J. Child Fsyihoi Ps}xhm!.\o\. 38, No. 2. p. 207 217. 1997Cambridge Universilj Press

(Jj 1997 Associalion for Child Psychology and PsychiatryPrimed in Gteat Britain All rights reserved

0021 9630/97 515.00 + 0.00

Asperger Syndrome, Autism and Attention Disorders: A ComparativeStudy of the Cognitive Profiles of 120 Children

Stephan Ehlers, Agtieta Nyden, Christopher Gillberg, Annika Dablgren Sandberg, Sven-Olof Dahlgren,Erland Hjelmquist and Anders Oden

University of Goteborg, Sweden

The Wechsler Intelligence Scale for Children-Revised (WISC-R) was applied (in a Swedishversion) in 120 children with Aspcrger syndrome, autistic disorder, and attention disorders.Using stepwise logistic regression analysis, the WISC's discriminating ability was inves-tigated. The overall rate of correct diagnostic classification was 63%. Further. WISCprofiles were analysed within each group. The group with autistic disorder was characterisedby a peak on Block Design. The Asperger syndrome group had good verbal ability andtroughs on Object Assembly and Coding, The group with attention disorders had troughs onCoding and Arithmetic, The results suggest that Kaufman's Verbal Comprehension,Perceptual Organisation and Freedom from Distractibility factors rather than verbal !Q andperformance IQ account for the variance on the WISC, Furthermore, the Aspergersyndrome and autistic disorder groups differed in respect of "fluid" and "crystallised"cognitive ability.

Keywords: Autism, Asperger syndrome, ADHD, WISC-R, children.

Abbreviations: ADHD: attention deficit hyperactivity disorder; DAMP: deficits in atten-tion, motor control and perception; PLSD: Fisher's Protected Least SignificantDifference test; WAIS-R: Wechsler Adult Intelligence Scale-Revised: WISC R:Wechsler Intelligence Scale for Children-Revised.

IntroductionOver the last 20 years, interest in the differential

diagnosis of "mild Kanner autism" has grown, par-ticularly after Wing (1981) pubhshed her account of 34cases with Asperger syndrome. Today, autism (AmericanPsychiatric Association, 1987, 1994; World Health Or-ganisation, 1992) is no longer conceptualised as onedisease entity with a very narrow phenotype and onedistinct etiology (Bailey, 1993; C, Gillberg 8i Coleman,1992; Wing, 1989). Rather, it is believed to constitute a"spectrum disorder", with Asperger syndrome (Asper-ger, 1944; Wing, 1981) and autism (Kanner. 1943)representing subgroups of a larger population of childrenwith social impairment, Asperger syndrome is believed tobe associated with no or mild global intellectual im-pairment and with better language skills than the syn-drome described by Kanner (Bowler, 1992; Ratnberg,Ehlers, Nyden, Johansson & Gillberg, in press).

A major question iti current research in the field is thevalidity of Asperger syndrome, i.e. whether autism andAsperger syndrome differ qualitatively or only quan-

Requests for reprints to: Dr. Stephan Ehlers. Department ofClinical Neuroscience, Section of Child and Adolescent Psy-chiatry, University of Goteborg, Annedals Clinics, S-413 45Goteborg, Sweden.

titatively. A small body of neuropsychological studieshave investigated the possibility of discriminating sub-jects with Asperger syndrome from those with high-functioning autism according to hypotheses proposing"theory of mind" deficits (Baron-Cohen, Leslie & Frith.1985), weak "centra! coherence" (Happe. 1994a) and"executive" dysfunction (Ozonoff, Pennington &Rogers, 1991) with mixed results (see C. Gillberg &Ehlers, in press; Happe, 1994b, for overviews). Moststudies have found evidence of problems in all three ofthese areas in autism and of executive dysfunction inAsperger syndrome. The temporofroiHal regions havebeen implicated as possible brain areas subserving thesefunctions(Baron-Cohen, 1995). Klin, Volkmar, Sparrow,Cicchetti and Rourke (1995) found evidence for adistinction between Asperger syndrome and high-func-tioning autism in a comparative study on neuropsycho-logical profiles. The profile obtained for the Aspergersyndrome group coincided closely with a cluster ofneuropsychological assets and deficits captured by theterm "nonverbal learning disabilities". According toRourke (1988), such disabilities reflect right-hemispheredifficulties. Executive deficits are believed to reflectprefrontal dysfunction in many cases (Ozonoff et al,,1991), Such deficits may show as attentional problems,deficient planning and poor time concepts. Children withAsperger syndrome often have comorbid attention prob-lems (Ehiers & Gillberg, 1993). Executive functions areimpaired not only in Asperger syndrome and autism. In

207

208 S. EHLERS et al.

other syndromes associated with attention deficits, suchas attention deficit hyperactivity disorder (ADHD) (Am-erican Psychiatric Association, 1994) and the syndromeof deficits in attention, motor control and perception(DAMP) (C. Gillberg & Hellgren, 1996). executive dys-functions are present in many cases (Barkley, 1990).Children with attention disorders often have mild autistictraits (C. Gillberg, 1992). Thus, Asperger syndromeappears to share some neuropsychological dysfunctionsboth with autism and with ADHD/DAMP.

The Wechsler Scales

Psychometric assessment is a crucial part of thediagnostic work-up in autism spectrum and attentiondisorders. The Wechsler Intelligence Scale for Children(WISC) (Wechsler, 1974) is one of the most widely usedpsychometric instruments available. The WISC providesmeasures of global ability (full scale IQ) with verbal IQand performance IQ subscores. The verbal scale com-prises the following subtests: Information, Similarities,Arithmetic, Vocabulary, Comprehension and Digit Span.The performance scale comprises the following subtests:Picture Completion, Picture Arrangement, Block Design,Object Assembly and Coding. Cognitive "profiles" areoften presented in terms of whether or not verbal IQ isequal to, is (much) superior to or (much) inferior toperformance IQ. However, the Wechsler Scales wereconstructed (and many of the various results obtained"interpreted") before the advent of modern factoranalysis. Such multivariate studies of the WISC-Rand the WAIS-R on normal standardisation samples(Kaufman. 1990) have demonstrated that verbal IQ-performance IQ discrepancies may nol be the best way ofassessing intellectual strengths and weaknesses (Lincoln,Allen & Kilman, 1995). In computing verbal IQ orperformance IQ, one is adding together scores fromsubtests (e.g. Vocabulary and Arithmetic for verbal IQ)that may not really share much common variance.Instead, three other factors that better accounted for thevariance on the WISC emerged in the Kaufman studies:viz. Verbal Comprehension (Information. Comprehen-sion, Similarities and Vocabulary), Perceptual Organ-isation (Picture Completion, Picture Arrangement,Object Assembly and Block Design) and Freedom fromDistractihility (Coding, Arithmetic and Digit Span).

Further, factor analytic studies by Cattell (1971), andCattell and Johnson (1986) have suggested that theWechsler subscales may also be divided into two main"second order" factors: "fluid" and "crystallised"ability. Fluid intelligence appears to be the basic reason-ing ability, dependent ultimately on the neural efficiencyof the brain. Crystalhsed intelligence, on the other hand,is the set of skills, valued by our culture, in which thisability is invested, and is highly dependent on learningexperiences (Kline. 1991; Lincoln et a l . 1995). Cattell(1971) suggested that in the very young child, fluid abilityand crystallised ability are clearly correlated. As the childgrows older and is exposed to different environmentaland learning experiences, fluid ability and crystallisedability become less highly correlated.

There seems to be agreement that the WISC verbalsubtests Vocabulary and Comprehension measure crys-

tallised cognitive function, and that performance subtestsBlock Design and Object Assembly measure fluid ability(Happe, 1994a; Kaufman, 1990; Kline, 1991; Lincoln etal., 1995).

The Wechsler Scale in Asperger .syndrome, autism andADHD/ DAM P. Several investigators have examinedthe cognitive profiles of children with autism in attemplsto arrive at a better understanding of the isolated abilitiesand specific difficulties encountered in that syndrome.Allen, Lincoln and Kaufman (1991), Happe (1994a),Lincoln, Courchesne. Kilman, Elmasian and Allen(1988). Lockyer and Rutter (1970), Rumsey and Ham-burger (1988), Rutter (1978) and Shah and Frith (1993)have all reported cognitive studies of children with autismand have found low scores on verbal tests, specifically theComprehension subtest. and relatively high scores ontests tapping visuospatial abiUties, particularly BlockDesign. Low scores have also been reported on PictureArrangement, a subtest of the performance scale. Mostchildren diagnosed as autistic have general intellectualretardation with IQ < 70. However. 10-25% have a fullscale IQ of 70 or above. Individuals with these relativelyhigher IQ levels are sometimes said to suffer from "high-functioning" autism, although "high-functioning" isreally a misnomer, considering that they are usually high-functioning only within their own diagnostic group ofautism.

Psychometric assessments of children with Aspergersyndrome have yielded contradictory results. Wolff andBarlow's (1979) "schizoid" children, many of whom fitthe Asperger syndrome phenotype, had peaks andtroughs identical with those seen in autism. Pomeroy andFriedman (1987), on the other hand, found troughsprimarily in visuospatial ability. Szatmari, Tuff, Finlay-son and Bartolucci (1990) found that differences tendedlo disappear with IQ levels above 85. The differentfindings could be accounted for by the fact that each ofthe published studies have employed different diagnosticcriteria. However, in general, children with Aspergersyndrome are reported to have higher intelligence thanchildren with autism.

Considering the enormous literature on ADHD, rela-tively little psychometric study has pertained to thisgroup. However, Kaufman (1979) found that childrenwith attention deficits scored low on tests tapping theDistractibility factor. Children with DAMP/ADHDhave slightly-moderately lower performance IQ thannormal children of the same age group (C. Giliberg &Rasmussen. 1982). but the majority are not mentallyretarded.

Aim.s of the Present Study

The aim of the present study was twofold. First, wewanted to evaluate the discriminating abihty of the WISCin respect of autism. Asperger syndrome and attentiondisorders. Can the WISC aid in clinical diagnosis andhelp separate these different syndromes from each other?Second, we aimed to identify characteristic WISC sub-score profiles within each of the three diagnostic groups.In other words: Are there typical ability peaks andtroughs that are characteristic of each subgroup?

ASPERGER SYNDROME, AUTISM AND ATTENTION DISORDERS 209

Table 1Background Factors in Study Groups

Factor

Age in years: Mean (SD)Male/female casesClinical/population casesSocial class: A'(%)

I11III

Social adversity: N (%)Physical diagnoses": A' {%)Epilepsy: A'(%)EEG abnormal: A^(%)

Aspergersyndrome

A" = 40

9.8 (2.6)40/030/10

8(20)20 (50)12(30)0(0)1 (2.5)0(0)9/28 (32)

AutismA = 40

9.9 (2.9)34/636/4

3(7,5)14(35)23(57.5)2(5)5(12.5)4(10)

11/37(30)

DAMPA = 40

9,9(2.0)36/436/4

7(18)13(32)20(50)

7(18)2(5)3(8)

15/28(53)

"Status post meningitis, tuberous sclerosis. Turner syndrome, Goldenhar syndrome, ichtyosisand minimal cerebral paresis.

Materials and Methods

SubjectsThe Swedish WISC (Wechsler. 1977) was standardised on

Swedish children for ages 6-15,4 years.A total of 120 children were examined. Of these, 118 were

6.0 15.4 years. In addition, one boy of 5.3 years was included.The scoring in this case was based on norms for the 6.0-6.9 yearage band. One further boy of 15.8 years, of low normalintelligence, was also included. In this case the scoring wasbased on norms for 14.4-15.3 year-olds.

Of the 120 children. 40 boys had Asperger syndrome, 34 boysand 6 girls had autistic disorder/childhood autism and 36 boysand 4 girls had DAMP: see following for definitions of thesegroups.

The three diagnostic groups comprised a mixed sample of 102subjects referred to a statewide diagnostic clinic for autism andother neuropsychiatric disorders in western Sweden, plus 18subjects from population studies of Asperger syndrome (Ehlers& Gillberg. 1993) and autism (C, Gillberg. SteiTenburg &Schaumann, 1991), diagnosed at the same clinic. The dis-tribution ofcases according to source of recruitment and genderis shown in Table 1,

The Asperger syndrome group was recruited first. The autismand the DAMP groups were then selected from the clinic'sregister in order to resemble the Asperger syndrome group withregard to IQ level. Only subjects with an IQ > 70 on at least oneof full scale IQ. verbal IQ or performance IQ were included. Thereason for this was to ma.ximise the sample and yet minimisefloor effects.

The first 40 cases, recruited at population and clinic registerscreening, meeting I, C. Gillberg and Gillberg (1989) criteria forAsperger syndrome and who had completed a full scale WISC,lurncd out to be boys, A small number of girls were included inthe autism and DAMP groups because of difficulty recruiting anall-male sample with these diagnoses meeting full criteria forinclusion.

The 120 subjects had all been examined and diagnosed bychild psychiatrists at the clinic. All had undergone thoroughneuropsychiatric work-up for severe behaviour disorder com-prising evaluation by (a minimum oO an experienced childneuropsychiatrist and an experienced clinical psychologist.Clinical consensus diagnosis was achieved by the child neuro-psychiatrist after consultation with colleagues (psychiatristsand psychologists who had also examined the child). Thediagnoses were never based on the results obtained on theWise . All diagnoses had been made in keeping with the

following criteria: DSM-III (American Psychiatric Association,1980, 1987 [DSM-III-R]) for infantile autism/autistic disorder,I. C. Gillberg and Gillberg (1989). for Asperger syndrome andI. C. Gillberg. Gillberg and Groth (1989) for DAMP (see Table2).

Based on meticulous medical record review, a diagnosticrevaluation was made by the first author (SE) of all cases. Onlycases meeting full criteria, as specified in the following section,were included.

Study GroupsAsperger syndrome. The subjects' ages at psychological

assessment ranged from 5,3-15.0 years with a mean age of 9.8years ( ± 2.6 SD). All individual cases met the I. C. Gillberg andGillberg (1989) criteria- elaborated in C. Gillberg (1991)—forAsperger syndrome. Thirty-four cases also met the ICD-10draft research criteria for Asperger syndrome (World HealthOrganisation, 1990). Of the six cases not meeting full ICD-10criteria, three subjects had shown clearly delayed language

Table 2Diagnostic Criteria for DAMP (C. Gillberg & Hellgren,1996)

A, ADD (attention deficit disorder) as manifest by:(a) severe problems in at least one or moderate problems in

at least two of the following areas: attention span,activity level, vigilance and ability to sit still, and

(b) cross-situational problems in the areas mentioned under(a), documented at two or more of the following:psychiatric, neurological, psychological evaluation ormaternal or teacher report,

B, MPD (motor perception dysfunction) as manifest bymarked:

(a) gross motor dysfunction according to detailedneurological examination (see Rasmussen et al.. 1983). or

(b) fine motor dysfunction according to detailedneurological examination (see Rasmussen et al., 1983). or

(c) perceptual dysfunction according to testing, e.g. withSCSIT (Ayres, 1974) and defined in Gillberg et a!,.(1982).

C, Problems not accounted for or associated with MR(mental retardation) or CP (cerebral palsy)

All of A-C have to be met.

210 S. EHLERS et al.

development. In another three cases it was not possible todocument in retrospect that early language development hadbeen normal as required in the ICL>-10. Normal curiosity aboutthe environment before age 3 years could not be ascertainedwith certainty in any of the 40 cases.

One of the population screening pilot study Aspergersyndrome cases met 8 out of 16 criteria for DSM-IIl-R autisticdisorder (American Psychiatric Association, 1987) (in additionto meeting Asperger syndrome criteria). However, clinically.this subject presented with a very typical Asperger syndromebehavioural profile, and so was retained in the Aspergersyndrome group.

Autistic disorder/childhood autism. The subjects were 6.1-15.8 years with a mean age of 9.9 years ( ± 2.9 SD) at the time ofthe psychologica! assessment. They all met the DSM-lII criteriafor infantile autism, the DSM-III-R criteria for autisticdisorder, and the ICD-10 drat̂ t research criteria for childhoodautism (World Health Organisation. 1990). They did not meetfull criteria for Asperger syndrome according to I. C. Gillberg(1991).

Features that significantly distinguished the autism andA-Sperger syndrome groups were (1) the total mean symptomscore on the DSM-III-R [autism > Asperger syndrome, t (78)= 13.37, p < .0001], (2) presence of definitely abnormal de-velopment during the first three years oFlife [autism > Aspergersyndrome, / (78) = 8.15,/) < .0001], (3) narrow interest [Asper-ger syndrome > autism. /(78) = 3.85,/J - .0002], and (4) motorcoordination disorder [Asperger syndrome > autism, / (78) =13.26,/7 = .0016].

Deficits in attctition. motor control and perception (DAMP).This group ranged in age from 6.3-14.4 years at psychologicalassessment with a mean age of 9.9 years (±2.0 SD). All casesmet the criteria for DAMP outlined in Table 2. Thirty of the 40children met the DSM-III-R (American Psychiatric Associ-ation, 1987) criteria for attention deficit hyperactivity disorder(ADHD), and the remaining !0 met "only" DSM-III (Ameri-can Psychiatric Association, 1980) criteria for attention deficitdisorder. All except two individuals in addition met full C.Gillberg and Rasmussen (1982) criteria for age-inappropriatemotor perception dysfunction, corresponding to DSM-IV(American Psychiatric Association, 1994) developmental co-ordination disorder. They did not meet criteria for Aspergersyndrome or autism. However, a number of cases exhibitedsome autistic features. The ICD-10 draft version listed 11inclusion criteria for Asperger syndrome (which were identicalto the criteria for autism in the areas of social interaction andbehaviour). Three of the subjects with DAMP met 3 criteria, 7met 2 criteria, 18 met 1 criterion and the remaining 12 subjectsdid not meet any of the 11 Asperger syndrome criteria.

Background Factors

Some background factors (age, social class and associatedmedical disorders) are shown in Table L

Social class. Soeioeconomic status was assessed accordingto the principal breadwinner occupation (Swedish CentralBureau of Statistics, 1980). Group by group comparisonrevealed no significant differences.

Social adversity. Social adversity was defined here as eitherparental alcoholism/drug abuse, severe marital discord or both.Out of the nine cases with such adversity, live had been placedin a foster-home.

Methods

The WISC was administered individually in all cases. Theassessments were performed by clinical psychologists. The finalresults of the diagnostic assessments were not available/knownto the psychologists at the time of testing. In accordance with

the requirements of the Swedish WISC scoring criteria. DigitSpan was not included in the calculation of verbal and full scaleIQ in the present study. Raw scores were transformed intoscaled scores for the subtests and to IQ scores for full scale IQ,verbal and performance IQ.

Statistical Analysis

Intergroup comparison. In order to assess the discriminatingability of the WISC, a stepwise logistic regression analysis wasperformed. In contrast to basic discriminant function analysis,logistic regression analysis does not assume normal distributionof the covariates (Streiner, 1994). The logistic regression modelprovides an estimate of the conditional probability of belongingto one of two groups given the values of a set of variables. Theprobability is calculated for each individual. The analysisprovides information about not only which diagnostic group isthe most likely one, but also the uncertainty of the classification,based on the variables in the individual case.

The various WISC subtest scores were chosen as covariatesfor each one of the three pairs of diagnostic groups. Theanalyses resulted in three sets of discriminating scores (Aspergersyndrome vs. autism, Asperger syndrome vs. DAMP and aut-ism vs. DAMP). A high discriminating score indicated that itwas likely that the individual belonged in the latter of the twogroups in a pair. For each discriminating score, those variableswith coefficients significantly dilTerent from zero were takeninto account. Thus, only variables that contributed significantlyto the discrimination between the groups were included in thescore.

One-way analysis of variance (ANOVA) and Fisher's Pro-tected Least Significant Difference test (Fisher's PLSD) wereused for descriptive statistics and comparison of mean scaledscores across diagnostic groups. Also. ANOVAs, Fisher'sPLSD, and unpaired ^tcsts were used for analyses of differencesbetween clinical and population cases, between males andfemales and across subgroups (see following) in the analysis ofKaufman factors.

Intragroup comparison. Intragroup comparisons of subtestresults were performed using Fisher's nonparametrie test forpairwise comparisons (two-sided) (Bradley, 1968). This analysisidentifies specific WISC subscale differences characteristic ofeach of the three diagnostic groups. In addition, Bonferroni'scorrection was applied due to the large number of comparisons.Within each of the diagnostic groups, there were 55 possiblepairwise comparisons of the results on the 11 subscales of theWISC. In addition, Bonferroni's correction was applied so thatthe probability of one false significance did not exceed .05. TheWilcoxon Signed Rank Test was used for comparison on theKaufman factors within each of the closely selected subgroups.

Kaufman factors. Further exploration of the data relative toKaufman's Verbal Comprehension, Perceptual Organisationand Freedom from Distractibility (Kaufman. 1990; Lincoln etal., 1995) factors was performed. In order to achieve the bestpossible similarity for IQ, for these analyses we selected allsubjects in each of the diagnostic groups who had full scale IQsIn the 71-105 range. In addition, we included only male subjectswith no history of social adversity. These groups will be referredto as the "IQ-similar subgroups"'.

Results

Overall Findings

The mean scaled scores for al! the subtests and meansof verbal IQ, performance IQ and ful! scale IQ of thestudy groups are shown in Table 3. The table is shown forconvenience and should not be used as the ultimate basisfor conclusions regarding intergroup differences.

ASPERGER SYNDROME. AUTISM AND ATTENTION DISORDERS 211

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Table 41^-coefficient. Standard Error (SE), Level of SignificantIntergroup Difference and Discriminating Score Accordinglo Logistic Regression Analysis

Variable SE p-level"

AS vs. AU"Comprehension - 0.3478 0.0882 0.0001Picture Arrangement -0.1739 0.0806 0.0309Constant 4.2485 0.9850 0.0000

Discriminating score = 4.25 ^.35 Comprehension— .17 Picture Arrangement

AS vs. DAMPInformationConstant

Discriminating score

AU vs. DAMPComprehensionPicture ArrangementBlock DesignConstant

Discriminating score

-0.2463 0.0742 0.00092.5320 0.7979 0.0015

2.53 —.25 Information

0.4140 0.1123 0.00020.3090 0.1100 0.0050

-0.2327 0.1145 0.0422-3.6434 1.1871 0.0021

— 3.64-f.41 Comprehension-t-.31Picture Arrangement —.23 BlockDesign

''AS: Asperger syndrome: AU: autism; DAMP: deficits inattention, motor control and perception.

Percentile points of the discriminating score

Figure !. Asperger syndrome vs. autism according to logisticregression analysis. Note that 1 minus the probability ofbelonging to the autism group represents the probability of

belonging to the Asperger syndrome group.

When this analysis was applied to autism vs. DAMP,the same level of discriminating ability was obtained, i.e.a good guess was achieved for 49 % of the subjects (belowthe 25th and above the 76th percentile).

The Asperger syndrome vs. DAMP analysis, in con-trast, demonstrated a much poorer discriminating ability.A good guess was achieved for only 16 % of the subjects(below the 12th and above the 96th percentile).

Intergroup Differences., Diagnostic Groups Comparedin Pairs

Table 4 summarises the results of the stepwise logisticregression analysis and shows the discriminating vari-ables and scores across groups. Comprehension (As-perger syndrome > autism, DAMP > autism). PictureArrangement (DAMP > autism, Asperger syndrome >autism). Information (Asperger syndrome > DAMP)and Block Design (autism > DAMP) differentiated be-tween the groups.

Figure 1 depicts the result of the discriminant functionanalysis—using logistic regression—for Asperger syn-drome vs. autism. On the x-axis are the percentile pointsof the discriminating score. The 50th percentile point isthe median score, and the 80th percentile point corre-sponds to the score that 80 % of the sample do not exceed.On the y-axis are the probabilities of belonging to theautism group (note that I minus the probability ofbelonging to the autism group represents the probabihtyof belonging to the Asperger syndrome group). For in-stance. .20 on the y-axis means 20% probability ofbelonging to the autism group and 80% of belonging tothe Asperger syndrome group, and vice versa. Eightypercent probability is considered as an acceptable cut-offpoint for diagnostic separation. From the figure it can beseen that .20 and .80 on the y-axis corresponds to the 25thpercentile (25 % of subjects) and the 76th percentile (24 %of subjects) respectively.

Thus, for 49% of the Asperger syndrome and autismsubjects, a good guess was achieved by using the WISC.For the rest of the subjects (between the 25th and the 76thpercentile) the discriminating ability of the WISC wasrather poor.

Overall Diagnostic Prediction in a Three-groupAnalysis

The stepwise logistic regression analysis of the WISCsubscore results was also used to predict diagnostic groupproperty, by presenting the data in a manner that accordswith a three-group discriminant function analysis. Allcases for which there were no missing data in respect ofrelevant variables (Comprehension, Picture Arrange-ment. Information and Block Design) were included. Ineach intergroup comparison, the 50th percentile was usedfor cut-off, e.g. in the Asperger syndrome/autism cotn-parison. all cases above this level were predicted as autismwhereas all cases below this level were predicted asAsperger syndrome. The results of this analysis areshown in Table 5. The overall rate of correct diagnostic

Table 5Predicted Cla.ssification of Asperger Syndrome, Autismand DA MP Groups Based on Stepwise Logistic RegressionAnalysis Relative to Clinical Diagnosis

Prediction

Aspergersyndrome

Autism

DAMP

A'

Aspergersyndrome

2357.5%

717.5%

1025.0%

40

Clinical diagnosis

Autism

410.3%

3179.5%

410.3%

39

DAMP

1230.8%

717.9%

2051.3%

39

ASPERGER SYNDROME, AUTISM AND ATTENTION DISORDERS 213

Table 6Significant Inlragroup Differences"

Aspergersyndrome

OA < IN"OA <CMOA <SMOA < BDOA < VOOA < PCCD< INCD < CMCD <SMCD< VOCD< BDAR < INAR <SM

Autism

PA < BDCM < BDAR < BDOA < BDCD < BDCD<PC

DAMP

CD<CMCD<SMCD< VOCD< BDCD<PCAR<SMAR<CMAR< VOOA<CM

" Fisher's nonparametric test for pairwisc comparisons (two-sided) and including Bonferroni's correction.

"AR: Arithmetic: BD: Block Design: CD: Coding; CM:Comprehension; IN: Information; OA: Object Assembly; PA:Picture Arrangement; PC: Picture Completion; SM: Simi-larities; VO: Vocabulary.

classification was 63%. which should be compared withthe one-in-three chance rate of true prediction.

Intragroup Differences

The results of the intragroup analysis are shown inTable 6. The Asperger syndrome group was characterisedby significantly lower results on the performance subtestsObject Assembly and Coding. On the verbal subtests thisgroup showed significantly better results on Information,Similarities. Comprehension and Vocabulary, but poorerresults on Arithmetic.

The autism group was characterised particularly bysignificantly better results on Block Design.

The DAMP group was characterised by several signifi-cant differences between Coding and Arithmetic on theone hand (low scores), and verbal tests (Comprehension,Similarities and Vocabulary) on the other (higher scores).In addition. Block Design and Picture Completion wereat a significantly higher level than Coding.

Type of Sample (Population/ Clinic) and Gender

In the Asperger syndrome group, the populationsample (A' = 10) had significantly higher performance IQ{p = .04) than the clinical sample (N = 30). This dif-ference across the subgroups was accounted for by higherscores on Picture Arrangement in the population sample.No significant differences were found regarding verbal IQor full scale IQ.

A small number of population cases were included inthe autism (,V = 4) and DAMP (A' - 4) groups. Also, afew female cases (A' = 10) were included in these groups.No significant gender differences or differences acrossthese population and clinical samples were found re-garding verbal IQ, performance IQ or full scale IQ.

Kaufman Factors

The mean scaled scores for all the WISC subtests andthe means of verbal IQ, performance IQ and full scale IQof the IQ-similar subgroups are shown in Table 7. TheAsperger syndrome group demonstrated better verbalability than the autism group (reflected in significantlyhigher scores on verbal IQ and the verbal subtestsComprehension and Vocabulary). The same picture wasdemonstrated when comparing the two subgroups on theVerbal Comprehension factor {p < .001).

Overall, the Asperger syndrome group scored relativelyhigh on Verbal Comprehension, the autism group rela-tively high on Perceptual Organisation and the DAMPgroup relatively low on Freedom from Distractibility. Ofthe intragroup comparisons, only Verbal Comprehensionvs. Freedom from Distractibility in the Asperger syn-drome group (p = .005) and in the DAMP group (p =.0004) were significant.

Discussion

This study contrasts the cognitive performance ofschool-age children with Asperger syndrome with that ofhigh-functioning children with autism and with that ofchildren with attention disorders.

The results of the stepwise logistic regression analysisdemonstrated that thediscriminatingability of the WISCacross study groups was good for half of the cases in theseparation of autism from Asperger syndrome and autismfrom DAMP, but for only one in six cases in theseparation of Asperger syndrome from DAMP. Furtheranalysis of the data in a manner that accords with a three-group discriminant function analysis yielded an overallrate of correct diagnostic classification of 63 %. Aspergersyndrome—when contrasted with autism—was charac-terised by significantly better Comprehension and PictureArrangement. When compared with the DAMP group,the Asperger syndrome group had significantly higherInformation, but the discrimination of these two groupsfrom each other was considerably poorer. The DAMPgroup was distinguished from the autism group bysignificantly better Comprehension and Picture Arrange-ment, and lower Block Design.

The intragroup analysis showed Asperger syndrome tobe associated with good verbal ability, refiected in asubscale cluster identical with Kaufman's Verbal Com-prehension factor. Also, this group generally showedrather poor perceptual abihty. Low results were obtainedon the Object Assembly subtest, suggesting difficultiesattending to wholes rather than detail, and on the Codingand Arithmetic subtests, which are regarded to reflectdistractibility (Kaufman, 1990). However, a low result onCoding might also mirror the extreme slowness, circum-stantiality and drive for perfection often found in thisgroup. Unfortunately, analysis of qualitative charac-teristics such as these was beyond the scope of the presentstudy.

The autism group results clearly indicate relativelysuperior ability regarding visuospatial function, reflectedin a characteristic peak on Block Design. The findings inthe autism group tally well with previous research in the

214 S. EHLERS etai .

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ASPERGER SYNDROME, AUTISM AND ATTENTION DISORDERS 215

field (Frith, 1989; Happe, 1994a, b; Lmcoln et al., 1995;Lockyer & Rutter, 1970; Rumsey & Hamburger, 1988;Shah & Frith, 1993; Witkin & Goodenough, 1981).

The major—and striking—finding in the DAMP groupwas poor performance on Coding and Arithmetic, incomparison to results on most of the other scales. Thiscognitive profile suggests difliculties concerning tasksmeasuring attention and concentration and is in line withKaufman's (1979) finding of a characteristic distracti-bility factor in a group of children with ADHD.

We need to stress here that the conclusions pertain toaveraged group findings. Visual inspection of individualWISC profiles revealed that a minority of subjects in eachdiagnostic group conformed to the key profile charac-teristic of one of the other two. In addition, within-groupcomparison of the data in terms of "individual"^uncharacteristic—profiles on the one hand, and fre-quencies of subjects from each group showing the" uniform "—characteristic—profile pattern on the other,demonstrated a heterogeneous picture. This findingaccords with that of other studies that have examinedsamples of children with autism on individual differencesin cognitive profiles/deficits (Green. Fein, Joy & Water-house, 1995). In the present study, only a minority (albeita large one) of each diagnostic group showed the highlycharacteristic profile. This explains the relatively poordiscriminating ability even when using the most ad-vanced statistical analysis—and hence the far fromperfect diagnostic predictive ability.

It is important to emphasise that all three diagnosticgroups share some behavioural features, and that theAsperger syndrome and autism groups show a clearoverlap of symptoms in the areas of social interaction,communication and behaviour. In fact, there is goodconsensus regarding a continuum of Asperger syndromeand autism at the behavioural level. This consensus isreflected, for instance, in the ICD-10 draft and finalICD 10 (World Health Organisation, 1990, 1992, 1993)and the DSM IV (American Psychiatric Association,1994) criteria for the disorders. This overlap of behavi-oural symptoms and the lack of a true "gold standard"(Szatmari, 1992) entail the risk of diagnostic "mis-classification" and loss of statistical power. However, thepresent study applied a.s stringent a diagnostic procedureas possible using current operationalised criteria. TheAsperger syndrome and autism samples differed signifi-cantly, for instance on the numberof DSM-III-R criteriamet. Only one case meeting criteria for Asperger syn-drome also (just) met the DSM-III-R (American Psy-chiatric Association, 1987) criteria for autism, but wasclassified as Asperger syndrome because of his striking"behavioural phenotype ".Ten cases in the DAMP groupexhibited two or three autistic symptoms according to theICD-10 draft criteria.

The present findings bring out the important issue ofthe external validity of Asperger syndrome. The resultsindicate that Asperger syndrome and autism share certainWISC cognitive deficits. However, they differ on IQ leveland verbal ability. Overall, the findings do nol argueconvincingly fora clear association of "autism spectrumdisorder" and a specific cognitive profile on the WISC.Furthermore, the poor Asperger syndrome/DAMP dis-crimination ability (in spite of a relatively low frequency

of autistic features in this group) precludes firm con-clusions regarding the validation of Asperger syndromebased on the WISC only.

It could be argued that the higher IQ of the Aspergersyndrome group invalidates conclusions. However, in theAsperger syndrome, autism and DAMP IQ-similar sub-groups, we found that good verbal ability is characteristicof Asperger syndrome. These analyses also underscoredthe good visuospatial ability of the autism group and thedistractibility of the DAMP group. In short, even aftervery strict selection of cases according to full scale IQ, thethree subgroups demonstrated the same distinct patternof cognitive strengths and weaknesses as were reportedfor the larger groups.

The findings indicate that Kaufman's factor analyticconcept of Verbal Comprehension, Perceptual Organ-isation and Freedom from Distractibihty, rather than thetheoretically constructed concept of verbal IQ andperformance IQ, account for the variance on the WISC.The Asperger syndrome and the DAMP intragroupanalyses demonstrated a WISC subtest scatter, withArithmetic standing out as atypical among the WISCverbal subtests, and instead correlating strongly with theperformance subtest Coding. To quote Lincoln et al.,(1995) "In evaluating performance IQ or verbal IQscores, one is adding...apples and oranges".

Block Design and Object Assembly are considered tobe a good measure of fluid cognitive function, whereasVocabulary and Comprehension are regarded as goodmeasures of crystallised cognitive function (Happe,1994a; Lincoln et al., 1995). Fluid cognitive ability isbelieved to be innate and not influenced by learning orexperience. Crystallised ability, on the other hand, isthought to be dependent on acquired knowledge andinfluenced by experience and culture. Interestingly, thepresent findings showed that the Asperger syndrome andautism groups differed on subtests thought to measurecrystallised vs. fluid intelligence. The relatively poor crys-tallised cognitive function in the autism group accordswith the well-known difliculty for individuals with autismto be influenced by context—a failure to "seek outexperiences and make a coherent story out of them"(Wing, cited in Happe, 1994b, p. 127), or, in other words,to make use of "central coherence" (Frith. 1989). In theAsperger syndrome group of the present study, thisability did not appear to be as severely affected; instead itseemed that they had some capacity for learning fromother people (and hence for acquiring "crystallised"intelligence). Thus, our findings suggest that this di-chotomy in respect of intellectual abilities on the WISCmight serve as one useful tool for aiding in the separationof the two clinical diagnostic groups. Before conclusionscan be drawn, however, the results (particularly withregard to high Comprehension and Vocabulary and lowObject Assembly scores in Asperger syndrome, and highBlock Design scores in autism) need to be replicated inother large samples examined by other groups.

In conclusion, the present findings suggest that thesubtest resultsof the WISC may provide a useful basis forcomparison of cognitive peaks and troughs within andacross different clinically defined groups. However, thefar from perfect intergroup discrimination overall, evenwhen applying an optimal statistical method (stepwise

216 S. EHLERS etal.

logistic regression), precludes firm conclusions regardingthe validation of Asperger syndrome vis-a-vis autismbased on the WISC only.

Acknowledgements—This study was supported by grantsfrom the Wilhelm and Martina Lundgren Foundation, theRBU Research Foundation, the Sven Jerring Foundation andthe Clas Groschinsky Memorial Foundation, and the SwedishMedical Research Council (MFR grant no. B95-21X-11251-OlA).

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Accepted manuscript received 22 March 1996