The Specificity and Independent Contribution of Inhibition, Working Memory, and Reaction Time Variability in Relation to Symptoms of ADHD and ASD

Abstract

The aim of the present study was to investigate the specificity of inhibition, working memory (WM), and reaction time variability (RTV) in relation to symptoms of ADHD and autism spectrum disorder (ASD).
A community-based sample of schoolchildren aged 7 to 9 years (N = 200) completed tasks designed to measure inhibition, WM, and RTV.
All neuropsychological functions were related to symptoms of both ADHD and ASD. The results from regression analyses showed that inhibition and RTV were related specifically to ADHD symptoms when controlling for symptoms of ASD. Regarding WM, no specific association with either symptom domain was evident after controlling for the other. Furthermore, independent contributions of inhibition and RTV were found in relation to ADHD symptoms after controlling for ASD symptoms.
The present study underscores the relevance of controlling for ADHD symptoms when examining ASD symptoms in relation to neuropsychological functions.
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Journal of Attention Disorders
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DOI: 10.1177/1087054715587093
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Article
Introduction
ADHD and autism spectrum disorder (ASD) are two com-
mon childhood-onset psychiatric disorders that involve
deficits in neuropsychological functions related to fronto-
striatal and fronto-parietal networks (see review by Gargaro,
Rinehart, Bradshaw, Tonge, & Sheppard, 2011). High levels
of comorbidity have been demonstrated between the symp-
tom domains (Ronald, Simonoff, Kuntsi, Asherson, &
Plomin, 2008), and co-diagnosis of ADHD and ASD is now
permitted according to the current diagnostic criteria
(Diagnostic and Statistical Manual of Mental Disorders
[5th ed.; DSM-5]; American Psychiatric Association, 2013).
In addition, symptoms of both ADHD and ASD have been
shown to be dimensionally distributed in the population
(Marcus & Barry, 2011; E. B. Robinson et al., 2011). Twin
studies have shown that the heritability of clinical levels of
symptoms of ADHD and ASD is no different than the heri-
tability of scores across the normal range (Constantino &
Todd, 2003; Levy, Hay, McStephen, Wood, & Waldman,
1997). Children with symptom levels below the cutoff for
formal diagnosis display cognitive deficits and negative
consequences similar to those of children with the diagno-
sis, albeit less severe (Bauermeister et al., 2007; Sucksmith,
Roth, & Hoekstra, 2011). Therefore, taking a wide range of
symptom level into account is of relevance when studying
symptoms of the two childhood psychiatric disorders in
relation to other aspects, such as cognitive functioning.
ADHD is often accompanied by poor executive func-
tions (EF; Willcutt, Doyle, Nigg, Faraone, & Pennington,
2005) and increased reaction time variability (RTV; Kofler
et al., 2013). However, the degree to which these functions
are nosologically specific to ADHD symptoms in relation to
symptoms of other disorders is less well known. While it is
also proposed that ASD may be related to poor EF and
increased RTV (see Rommelse, Geurts, Franke, Buitelaar,
& Hartman, 2011, for a review), it is still unclear whether
these findings may be the result of co-occurring ADHD
symptoms in the samples studied. The main aim of the pres-
ent study was therefore to explore the specificity of inhibi-
tion, working memory (WM), and RTV in relation to
symptoms of ADHD and ASD in a community sample. An
additional aim was to explore independent contribution of
587093JAD
XXX10.1177/1087054715587093Journal of Attention DisordersTruedsson et al.
research-article2015
1
Uppsala University, Sweden
Corresponding Author:
Erik Truedsson, Department of Psychology, Uppsala University, P.O.
Box 1225, SE-751 42, Uppsala, Sweden.
Email: erik.truedsson@psyk.uu.se
The Specificity and Independent
Contribution of Inhibition, Working
Memory, and Reaction Time Variability in
Relation to Symptoms of ADHD and ASD
Erik Truedsson
1
, Gunilla Bohlin
1
, and Cecilia Wåhlstedt
1
Abstract
Objective: The aim of the present study was to investigate the specificity of inhibition, working memory (WM), and
reaction time variability (RTV) in relation to symptoms of ADHD and autism spectrum disorder (ASD). Method: A
community-based sample of schoolchildren aged 7 to 9 years (N = 200) completed tasks designed to measure inhibition,
WM, and RTV. Results: All neuropsychological functions were related to symptoms of both ADHD and ASD. The results
from regression analyses showed that inhibition and RTV were related specifically to ADHD symptoms when controlling
for symptoms of ASD. Regarding WM, no specific association with either symptom domain was evident after controlling
for the other. Furthermore, independent contributions of inhibition and RTV were found in relation to ADHD symptoms
after controlling for ASD symptoms. Conclusion: The present study underscores the relevance of controlling for ADHD
symptoms when examining ASD symptoms in relation to neuropsychological functions. (J. of Att. Dis. XXXX; XX(X) XX-XX)
Keywords
ADHD, ASD, specificity, independent contributions, executive functions
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2 Journal of Attention Disorders
the three neuropsychological functions in relation to each of
the symptom domains. This was in particular interest in
relation to ADHD symptoms following a common finding
that EFs (in particular inhibition) and RTV make indepen-
dent contributions to ADHD symptoms (Kuntsi, Oosterlaan,
& Stevenson, 2001; Sjöwall, Roth, Lindqvist, & Thorell,
2013; Sonuga-Barke, Bitsakou, & Thompson, 2010).
Inhibition
In relation to ADHD, poor inhibition has either been con-
ceptualized as the core deficit or as an independent pathway
to the disorder (Barkley, 1997; Sonuga-Barke et al., 2010).
With respect to ASD, there is currently no theoretical con-
sensus as to whether the disorder should be associated with
inhibition deficits, and thus far, studies concerning inhibi-
tion in relation to ASD have yielded mixed results
(Rommelse et al., 2011)
Although it has been proposed that poor inhibition is
related to repetitive, stereotyped patterns of behavior (i.e.,
core symptoms of ASD; Mosconi et al., 2009), it has also
been suggested that inhibition might be a neuropsychologi-
cal function that could distinguish children with ASD from
children with ADHD (i.e., children with ASD have pre-
served functions whereas children with ADHD display defi-
cits; Sinzig et al., 2008). When direct comparisons have
been made of children with ADHD and children with ASD
in relation to inhibition, the results have been mixed.
However, in the studies in which ADHD symptoms in chil-
dren with ASD have been considered, the findings support
that inhibition is related mainly to ADHD (Bühler,
Bachmann, Goyert, Heinzel-Gutenbrunner, & Kamp-
Becker, 2011; Happé, Booth, Charlton, & Hughes, 2006;
Sinzig et al., 2008).
WM
Several models have been presented to explain the role of
WM in relation to ADHD. Whereas some consider WM
deficits to be a core component or endophenotype (Alderson,
Rapport, Hudec, Sarver, & Kofler, 2010), others view WM
deficits as more peripheral (Willcutt et al., 2005). In relation
to ASD, it has been proposed that WM has a great influence
because of its importance for social cognition and interper-
sonal interaction (Barendse et al., 2013). The results of the
few studies that have directly investigated WM perfor-
mance in relation to ADHD and ASD, while considering
symptom overlap, suggest that WM deficits are associated
primarily with ADHD (Happé et al., 2006; Sinzig et al.,
2008; van der Meer et al., 2012; Yerys et al., 2009).
However, in the study by Happé et al. (2006), the findings
differed depending on age. At young ages (8-10), both chil-
dren with ADHD and ASD performed worse than typically
developing children on a spatial WM task whereas at older
ages (11-16), only children with ADHD displayed impaired
performance.
RTV
In the field of ADHD research, there has been a growing
interest in RTV following its robust phenotypic link to the
disorder (Kofler et al., 2013). Interestingly, during recent
years, RTV has also been shown to be related to ASD
(Rommelse et al., 2011, for a review). The results from
studies that have directly compared RTV in children with
ADHD and children with ASD are contradictory. However,
a recent meta-analysis suggests that increased RTV is
related to ASD only when children with comorbid ADHD
are included (Karalunas, Geurts, Konrad, Bender, & Nigg,
2014). The exact mechanisms behind the increased RTV are
not yet known. According to Castellanos, Sonuga-Barke,
Milham, and Tannock (2006), increased RTV may be con-
nected to a pathophysiological processing common to dif-
ferent disorders. However, neuropsychological models
concerning ADHD have also presented possible explana-
tions for increased RTV in relation to ADHD. According to
the state regulation model (Börger & van der Meere, 2000)
and the cognitive energetic model (Sergeant, 2000), the
increased RTV seen in ADHD may be connected to ener-
getic deficits (e.g., an incapacity to mobilize an adequate
amount of energy in relation to specific task requirements).
Aim of the Present Study
The main aim of the present study was to examine the speci-
ficity of inhibition, WM, and RTV in relation to symptoms
of ADHD and ASD in a community sample. This is impor-
tant for identifying neuropsychological characteristics that
are either specific to or shared between the two symptom
domains. Although the literature (see Rommelse et al.,
2011, for a review) suggests that both ADHD and ASD are
related to poor inhibition, WM, and increased RTV, it is
unclear whether ASD symptoms are specifically associated
to these functions when accounting for co-existing ADHD
symptoms. An additional aim was to examine the indepen-
dent contribution of the three functions in relation to each of
the symptom domains. This was of particular interest in
relation to ADHD symptoms because previous research has
shown that EFs (in particular inhibition) and RTV make
independent contributions to the symptom domain (Sjöwall
et al., 2013; Sonuga-Barke et al., 2010). In addition, it has
not been explored whether independent contributions of
these functions remain after controlling for symptoms of
ASD.
First, relations between the neuropsychological func-
tions and the two symptom domains were examined (prior
to controlling for the other symptom domain). Based on
findings from previous studies (see Rommelse et al., 2011,
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Truedsson et al. 3
for a review), we expected that WM and RTV would be
associated with symptoms of both ADHD and ASD.
Regarding inhibition, we predicted an association to ADHD
symptoms (Willcutt et al., 2005), whereas whether ASD
symptoms would be associated was left open following the
inconsistency of previous findings (Christ, Kester, Bodner,
& Miles, 2011; Mosconi et al., 2009; S. Robinson, Goddard,
Dritschel, Wisley, & Howlin, 2009; Sanderson & Allen,
2013; Sinzig et al., 2008). Second, independent contribu-
tions were analyzed separately for ADHD and ASD symp-
toms (dependent on whether multiple neuropsychological
functions were associated with a symptom domain). In line
with previous findings (Sjöwall et al., 2013; Sonuga-Barke
et al., 2010), we predicted that inhibition and RTV would
make independent contributions to ADHD symptoms,
whereas no prediction was made concerning WM.
Regarding independent contributions of inhibition, WM,
and RTV to ASD symptoms, no a priori hypotheses were
formulated.
Thereafter, the specificity of each neuropsychological
function in relation to ADHD or ASD symptoms was ana-
lyzed after dimensionally controlling for the other symptom
domain. Findings from research that have accounted for
ADHD symptoms in children with ASD suggest that
impaired EF and increased RTV is primarily associated
with ADHD symptoms (Bühler et al., 2011; Happé et al.,
2006; Karalunas et al., 2014; Sinzig et al., 2008; van der
Meer et al., 2012). We therefore predicted that inhibition,
WM, and RTV would specifically associate to ADHD
symptoms even after controlling for symptoms of ASD
whereas the associations between ASD symptoms and the
functions would disappear after controlling for symptoms
of ADHD. Finally, independent contributions of neuropsy-
chological functions in relation to ADHD symptoms were
analyzed while dimensionally controlling for ASD symp-
toms and vice versa, if applicable. Whether the different
neuropsychological functions would make independent
contributions to symptoms of ADHD after controlling for
ASD, and vice versa, was left as an open question.
Method
Participants and Procedures
Participants in the current study were part of a larger longi-
tudinal study (N = 650). The children were initially recruited
from randomly selected child health care centers in Uppsala
County, Sweden, in connection with an health control at the
age of 5 offered to all families by the public health service.
Approximately 3 years later, the parents of all children in
the sample were contacted and asked to participate in a fol-
low-up study. About 76% of the parents answered the ques-
tionnaire as well as gave permission for the child’s teacher
to complete the same questionnaire; approximately 75% of
the teachers answered. The questionnaire to the parents also
included questions about developmental and medical his-
tory. Children with significant developmental or medical
problems were excluded from further participation (<1%).
This resulted in a sample of 357 children.
From the sample, we contacted the children with the
highest levels of symptoms (highest 30%: n = 121) on at
least one of the two domains (ADHD or ASD) based on
aggregated parent and teacher ratings (the questionnaires
are described in detail below, see “Ratings of Behavioral
Symptoms” section). In addition, 57% (n = 123) of the chil-
dren were randomly selected from the group with low-to-
moderate ratings on both symptom domains (lowest 70%:
n = 217). This resulted in a subsample of 200 children (93
girls) aged 8 to 9 years (M = 8.5 years; SD = 2.2 months; see
Table 1 for more information). A large part of the present
sample has been included in two earlier studies (Wåhlstedt
& Bohlin, 2010; Wåhlstedt, Thorell, & Bohlin, 2009) that
concerned research issues other than those addressed in the
current study.
Information on parental educational status (on a 5-step
scale) was collected as a measure of socioeconomic status
(SES). The tasks included in the current study were part of
a larger assessment battery intended to capture various neu-
ropsychological and behavioral functions. All children were
tested individually in a separate room at their school; the
whole assessment lasted for approximately 1 hr. The tests
were given in random order except for the go/no-go tasks,
where the normal condition was always the first and the
slow condition always the last. None of the children in the
Table 1. Descriptive Data on All Variables.
M SD Minimum-maximum
Behavioral ratings
ADHD 8.9 7.0 0.0-30.0
ASD 10.7 2.7 8.0-19.0
Neuropsychological measures
Inhibition
a
0.0 0.8 −1.2-2.5
WM
b
0.0 0.8 −2.2-2.6
RTV 175.0 51.0 82.4-328.6
Control variables
Conduct problems 7.2 2.1 5.0-17.5
Internalizing problems 8.1 2.8 5.0-16.5
Intelligence
c
11.0 2.3 5.5-17.5
SES 3.9 1.1 1.5-5.0
Note. N = 200 (boys = 107, girls = 93). ASD = autism spectrum disorder;
WM = working memory; RTV = reaction time variability; WISC-III =
Wechsler Intelligence Test for Children−3rd edition.
a
This variable is an aggregate based on standardized scores from two
inhibition tasks.
b
This variable is an aggregate based on standardized scores from two
WM tasks.
c
This variable is an aggregate based on standard points from two sub-
tests from WISC-III.
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4 Journal of Attention Disorders
study were receiving psychostimulant medication for
ADHD at the time of testing. The study was approved by
the local ethics committee.
Measures
Inhibition. Inhibition was studied using two validated tasks
based, respectively, on the go/no-go and the Stroop para-
digm, which have been widely used in ADHD and ASD
research (Christ, Holt, White, & Green, 2007; Crosbie et al.,
2013; Goldberg et al., 2005; Happé et al., 2006; S. Robin-
son et al., 2009; Sinzig et al., 2008). Furthermore, the tasks
have been used to discriminate between children with
ADHD and normally developing controls and have shown
adequate (go/no-go task: r = .84; Opposite Pairs Stroop
Task: r = .62, ps < .001) test–retest reliability (Berlin &
Bohlin, 2002; Brocki, Nyberg, Thorell, & Bohlin, 2007;
Thorell, 2007).
In the present go/no-go task (Berlin & Bohlin, 2002),
four different stimuli were presented: a blue square, a blue
triangle, a red square, and a red triangle. During the first
part of the task, the child was instructed to press a key
(“go”) when a blue figure appeared on the screen, but to
make no response (“no-go”) when an infrequent stimulus (a
red figure) appeared. The same stimuli were used for the
second part of the task, but the child was now instructed to
press a key every time he or she saw a square, irrespective
of color. The whole task was administered in two conditions
that differed in interstimulus interval (ISI; normal and
slow). In the normal condition, each stimulus was presented
for 800 ms with an ISI of 2,500 ms, whereas in the slow
condition, each was presented for 800 ms with an ISI of
8,000 ms. In total, there were 120 trials, 60 in each condi-
tion. The response inhibition score derived from the task
was the number of commission errors (i.e., pressing the key
when a “no-go” target was presented) across all trials.
In the Opposite Pairs Stroop Task (Berlin & Bohlin,
2002) the child was presented with four pairs of pictures,
where the pictures in each pair were opposites (e.g., day–
night, boy–girl, large–small, and up–down). After ensuring
that the child understood what each picture represented, he
or she was instructed to say the opposite as fast as possible
every time a picture was shown on the computer screen
(i.e., to say “boy” every time he or she saw a girl). The task
included two parts. In the first part, the child was presented
with each picture three times in random order, but the pairs
were not mixed (i.e., the first eight pictures were either a boy
or a girl; the next six pictures were either large or small, and
so on). In the second part, the instructions were the same and
each picture was also presented three times, but the eight pic-
tures were now presented in a fixed random order. Each stim-
ulus was displayed for 1,500 ms with an ISI of 1,500 ms.
Errors on this task were registered when the child named, or
started to name, the picture instead of saying the opposite or
when no answer was given. The interference control score
derived was the total number of errors on the Opposite Pairs
Stroop Task. To form a broad measure of inhibition, the two
inhibition measures were standardized and aggregated (r =
.42, p < .001). High values indicate poor inhibition.
WM. WM was assessed using two measures: Children’s
Size-Ordering Task (CSOT; McInerney, Hrabok, & Kerns,
2005) and a backward word span task (Thorell & Wåhlst-
edt, 2006). The tasks have been used considerably in both
clinical and non-clinical samples (Brocki & Bohlin, 2006;
Thorell, 2007; Thorell & Wåhlstedt, 2006; Diamantopou-
lou, Rydell, Thorell, & Bohlin, 2007; Sjöwall et al., 2013).
The CSOT has been shown to discriminate well between
children with ADHD and normally developing controls
(McInerney et al., 2005), and the backward word span task
is almost identical to the digit span task included in the
Wechsler Intelligence Test for Children−3rd edition (WISC-
III; Wechsler, 1991). Test–retest reliability has been shown
to be adequate for the backward word span task (r = .82,
p < .001). Regarding the CSOT, a previous study (Tillman,
Eninger, Forssman, & Bohlin, 2011) has reported adequate
split-half reliability for this task, .63.
In the CSOT, the child was presented with a list of com-
mon objects (e.g., pencil, mountain, train) read aloud at a
rate of one item per second; the child was then asked to
repeat the object names in order of size from smallest to
largest. The task began with two items per trial and became
gradually more difficult, concluding with a maximum of
seven items per trial. All children were administered all tri-
als, regardless of performance. The test was not time lim-
ited. The WM score derived from the task was the total pairs
of items ordered correctly across all trials.
In the backward word span task (Thorell & Wåhlstedt,
2006), the experimenter read unrelated nouns (e.g., flower,
dog, chair) to the child (ranging from 2 to 9) with a speed of
1 word/s. The child was first presented with two words, and
the span length thereafter increased by one word on every
other trial until the child failed to repeat at least one trial
correctly at a particular sequence length. After the experi-
menter said the words, the child had to say them in the
reverse order. One point was given to the child for each cor-
rect trial. The WM score derived from the task was the total
number of correct trials. To form a broad measure of WM,
scores from the two tasks were standardized and aggregated
(r = .34, p < .001). Low values indicate poor WM.
RTV. Data for the measures were obtained from the comput-
erized go/no-go task. Reaction times less than 150 ms on the
task were viewed as anticipatory errors and were discarded,
in line with previous research (Geurts et al., 2008). RTV (SD
in reaction time) was based on all correct “go trials” from
both parts of the two conditions (normal and slow) in the go/
no-go task. High values indicate increased RTV.
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Truedsson et al. 5
Intelligence. Two subtests from the WISC-III (Wechsler,
1991)—information and block design—were used to tap
general intelligence. Both subtests are considered reliable
and have been shown to correlate highly with full-scale IQ
(information: r = .95; block design: r = .93; Groth-Marnat,
1997). An aggregated mean score of standard points was
used as a measure of intelligence (r = .36, p < .001).
Ratings of Behavioral Symptoms
Measures regarding symptoms of ADHD, ASD, internaliz-
ing problems, and conduct problems were based on
aggregated mean scores of parent and teacher ratings
(ADHD: r = .46, p < .001; ASD: r = .31, p < .001; inter-
nalizing problems: r = .33, p < .001; conduct problems:
r = .39, p < .001) to obtain measures that reflected the
child’s behavior across contexts. Internal consistency as
measured by Cronbach’s alpha was adequate to high with
regard to both parent and teacher ratings of the behav-
ioral symptoms, with coefficients ranging from .68
to .94.
ADHD symptoms were measured using the ADHD
Rating Scale−IV (Dupaul, Thomas, & Anastopoulos,
1998). The scale includes 18 items (9 items assess symp-
toms of inattention, 9 items assess impulsivity/hyperactiv-
ity symptoms). Each item on the scale was scored on a
4-point scale ranging from 0 (never or rarely) to 3 (very
often). This measure has been validated and is frequently
used in ADHD research (Dupaul et al., 1997). ASD symp-
toms were assessed using 8 items from the Autism
Spectrum Screening Questionnaire (ASSQ; Ehlers &
Gillberg, 1993), a well validated instrument for screening
high-functioning ASD in both clinical and non-clinical
samples (Ehlers, Gillberg, & Wing, 1999; Posserud,
Lundervold, & Gillberg, 2006; Mattila et al., 2009). The 8
items tap core symptoms of the broad autism phenotype
(i.e., social interaction, communication problems, and
restricted and repetitive behavior) and has previously been
used to assess ASD symptoms in school-aged children
(Bohlin, Eninger, Brocki, & Thorell, 2012). Examples of
items are “can be with other children, but only on his/her
terms,” “uses language freely, but fails to make adjust-
ments to fit social contexts and the needs of different lis-
teners,” and “has special routines: insists on no change.”
Each item was scored on a 5-point scale ranging from 1
(does not apply at all) to 5 (applies very well). Conduct
problems and internalizing problems were measured with
the Strengths and Difficulties Questionnaire (SDQ;
Goodman, 1997), subscales conduct problems respectively
emotional problems. Each subscale includes 5 items, and
ratings were made on a 5-point scale ranging from 1 (does
not apply at all) to 5 (applies very well). The SDQ has been
shown to have adequate reliability and validity (Malmberg,
Rydell, & Smedje, 2003).
Data Management and Statistical Analyses
Prior to analysis, data were screened for outliers, defined as
values ±3 SD and replaced with the value that was the next
most extreme, in line with the Winsorizing procedure
(Chen, Welsh, & Chan, 2001). No multivariate outliers
defined by Cook’s D > 1 (Cook & Weisberg, 1982) were
identified in any of the subsequent analyses. Relations
between the neuropsychological functions and ADHD and
ASD symptoms were first analyzed using correlations. To
examine the independent contributions of neuropsychologi-
cal functions in relation to symptoms of ADHD and ASD
separately, multiple regression analyses were used in which
all neuropsychological functions that significantly corre-
lated with the analyzed symptom domain were entered
together. Thereafter, to explore specificity, regression anal-
ysis was used in which each neuropsychological measure
was entered as a predictor variable together with the other
symptom domain (ADHD or ASD) in separate analyses.
Finally, independent contributions in relation to ADHD and
ASD symptoms were examined while controlling for the
other symptom domain. All analyses were made adjusting
for sex because the variable was associated with both pre-
dictors and outcomes. In addition, to assess whether intel-
ligence or common co-occurring behavioral problems
(conduct problems and internalizing problems, respec-
tively) would influence the relations between the symptom
domains and the neuropsychological functions, we re-ran
the analyses with control for each of these variables. All
instances in which this led to changed conclusions are
reported.
Results
Descriptive statistics for behavioral symptoms, neuropsy-
chological functions, and control variables are presented in
Table 1. Intercorrelations among all variables, adjusted for
sex, are shown in Table 2. Behavioral ratings of ADHD and
ASD symptoms were highly correlated (r = .62, p < .001).
Regarding the relation between the neuropsychological func-
tions and the symptom domains, we found that poorer perfor-
mance on inhibition and WM as well as increased RTV were
associated with higher levels of both ADHD and ASD symp-
toms. Conduct problems as well as internalizing problems
were positively correlated with symptoms of both ADHD
and ASD. Higher levels of conduct problems were associated
to poorer performance on inhibition. Furthermore, lower
intelligence was associated with higher levels of conduct
problems and to poorer performance on inhibition and WM.
Multiple regression analyses were performed to investi-
gate the independent contributions of neuropsychological
functions in relation to the two symptom domains. Sex and
the three neuropsychological measures were entered together
as predictor variables. As seen in Table 3, inhibition and
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6 Journal of Attention Disorders
RTV contributed independently to ADHD symptoms
whereas inhibition and WM contributed independently to
ASD symptoms. While re-analyzing and controlling for con-
duct problems, the independent contributions of inhibition
and WM to symptoms of ASD became non-significant (β =
.04, p = .53 and β = −11, p = .06, respectively, ΔR
2
= .39).
To examine specificity, regression analyses were used in
which each neuropsychological measure was entered as a
predictor together with sex and the other symptom domain
(ADHD or ASD) in separate analyses. We found that inhibi-
tion and RTV remained associated to ADHD symptoms
after controlling for symptoms of ASD (β = .31, p < .001 and
β = .22, p < .01, respectively), whereas WM did not remain
related to ADHD symptoms (β = −08, p = .13). No associa-
tions between the separate neuropsychological functions and
ASD symptoms remained after controlling for ADHD symp-
toms (inhibition: β = −07, p = .26; WM: β = −08, p = .17;
RTV: β = −03, p = .64). Thus regarding WM, no association
remained with either ADHD or ASD symptoms after con-
trolling for symptoms of the other domain. Given that sev-
eral neuropsychological measures were specifically related
to ADHD symptoms, independent contributions were also
investigated while controlling for ASD symptoms. Sex,
ASD symptoms, and the neuropsychological measures were
entered together as predictor variables with ADHD symp-
toms as the dependent variable. Both inhibition and RTV
showed independent contributions to ADHD symptoms
(β = .27, p < .001 and β = .15, p < .01, respectively, ΔR
2
= .53).
Discussion
The present study set out to examine the specificity of inhi-
bition, WM, and RTV in relation to symptoms of ADHD
and ASD in a community sample. Prior to controlling for
the other symptom domain, all neuropsychological mea-
sures were related to both ADHD and ASD symptoms.
Independent contributions of inhibition and RTV were evi-
dent in relation to symptoms of ADHD, whereas inhibition
and WM made independent contributions to symptoms of
ASD. Regarding specificity, inhibition and RTV were spe-
cifically related to ADHD symptoms. However, WM was
not specifically associated with ADHD symptoms, which
runs counter to our prediction. Furthermore, the indepen-
dent contributions of inhibition and RTV to ADHD symp-
toms remained after controlling for ASD symptoms.
Independent Contributions of Neuropsychological
Functions to ADHD and ASD Symptoms
In accordance with our hypotheses, we found that ADHD
symptoms were independently associated with both inhibition
and RTV. More interestingly, the independent contributions
Table 2. Intercorrelations Between Behavioral Ratings, Neuropsychological Measures, and Intelligence Adjusting for Sex (N = 200).
1 2 3 4 5 6 7 8
1. ADHD — .62*** .61*** .25*** .43*** −.21** .34*** −.14
2. ASD — .60*** .49*** .21** −.20** .18* −08
3. Conduct problems — .22** .23** −13 .13 −16*
4. Internalizing problems — .03 −.07 .07 .03
5. Inhibition — −.14* .29*** −20**
6. WM — −.22** .41***
7. RTV — −.02
8. Intelligence —
Note. ASD = autism spectrum disorder; WM = working memory; RTV = reaction time variability.
*p < .05. **p < .01. ***p < .001.
Table 3. Results of Regression Analyses Studying Independent Contributions of Neuropsychological Functions in Relation to ADHD
and ASD Symptoms, Adjusting for Sex.
ADHD ASD
ΔR
2
β ΔR
2
β
Inhibition .29 .34*** .09 .16*
a
WM −.11 −.16*
a
RTV .20** .10
Note. The neuropsychological measures are entered together as predictor variables with respective symptom domain (ADHD or ASD) as the depen-
dent variable. ASD = autism spectrum disorder; WM = working memory; RTV = reaction time variability.
a
The associations were non-significant when controlling for conduct problems.
*p < .05. **p < .01. ***p < .001.
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Truedsson et al. 7
remained after controlling for ASD symptoms. Our findings,
therefore, extend previous knowledge by demonstrating that
the two functions contribute with unique variance to ADHD
symptoms even in the presence of ASD symptoms. Inhibition
and RTV, therefore, seem to be of greater importance than
WM in relation to ADHD symptoms in the age group studied.
This is supported by a recent clinical study with school-aged
children (Sjöwall et al., 2013) in which inhibition and RTV,
whereas not WM, independently predicted group status
(ADHD vs. control).
In relation to ASD symptoms, inhibition and WM made
independent contributions. The two EFs have been dis-
cussed in relation to different core characteristics of ASD:
inhibition to repetitive, stereotyped patterns of behavior
(Mosconi et al., 2009) and WM to theory of mind and social
cognition (Barendse et al., 2013). However, when control-
ling for ADHD symptoms or conduct problems, the inde-
pendent contributions of inhibition and WM to symptoms
of ASD did not remain. This suggests that it is relevant to
consider both symptoms of ADHD and conduct problems
when examining neuropsychological functions in relation
to ASD symptoms. However, the relation between conduct
problems and neuropsychological functions may also pos-
sibly be explained by the large overlap between conduct
problems and ADHD symptoms. Studies examining ADHD
and conduct problems in relation to EFs have shown that
poor EF is primarily related to ADHD (Berlin & Bohlin,
2002; Brocki & Bohlin, 2006; Oosterlaan, Scheres, &
Sergeant, 2005; Sergeant, Geurts, & Oosterlaan, 2002). In
addition, the fact that the independent contribution of inhi-
bition remained to ADHD symptoms when controlling for
conduct problems as well as ASD symptoms further sup-
ports that it is ADHD symptoms and not conduct problems
that are primarily associated to EFs.
Specificity of Neuropsychological Functions in
Relation to ADHD and ASD Symptoms
Inhibition. Inhibition was significantly associated with
symptoms of ADHD and ASD. Furthermore, consistent
with our predictions, inhibition was specifically related to
ADHD symptoms. The present findings are thus in line
with results from clinical studies in which the other symp-
tom domain has been taken into account (Bühler et al.,
2011; Happé et al., 2006; Sinzig et al., 2008) as well as with
a recent study that gives strong support to the validity of
inhibition as an endophenotype of ADHD (Crosbie et al.,
2013). Thus, it is possible that previous findings from clini-
cal studies documenting inhibition deficits in children with
ASD can be explained by elevated levels of ADHD
symptoms.
WM. In line with our expectations, prior to controlling for
the other symptom domain, WM was associated with symp-
toms of both ADHD and ASD. However, our prediction that
WM would be specifically related to ADHD symptoms was
not confirmed in the current study. Although clinical studies
comparing WM capacity in children with ADHD and chil-
dren with ASD have found lower WM performance related
primarily to ADHD (Happé et al., 2006; Sinzig et al., 2008;
van der Meer et al., 2012), the current findings suggest that
lower WM performance may be a shared cognitive charac-
teristic for symptoms of the two disorders. Interestingly, it
has been suggested that the association between ADHD and
ASD symptoms is a consequence of genetic overlap with
attention difficulties (Polderman et al., 2013). In addition,
when ADHD symptoms have been further divided into
symptoms of inattention and hyperactivity/impulsivity,
WM has been shown to be related primarily to symptoms of
inattention (Brocki, Eninger, Thorell, & Bohlin, 2010;
Chhabildas, Pennington, & Willcutt, 2001). Therefore,
given the strong association between WM and attention
(Unsworth & Engle, 2007), it is plausible that WM could be
a common factor reflecting deficits in attention shared by
symptoms of ADHD and ASD. Of the previous studies on
this issue that found WM to be primarily related to ADHD,
two included only a single spatial task to tap WM (Happé
et al., 2006; Sinzig et al., 2008) whereas in the current study,
two verbal WM tasks were used. The discrepancy between
the current findings and the previous ones may therefore be
influenced by modality, although other factors may also be
of relevance, such as executive demand (Kasper, Alderson,
& Hudec, 2012). Thus, future studies aimed at further
understanding the specific and shared associations between
WM and the two symptom domains would benefit from
including several measures designed to tap various sub-
functions of WM. Furthermore, the associations between
WM and the symptom domains found in the current study
may be influenced by age-dependent factors. In the study
by Happé et al. (2006) children with ADHD and ASD per-
formed worse than typically developing children on a spa-
tial WM task at young ages (8-10 years) while at older ages
(11-16 years), only children with ADHD displayed impaired
performance. Thus, research on the issue from a develop-
mental perspective is warranted.
RTV. Increased RTV was significantly associated with
symptoms of both ADHD and ASD. Although increased
RTV previously has been suggested to be associated to symp-
toms of both disorders (Rommelse et al., 2011), our findings,
in line with a recent meta-analysis on clinical samples
(Karalunas et al., 2014), propose that increased RTV may be
specific to ADHD symptoms. Thus, although it has been sug-
gested that increased RTV reflects non-specific brain pathol-
ogy associated to a number of neurodevelopmental disorders
(Castellanos et al., 2006), the current findings support the
hypothesis that increased RTV is primarily associated to
ADHD symptoms. Hence, the results support models in
which increased RTV is considered to be connected to
domain-specific deficits, such as the state regulation model
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8 Journal of Attention Disorders
or the cognitive energetic model (Börger & van der Meere,
2000; Sergeant, 2000). In addition, in the present study,
increased RTV continued to make independent contribu-
tions to symptoms of ADHD together with inhibition after
controlling for ASD. The findings therefore provide further
support for the role of RTV as a cognitive characteristic of
ADHD symptoms.
Limitations
Although the present study is the first to dimensionally study
the specificity of neuropsychological functions in relation to
symptoms of ADHD and ASD, a few limitations should be
noted. Although community-based studies on symptoms of
ADHD and ASD should be viewed as a valuable comple-
ment to clinical research, it should also be noted that there
may be constraints on the generalizability of the findings to
clinical populations. Thus, research with children formally
diagnosed with ADHD and ASD is warranted.
Furthermore, the study is cross-sectional, it only pro-
vides information about a particular point in development.
In addition, future studies would benefit from including a
broader battery of neuropsychological, motivational, and
emotional measures (in particular to tap functions that have
been shown to be primarily related to ASD, such as central
coherence and theory of mind) to provide more knowledge
of unique and shared endophenotypes of the two symptom
domains. In the current study, behavioral symptom levels
were based on questionnaires only, which, compared with
interviews, might overestimate the degree of comorbidity
given that questionnaires do not allow further explanation
of questions. However, by including both parent and teacher
ratings, symptoms from two different situations and sources
were captured.
Conclusion
In the current study, symptoms of both ADHD and ASD
were associated with lower performance on several neuro-
psychological tasks. However, the dimensional analysis
regarding specificity showed that poor inhibition and
increased RTV were specifically associated with symptoms
of ADHD, whereas for WM, no specific association was
found with either symptom domain. The current study dem-
onstrates that dimensional approaches are useful in under-
standing the relationship between neuropsychological
functions and symptoms of ADHD and ASD in more depth.
Our study also extends previous findings by showing not
only the specificity but also independent contributions of
inhibition and RTV in relation to symptoms of ADHD when
controlling for symptoms of ASD. The findings, therefore,
further strengthen the importance of both inhibition and
RTV as neuropsychological characteristics of ADHD
symptoms.
Authors’ Note
This study has been designed in accordance with the ethics rules of
the Swedish Research Council.
Declaration of Conflicting Interests
The author(s) declared no potential conflicts of interest with
respect to the research, authorship, and/or publication of this
article.
Funding
The author(s) disclosed receipt of the following financial support
for the research, authorship, and/or publication of this article: This
study was supported by Grant 2009-0869 from the Swedish
Research Council for Health, Working Life and Welfare.
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Author Biographies
Erik Truedsson is a PhD student at the Department of Psychology
at Uppsala University.
Gunilla Bohlin is a professor in psychology at the Department of
Psychology, Uppsala University.
Cecilia Wåhlstedt is a researcher at the Department of Psychology,
Uppsala University.
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