JOURNAL OF LEARNING DISABILITIESVOLUME 40, NUMBER 3, MAY/JUNE 2007, PAGES 194–202

Cognitive Functioning in Children With and Without Attention-Deficit/HyperactivityDisorder With and Without Comorbid Learning Disabilities

Angela Jakobson and Eve Kikas

Abstract

This study attempted to determine whether children with the combined subtype of attention-deficit/hyperactivity disorder (ADHD)have impairments in cognitive functioning and motor skills. The specific effect of the comorbidity of learning disabilities (LD) was alsoinvestigated. A battery of cognitive tests was administered to 26 children with a clinical diagnosis of ADHD–combined subtype(ADHD/C), to 24 children with ADHD/C with a comorbid diagnosis of LD (ADHD/C+LD), and to 102 participants without disabili-ties, all between ages 7 and 10. The testing battery consisted of tasks assessing memory, visuospatial and verbal abilities, and fine motorskills. In general, the test results of children with ADHD/C were poorer than those of the control group but better than the results of chil-dren with a combined ADHD/C+LD diagnosis (with the exception of motor skills). The predictive accuracy of the testing battery tasksin children with ADHD/C and ADHD/C+LD was examined. The results of a standard procedure of discriminant function analysesrevealed that the measures correctly classified 73.6% of the children.

Attention-deficit/hyperactivitydisorder (ADHD) is one of themost prevalent childhood dis-

orders, occurring in 3% to 5% of school-age children (American Psychiatric As-sociation, 1994). Symptoms of ADHDinclude high levels of activity, impul-sivity, and inattention, which may leadto difficulties in scholastic, social, andfamily contexts. In the current nomen-clature, the fourth edition of the Diag-nostic and Statistical Manual of MentalDisorders (DSM-IV; American Psychi-atric Association, 1994), ADHD is sub-typed into (a) the predominantlyhyperactive–impulsive type, (b) thepredominantly inattentive type, and(c) the combined type. Most childrenwith ADHD have symptoms of bothinattention and hyperactivity—thus,of the combined type (American Psy-chiatric Association, 1994). Chhabil-das, Pennington, and Willcutt (2001)found that the DSM-IV subtypes ofADHD differ in demographic charac-

teristics, types of functional impair-ment, and profiles of comorbidity withother childhood disorders. However, itis unclear whether the DSM-IV sub-types differ neuropsychologically. Bark-ley (2001) argued that the inattentivesubtype of ADHD may not be a sub-type at all, but an entirely distinct dis-order having a qualitatively differentform of inattention from that seen inADHD. However, Hinshaw (2001) andLahey (2001) argued that this distinc-tion between the predominantly in-attentive and the combined types ofADHD may be premature. Relativelyconsistent across viewpoints, however,is the opinion that a subset of childrenwith only high levels of inattentionprobably has a qualitatively differentproblem in attention than is seen inchildren with ADHD.

ADHD shows frequent comor-bidity with learning disabilities (LD;Kamphaus & Frick, 1996; Stanford &Hynd, 1994); reported comorbidity

rates range from 10% (Epstein, Shay-witz, Shaywitz, & Woolston, 1991) to ashigh as 80% (McGee & Chare, 1988)The learning problems that co-occurwith ADHD have been identified asclinically significant due to the findingthat they cause more disruption for thechild and suggest poorer assessmentand intervention outcomes than thediagnostic features of ADHD alone(Kamphaus & Frick, 1996).

Earlier studies have shown quitecontroversial results in the cognitivefunctioning of children with ADHD,which may be caused by variousmethodological problems, of whichsmall sample sizes, inconsistent selec-tion criteria, and the failure to controlfor potentially confounding comorbiddisorders are the most common (Bark-ley, 1997). The comorbidity of LD mightbe a confounding factor in many stud-ies, but its effects have not been exten-sively studied. The goal of this studywas to examine the cognitive profiles

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of children with ADHD–combinedsubtype (ADHD/C) and children withADHD–combined subtype and comor-bid LD (ADHD/C+LD).

Children with ADHD have beenshown to achieve poor results in visuo-spatial tasks, which, are usually com-plex, involving the integration ofmultiple cognitive operations (e.g., plan-ning, organization, working memory,and sustained attention). For example,the copied products of children withADHD on the Rey-Osterrieth ComplexFigure are significantly less organizedthan those of control children (Grod-zinsky & Diamond, 1992; Shin, Kim,Cho, & Kim, 2003).

A number of studies have demon-strated that children with ADHD havepoorer working memory than those in control groups (Barkley, 1997; Mc-Innes, Humphries, Hogg-Johnson, &Tannock, 2003). Working memory per-formance is influenced by the mean-ingfulness of the task: When memorytasks are semantically and spatiallymeaningful, the performance of chil-dren with ADHD may be similar tothat of children without disabilities(Adams, Sheslow, Robins, Payne, &Wilkinson, 1991). Furthermore, chil-dren with both ADHD and LD showconsiderably poorer results in memorytasks than those with “pure” ADHD(Johnson, Altmaier, & Richman, 1999;Shin et al., 2003).

Children with ADHD are physi-cally more active (American Psychi-atric Association, 1994) but less able to control their motorics (Denckla &Rudel, 1978) and less coordinated infine-motor tasks (Barkley, 1997) thanchildren wthout disabilities. Kalff et al.(2003) found that children at risk forADHD were in general less accurateand more variable in their movementsthan children with other psychopa-thologies and healthy controls. How-ever, immature motor coordination oc-curs less frequently than cognitivedeficits (Njiokiktjien, 1989).

Deficits in language skills mostlyinvolve the use of language to accom-plish social action (i.e., the pragmaticfunction of language). For example,

children with ADHD lack the ability touse self-talk, which is critical for beingable to mediate their behavior and fol-low prescribed rules; the topic of con-versation is frequently switched, andtheir responses are not related to ques-tions asked (Heyer, 1995). These defi-cits in language skills may cause com-munication difficulties, which mayresult in social problems. Childrenwith ADHD also lack the ability tokeep their thoughts in a logical order,to organize and to categorize linguisticinformation (Heyer, 1995). Lorch et al.(2000) found that children with ADHDcomprehended factual details fromtelevised stories as well as typical chil-dren did, but perform poorer in un-derstanding and recalling causal rela-tionships among events in the story.Children with LD have problems bothin analyzing spoken language and inretelling the narratives (Korkman &Pesonen, 1994).

Observations in multiple environ-ments, behavioral rating scales, struc-tured interviews, achievement testing,and medical evaluations are used to as-sess ADHD (Barkley, 1998; Kamphaus& Frick, 1996). Whereas self-reportsand rating scales may be influenced byrecall biases, halo effects, and so on,cognitive measures directly assess per-formance, providing validating criteriafor ADHD and information for inter-vention. Neuropsychological tests aiddiagnosis and treatment (by identify-ing adaptive capacities, strengths, andlimitations) but are also used to assesschange over time (Seidman, 1997). Ifthe question about LD in a child withADHD arises, a comprehensive assess-ment of cognitive and academic skillsis essential (Dewey, Kaplan, Crawford,& Fischer, 1998)

There are numerous different testbatteries that evaluate cognitive defi-cits of children with ADHD and alsodiscriminate individuals with ADHDfrom children without disabilities. Bark-ley (1998) did not establish a basis forsupporting the routine administrationof neuropsychological batteries in atypical ADHD evaluation. He reportedthat no single subtest or combination

of subtests within the Halstead-ReitanNeuropsychological Test Battery and theLuria-Nebraska Neuropsychological Bat-tery, which are two formal neuropsy-chological assessment batteries, dis-played sufficient predictive validity.Doyle, Biederman, Seidman, Weber,and Faraone (2000) have also foundthat a single neuropsychological testhad limited discriminating ability. Thetest battery used in the current studyevaluates the working memory, finemotor skills, and visuospatial and ver-bal abilities of participating childrenand has been previously used formeasuring cognitive abilities of chil-dren without disabilities in elementaryschool. The battery tasks were devel-oped based on the theoretical viewsand neuropsychological tests of Luria(1969). As the battery measures abili-ties that are expected to be poorer inthe case of children with ADHD andLD, we decided to evaluate whetherthe battery is useful in discriminatingchildren with disabilities.

When assessing people withADHD, the selection and sequence ofpresenting tasks is of importance. Therepetitive and boring nature of manycognitive tasks may not elicit maxi-mum cognitive performance, and find-ings may reflect motivational issuesrather than psychological deficits (Le-zak, 1995). Therefore, in the presentstudy, the order of presentation oftasks was randomized. The childrenappeared to enjoy the test and readilyengaged in the tasks. The present studywas based on a sample of children withADHD/C. Elementary school childrenwere selected because children experi-ence more problems in elementaryschool than before; more schoolchild-ren are referred to psychiatrists and di-agnosed than younger children. Also,there is a need for intervention plans,for which developing a knowledge of achild’s cognitive profile is necessary.This study controlled for comorbid di-agnoses. Moreover, groups did not dif-fer in sex, age, or ethnicity.

On the basis of earlier studies,predictions were that (a) the general taskperformance of the ADHD/C group

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would be poorer than that of the con-trol group, whereas (b) children withcomorbid LD would show poorer per-formance than those with “pure”ADHD/C, with the exception of tasksmeasuring fine motor movements,where differences were not expected.Specifically, we hypothesized the fol-lowing:

1. The performance of children withADHD/C on the tasks measuringworking memory would be poorerthan that of the control group; theperformance of children withcomorbid LD on tasks measuringmemory would be poorer than thatof children with pure ADHD/C.Children with ADHD/C, includingthose with comorbid LD, wouldperform as well as the controlgroup children in tasks that aresemantically meaningful.

2. The visuospatial abilities of chil-dren with ADHD/C would bepoorer than those of the controlgroup; the performance of ADHD/C+LD and pure ADHD/C groupswas expected to be similar.

3. The performance of children withADHD/C on tasks of fine motorskills would be poorer than that ofthe control group.

4. The results of children with ADHD/C+LD on tasks measuring verbalabilities would be considerablypoorer than that of the ADHD/Cand the control groups.

Method

Procedure and Inclusion Criteria

The ethics committee of the Universityof Tartu approved the study. First, let-ters explaining the aims of the studywere sent to the parents. Signed in-formed consent was obtained fromparents, and children gave verbal as-sent.

Children with ADHD/C andADHD/C+LD were diagnosed by apsychiatrist and a psychologist; diag-noses were in accordance with the cri-

teria in DSM-IV (American PsychiatricAssociation, 1994). The psychiatricevaluations involved a semistructuredinterview with the parents and withthe child (including an assessment ofthe child’s history and current symp-toms), clinical observations of thechild, and an analysis of the ratingscales and questionnaires completedby the parents and teachers. The chil-dren in the ADHD/C group met theDSM-IV criteria for ADHD–combinedtype, and the comorbid LD group metthe criteria for a reading disorder or adisorder of written expression. The di-agnosis of comorbid LD was based on(a) the evaluations of a school psychol-ogist or a speech therapist who testedand interviewed all participants sepa-rately; (b) interviews with teachers onthe child’s behavioral, academic, andsocial functioning; and (c) interviewswith parents that provided a detaileddescription of each child’s behavioraldifficulties together with his or her de-velopmental and medical history. Asthere are no current standardized testsmeasuring LD, the tests measuringchildren’s academic achievement inthe Estonian language and mathemat-ics are generally used. In our study, theLD group included children whosereading aptitude or writing skills wereremarkably lower than the level com-mon to their age group, general intelli-gence, and school class.

Children were individually testedin a quiet room in psychiatric hospitals(clinical groups) or schools (controls).A group of postgraduate students withspecific training and supervision forthe task carried out the testing inschools. Clinical groups were tested bythe first author. Depending on disorderand age, the duration of the task forone participant ranged from 45 to 150min.

Participants

The sample consisted of 152 childrenages 7 to 10 years. The ADHD/C groupconsisted of 26 children (25 boys, 1 girl;age M = 8.5 years, SD = 1.10) who metthe DSM-IV criteria for ADHD–com-

bined type. The ADHD/C+LD groupincluded 24 children (23 boys, 1 girl;age M = 8.7 years, SD = 0.99) who met the DSM-IV criteria for ADHD–combined type and also the criterionfor LD. Participants on stimulant med-ications (3 children in the ADHD/Cgroup were prescribed Ritalin) discon-tinued their medication at least 24hours prior to participating in thisstudy. The ratio of male and femaleparticipants in clinical groups reflectedthe gender prevalence estimates sug-gesting that ADHD is more frequentlydiagnosed in male than in female indi-viduals (American Psychiatric Associ-ation, 1994). The participants diag-nosed with ADHD/C and ADHD/C+LD were from municipal schools inEstonia.

Each clinical group child was in-dividually matched on age (±1months) and gender with two childrenin the control group (age M = 8.6 years,SD = 1.05). The control group childrenwere recruited from a municipal ele-mentary school in Estonia. The controlgroup was originally larger, but a sam-ple of 102 children was selected by ran-domly taking into account the indi-vidual matching with clinical groupchildren. The criteria for inclusionwere the absence of neurological symp-toms and lack of history of behavioralor emotional problems in school or athome. This information was obtainedby interviewing parents and teachers.Moreover the results of the academicachievement test did not meet the cri-teria for LD. Also, teachers assessedthe children’s achievement in all sub-ject areas average and above as com-pared to their classmates’.

There was no significant groupdifference for age, F(2, 149) = 0.86, p <.42. The majority of the children camefrom families in the middle range ofsocioeconomic status (SES). Also, thegroups did not differ in ethnicity.

Instruments

The tasks were developed by a re-search team of the Departments of Spe-cial Education and Psychology of the

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University of Tartu (members includedAaro Toomela, Marika Padrik, andKarl Karlep). These tasks were devel-oped based on the theoretical views ofLuria (1969), on his neuropsychologi-cal test battery, and on later neuro-psychological studies (see Luria, 1969;Scribner, 1997; Toomela, 2002; Toomela,Tomberg, & Orasson, 1999). More than250 typical elementary school childrenand 200 preschool children have beentested with these tasks in another,larger study. The reliability and valid-ity of the tasks has been shown to begood (e.g., Karlep, 2000; Toomela,2002; Toomela et al., 1999). As in sev-eral other neuropsychological batteries(see Lezak, 1995), we used separateitems, not longer scales; therefore, theinternal consistency cannot be calcu-lated. The battery consisted of 15 tasks.The sequence of task presentation wasnot fixed. Ten different orders of testswere created pseudorandomly and as-signed to participants consecutively inthe order that the participants weretested.

Visuospatial abilities were as-sessed with five tests. In two ContourExtraction tests, the child was pre-sented with a contour picture of ahouse in a garden. Six parts of the samepicture were presented to the child,and she or he was asked to indicatewhere the part was located in the pic-ture. Half the parts could be easilynamed (e.g., chimney); the rest includedparts of the contours of several objectsand were not so easily verbalized. Thenumber of correctly identified abstract(with a maximum of 3 points) and con-crete (maximum 3 points) objects wasrecorded.

In the Block Construction test(Toomela, 2002), a child was givenLego™ blocks of two different sizes,and she or he was asked to build con-structions presented on line drawings.There were four such drawings. Allconstructions were extended in threedimensions. The number of correctconstructions was recorded (with amaximum of 4 points). This task re-quired the integration of visuospatialskills with motor output.

There were two types of MentalRotation tasks (see Luria, 1969; Scrib-ner, 1997; Toomela, 2002). In theMoomies test (4 tasks), a Moomie™ waspresented together with three rotatedMoomies. A child was asked, “Look atthis Moomie. If this Moomie fell downinto the snow, which trace would therebe—this, this, or this?” The number ofcorrect answers was recorded (maxi-mum 4 points). In the Hands test, fourline drawings of hands (two left andtwo right hands) were presented. Thechild was asked to indicate or saywhether the hand represented in thepicture was left or right. It was not re-quired for children to give verbal an-swers. It was sufficient to raise thehand that corresponded to that on thepicture. The number of correct answerswas recorded (maximum 4 points).

Working memory tasks were bothverbal and visuospatial. Memory forWords was tested by means of two ver-bal free recall tasks (see Toomela, 2002;Toomela et al., 1999). Two lists of 16 rel-atively unrelated verbs and adjectiveswere read out to the child. The numberof correctly recalled words was re-corded (maximum 32 points).

Memory for Sentences was testedby means of two sets of sentences. Eachset comprised 11 sentences. The first ofthe sentences was 2 words long; oneword was added to every next sen-tence, so that the longest sentence was12 words long. Sentences were pre-sented in the same order, from theshortest to the longest. Sentences wereread out to the child. The number ofwords in the longest sentence recalledcorrectly was recorded. The results ofperformance in two sets of sentencetasks were summed (maximum 24points).

Memory for Objects was tested bymeans of two sets of four geometricshapes. Geometric shapes were createdin two steps. First, a set of 40 shapeswas created. After that, 10 undergrad-uate students of psychology wereasked to name the shapes. The shapesthat were the hardest to name werechosen. The number of correctly recog-nized shapes out of 9 (in the second

task, out of 12) was recorded. The re-sults of performance over the two setsof shapes were summed (maximum 8points).

Memory for Spatial Relations wasmeasured by means of a test in whichchildren were presented with a displayof six cylindrical (2 × 0.5 cm) pegsplaced on a 20 × 20 cm piece of paperwith landmarks in predetermined po-sitions. The child was required to re-member the positions of the pegs. Pegswere removed from the paper, and thechild was asked to put them back in ex-actly the same places. The same proce-dure was repeated with a white card(no landmarks). Numbers of correctlyplaced pegs were recorded (maximum12 points). The placement was scoredcorrect when the center of a peg placedby the child was in an area covered bya peg presented by the experimenter.

Fine motor skills were measuredby means of two tasks: Tower Buildingand Necklace Making (Toomela, 2002).First, the child was instructed to builda tower as high as possible out of smallelements in 20 seconds. Second, thechild was given a small rod with a cordconnected to one end of the rod. He orshe had to put small pearls onto thecord in 20 seconds. These tasks assessthe child’s ability to produce rapid, ac-curate, and coordinated finger move-ments.

Verbal abilities were assessed bymeans of four tests. Verbal Reasoningwas measured by means of six syllo-gisms (see Scribner, 1997). Three of thesyllogisms were commonplace with re-gard to subject matter (e.g., “A rabbitand a mouse are friends and always eattogether. The rabbit is eating now. Isthe mouse eating now?”), the otherthree were unusual in content (e.g., “InDotland no dog can bark. Spot lives inDotland. Can Spot bark?”) All syllo-gisms required a yes or no answer. Thenumber of correct answers wasrecorded (maximum 6 points).

In the Picture Naming task (Too-mela, 2002), two sets of 16 pictures de-picting common objects (e.g., tools)were presented to the child. Objectswere presented from two perspec-

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tives—from the front and sideways.The number of correctly named pic-tures was recorded (maximum 32points).

Understanding Sentences (Karlep,2000) was measured by means of a testin which children were required to in-dicate a picture matching the pre-sented sentence in a forced-choice par-adigm. In the sentences, differentrelations between objects were ex-pressed. Some relations were usual,and some were unusual (e.g., “A lampis on the table” versus “A table is onthe lamp”). There were four pictureswith the same objects in different rela-tions. Four sentences were presented.The number of correctly identified pic-tures was recorded (maximum 8 points).

In the Categorization task (Karlep,2000) children were presented with agroup of pictures of objects. They wereasked to select objects on various levelsof generality (e.g., “Give me all mo-

biles”). The number of correctly se-lected pictures was recorded (maxi-mum 46 points).

Data Analyses

First, an analysis of variance (ANOVA)and planned comparisons to checkspecific differences among the threegroups were carried out (with the LSDtest used in post hoc analyses). Second,a discriminant function analysis wasconducted to determine the suitabilityof the tasks for predicting membershipof the ADHD/C, ADHD/C+LD, andcontrol groups. Because of the rela-tively small number of participants,the effects caused by age were notanalyzed.

Results

Means, standard deviations, and theresults of ANOVAs are presented in

Table 1. Here we describe only thegroup differences revealed by LSDtests.

Visuospatial Abilities

Post hoc tests revealed that childrenwith ADHD/C+LD performed signifi-cantly lower than the control group inContour Extraction–Abstract Objects(p = .02), Block Construction, and Men-tal Rotation–Moomies (p < .001). More-over, children with ADHD/C per-formed lower than controls in BlockConstruction (p = .007) and Mental Ro-tation–Moomies (p < .001). Clinicalgroups did not differ significantly fromeach other on any test.

Memory

LSD tests showed that children withADHD/C+LD performed significantlylower than the control group in Mem-

TABLE 1Means and Standard Deviations of Cognitive Tasks Used in the Study, with ANOVA Results and Effect Sizes

Controla ADHD/Cb ADHD/C+LDc

Task M SD M SD M SD F Planned comparisons Cohen’s d

Visuospatial AbilitiesContour Extraction

Concrete Objects 2.82 0.38 2.62 0.50 2.79 0.41 2.69* C > ADHD/C 0.45Abstract Objects 0.73 0.95 0.54 0.71 0.25 0.44 3.19** C > ADHD/C+LD 0.65

Block Construction 2.59 1.14 1.92 1.09 1.42 0.93 12.75*** C > ADHD/C; ADHD/C+LD 0.60; 1.12Mental Rotation

Moomies 3.32 0.86 2.65 1.16 2.63 0.92 9.14*** C > ADHD/C; ADHD/C+LD 0.02; 0.78Hands 2.72 1.20 2.62 1.20 2.33 1.09 1.00

Working MemoryMemory for Words 8.60 2.55 7.35 3.02 6.32 2.40 8.02*** C > ADHD/C; ADHD/C+LD 0.45; 0.92Memory for Sentences 13.59 3.14 12.35 2.90 11.61 2.31 5.05*** C > ADHD/C+LD 0.72Memory for Objects 5.49 1.38 4.73 1.69 4.57 1.20 5.88*** C > ADHD/C; ADHD/C+LD 0.49; 0.71Memory for Spatial Relations 6.87 2.08 4.58 2.27 4.42 2.54 19.81*** C > ADHD/C; ADHD/C+LD 1.05; 1.05

Fine-Motor SkillsTower Building 14.50 2.23 13.31 2.31 13.58 1.86 3.98* C > ADHD/C 0.52Necklace Making 10.59 2.66 9.62 2.08 10.48 2.50 1.52

Verbal AbilitiesVerbal Reasoning 3.43 1.69 2.65 1.50 2.21 1.25 6.93*** C > ADHD/C; ADHD/C+LD 0.49; 0.82Understanding Sentences 7.57 0.88 7.23 1.27 7.08 0.88 3.18* C > ADHD/C+LD 0.56Picture Naming 25.31 3.46 25.27 2.60 24.21 2.62 1.18Categorization 40.98 3.89 41.52 2.84 36.79 5.41 11.96*** C > ADHD/C; ADHD/C+LD 0.89; 1.09

Note. C = control group; ADHD/C = children with attention-deficit/hyperactivity disorder–combined type; ADHD/C+LD = children with attention-deficit/hyperactivitydisorder–combined type and comorbid learning disabilities.an = 102. bn = 26. cn = 24.*p < .05. **p < .01. ***p < .001.

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ory for Sentences (p = .005), Memoryfor Words, Memory for Objects, andMemory for Spatial Relations (p <.001). Moreover, the ADHD/C groupperformed lower than controls inMemory for Words (p = .03), Memoryfor Objects (p = .01), and Memory forSpatial Relations (p = .005). Clinicalgroups did not differ significantly fromeach other on any test.

Fine-Motor Skills

In the Tower Building task, the controlgroup was significantly faster andmore precise than the ADHD/C group(p = .01). All the other differences werenonsignificant.

Verbal Abilities

Post hoc tests revealed that theADHD/C+LD group performed sig-nificantly lower than the control groupin Verbal Reasoning (p < .001), Under-standing Sentences (p = .02), and Cate-gorization (p < .001). The ADHD/Cgroup performed significantly worsethan the control group in Verbal Rea-soning (p < .001). The ADHD/C+LDand ADHD/C groups differed fromeach other on the Categorization task(p < .001).

Discriminant Function Analysis

The discriminant function calculatedwas found to be significant, Wilks’ λ =.50, F(30, 262) = 3.67, p < .001, indicat-ing that a reliable relation existed be-tween the groups and the predictortasks. The loading matrix of correla-tions between predictor tasks and dis-criminant functions is depicted inTable 2. The best predictors for distin-guishing control group children fromchildren with ADHD/C+LD are theBlock Construction, Mental Rotation–Moomies, Categorization, and allmemory tasks. The best predictors fordistinguishing children with ADHD/Cfrom children with ADHD/C+LD arethe Contour Extraction–Concrete Ob-jects and Categorization tasks.

The discriminant analysis used 15tasks as predictors to classify partici-pants into one of three groups: theADHD/C, ADHD/C+LD, and non-ADHD group. The classification re-sults presented in Table 3 indicate that113 out of 152 children were classifiedcorrectly. All in all, 73.6% of the chil-dren were correctly classified: 80% ofthe ADHD/C group, 72.7% of theADHD/C+LD group, and 72.3% of thecontrol group (see Table 3). The dis-criminant function classified 21 out of26 children with ADHD/C as belong-ing to the ADHD/C group, with 3 chil-dren classified as controls and 2 asADHD/C+LD. Children without dis-

abilities were most commonly misclas-sified as having ADHD/C.

DiscussionVisuospatial abilities, working mem-ory, fine motor skills, and verbal abili-ties were assessed in three groups of el-ementary school children. As opposedto many earlier studies, we differenti-ated between children with a “pure”diagnosis of ADHD/C and a diagnosisof ADHD/C+LD (see Barkley, 1997;Riccio, Gonzalez, & Hynd, 1994). Bothabstract and meaningful materialswere used (see Adams et al., 1991).Children appeared to enjoy the test,

TABLE 2Structure Matrix of Correlations Between Predictor Variables and

the Discriminant Function

Task Function 1 Function 2

Contour Extraction–Concrete Objects .128 .347

Contour Extraction–Abstract Objects .268 −.041

Block Construction .547 −.044

Mental Rotation–Moomies .421 .230

Mental Rotation–Hands .141 −.101

Tower Building .252 .227

Necklace Making .128 .291

Memory for Words .428 −.068

Memory for Sentences .365 −.047

Memory for Objects .361 .095

Memory for Spatial Relations .689 .273

Verbal Reasoning .396 .042

Understanding Sentences .288 −.010

Picture Naming .162 −.136

Categorization .392 −.564

TABLE 3Classification Results of Discriminant Function Analysis

Actual membership Predicted membership

Group n Control ADHD/C ADHD/C+LD

Control 102 74 18 10

ADHD/C 26 3 21 2

ADHD/C+LD 24 3 3 18

Total 152 80 42 30

Note. ADHD/C = children with attention-deficit/hyperactivity disorder–combined type; ADHD/C+LD = children with attention-deficit/hyperactivity disorder–combined type and comorbid learning disabilities.

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and they had no difficulties in under-standing the instructions. None of thetasks in this study have been used toassess children with ADHD before.

As expected, children withADHD/C and ADHD/C+LD had dif-ficulties in tasks demanding sophisti-cated cognitive integration (see Grod-zinsky & Diamond, 1992; Shin et al.,2003). The ability of children with acombined diagnosis of ADHD/C+LDto process visual information (e.g., ro-tating objects in the mind, distinguish-ing between objects on the basis of con-tours) was considerably lower thanthat of the control group. Tasks involv-ing operations with abstract constructswere especially difficult for the com-bined diagnosis group. Similar diffi-culties were observed in workingmemory tasks requiring the organiza-tion of nonsignificant material. Themore successful performance of chil-dren with ADHD/C only compared tothe combined diagnosis group couldbe explained by a somewhat loweramount of distractors in tasks (see alsothe results of the nonverbal Memoryfor Objects task). In tasks demandingmore sophisticated cognitive integra-tion (e.g., Block Construction), the per-formance of the clinical groups waspoorer than that of the controls.

In concordance with earlier stud-ies (Barkley, 1997; McInnes et al., 2003)and with our second hypothesis, animpairment in working memory taskswas found. In the visuospatial andword memory tasks, both clinicalgroups did worse than the controlgroup but did not differ between them-selves. Poorer performance in the non-verbal memory tasks demonstratedthat the concurrent visual processingabilities were hampered by the distrac-tion of stimuli and by a poorer abilityto relate objects to each other. Weakerperformance in the Memory for Ob-jects task demonstrated that thesechildren were not able to distinguishpreselected objects among a number ofdistracting objects. The poorer perfor-mance on both these tasks could becaused by weaker problem-solvingskills, such as the inability to use land-

marks to memorize the position (seeHeyer, 1995). The tendency of childrenwith ADHD/C+LD to show worse re-sults on memory tasks compared tothose with pure ADHD/C (Johnson etal., 1999; Shin et al., 2003) was seen inour study as well; however, these dif-ferences were not statistically signifi-cant.

Whereas the concreteness andmeaningfulness of the task seemed tohelp children with ADHD/C, it did notalways help children with ADHD/C+LD. For example, the Memory forSentences results of the ADHD/C andthe control group did not differ signif-icantly, whereas the results of childrenwith ADHD/C+LD differed from thoseof children without disabilities. Also,the differences between groups couldbe explained by a deficit of verbal abil-ities caused by comorbid LD. Whereaschildren in the ADHD/C and the con-trol group were using meanings cre-ated by interverbal relations as a mne-monic tool, the ADHD/C+LD groupfailed to do so because of the saiddeficit in verbal abilities. The results ofnaming the concrete colorful objects inpictures (in the Picture Naming task)were similar for all three groups. Ear-lier studies have also revealed that theperformance of the ADHD and thecontrol groups in semantically andspatially meaningful tasks are moresimilar than in meaningless tasks(Adams et al., 1991).

Our third hypothesis was basedon the assumption that lower verbalabilities have been seen specifically inchildren with combined ADHD andLD (see World Health Organization,1992). Difficulties in understandingsyntax were seen in the UnderstandingSentences task. This task assesses theunderstanding of words describing re-lations between objects (e.g., front,back, above, beside). Difficulties in ab-stract thinking were seen in the Cate-gorization task. Children with ADHD/C+LD also performed poorly in tasksinvolving operations with abstractconstructs and organizing non-experi-ential or not easily interpretable mate-rial (e.g., Contour Extraction–Abstract

Objects). In the Verbal Reasoning task,both clinical groups gave significantlymore false answers than the controlsdid. Performing this task demandedthat children were precise in followinginstructions and suppressed previousknowledge and experience. To providean answer to the riddle, the child hadto follow new information provided inthe task. Earlier studies have similarlyshown that children with ADHD havea poorer understanding of causal rela-tions (Lorch et al., 2000).

It has been stressed that childrenwith ADHD are active and quick butnot able to control their motorics(Barkley, 1997; Denckla & Rudel, 1978).It seems that as speed was of more im-portance than accuracy in the NecklaceMaking task, no significant differencesoccurred between groups. In contrast,Tower Building demands good coordi-nation, and the ADHD/C group didworse than the control and ADHD/C+LD groups. Still, one striking find-ing was that the ADHD/C+LD group’sperformance didn’t differ significantlyfrom either group. Children withADHD/C had more difficulties thanthe ADHD/C+LD group with finemotor tasks, but the differences werenot statistically significant. To clarifythe results, more studies are neededwith children with pure LD.

One of the purposes of this studywas to evaluate the usefulness of thecognitive battery in children withADHD. The discriminant analysisshowed that the test battery correctlyclassified 73.6% of the cases. The dis-criminant function classified 21 out of26 children with ADHD/C as belong-ing to the ADHD/C group, and 18 outof 24 children with ADHD/C+LD asbelonging to the ADHD/C+LD group.Children without disabilities weremost commonly misclassified as hav-ing ADHD/C. The battery cannot beused in isolation when diagnosingADHD/C, but it may be used as an ad-ditional tool in assessment. Also, theresults provide a child’s cognitive pro-file—a picture of his or her strengthsand weaknesses, which is valuable inplanning interventions.

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Clearly, one of the limitations ofthis study is the lack of control for chil-dren’s cognitive abilities. This was dueto the current situation in Estonia: Wedo not yet have adapted and standard-ized IQ tests or norms for elementaryschool children. In the current study,we checked the results of children’sstate-level tests in Estonian and math-ematics. Also, it should be noted thatall the children studied in general edu-cation municipal schools and not inspecial education schools. However,there still is a possibility of potentialconfounds.

The present findings have broadimplications for the school system.Knowledge of the cognitive processingcapabilities of children with ADHD iscritical for understanding and stimu-lating the academic performance ofthese children. The study results mayprovide some important insights in con-structing appropriate educational in-terventions for children with ADHD/C.

ABOUT THE AUTHORS

Angela Jakobson, MSc, is a PhD student inthe Faculty of Education, University of Tartu,Estonia. Her current interests include cognitiveabilities of children with ADHD and occupa-tional health psychology. Eve Kikas, PhD, is aprofessor of preschool and primary school edu-cation in the Faculty of Education, Universityof Tartu. Address: Angela Jakobson, MSc, Hiie38, Tartu 51006, Estonia; e-mail: jakobson@mk.ee

AUTHORS’ NOTES

1. This article was written with the support ofa grant from the Estonian Science Founda-tion (Grant No. 5371).

2. We thank Aaro Toomela for helpful com-ments, and Heikki Kalle and Ott Heinapuufor help with the language.

REFERENCES

Adams, W., Sheslow, D., Robins, P., Payne,H., & Wilkinson, G. (1991, August). Mem-ory abilities in children with attention deficithyperactivity disorder. Paper presented atthe meeting of the American Psychologi-cal Association, San Francisco, CA.

American Psychiatric Association. (1994).Diagnostic and statistical manual of mental

disorders (4th ed.). Washington, DC: Au-thor

Barkley, R. (1997). Behavioural inhibition,sustained attention, and executive func-tions: Constructing a unifying theory ofADHD. Psychological Bulletin, 121, 65–94.

Barkley, R. (1998). Attention deficit hyperac-tivity disorder: A handbook for diagnosis andtreatment. New York: Guilford.

Barkley, R. (2001). The inattentive type ofADHD as a distinct disorder: What re-mains to be done. Clinical Psychology Sci-ence and Practice, 8, 489–493.

Chhabildas, N., Pennington, B. F., & Will-cutt, E. G. (2001). A comparison of theneuropsychological profiles of the DSM-IV subtypes of ADHD. Journal of Abnor-mal Psychology, 29, 529–551.

Denckla, M., & Rudel, R. (1978). Anomaliesof motor development in hyperactiveboys. Annals of Neurology, 3, 231–233.

Dewey, D., Kaplan, B., Crawford, S., &Fisher, G. (1998) Predictive accuracy ofthe wide range assessment of memoryand learning in children with attentiondeficit hyperactivity disorder and read-ing difficulties. Developmental Neuro-psychology, 19, 173–189.

Doyle, A., Biederman, D., Seidman, L.,Weber, W., & Faraone, S. (2000). Diagnos-tic efficiency of neuropsychological testscores for discriminating boys with andwithout attention deficit–hyperactivitydisorder. Journal of Consulting and ClinicalPsychology, 68, 477–488.

Epstein, M., Shaywitz, S., Shaywitz, B., &Woolston, J. (1991). The boundaries of at-tention deficit disorder. Journal of Learn-ing Disabilities, 24, 78–86.

Grodzinsky, G., & Diamond, R. (1992).Frontal lobe functioning in boys with at-tention-deficit hyperactivity disorder.Developmental Neuropsychology, 8, 427–445.

Heyer, J. (1995). The responsibilities ofspeech–language pathologists towardchildren with ADHD. Seminars in Speechand Language, 16, 275–288.

Hinshaw, S. (2001). Is the inattentive type ofADHD a separate disorder? Clinical Psy-chology Science and Practice, 8, 498–501.

Johnson, B., Altmaier, E., & Richman, L.(1999). Attention deficit and reading dis-abilities: Are immediate memory defectsadditive? Developmental Neuropsychology,15, 213–226.

Kalff, A., De Sonneville, L., Hurks, P., Hen-driksen, J., Kroes, M., Feron, F., et al.(2003). Low- and high-level controlled

processing in executive motor controltasks in 5–6-year-old children at risk ofADHD. Journal of Child Psychology andPsychiatry and Allied Disciplines, 44,1049–1057.

Kamphaus, R., & Frick, P. (1996). Clinical as-sessment of child and adolescent personalityand behavior. Boston: Allyn & Bacon.

Karlep, K. (2000). Ruumisuhete verbaliseer-imise oskused erikooli õpilastel [Verbal-ization skills of spatial relationships ofspecial school children]. In Haridus ja sot-siaalne tegelikkus [Education and social re-ality] (pp. 33–39). Tartu, Estonia: TartuÜlikooli Kirjastus.

Korkman, M., & Pesonen, A. (1994). A com-parison of neuropsychological test pro-files of children with attention deficit–hyperactivity disorder and/or learningdisorder. Journal of Learning Disabilities,27, 383–392.

Lahey, B. (2001). Should the combined andpredominantly inattentive types of ADHDbe considered distinct and unrelated dis-orders? Not now, at least. Clinical Psy-chology Science and Practice, 8, 494–497.

Lezak, M. (1995). Neuropsychological assess-ment. New York: Oxford University Press.

Lorch, E., Milch, R., Sanchez, R., Van denBroek, P., Baer, S., Hooks, K., et al. (2000).Comprehension of televised stories inboys with attention deficit/hyperactivitydisorder and non-referred boys. Journal ofAbnormal Psychology, 109, 321–330.

Luria, A. (1969). Vyshije korkovyje funktsiitsheloveka i ikh narushenija pri lokal’nykhporazenijakh mozga [Higher cortical func-tions in man and their disturbances inlocal brain lesions]. Moscow: Izdatel’stvoMoskovskogo Universiteta.

McGee, R., & Chare, D. (1988) Attentiondeficit disorder–hyperactivity and aca-demic failure: Which comes first andwhat should be treated? Journal of theAmerican Academy of Child and AdolescentPsychiatry, 27, 318–325.

McInnes, A., Humphries, T., Hogg-Johnson, S., & Tannock, R. (2003). Listen-ing comprehension and working mem-ory are impaired in attention-deficithyperactivity disorder irrespective oflanguage impairment. Journal of AbnormalChild Psychology, 8, 640–649.

Njiokiktjien, C. (1989). Pediatric behav-ioural neurology. Clinical Principles, 1,254–265.

Riccio, C., Gonzalez, J., & Hynd, G. (1994).Attention-deficit hyperactivity disorder(ADHD) and learning disabilities. Learn-ing Disability Quarterly, 17, 311–319.

JOURNAL OF LEARNING DISABILITIES202

Scribner, S. (1997). Modes of thinking andways of speaking: Culture and logic re-considered. In E. Tobach, R. Falmagne, M. Parlee, L. Martin, & A. Scribner Kapel-man (Eds.), Mind and social practice:Selected writings of Sylvia Scribner (pp. 125–144). Cambridge, UK: Cambridge Uni-versity Press.

Seidman, L. (1997). Neuropsychologicaltesting. In A. Tasman, J. Kay, & J. Liber-man (Eds.), Psychiatry (pp. 498–508).Philadelphia: Saunders.

Shin, M., Kim, Y., Cho, S., & Kim, B. (2003).Neuropsychological characteristics of

children with attention-deficit hyper-activity disorder (ADHD), learningdisorder, and tic disorder on the Rey-Osterreith Complex Figure. Journal ofChild Neurology, 18, 835–844.

Stanford, L., & Hynd, G. (1994). Congru-ence of behavioral symptomatology inchildren with ADD/H, ADD/WO, andlearning disabilities. Journal of LearningDisabilities, 27, 91–103.

Toomela, A. (2002). Drawing as a verballymediated activity: A study of relation-ships between verbal, motor, and visu-ospatial skills and drawing in children.

International Journal of Behavioral Develop-ment, 26, 234–247.

Toomela, A., Tomberg, T., & Orasson, A.(1999). Paradoxical facilitation of a freerecall of nonwords in person with trau-matic brain injury. Brain and Cognition, 39,187–201.

World Health Organization. (1992). TheICD-10 classification of mental and behav-ioural disorders: Clinical descriptions and di-agnostic guidelines. Geneva, Switzerland:Author.