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Developmental Neuropsychology
ISSN: 8756-5641 (Print) 1532-6942 (Online) Journal homepage: https://www.tandfonline.com/loi/hdvn20
Executive Functions in Autism and Asperger's
Disorder: Flexibility, Fluency, and Inhibition
Natalia Kleinhans , Natacha Akshoomoff & Dean C. Delis
To cite this article: Natalia Kleinhans , Natacha Akshoomoff & Dean C. Delis (2005) Executive
Functions in Autism and Asperger's Disorder: Flexibility, Fluency, and Inhibition, Developmental
Neuropsychology, 27:3, 379-401
To link to this article: https://doi.org/10.1207/s15326942dn2703_5
Published online: 08 Jun 2010.
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Executive Functions in Autism and
Asperger’s Disorder: Flexibility,
Fluency, and Inhibition
Natalia Kleinhans
Joint Doctoral Program in Clinical Psychology
San Diego State University and University of California, San Diego
Natacha Akshoomoff
Department of Psychiatry
University of California, San Diego
Child and Adolescent Services Research Center
Children’s Hospital, San Diego
Dean C. Delis
School of Medicine
University of California, San Diego
The Color-Word Interference Test, TrailMaking Test, Verbal Fluency Test, and Design
Fluency Test from the Delis–Kaplan Executive Function System (Delis, Kaplan, &
Kramer, 2001) were administered to 12 high-functioning adults and adolescents with
autistic disorder or Asperger’sdisorder. Each test included a switching condition in ad-
dition to baseline and/or other executive-function conditions. Participants performed
significantly belowaverage on a composite measure of executive functioning adjusted
for baseline cognitive ability. Complex verbal tasks that required cognitive switching
and initiation of efficient lexical retrieval strategies produced the most consistent defi-
cits, whereas cognitive inhibition was intact. We discuss implications of these findings
for understanding the neurocognitive substrates of autistic spectrum disorders.
Autism spectrum disorders (ASD), which include autistic disorder and Asperger’s
disorder, are characterized by atypical communication and social development.
DEVELOPMENTAL NEUROPSYCHOLOGY, 27(3), 379–401
Copyright © 2005, Lawrence Erlbaum Associates, Inc.
Requests for reprints should be sent to Natalia Kleinhans, Joint Doctoral Program in Clinical Psy-
chology, San Diego State University and University of California, San Diego, 8110 La Jolla Shores
Drive, Suite 201, La Jolla, CA 92037. E-mail: nmk_353@yahoo.com
Restricted, repetitive, and stereotyped patterns of behavior are also key features of
the disorder and are thought to reflect a failure of inhibition, cognitive rigidity, and
a generativity impairment (Turner, 1997). The restricted and repetitive behaviors
observed in ASD are heterogeneous and appear to differ according to developmen-
tal level and cognitive capacity (Militerni, Bravaccio, Falco, Fico, & Palermo,
2002). For example, higher functioning individuals with autistic disorder may dis-
play more insistence on sameness than individuals with Asperger’s disorder who
alternatively demonstrate a higher level of circumscribed interests (Ozonoff,
South, & Miller, 2000). However, younger, lower functioning individuals with au-
tistic disorder more commonly exhibit motor stereotypies such as hand flapping
(Militerni et al., 2002). Such behaviors are thought to reflect executive dysfunction
in individuals with ASD.
Individuals with autistic disorder and Asperger’s disorder have been found to be
impaired on neuropsychological tests of executive functioning such as the Wiscon-
sin Card Sorting Test (WCST; Berg, 1948), Tower of Hanoi (Boyrs, Spitz, &
Dorans, 1982), and Trail Making Test (Army Individual Test Battery, 1944;
Bennetto, Pennington, & Rogers, 1996; Liss et al., 2001; Minshew, Meyer, &
Goldstein, 2002; Ozonoff & Jensen, 1999; Ozonoff & McEvoy, 1994; Ozonoff,
Pennington, & Rogers, 1991; Pascualvaca, Fantie, Papageorgiou, & Mirsky, 1998;
Prior & Hoffmann, 1990; Rumsey, 1985; Rumsey & Hamburger, 1988, 1990; Shu,
Lung, Tien, & Chen, 2001; Szatmari, Tuff, Finlayson, & Bartolucci, 1990). How-
ever, the factors related to poor performance on these tests are unclear. Execu-
tive-function tests typically tap multiple fundamental and higher order cognitive
skills, rendering them highly sensitive clinical measures but lacking specificity for
delineating distinct cognitive processes. For example, Part B of the traditional
Trail Making Test requires complex visual scanning, motor speed, number se-
quencing skills, and letter sequencing skills in addition to cognitive flexibility
(Lezak, 1995). As such, difficulties with letter sequencing, a common weakness in
individuals with verbal learning disabilities, may contribute to the impairments on
this task that have been found in high-functioning individuals with autistic disor-
der (HFA; non-mentally retarded individuals) (Minshew et al., 2002; Rumsey &
Hamburger, 1988).
Determining the significance of poor performance on executive-function tests is
further complicated by the impairments in other neurocognitive functions in individu-
als with ASD. Several of the basic cognitive processes needed for successful perfor-
mance on multicomponent executive-function tests are affected in HFA. For example,
shifting attention, adjusting the spatial distribution of attention, orienting attention,
sensory modulation, and visual filtering have all been found to be deficient in autistic
disorder (Akshoomoff, Courchesne, & Townsend, 1997; Allen & Courchesne, 2003;
Belmonte, 2000; Bryson, Landry, & Wainwright, 1997; Casey, Gordon, Mannheim, &
Rumsey, 1993; Ciesielski, Courchesne, & Elmasian, 1990; Courchesne et al., 1994;
Harris, Courchesne, Townsend, Carper, & Lord, 1999; Pierce & Courchesne, 2001;
380 KLEINHANS, AKSHOOMOFF, DELIS
Rinehart, Bradshaw, Moss, Brereton, & Tonge, 2001; Townsend et al., 1999;
Townsend, Harris, & Courchesne, 1996). Thus, it may be that individuals with ASD
perform poorly on tests of executive functioning not because of high-level cognitive
dysfunction but because of deficits in more fundamental cognitive abilities.
Certain components of executive functioning may be spared in ASD. For exam-
ple, a number of studies have failed to detect a deficit in both the cognitive and m otor
aspects of inhibition as assessed by the Go-NoGo (Ozonoff, Strayer, McMahon, &
Filloux, 1994), NegativePrim ing (Tipper, 1985), Stop Signal (Logan, Cowan, & Da-
vis, 1984), and Stroop (1935) tasks (Brian, Tipper,Weaver, & Bryson, 2003; Bryson,
1983; Eskes, Bryson, & McCormick, 1990; Ozonoff & Jensen, 1999; Ozonoff &
Strayer,1997). Ozonoff and Strayer (1997) studied motor and cognitive inhibition in
17 nonretarded children and adolescents with autistic disorder. Theywere sim ilar to
control participants in inhibiting motor responses to neutral and prepotent stimuli on
the Stop Signal motor inhibition task. On a cognitive inhibition task (Negative
Priming; Tipper, 1985), individuals with autisticdisorder dem onstrated intact nega-
tive priming such that, similar to matched control participants, their responses were
slower and less accurate to targets that were presented previously as distracters. A
follow-up study by Ozonoff and Jensen (1999) found that performance on a Stroop
task, another measure of cognitive inhibition, was intact relative to the normal con-
trol sample despite significantly impaired performances on the WCST and Tower of
Hanoi task (Ozonoff & Jensen, 1999).
Semantic fluency has also been found to be an area of strength for ASD. Children
with autistic disorder and Asperger’s disorder generate as manywords from a given
category as typically developing children (Boucher, 1988; Dunn, Gomes, &
Sebastian, 1996; Manjiviona & Prior, 1999), although they may include a higher
number of uncommon category members (e.g., “yak” for animal) than expected
(Dunn et al., 1996). One study (Turner, 1999) found that individuals with HFA were
also adequate in their performance on a nonverbal fluencytest, which is a m easure of
the ability to generate novel designs; however, in contrast with other studies, this
study reported that the HFA individuals were impaired on category fluency.
Findings on phonological fluency tasks (generating words that begin with a
designated letter) have been more variable. Phonological fluency requires greater
organization and effort to retrieve appropriate lexical items than semantic fluency,
which relies more on overlearned semantic knowledge. Phonological fluency (“F,”
“A,” and “S”) has been found to be intact in high-functioning autistic participants
relative to control participants by Minshew, Goldstein, and Siegel (1997) and pub-
lished norms by Manjiviona and Prior (1999). Conversely, other investigators have
found that adults with HFA were impaired on phonological fluency relative to nor-
mal controls (Rumsey & Hamburger, 1988, 1990; Turner, 1999) and to adults with
severe dyslexia (Rumsey & Hamburger, 1990).
The goals of this study were to identify a pattern of strengths and weakness in
certain aspects of executive functioning (cognitive switching, verbal and nonverbal
ASPERGER’S DISORDER 381
fluency, and inhibition) and determine whether fundamental cognitive skill deficits
could account for previously reported deficits on tests of executive functioning in
ASD. The tasks chosen in this study are from the Delis–Kaplan Executive Func-
tion System (D–KEFS; Delis et al., 2001). The tests selected are adaptations of
commonly used clinical neuropsychological tests that are widely regarded to be
sensitive to executive dysfunction: a Stroop task, Trail Making Test, Verbal Flu-
ency Test, and Design Fluency Test. The D–KEFS tests include new procedures
that are designed to increase the sensitivity to subtle executive-function deficits.
For example, new switching conditions have been added to the Color-Word Inter-
ference Test (a Stroop task), Verbal Fluency Test, and Design Fluency Tests. In ad-
dition, new baseline conditions have been added to the D–KEFS Trail Making Test
that enable the clinician to distinguish between key basic or component skills and
higher level switching skills tapped by these tests. This study is the first to compare
the performance of individuals with ASD to data from an extensive normative
sample on these particular measures. As such, stronger conclusions can be drawn
about relative strengths and weaknesses than are possible when comparing across
previous studies of ASD, which due to small sample sizes (but see Minshew et al.,
2002), utilized potentially nonequivalent control groups. The normative data avail-
able within the D–KEFS also enabled us to report level of clinical impairment as
well as statistical differences from normal performance.
The ASD group was predicted to have difficulty with cognitive switching based
on studies that have shown poorer performance on the WCST (Minshew et al.,
1992; Liss et al. 2001) and abnormal performance in experimental studies of
switching attention (Courchesne et al., 1994; Townsend et al., 1999). Based on pre-
vious studies, we also hypothesized that the ASD profile would exhibit relative
strengths in inhibition, semantic fluency, and nonverbal fluency, with relative defi-
cits in phonological fluency.
METHOD
Participants
Twelve adult and adolescent males (ages 14–42 years, M=26.4,SD = 7.7) partici-
pated in this experiment. Of the participants, 6 met criteria for autistic disorder and 6
met criteria for Asperger’s disorder. All had participated previously in functional
neuroimaging studies of behavior and cognition in ASD in our laboratory.Nine were
right handed and 1 was left handed. Two reported no strong hand preference but used
the right hand for all tests. Handedness was ascertained by self-report. Participants
were recruited regardless of race or gender. All participants were White.
Participants were diagnosed with autistic disorder if they met criteria for autistic
disorder on the Autism Diagnostic Interview–Revised (ADI–R; Lord, Rutter, & Le
Couteur, 1994), the Autism Diagnostic Observation Schedule (ADOS; Lord et al.,
382 KLEINHANS, AKSHOOMOFF, DELIS
2000), and the Diagnostic and Statistical Manual of Mental Disorders (4th ed.
[DSM–IV]; American Psychiatric Association, 1994) criteria for autistic disorder.
Participants were diagnosed with Asperger’s disorder if they met criteria for autistic
disorder or ASD on the ADI–R and ADOS and DSM–IV criteria for Asperger’s dis-
order. The DSM–IV requires that there are significant deficits in social, communica-
tion, and restrictive and repetitive interests in the absence of a clinically significant
history of delay in language, cognitive functioning, or adaptive skills and the ab-
sence of a DSM–IV diagnosis of autistic disorder. Based on the questions from the
ADI–R that ask the parent about the child’s behavior between the ages of 4 and 5, all
the individuals with Asperger’s disorder had difficulty looking their parents in the
face and did not smile when greeting people, offer to share, engage in imaginative
play with peers, or show interest in other children. In addition, all reportedly did not
use “coordinated eye gaze with accompanying vocalization” in situations in which
they were motivated to communicate and did not respond in a socially appropriate
manner when others wouldapproach them in a friendly manner.The ADI–R and past
medical records were used to ascertain whether participants had a history of clini-
cally significant language delay according to DSM–IV criteria (i.e., single words by
age 2 years and communicative phrases by age 3 years). Five of the participants diag-
nosed with Asperger’s disorder reportedlywere using single words by 24 months of
age and phrase speech by 36 months. The parents of the sixth participant reported
thattheycouldnot recall when their son had met specific languagemilestones butdid
notremember havinganyconcerns at that time. All individuals diagnosed withautis-
tic disorder met criteria for language delay. At the tim eof testing, one individual with
Asperger’s disorder was taking Adderall®, one individual with Autism was taking
Zoloft®, and one individual with Autism was taking Ritalin®and Paxil®. Full Scale
IQ (FSIQ), evaluated by the Wechsler Abbreviated Scale of Intelligence (WASI;
Wechsler, 1999), ranged from 80 to 117 in our sample (M= 100); see Tables 1 and 2
for further information about the participants. This study was approved by the Uni-
versity of California, San Diego and Children’s Hospital of San Diego Institutional
Review Boards. Informed written consent was obtained from the participants and
their parents, and participants were paid $20 per hour of testing.
Participants with HFA and Asperger’s disorder were combined into one ASD
group. Although they are considered separate disorders in the DSM–IV, the validity
of separate diagnostic categories is currently a matter of considerable debate (e.g.,
Mayes, Calhoun, & Crites, 2001; Miller & Ozonoff, 1997, 2000). Significant dif-
ferences in neuropsychological test performance have not been found between
HFA and Asperger’s disorder, particularly when one controls for IQ (Manjiviona
& Prior, 1999; Miller & Ozonoff, 2000). Preliminary evidence supports the con-
tention that autistic disorder and Asperger’s disorder lie along a spectrum and that
the current diagnostic distinction, in practice, is based primarily on symptom se-
verity and early language development history rather than distinct cognitive and
behavioral profiles (Mayes & Calhoun, 2001).
ASPERGER’S DISORDER 383
384
TABLE 1
High-Functioning Autism (HFA)
Vari a bl e HFA1 HFA2 HFA3 H FA 4 HFA5 HFA6 Ran g e M SD
Age (years) 42 15 21 26 18 14 14–42 22.7 10.4
ADI–R
Social (cutoff = 10; max = 30) 22 27 18 30 27 26 18–30 25.0 4.3
Verbal (cutoff = 8; max = 26) 19 19 21 16 22 19 16–22 19.3 2.1
Restricted interests and repetitive behavior (cutoff = 3; max = 12) 6 7 10 11 12 6 6–12 8.7 2.7
Age at first concern (months) 42 17 7 7 30 30 7–42 22.2 14.2
Age single words acquired (months) 13 na 15 36 36 72 13–72 28.7 23.7
Age phrase speech acquired (months) 54 30 na 108 48 96 30–108 56.0 33.2
Wechsler Abbreviated Scale of Intelligence
Verbal IQ 86 76 115 76 86 108 76–115 91.2 16.5
Performance IQ 115 89 116 127 109 108 89–127 110.7 12.6
Full Scale IQ 100 80 117 99 98 108 80–117 100.3 12.3
Note. ADI–R = Autism Diagnostic Interview–Revised.
385
TABLE 2
Asperger’s Disorder (AD)
Variable AD1 AD2 AD3 AD4 AD5 AD6 Range M SD
Age (years) 14 19 19 21 15 21 14–21 18.2 3.0
ADI–R
Social (cutoff = 10; max = 30) 18 21 13 23 8 22 13–23 17.5 5.9
Verbal (cutoff = 8; max = 26) 11 20 12 18 7 16 7–20 14.0 4.9
Restricted interests and repetitive behavior (cutoff = 3; max = 12) 6 7 3 6 4 8 3–8 5.7 1.9
Age at first concern (months) 24 20 30 4 na 114 4–114 38.4 43.3
Age single words acquired (months) 13 18 9 36 na 10 9–36 17.2 11.1
Age phrase speech acquired (months) 24 24 13 36 na 19 13–36 23.2 10.8
Wechsler Abbreviated Scale of Intelligence
Verbal IQ 102 111 96 105 87 106 87–111 101.2 8.5
Performance IQ 117 99 105 101 96 118 96–118 106.0 9.4
Full Scale IQ 109 106 101 103 90 112 90–112 103.5 7.7
Note. ADI–R = Autism Diagnostic Interview–Revised.
Procedure
Participants were administered the Trail Making Test, Verbal Fluency Test, Design
Fluency Test, and Color-Word Interference Test of the D–KEFS (Delis et al., 2001)
as part of a larger neuropsychological battery. Administration procedures adhered
to the instructions detailed in the D–KEFS manual, and tests were administered in
the order just listed.
D–KEFS Trail Making Test.
This test consists of five testing conditions: vi-
sual scanning, number sequencing, letter sequencing, number-letter switching,
and motor speed. The primary executive-function condition is number-letter
switching, a visuomotor sequencing task that measures flexibility of thinking
(analogous to Part B of the traditional test). This condition requires examinees to
switch back and forth between connecting numbers and letters in sequence. It dif-
fers from Part B of the traditional Trail Making Test in that the scanning demands
have been increased (i.e., the stimuli span two pages instead of one) and the inclu-
sion of capture stimuli (i.e., pairs of consecutive numbers or consecutive letters
placed near each other).
The four remaining conditions of this test measure baseline components of the
number-letter switching task. In the visual scanning task, the participant is asked to
cancel out all of the 3s. This condition measures the visual scanning and attention
component of performing the higher level number-letter switching condition. The
number sequencing condition is similar to Part A of the traditional Trail Making
Test in that the examinee is asked to sequence numbers quickly and accurately;
however, both numbers and letters are present in the stimulus array. In the letter se-
quencing condition, the examinee is asked to sequence letters presented in a stimu-
lus array comparable to that used in the number sequencing condition. This condi-
tion is particularly useful in interpreting number-letter switching performance in
examinees with developmental verbal learning difficulties for whom alphabet se-
quencing problems represent a long-standing cognitive weakness. Finally, in the
motor speed condition, the examinee is presented with blank circles connected by
a dashed line. The examinee is instructed to draw a line over the dotted line as fast
as he or she can. This condition is designed to assess the contribution of motor dys-
function to test performance. Scaled scores were based on the total amount of time
taken to complete each task.
Verbal Fluency Test.
The Verbal Fluency Test comprises three conditions:
letter fluency, category fluency, and category switching. Participants were given 60
sec to generate as many items as possible per trial. In the letter fluency condition,
the examinee is asked to generate as many words as they can think of that begin
with the letters “f,” “a,” and “s.” in the category fluency condition, the examinee is
asked to generate as many different words as possible from the categories “ani-
386 KLEINHANS, AKSHOOMOFF, DELIS
mals” and “boy’s names.” In the category switching condition, the examinee is
asked to alternate between naming a type of fruit and a piece of furniture. Scaled
scores were based on the total number of valid words generated per condition.
Design Fluency Test.
Design Fluency also comprises three conditions:
filled dots, empty dots, and switching. In each condition, the examinee is asked to
draw different designs for 60 sec using only four straight lines to connect the dots.
For all of the conditions, a design is not given credit if it contains greater or fewer
than four lines or is a repetition of a previous design. In the filled dots condition,
the examinees are presented squares containing five filled (i.e., black) dots, and
they are asked to draw their designs by connecting the dots. The squares in the
empty dots condition contained five empty dots and five filled dots. Examinees are
instructed to connect only the empty dots. In addition to the scoring guidelines
listed previously, designs in which a line connected to a filled dot were not given
credit. The switching condition also contained five empty dots and five filled dots;
however, in this condition, the examinees are instructed to alternate between con-
necting empty dots and filled dots. Credit is not given for designs in which
examinees did not switch correctly.
Color–Word Interference Test.
This Stroop task is composed of four con-
ditions: color naming, word reading, inhibition, and inhibition/switching. In the
color naming condition, the examinee is presented with a stimulus page displaying
rows of colored patches (red, blue, green, yellow) and asked to name each color out
loud as quickly as possible. In the word reading condition, the examinee is pre-
sented with color words printed in black ink and asked to read each word aloud as
quickly as possible. In the inhibition condition, the examinee is presented with
color names that are written in an incongruent ink color. The examinee is asked to
name the ink color and ignore the written word, which requires the examinee to in-
hibit the more automatic word reading response (i.e., the “Stroop” effect). The in-
hibition/switching condition also contains names of colors written in incongruent
ink colors. However, half of the words appear in a box. Examinees are instructed to
name the ink color if it does not appear in a box or read the word if it appears in a
box. The scaled scores for each condition were based on the total number of sec-
onds required to complete the task.
Composite measures.
Two composite measures were used in this study to
reduce the data into specific domains (baseline and executive functioning) to lower
the number of ttests required to test our main hypotheses. Further, given the
across-test individual variability that has been found in previous studies (Prior &
Hoffmann, 1990; Rumsey, 1985), composite measures may more accurately cap-
ture executive functioning because several tests are combined. However, specific
strengths and weaknesses relative to the normal population cannot be inferred. Be-
ASPERGER’S DISORDER 387
cause norms for these composite measures were not available, the predicted aver-
age score for the composite measures was based on the premise that average per-
formance in the domain represented by the composite score should equal the num-
ber of measures included in the composite score multiplied by 10, for example,
baseline composite score = 6 (number of tasks) × 10 (normal mean) = 60. The pre-
dicted average score was 60 for the baseline composite measure and 80 for the ex-
ecutive functioning composite measure. The tests included in the composite mea-
sures are listed in Table 3.
Contrast scaled scores.
Contrast scaled scores were included in the com-
posite measures in addition to standard scaled scores. Contrast scaled scores are
normed variables included in the D–KEFS manual. They were derived by first sub-
tracting the scaled score on one cognitive measure from the scaled score of another
cognitive measure (often a baseline cognitive function from a higher level cogni-
tive function) then converting the scale-score difference to a new scaled score.
Scaled scores for the contrast measures also have a mean of 10 and standard devia-
tion of 3. Contrast scores minimize overestimates or underestimates of executive
dysfunction that are due to factors other than the cognitive operation of interest
(Delis et al., 2001). An example of the effect of contrast scores on results and inter-
pretation in this sample is depicted in Figure 1.
RESULTS
We compared the performance of the ASD group to the age-corrected scaled
scores provided by the D–KEFS national normative database. The D–KEFS nor-
mative sample included, in part, 100 fourteen-year-olds, 100 fifteen-year-olds, 175
sixteen- to nineteen-year-olds, 175 twenty-year-olds to twenty-five-year-olds, and
100 forty-year-olds to forty-nine-year-olds (Delis et al., 2001). The mean scaled
388 KLEINHANS, AKSHOOMOFF, DELIS
TABLE 3
Composition of the Composite Scores
Baseline Composite Executive Function Composite
Color naming Inhibition versus color naming
Word reading Inhibition switching versus inhibition
Visual scanning Number–letter switching versus combined number + letter sequencing
Number sequencing Design fluency filled + empty dots combined
Letter sequencing Design fluency switching versus filled + empty dots combined
Motor speed Letter fluency
Category fluency
Category switching versus category fluency
score for each condition is 10 with a standard deviation of 3. Raw scores were con-
verted to scaled scores for each condition.
Baseline Cognitive Functioning
Each participant’s baseline composite score equaled the sum of their age-corrected
scaled scores on the baseline composite measures. Overall performance on base-
line measures of cognitive functioning in ASD was not significantly below the pre-
dicted average score (60; M=53,SD =14.2),t(11) = –1.71, p=.12.However,50%
of the participants were impaired on two or more baseline measures. The most con-
sistent deficit occurred on visual scanning (M=7.25,SD = 4.41), which was
largely due to the HFA participants (see following).
Executive Functions
We tested whether a generalized deficit in executive functioning was present in
ASD. Contrast measures were included when available to account for possible im-
pairments on baseline measures or on less effortful executive-function tasks. Each
ASPERGER’S DISORDER 389
FIGURE 1 Example of two participants with contrasting profiles of strengths and weak-
nesses on the Color-Word Interference Test. Participant high-functioning autism Number 3
(HFA3) performed well on the baseline and inhibition tasks but demonstrated a relative decline
on the inhibition/switching condition. Participant Asperger’s disorder Number 4 (AD4), on the
other hand, was mildly impaired on all tasks, but the contrast scaled scores indicated that inhibi-
tion and inhibition/switching were not specifically impaired relative to fundamental naming
and reading abilities. Note that despite marked difference in fundamental cognitive abilities,
cognitive inhibition was equivalent in these two participants (contrast scaled score = 11).
participant’s executive function composite score equaled the sum of their age cor-
rected scaled scores on the executive-function measures. The ASD group per-
formed significantly below the predicted average score (80) on this measure of ex-
ecutive functioning (M=75.2,SD =6.0),t(11) = –2.79, p= .02. Individual tests
were evaluated to characterize the pattern of strengths and weaknesses on particu-
lar aspects of executive functioning (see Table 4). Figures 2 through 5 depict indi-
vidual performances on the executive-function tests. The participants performed
below average on letter fluency (M=8,SD = 2.5) and category switching fluency
(M= 6.4, SD = 2.7).
Errors
Scaled scores and cumulative percentiles of errors rates were analyzed. Cumula-
tive percentile ranks reflect the percentage of the normative sample that obtained
390 KLEINHANS, AKSHOOMOFF, DELIS
TABLE 4
Individual Conditions
ASDaNormal Average
Condition M SD M
Verbal Fluency Test
Letter fluency 8.1 2.4 10
Category fluency 10.2 3.6 10
Category switching fluency 7.1 3.2 10
Category switching contrast 7.1 3.9 10
Design Fluency Test
Filled + empty dots combined 9.5 2.5 10
Design fluency switching 9.8 3.5 10
Design fluency switching contrast 10.3 2.8 10
Color-Word Interference Test
Color naming 8.2 2.8 10
Word reading 9.3 3.5 10
Inhibition 8.1 3.7 10
Inhibition contrast 9.8 2.6 10
Inhibition switching 9.1 2.8 10
Inhibition switching contrast 11.1 2.7 10
Trail Making Test
Visual scanning 7.3 4.4 10
Number sequencing 8.9 3.7 10
Letter sequencing 8.8 3.9 10
Line drawing 10.7 2.9 10
Number-letter switching 8.2 3.7 10
Number-letter switching contrast 9.2 2.5 10
Note. ASD = autism spectrum disorders.
aN= 12.
391
FIGURE 2 Scatter plot of individual performance on the Trail Making Test. Bars depict the
autism spectrum disorders group mean.
FIGURE 3 Scatter plot of individual performance on the Color-Word Interference Test. Bars
depict the autism spectrum disorders group mean.
392
FIGURE 4 Scatter plot of individual performance on the Verbal Fluency Test. Bars depict the
autism spectrum disorders group mean.
FIGURE 5 Scatter plot of individual performance on the Design Fluency Test. Bars depict
the autism spectrum disorders group mean.
raw scores that were equal to or worse (i.e., greater number of errors) than the raw
score obtained by the examinee. Table 5 displays a summary of the average stan-
dard error scores of the ASD group relative to the normative sample on the baseline
and executive-function tasks. The average error rates on all the measures were
within normal limits.
HFA Versus Asperger’s Disorder
Mann–Whitney U tests were conducted to determine if the participants with HFA
differed significantly from the participants with Asperger’s disorder on demo-
graphic and cognitive variables. Significant group differences were not found on
age, Verbal IQ, Performance IQ, FSIQ, or the baseline and executive composite
measures. In addition, no significant differences were found between the HFA and
Asperger’s disorder participants on the executive-function conditions of the indi-
vidual tests. However, the HFA participants performed significantly worse on the
visual scanning condition of the Trail Making Test than the Asperger’s disorder
participants (Mdifference = 6.17, SD =1.83),U=3.5,p= .015. The poor perfor-
mance by the HFA group was likely due to slow scanning. None of the HFA partic-
ipants made any commission errors, and only one participant made an omission er-
ASPERGER’S DISORDER 393
TABLE 5
Group Mean Error Scores
Variable Performance
Color-Word Interference Test Errors (corr. + uncorr.)
Color naming cum% = 56.92 ± 45.25
Word reading cum% = 84.75 ± 35.72
Inhibition SS = 10 ± 2.97
Inhibition switching SS = 10.4 ± 2.27
Verbal Fluency Test Set loss Repetition
Combined (letter, category, category
switching)
SS = 9.3 ± 2.60 SS = 10.3 ± 2.50
Design Fluency Test Set loss Repetition
Combined (filled, empty, switching) SS = 11.5 ± 2.91 SS = 11.5 ± 2.15
Trail Making Test Ommission Commission
Visual scanning cum% = 75.36 ± 42.21 cum% = 100 ± 0
Set loss Sequencing
Number sequencing cum% = 100 ± 0 cum% = 100 ± 0
Letter sequencing cum% = 100 ± 0 cum% = 91.2 ± 29.25
Number-letter switching cum% = 69364 ± 40.2 cum% = 71.54 ± 42.47
Line drawing na na
Note. Higher cumulative percentile reflects better performance. corr. = corrected; uncorr. = un-
corrected; cum% = cumulative percentile; SS = scaled score.
ror on the visual scanning condition. Significant group differences were not found
on any of the other measures. A summary of the between-group statistical compar-
isons can be found in Table 6.
DISCUSSION
The high-functioning adults and adolescents with ASD in this study performed
significantly below average on our composite measure of executive functioning
over and above their seemingly intact fundamental cognitive skills. Potential
confounds of baseline impairments in cognitive processes were controlled for
with contrast measures, which are scaled scores based on the relative difference
between lower level tasks and higher level tasks rather than the individual, un-
corrected scores. Even with this conservative method of analysis, 11 of 12 par-
ticipants (92%) were impaired (scaled score ≤7) on at least one executive-func-
tion condition, and the group averaged two impaired scores per individual (range
= 0–3).
Executive dysfunction in ASD was generally mild, with 78% of the impaired
scores falling between 1 and 2 SDs below average. The most consistent deficit was
found on measures of verbal fluency that required cognitive switching and initia-
tion of efficient lexical retrieval strategies. This finding was likely not due to diffi-
culties exhibited by only a few participants: 10 of 12 participants were impaired on
either letter fluency or category switching fluency. In addition, these deficits can-
not be accounted for by a general difficulty with language-based tasks given the
strong overall performance by the group on category fluency.
394 KLEINHANS, AKSHOOMOFF, DELIS
TABLE 6
Between-Group Comparisons of Demographic and Neuropsychological
Variables
HFAaADa
Variable M SD M SD pb
Age 22.7 10.4 18.2 2.9 ns
VIQ 91.2 16.5 101.2 8.5 ns
PIQ 110.7 12.6 106.0 9.4 ns
FSIQ 100.3 12.3 103.5 7.7 ns
Baseline composite 45.67 13.91 60.3 10.91 ns
Executive function composite 75 4.15 75.33 7.87 ns
Visual scanning 4.17 3.49 10.33 2.81 .015
Note. HFA = high-functioningautism ; AD = Asperger’s disorder; VIQ = Verbal IQ; PIQ = Perfor-
mance IQ; FSIQ = Full Scale IQ.
an = 6 for each group. bBased on Mann–Whitney U test.
Although we did not find a significant deficit in overall baseline functioning,
50% of the participants had mild to moderate deficits on two or more baseline mea-
sures. Many of the deficits were mild and occurred on color naming, number se-
quencing, and letter sequencing. However, 4 participants (all HFA) performed at
the first percentile on the visual scanning task. This finding suggests that attention
difficulties may be present in individuals with ASD. Visual attention problems of
this type may be more common in individuals with autism than Asperger’s disor-
der. Determining similarities and differences in executive skills was not the focus
of this study; therefore, additional testing is needed to determine to what degree
differences in diagnostic categorization (severity of symptoms, history of lan-
guage delay, etc.) may affect the development of these skills. One prior study
(Pascualvaca et al., 1998) reported that young children with autistic disorder were
unimpaired on a similar visual scanning test, although they did perform the task
more slowly than the control children. Because the control children in the
Pascualvaca et al. study were 2 to 3 years younger than the autistic disorder group,
age differences likely attenuated any potential group differences. Unfortunately,
the other published autism studies that have included a similar visual scanning
measure have not reported completion time data (Liss et al., 2001; Minshew et al.,
1997), precluding comparisons to this study.
Executive dysfunction in our group of high-functioning individuals with ASD
was not characterized by a preponderance of classic “frontal” type errors. For ex-
ample, some non-ASD patients with frontal lobe lesions continue to make
perseverative errors on the WCST despite verbalizing the correct sorting principles
(Milner, 1964). This failure may be related to an inability to translate knowledge
into action because they cannot inhibit a prepotent response (i.e., a response ten-
dency built up through repetition). Difficulty disengaging from a prepotent re-
sponse was not noted in this study. The ASD group made very few set loss or
perseveration errors, both of which were within the normal ranges. Instead, slow-
ness was the typical cause of poor performances on the executive-function tests.
However, it is possible that when individuals with ASD are young children, these
types of errors may be more common than in matched control children (McEvoy,
Rogers, & Pennington, 1993; Pascualvaca et al., 1998; Prior & Hoffmann, 1990).
Further, parent report from the ADI–R indicated that these types of behaviors were
more prominent when the participants in this study were very young.
In this study, the individuals with ASD consistently had difficulty only on com-
plex verbal tasks that required generating and initiating efficient cognitive search
strategies and problem-solving techniques to improve performance. Interestingly,
although impaired in verbal fluency, the ASD participants were normal on design
fluency. These findings suggest ASD may be associated with a modality-specific,
executive-function deficit. The ASD group may have performed well on the non-
verbal fluency task because of the inherent strength in visuospatial processing that
many ASD individuals posses (Joseph, Tager-Flusberg, & Lord, 2002; Lincoln,
ASPERGER’S DISORDER 395
Courchesne, Kilman, Elmasian, & Allen, 1988; O’Riordan, Plaisted, Driver, &
Baron-Cohen, 2001; Plaisted, O’Riordan, & Baron-Cohen, 1998a, 1998b).
Minshew and colleagues (Goldstein, Johnson, & Minshew, 2001; Minshew et
al., 1997) have posited that cognitive dysfunction in ASD reflects the complexity
of the information-processing demands in cognitive tasks with the exception of
those in the visuospatial domain. Although the performance by the ASD group on
the Verbal Fluency Test was consistent with this view, the other results in this study
were not, such as their normal performance on number-letter switching, inhibition,
and inhibition/switching relative to baseline tasks, which calls into question the
generalizability of this theory. The theory that fundamental motor, sensory, and at-
tention processing are intact in autistic disorder and that deficits only emerge once
the cognitive demands of a task increase would predict that the ASD group would
perform best on the baseline cognitive measures, followed by nonswitching execu-
tive functions, and worst on the switching tasks. This pattern was not observed in
three of the four separate tests employed in this study. On the Color-Word Interfer-
ence and Design Fluency Tests, the ASD participants achieved higher scaled
scores on the switching conditions than on the nonswitching executive functioning
conditions (i.e., inhibition and filled + empty dots). This inverse pattern was even
more pronounced on the Trail Making Test in that participants performed worse on
visual scanning than on number-letter switching. As such, the complexity of the
task may not be the critical factor in determining success or failure on these cogni-
tive tests. Instead, greater success may be predicted on tests with greater inherent
structure, even if they are complex.
An alternative view is that children with autistic disorder are challenged by
some tests of executive functioning because they are unlikely to encode rules in a
verbal form (Russell, Jarrold, & Hood, 1999). Russell et al. hypothesized that this
purported feature makes it difficult for children with autistic disorder to perform
well on tasks that involve arbitraryrules and require a nonverbal response (e.g., the
WCST) but that autistic children are not impaired relative to controls when one of
these requirements is lifted. The Letter Fluency and Design Fluency Tests are ide-
ally suited to investigate whether individuals with ASD are differentially impaired
relative to normal controls on verbal and nonverbal tasks with arbitrary rules. Both
fluency tasks require the participant to follow several arbitrary rules and generate
as many different exemplars as possible within 60 sec. However, the ASD partici-
pants exhibited deficient performance only on letter fluency, a task that requires a
verbal response. The group average for design fluency was within the average
range (Mscaled score = 9.5).
A clear pattern of executive-function strengths and deficits is beginning to
emerge, with considerable evidence pointing to intact abilities on tests of cognitive
inhibition in ASD. Inhibition and inhibition/switching were intact in this study, in
agreement with previous studies (Brian et al., 2003; Ozonoff & Jensen, 1999;
Ozonoff & Strayer, 1997; Russell et al., 1999). However, the validity of the Stroop
396 KLEINHANS, AKSHOOMOFF, DELIS
paradigm as a measure of cognitive inhibition in ASD has been questioned because
it relies on the automaticity of the reading process (e.g., Russell et al., 1999). As
such, an average or above average score on the interference condition could result
from either intact inhibitory processing or lack of a reading-related interference ef-
fect. Given that ASD individuals have atypical language development, these possi-
bilities were investigated by analyzing the results of the component measures. We
contend that the intact performance on the inhibition condition of the Color-Word
Interference Test indicates intact inhibitory processing because participants per-
formed in the average range on the word reading condition. In addition, the com-
pletion time on the inhibition condition was double the completion time on the
color naming condition (60.75 sec vs. 33.5 sec, respectively), which suggests that
the written words caused an interference effect. Although it is clear that individuals
with ASD perform well on tests of cognitive inhibition, it is possible that these
tests are not sensitive to the processes that lead to the behavioral disinhibition
noted clinically in individuals with ASD.
The wide age range (adolescents to adults) of the individuals who were in-
cluded in this study coupled with the relatively small sample size needs to be taken
into account when considering the results of this study. However, it is important to
note that each participant’s scores were transformed to scaled scores and compared
to the normative sample that purportedly provided a much stronger basis for infer-
ence than a traditional control group would have. We also did not address the topic
of age-related changes in executive functioning in this study, and it is possible that
the patterns of executive-function abilities reported would not be observed to the
same degree at all ages. Twenty-five percent of the sample was taking psychotropic
medication at the time of testing to manage symptoms of anxiety, inattention, and
depression. Theoretically, none would be expected to adversely impact
neuropsychological test performance, although little to no information is available
on the effects on psychotropic medications on neuropsychological performance in
individuals with ASD. Because of the limitations noted previously, replication
with a larger sample is warranted to determine the degree that these results general-
ize to the full range of individuals with ASD.
Overall, executive dysfunction is present in HFA and Asperger’s disorder that
cannot be accounted for by impairments in fundamental cognitive processes. How-
ever, executive functioning in this group was not characterized by an overall de-
pressed executive-function profile. Rather, deficits in executive functions were
mild and circumscribed. The most consistent deficits were observed in the verbal
domain on tests that required high-level executive functioning such as cognitive
switching and initiation of efficient lexical retrieval strategies. Given the strong
overall performance by the group on category fluency, impairment on these tasks
cannot be attributed to a generalized difficulty with language-based tasks. Diffi-
culty with certain fundamental cognitive processes was also prevalent in the ASD
group. In particular, a fundamental impairment in visual attention was observed,
ASPERGER’S DISORDER 397
which appears to be more prominent in HFA than in Asperger’s disorder. This find-
ing highlights the importance of accounting for baseline functioning when inter-
preting tests of complex cognitive operations, particularly when studying patient
populations characterized by heterogeneous cognitive deficits.
ACKNOWLEDGMENTS
This work was supported by National Institute of Mental Health Grant
RO1–MH36840 awarded to Eric Courchesne. Portions of this work were pre-
sented at the Meeting for the International Neuropsychological Society, Honolulu,
Hawaii, February 2003.
We thank Jeanne Townsend for helpful comments on the article.
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