsymptoms has been established in previous between-groups research (e.g.,see Losier, McGrath, & Klein, 1996, for a meta-analytic review) as well as inmore recent studies examining continuous ADHD symptom scores inepidemiological samples (e.g., Epstein et al., 2003), with higher ADHDsymptom scores being associated with poorer performance in terms of moreerrors, and more variable reaction times.

Despite empirical data showing developmental changes in the manifesta-tion of hyperactivity/impulsivity and inattention (e.g., Hart, Lahey, Loeber,Applegate, & Frick, 1995; Lahey et al., 1998), age-related change in the

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relation between neuropsychological deficits and ADHD behaviouralsymptoms have been largely ignored in extant research. As Nigg (2006,p. 25) so aptly puts it, ‘‘Nearly all research treats ADHD in a static fashion,rather than as an unfolding developmental pathway’’. Brocki and Bohlin(2006) examined this developmental issue by investigating age effects in therelation between executive functions and ADHD symptoms. The resultsfrom this study, based on a normal sample ranging in age from 6 to 13 years,showed that poor inhibition was most clearly associated with ADHDsymptoms (both hyperactivity and inattention) for younger children,whereas poor functioning with regard to later developing and more complexexecutive functions such as working memory was associated with inattentionsymptoms for older children. Indeed, these findings stress the importance offurther investigating how neuropsychological dysfunction in relation toADHD symptoms might change with a child’s developmental level.Reichenbach and colleagues established that age accounted for nearly athird of the variance in children’s activity level and nearly a quarter of thevariance in children’s CPT performance, whereas Teicher et al. (1996)showed only marginally significant effects of age on activity level. In neithercase did the authors go on to examine how the relation between activity leveland ADHD symptoms changes with age.

Relatively little research has been directed to the question of whether thetwo main dimensions of ADHD symptoms (i.e., inattention and hyper-activity/impulsivity) are specifically associated with different neuro-psychological impairments and with regard to CPT performance the fewextant results reveal inconsistencies. For example, Epstein et al. (2003)found that CPT performance did not demonstrate symptom specificity in alarge epidemiological sample. In contrast, based on a clinical ADHD sampleaged 8–18, Chhabildas, Pennington, and Willcutt (2001) found thatsymptoms of inattention, best predicted impaired neuropsychologicalfunction (including CPT performance). Considering the developmentalhypothesis of ADHD the authors note that developmental change in themanifestation of ADHD may have had implications for the results, thusunderscoring the need to examine how the relation between each of the twoADHD symptom domains and cognitive impairment might change withage. As for specific relations between the two ADHD symptom domains andmotor activity, findings by Dane, Schachar, and Tannock (2000) did notdemonstrate the predicted domain specificity with regard to hyperactivity;there was no difference between the predominantly inattentive andcombined subtypes in level of motor activity as measured with actigraphyin a clinical ADHD sample aged 7–12 years. No information is available ondevelopmental change in the relation between objective measures of motoractivity and the two specific ADHD symptom dimensions.

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Aim of study

We wished to: (a) describe normative age and sex distributions on motoractivity as measured by an actigraphic-based motion tracking system (MTS)and on CPT performance, in a large sample representative of the latepreschool years, middle childhood, and late childhood; (b) provide noveldevelopmental data representing performance decrements on time on taskon all CPT and MTS variables; (c) examine specificity in relations betweenratings of the two ADHD symptom domains (inattention and hyperactivity/impulsivity) and motor activity and CPT performance; and, importantly, (d)study how these posited relations might change with age. All relationsbetween ADHD ratings and motor activity and CPT performance were alsoanalysed using time-on-task measures.

Based on previous findings (e.g., Conners et al., 2003; Lin et al., 1999) wepredicted that CPT performance would improve and level of activity woulddecrease with age. Despite paucity in extant data describing developmentaleffects on the decline in performance over time on task, a smallerdeterioration in performance over time on task with increasing age on allCPT and MTS variables was predicted. Further, based on previous data(Brocki & Bohlin, 2006; Chhabildas et al., 2001; Epstein et al., 2003) and thedevelopmental hypothesis of ADHD, we hypothesized that CPT perfor-mance would be related to both symptom domains, but that the effect of ageon these relations could differ between the two domains. As for motoractivity, there is little guidance in previous literature, but from a theoreticalpoint of view relations were primarily expected to ratings of hyperactivity.Finally, the scarcity of dimensional data and the inconsistency in previousfindings offered no basis for predictions with regard to specific relationsbetween the two ADHD symptom domains and time on task on motoractivity and CPT performance.

METHOD

Participants

Five-hundred ninety-four children were asked to participate in the study,401 (68%) consented to participation, 10% declined participation, and theremaining 22% did not respond. The participating children (49% boys) werebetween 6 and 12 years of age and came from six different schools situated incities of different size and level of urbanization in the middle parts ofSweden. Participants were recruited from grade level 0 to 6 (grade level 0 inSweden corresponds to kindergarten in the United States). For means andstandard deviations of age for the different grade levels, see Table 1. Asindicated by the teachers’ answers to a question about diagnostic status with

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regard to ADHD included in the questionnaire, two children in the samplewere diagnosed with ADHD. Because it could not be assured that thesechildren were not on medication, they were excluded from the sample. Onlychildren in regular school classes were included in the recruitment. However,14 children received various types of educational support at school (e.g.,mathematics, reading and writing). Analysis of interview data from theparticipating children showed the sample to be representative of Swedishdemographics in terms of parent ethnicity, parent marital status, possessionof car in the household, and the free-time activities that the child indulged in(data files from Statistics Sweden, http://www.scb.se, retrieved 20 February2006. Participants were recruited from preschools and elementary schoolsafter verbal consent from head principals and teachers and written consentfrom parents. In addition, verbal assent was obtained from participatingchildren prior to the assessment session. Each school class collectivelyreceived an amount of e55 to 77, depending on the number of children thatactively took part in the study.

Apparatus and measures

Continuous Performance Test (CPT). The CPT paradigm is thought toassess attention abilities, impulsivity and vigilance. The participants wereinstructed to respond as quickly as possible by pressing a hand-held buttonelectronically attached to the computer each time a go stimulus (a filled greycircle) was presented on the computer screen and not to press when a no-gostimulus (a filled grey circle with a black cross) appeared. The instructionsemphasized both speed and accuracy. The stimulus duration was 100 ms,with an inter-stimulus interval (ISI) of 1900 ms. The task consisted of a totalof 450 trials, with an equal number of go and no-go stimuli. The testduration was 15 minutes, with the order of presentation of go and no-gostimuli randomized.

Measures derived from the CPT included omission errors (missed gostimuli) and commission errors (responses to no-go stimuli), which weremeasured as a percentage of the total number of go stimuli and no-gostimuli, respectively. Reaction time (RT) on hits and the standard deviationof RT (RTSD) were recorded in milliseconds. Analysis of test–retest datafor 24 children (age: M¼ 10.8, SD¼ 1.7; retest interval: M¼ 9.5 days,SD¼ 10.7) showed r¼ .86 for omissions, .85 for commissions and .65 forRT (Fredrik Ulberstad, personal communication, November 22, 2007).

Motion tracking system (MTS). Activity was recorded during the CPTusing a high-precision infrared MTS. The MTS together with the CPTconstitute the ‘‘QbTest’’, developed by QbTech AB in Gothenburg, Sweden(Bergfalk, 2003). The MTS records movements via a reflector attached to a

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head band worn by the participant during the CPT. The position ofthe reflector was sampled 50 times per second, with a spatial resolution of1/27 mm per camera unit (Bergfalk, 2003). In order for the camera to be ableto record movements towards and away from the camera, the head bandreflector on the child was to be situated approximately 100 cm in front of, and40 cm below, the camera lens. Test–retest data for the MTS variables wereobtained in the same study as the CPT data (Fredrik Ulberstad, personalcommunication, November 22, 2007) and showed r¼ .78 for distance, .81 forarea, and .57 for complexity.

Measures derived from the MTS included average distance in meterstravelled by the reflector each minute of the test, immobility (the averagetime in milliseconds that the participant moved less than 1 mm), andcomplexity of movements. The complexity of movements was measured bythe fractal dimension of the path travelled. For very simplistic movements(e.g., along a line) the fractal dimension is one, and for extremely complexmovements (total chaos) the path will fill the complete area (twodimensions) and the fractal dimension will then be close to two.

The mean across time for each variable from the CPT and MTS was usedas an overall measure for each participant. In addition, all variables fromthe CPT and MTS, except complexity of movements,1 were obtained as amean per minute during the 15 minutes CPT. This enabled the calculationof individual performance trend values, representing changes in perfor-mance over time on task (see, statistical analyses section). The participantswere tested individually in a separate room provided by the school.Participants were given test instructions, concerning both the CPT and theMTS, through a standardized video. After a practice session of 20 stimulithe test session started. The children were seated on a stool without backsupport or armrests, in order to assure that they would not adopt a recliningbody position.

Teacher ratings. ADHD behaviours were measured by letting teachersfill out a rating scale constituted by the items from the DSM-IV criteria forADHD (American Psychiatric Association, 1994). This measure has beenwell validated and is frequently used within ADHD research (e.g., DuPaul,Power, Anastopoulos, & Reid, 1998). A total of 9 items were composed toassess inattention and 9 items were composed to measure hyperactivity/impulsivity. Internal consistency of both scales in our study was a¼ .94.Each item was scored on a 4-point scale ranging from 0 (never/seldomoccurring) to 3 (very frequently occurring). The sum of scores on the twoscales, Inattention and Hyperactivity/Impulsivity, were used in the analyses.

1The composition of the measure of complexity of movement did not allow for temporal

division.

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In addition, the teacher questionnaire included an item referring to anyeducational support at school that the child received as well as an itemreferring to diagnostic status with regard to ADHD.

The questionnaires were distributed during a visit by the test leader to theschool class. After completion, the teachers were asked to mail thequestionnaires back to the test leader in the stamped envelope provided.The teacher received a gift certificate of e10 to 30, depending on the numberof participating children from their class and thus work load in terms offilling out questionnaires.

Statistical analyses

As a result of minor technical problems, CPT and MTS results were notobtained for 11 and 16 participants, respectively. The missing data wereevenly distributed between the grade levels. Data was checked for outliers onthe CPT and MTS, which were defined as values deviating more than 4 SDsfrom the mean. Participants having outlying values on one or more CPT orMTS measures were removed from the data set. For time-on-task values,participants with outlying values were only excluded on those particularmeasures. In total, 15 participants were removed from the data set, and upto three participants on each particular time-on-task measure were excludedfrom the analyses. To assure that educational support, which possibly couldreflect learning disabilities, did not affect the results, all analyses wereconducted both with and without the 14 children who received educationalsupport. The results were very similar. Therefore, the presented resultsinclude the children with educational support.

Two-way analyses of variance (ANOVAs) using the GLM procedurewere used to investigate effects of grade and sex on the overall CPT andMTS measures. To further examine developmental shifts in these measures,post hoc analyses using Tukey’s HSD test were conducted. Developmentalshifts were defined as statistically significant differences between nearestconsecutive grade levels in the post hoc test.

Individual linear trend values representing changes in performance overtime on task were calculated. In these calculations, the means for each of five3-minute time blocks for each CPT and MTS measure were used. Morespecifically, for a particular measure, the means from the first through thefifth time block were multiplied by 72, 71, 0, 1, and 2, respectively. Theseproducts were then summed to obtain the final time-on-task value used inthe analyses. In this way, a positive time-on-task value indicated an increasein value over time on task, and a negative time-on-task value implied adecrease in value over time on task. Individual time-on-task values wereadopted in the present study, instead of the more common way of examiningtime-on-task effects (i.e., using time blocks in a repeated-measures

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ANOVA). This alternative was preferred to enable the investigation ofcontinuous relations between time-on-task performance and ADHDbehaviours, as well as the effect of age on these relations. Hereafter theterms ‘‘overall measure’’ and ‘‘time-on-task measure’’ will be used wheredifferentiation of the two types of measures is necessary.

To investigate effects of age and sex on sustained attention, as measured bythe CPT andMTS time-on-task measures, two-way ANOVAs using the GLMprocedure were conducted. Post hoc analyses were conducted and interpretedfor the time-on-task measures in the same way as for the overall measures.

To explore relations between the overall CPT and MTS parameters andratings of inattention and hyperactivity/impulsivity, respectively, as well asthe effect of age on these relations, separate regression analyses wereconducted. First, each CPT and MTS parameter was entered as separatepredictor variables with Inattention or Hyperactivity/Impulsivity as out-come variable. Second, to explore age effects on these relations, the centredage variable, was entered together with each centred CPT and MTSparameter and an interaction term between each centred CPT and MTSmeasure and the centred age variable. Identical analyses were performedwith the time-on-task measures as predictor variables.

RESULTS

Age effects

CPT performance measures. For descriptive data on age and the overallCPT measures for the seven grade levels see Table 1, and for ANOVAresults see Table 2. The results showed significant developmental effects onall CPT measures. In other words, younger children made more errors ofcommission and omission (see Figure 1A) and had longer and more variableRT (see Figure 1B) compared to older children. There were significanteffects of sex on errors of commission and omission and RTSD with boysshowing poorer performance compared to girls.

The post hoc analyses of grade elucidated the obtained developmentaleffects on the CPT measures. Regarding errors of commission the resultsshowed significant developmental improvement between grade levels 1 and 3(i.e., between 7 and 9 years of age). Concerning errors of omission there wassignificant developmental improvement between grade levels 0 and 2 (i.e.,between 6 and 8 years of age), and further significant progress between gradelevels 2 and 5 (i.e., between 8 and 11 years of age). The results regarding RTrevealed three active periods of developmental improvement between gradelevels 0 and 4 (i.e., between 6 and 10 years of age), more specifically betweengrade levels 0 and 1, between grade levels 1 and 3, and between grade levels 3and 4. Regarding RTSD there were two active periods of developmental

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TA

BL

E1

Me

an

an

dst

an

da

rdd

ev

iati

on

for

ag

ea

nd

ea

chco

nti

nu

ou

sp

erf

orm

an

cete

st(C

PT

)m

ea

sure

an

dm

oti

on

tra

ckin

gsy

ste

m(M

TS

)m

ea

sure

,a

sa

fun

ctio

no

fg

rad

ele

ve

l

Gradelevel

0(n¼48)

1(n¼54)

2(n¼55)

3(n¼48)

4(n¼65)

5(n¼51)

6(n¼55)

Measures

MSD

MSD

MSD

MSD

MSD

MSD

MSD

Age(years)

6.41

0.25

7.61

0.36

8.68

0.42

9.64

0.37

10.68

0.40

11.86

0.34

12.64

0.30

Continuousperform

ance

test

Commissionerrors

16.15

14.51

21.63

13.61

16.75

12.99

14.30

10.98

14.14

10.80

11.91

10.28

11.12

10.47

Omissionerrors

13.53

9.28

10.37

8.55

7.52

9.41

3.91

5.27

3.47

6.68

2.83

4.66

3.78

7.38

RT

635.05

109.45

530.94

79.93

492.05

64.65

467.87

83.44

413.13

63.32

404.69

46.58

375.40

52.17

RTSD

198.85

47.60

172.94

56.64

146.93

58.39

131.54

45.71

113.94

59.47

107.04

30.71

105.06

52.60

Motiontrackingsystem

Distance

1.27

0.65

1.22

0.55

1.08a

0.71a

0.71

0.38

0.75

0.59

0.58

0.31

0.62a

0.37a

Immobility

91.52

44.64

93.80

47.47

115.60a

63.52a

162.67

81.86

194.09

126.98

210.82

135.35

214.21a

120.25a

Complexity

1.35

0.05

1.36

0.05

1.37a

0.06a

1.39

0.07

1.38

0.09

1.40

0.07

1.41a

0.08a

Note:RT¼reactiontime;

RTSD¼standard

deviationofreactiontime.

an¼53.

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improvement between grade levels 0 and 4 (i.e., between 6 and 10 years ofage), more specifically between grade levels 0 and 2, and between gradelevels 2 and 4. Although there was further improvement in the higher gradelevels on most CPT measures, this improvement did not reach significance.

MTS measures. For descriptive data on the overall MTS measures forthe seven grade levels see Table 1, and for ANOVA results see Table 2.There were significant developmental effects on all of the MTS measures (seeFigure 1C), with younger children moving a longer distance, being lessimmobile, and having a less complex movement pattern compared to olderchildren. Significant effects of sex were obtained on the measures of distanceand immobility, indicating a higher motor activity level in boys compared togirls.

Post hoc analyses further investigated the obtained developmental effectsof the MTS measures. Regarding the measure of distance, the most activedevelopmental period was between grade levels 2 and 3 (i.e., between 8 and 9years of age). A significant period of development concerning the immobilitymeasure occurred between grade levels 1 and 3 (i.e., between 7 and 9 years ofage). Complexity of movement, on the other hand, showed a moreprotracted period of developmental progress, occurring between gradelevels 0 and 5 (i.e., between 6 and 11 years of age).

CPT and MTS time-on-task measures. For ANOVA results, see Table 3.Regarding the time-on-task measures on the CPT and MTS, significantdevelopmental effects were obtained on errors of omission, RT, RTSD, anddistance. More precisely, the higher positive time-on-task value for younger

TABLE 2Effects of grade and sex on each overall measure on the continuous performance test

(CPT) and motion tracking system (MTS)

Measures

Grade Sex

df F df F

Continuous performance test

Commission errors 6,362 5.56*** 1,362 40.06***

Omission errors 6,362 16.07*** 1,362 9.07**

RT 6,362 67.35*** 1,362 1.39

RTSD 6,362 26.36*** 1,362 19.30***

Motion tracking system

Distance 6,358 18.38*** 1,358 31.62***

Immobility 6,358 17.89*** 1,358 17.52***

Complexity 6,358 4.60*** 1,358 0.41

Note: RT¼ reaction time; RTSD¼ standard deviation of reaction time. *p5 .05; **p5 .01;

***p5 .001.

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children, compared to older children, demonstrated a greater deteriorationin CPT performance and an increase in motor activity level over time ontask. The results further showed significant effects of sex on the time-on-taskmeasure of distance, with boys showing a higher positive time-on-task valuecompared to girls, implying a larger increase in activity level over time.

Post hoc analyses of the CPT and MTS time-on-task measures furtherelucidated the developmental effects found in the ANOVA. Concerningerrors of omission the most active period of development occurred between

Figure 1. (A) Means of standardized scores for errors of omission and commission on the

Continuous Performance Test (CPT) for each grade level. (B) Means of standardized scores for

reaction time (RT), and standard deviation of reaction time (RTSD) on the Continuous

Performance Test (CPT) for each grade level. (C) Means of standardized scores of distance,

immobility, and complexity of movements derived from the motion tracking system (MTS) for

each grade level.

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grade levels 1 and 2 (i.e., between 7 and 8 years of age). RT and RTSDshowed similar developmental courses, with significant developmentalimprovement between grade levels 2 and 3 (between 8 and 9 years of age).The measure of distance showed developmental improvement between gradelevels 0 and 3 (i.e., between 6 and 9 years of age).

Age effects and specificity in the relations between CPTand MTS measures and ratings of hyperactivity/impulsivityand inattention

Results from regression analyses relating the overall and time-on-taskmeasures on the CPT and MTS to inattention and hyperactivity/impulsivityscores, and effects of age on these relations, are presented in Table 4. Theresults of the overall measures showed that a higher rate of commissionerrors was linked to higher scores on both symptom domains. A higher rateof omission errors and a higher activity level (as indicated by the immobilityand distance measures) were associated with higher scores on thehyperactivity/impulsivity dimension.

Regarding the effect of age in the relations between the symptom domainsand CPT and MTS performance, the results showed that the links betweenboth errors of commission and omission, and inattention were dependent onage. Also the relation between RTSD and inattention appeared to beaffected by child age. In order to interpret the interactions with age wefollowed the recommendations by Cohen and Cohen (1983) and dicho-tomized the two predictor variables in each equation by entering valuesrepresenting+1 SD. In interpreting the plots of these values, it was found

TABLE 3Effects of grade and sex on each time-on-task measure on the continuous performance

test (CPT) and motion tracking system (MTS)

Measures

Grade Sex

df F df F

Continuous performance test

Commission errors 6,361 1.60 1,361 0.37

Omission errors 6,360 11.78*** 1,360 0.03

RT 6,359 8.86*** 1,359 0.01

RTSD 6,360 5.29*** 1,360 1.38

Motion tracking system

Distance 6,357 11.12*** 1,357 20.41***

Immobility 6,358 0.25 1,358 0.70

Note: RT¼ reaction time; RTSD¼ standard deviation of reaction time. *p5 .05; **p5 .01;

***p5 .001.

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that, particularly at older ages, higher rates of errors and more variableresponses were associated with higher inattention scores.

Regarding the time-on-task measures, the results showed a significantnegative relation between immobility and hyperactivity/impulsivity scores.This implied a greater decrease in immobility (i.e., increase in movement)over time on task in children with higher ratings of hyperactivity/impulsivity. Furthermore, there was a significant positive relation be-tween distance and the hyperactivity/impulsivity dimension, also indi-cating a greater increase in motor activity over time on task in childrenwith higher hyperactivity/impulsivity scores. Age did not have any effecton the relationships between the time-on-task measures and ADHDbehaviours.

DISCUSSION

This study extends our understanding of the normal developmental courseof CPT performance and motor activity by providing data for a largenormative sample. As predicted, our results show that CPT performanceimproves, and level of activity as measured by the MTS decreases, with age.Several effects of sex were obtained on measures of both CPT and

TABLE 4Beta values from the analyses investigating relations between the CPT and MTSmeasures and inattention and hyperactivity/impulsivity, and the effects of age on these

relations (results of the time-on-task measures are presented within parentheses)

Measures Inattention Hyperactivity/impulsivity

Continuous performance test

Commission errors .12* (.01) .12* (.00)

Commission errors6Age .11* (.10) .04 (.07)

Omission errors .08 (7.08) .12* (7.00)

Omission errors6Age .12* (7.06) .01 (7.05)

RT 7.09 (7.02) 7.03 (7.02)

RT6Age .02 (7.01) .02 (.03)

RTSD .04 (7.08) .07 (7.02)

RTSD6Age .11* (7.02) .04 (7.05)

Motion tracking system

Distance .10 (.10) .14** (.19***)

Distance6Age .00 (7.00) .06 (.05)

Immobility 7.01 (7.09) 7.12* (7.12*)

Immobility6Age .10 (.00) .05 (.01)

Complexity 7.05 (—) 7.10 (—)

Complexity6Age 7.01 (—) 7.02 (—)

Note: *p5 .05; **p5 .01; ***p5 .001.

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MTS, with boys performing at a lower level compared to girls. Analyses oftime-on-task performance showed improvement in the ability to sustainattention with increasing age, as reflected in a smaller deterioration inperformance over time for the majority of CPT measures, and for one of theMTS measures. Further, several relations were found between the CPT andMTS measures and the two ADHD symptom domains. A number of therelations with the inattention symptom dimension were also found to changewith age. We discuss these findings in turn.

Age effects on CPT and MTS measures

Interestingly, the results revealed comparable developmental transitions onthe CPT and MTS-derived measures. Active age-related improvementsoccurred particularly between 6 or 7 and 10 years of age, with furtherimprovement levelling off between the older age groups. Our results are inline with the findings from previous studies of CPT performance reportingactive developmental periods between the age of 6 and 12 years withprogressively diminishing improvement in performance with age (e.g.,Greenberg &Waldman, 1993; Levy, 1980; Lin et al., 1999; Seidel & Joschko,1990).

With regard to the developmental course of motor activity we found adownward trend, which is in line with the data reported by Eaton et al.(2001), although the decline there seemed to start somewhat later. Ourfindings also agree with Reichenbach et al. (1992), who showed a negativecorrelation of .57 between activity level and age in a sample of 6- to 13-year-olds. Much smaller age effects were reported by Teicher et al. (1996), but thesample size in this study and the possible confounding effects by theinclusion of children with ADHD in the analysis (18 children with ADHDand 11 controls) make these results less informative.

The development of sustained attention in this study was captured bytime-on-task measures (cf. Nigg, 2006; van der Meere & Sergeant, 1988),which have not been extensively used in the developmental literature onCPT performance and to our knowledge not at all on motor activity. Theresults showed improvement in the ability to sustain attention with age, asreflected in a smaller deterioration in performance over time for the majorityof CPT measures, as well as for the MTS measure distance, in oldercompared to younger children. CPT performance over time on taskgenerally improved between 7 and 9 years of age. Our results contradictthe results obtained by McKay et al. (1994) showing no significantdevelopment in sustained attention based on time-on-task measures ofCPT performance in children aged 7 to 11 years. The inconsistency indevelopmental effects between our work and the McKay et al. study couldbe due to factors such as differences in target frequency used in the CPT,

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which were 50% and 10%, respectively. The analyses of time-on-task effectson motor activity showed that younger children moved a longer distancewith increasing time on task compared to older children, with an activeperiod of development between 6 and 9 years of age.

Specificity and age effects in the relations between CPT andMTS measures and ratings of hyperactivity/impulsivity andinattention

The CPT measures commission errors and omission errors and most of theMTS measures predicted at least one of the ADHD symptom dimensionsacross age. In general, these results are in line with both clinical and non-clinical studies demonstrating a relationship between CPT performance,motor activity, and ADHD symptoms (e.g., Chhabildas et al., 2001; Daneet al., 2000; Epstein et al., 2003; Losier et al., 1996; Teicher et al., 1996).Commission errors were related to both ADHD symptoms domains,whereas omission errors, distance, immobility and complexity were relatedonly to hyperactivity/impulsivity.

The fact that commission errors were related to both symptomdimensions, can be interpreted to mean that this is the CPT parametermost consistently related to ADHD symptoms. This is partly in line withthe study by Epstein et al. (2003), which showed that only commissionerrors and RT variability significantly distinguished between childrenfulfilling diagnostic levels of ADHD and controls. Although our resultsfor omission errors, showing a significant relation only to hyperactivity/impulsivity, are counterintuitive, they also agree with the study byEpstein et al. (2003), which showed that omission errors were signifi-cantly related to only two specific inattention symptoms but to sevenhyperactivity/impulsivity items. The lack of a relation between RT andADHD symptoms in our study is consistent with the few studies thathave examined differences between children with ADHD and controls onRT in the CPT paradigm (Chee, Logan, Schachar, Lindsay, & Wachsmuth,1989; Schachar, Logan, Wachsmuth, & Chajczyk, 1988). Our results withregard to RT can also be compared to the findings reported by Epsteinet al. (2003), which showed little relationship between mean hit RT andany of the separate ADHD symptoms. As to the lack of relation betweenvariability in RT and ADHD symptoms in the current study, findings inthe literature are inconsistent (e.g., Epstein et al., 2003, vs. Schachar et al.,1988) and although factors such as sample selection criteria and variationsin particular task parameters used (i.e., target ratio, ISI, and stimulusdisplay) could be important we could not find any systematic relationsbetween task parameters and positive vs. negative findings in a review of theliterature.

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The MTS measures distance and immobility specifically predicted levelof hyperactivity in that children who were rated as more hyperactive/impulsive also had a higher level of motor activity. This is in line with ourpredictions, based upon the logical assumption that MTS would primarilybe related to hyperactivity/impulsivity, as this DSM scale arguably describesbehaviours that reflect activity. Thus, this result indicates ‘‘reciprocal’’construct validity of our motor activity parameters and the Hyperactivity/Impulsivity scale.

Analyses of the time-on-task measures showed relations to ADHDsymptoms for MTS measures; increased motor activity (assessed by distanceand immobility) with time on task was associated with higher ratings ofhyperactivity/impulsivity. For CPT performance there were no significantassociations between time-on-task measures and either ADHD symptomdimension. These CPT findings are compatible with Nigg’s (2006) conclusionthat this aspect of performance (i.e., sustained attention as measured by time-on-task decrements) is spared in most cases of ADHD. However, thisconclusion and our results disagree with findings of associations betweenADHD symptoms and time-on-task measures on CPT accuracy (Swaab-Barnewald et al., 2000) and on RT variability (Epstein et al., 2003). Suchdisparities across studies might be due to differences in sample selectioncriteria or particular ADHD subtype studied or, as discussed earlier,variations in task parameters. Further studies are needed to resolve this issue.

An interesting question in this study concerned the effect of age onassociations between objective measures and symptoms of ADHD. Inaccordance with the developmental hypothesis of ADHD (e.g., Barkley,2003; Hart et al., 1995; Lahey et al., 1998), predicting that theneuropsychological and behavioural manifestation of ADHD should differdepending on a child’s developmental level, the relations between severalCPT parameters (i.e., commission errors, omission errors, and variability inRT) and inattention were dependent on age. It is interesting that it was onlyfor the inattention dimension that relations to CPT parameters weredependent on age, particularly in view of previous findings indicating thatratings of inattention in younger and older children do not reflect exactly thesame neuropsychological functions (see Brocki & Bohlin, 2006). In thisstudy, the two error types and variability in RT were related to inattentionprimarily in older children. It should be noted, however, that although ourdevelopmental data are intriguing, the effects of age were relatively small.This was true also for the relations between CPT and MTS measures and theADHD dimensions themselves, a conclusion shared with Epstein et al.(2003). Nonetheless, the age effects were significant and may best serve as animpetus for future research treating ADHD as an unfolding developmentalpathway, rather than as a static disorder.

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Summary and conclusions

In addition to providing normative developmental data on measures of CPTperformance, this study is one of the first to provide developmental data onmotor activity as measured with actigraphy. Further, the presented data onage-related changes in motor activity and CPT performance as a function oftime on task are very uncommon, particularly so for the measures of motoractivity. Further, the results showing continuous linear associations betweenthe two ADHD symptom domains and CPT and MTS parameters, addfurther support to available research describing the nature of ADHD as acontinuous dimension with variable expression throughout the generalpopulation. The results indicating that increasing motor activity over timeon task is related to hyperactivity/impulsivity, while deterioration of CPTperformance over time is related to neither ADHD symptom dimension, areintriguing and should stimulate further research. Moreover, our data givepromise to the developmental hypothesis of ADHD, by showing that agematters in the relation between neuropsychological function, as measured byCPT, and ADHD behaviours in a large normative sample.

Manuscript received 20 June 2007

Revised manuscript accepted 12 December 2007

First published online 11 April 2008

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