Abilities to Explicitly and Implicitly Infer Intentions from Actions in Adults with Autism Spectrum Disorder
Journal of Autism and Developmental Disorders
Abilities to Explicitly and Implicitly Infer Intentions from Actions in Adults with Autism Spectrum Disorder
Eleanor J. Cole 0
Katie E. Slocombe 0
Nick E. Barraclough 0
0 The Department of Psychology, The University of York , Heslington, York YO10 5DD , UK
1 Eleanor J. Cole
Previous research suggests that Autism Spectrum Disorder (ASD) might be associated with impairments on implicit but not explicit mentalizing tasks. However, such comparisons are made difficult by the heterogeneity of stimuli and the techniques used to measure mentalizing capabilities. We tested the abilities of 34 individuals (17 with ASD) to derive intentions from others' actions during both explicit and implicit tasks and tracked their eye-movements. Adults with ASD displayed explicit but not implicit mentalizing deficits. Adults with ASD displayed typical fixation patterns during both implicit and explicit tasks. These results illustrate an explicit mentalizing deficit in adults with ASD, which cannot be attributed to differences in fixation patterns.
Autism spectrum disorder; Intentions; Mentalizing; Autistic traits; Action perception; Eye-tracking
Autism Spectrum Disorder (ASD) is the term used in the
most recent edition of the Diagnostic and Statistical
Manual of Mental Disorders (DSM-V) to describe a range of
neurodevelopmental disorders, classified according to core
deficits in social communication and interaction as well as
restricted and repetitive interests
. One of the social communication
difficulties associated with ASD is difficulty inferring the
internal states of others including their intentions, mental states
(Baron-Cohen et al. 1997; Chung et al. 2014;
Frith 2001; Holt et al. 2014a, b; Jolliffe and Baron-Cohen
, collectively referred to as mentalizing deficits.
Mentalizing deficits are so strongly associated with ASD
that mentalizing abilities are even assessed in diagnostic
and screening tools such as the Autism Diagnostic
Observation Schedule (ADOS-2) and the Autism Quotient (AQ)
(Baron-Cohen et al. 2001; Lord et al. 2000)
. Due to the
spectral nature of ASD, individuals without a diagnosis also
display varying degrees of autistic traits. Individuals with
relatively high but not clinically significant levels of
autistic traits have been shown to display subtler versions of the
behavioural and neurological characteristics associated with
(Best et al. 2015; Di Martino et al. 2009; Lindell et al.
2009; Ridley et al. 2011; van Boxtel and Lu 2013)
(Baron-Cohen et al. 2001; Chung et al.
2014; Happé 1994; Kana et al. 2014; Moran et al. 2011)
Despite the strong association between ASD and
mentalizing deficits, experimental evidence regarding the nature
of these deficits is inconsistent, with some studies finding
that adults with ASD are impaired at inferring intentions,
emotions and mental states of others
(Baron-Cohen et al.
2001; Castelli et al. 2002; Happé 1994; Kana et al. 2014;
Moran et al. 2011; Senju et al. 2009)
and others reporting
adults with ASD
(Kana et al. 2009; Kirkovski et al. 2015;
Ponnet et al. 2004; Roeyers et al. 2001; Spek et al. 2010)
high levels of autistic traits
(Nijhof et al. 2016)
performances on mentalizing tasks. A number of factors may
have contributed to these inconsistent findings, including
whether task instructions explicitly stated that participants
should mentalize, the stimuli used, the type of
mentalizing assessed and the method used to measure mentalizing
The majority of previous studies have explicitly asked
participants to make inferences about the internal states of
(e.g. Baron-Cohen et al. 2001; Gallagher et al. 2000;
Happé 1994; Holt et al. 2014a, b; Jolliffe and Baron-Cohen
1999a, b; Kana et al. 2009, 2014; McAleer et al. 2011;
Roeyers et al. 2001)
. Only a small number of studies have
examined the capabilities of adults with ASD to infer the internal
states of others when not specifically told to do so; this is
known as ‘implicit mentalizing’. The existing adult
literature shows consistent implicit mentalizing deficits associated
(Castelli et al. 2002; Rosenblau et al. 2015;
Schuwerk et al. 2014; Senju et al. 2009)
but the explicit
mentalizing data are inconsistent
(Baron-Cohen et al. 1997; Castelli
et al. 2002; Kana et al. 2009; Kirkovski et al. 2015; Ponnet
et al. 2004)
. It may be that the instructions given concerning
which elements should be attended to during explicit tasks,
allow some high functioning adults with ASD to perform at
a typical level, which they would be unable to do without
the explicit instructions.
The apparent existence of a consistent implicit
mentalizing deficit but lack of a consistent explicit mentalizing
deficit in adults with ASD in the existing literature may,
however, be attributable to other confounding factors, including
stimuli differences. Most studies that have reported implicit
mentalizing deficits in adults with ASD have used movie
(e.g. Rosenblau et al. 2015; Schuwerk et al. 2014;
Senju et al. 2009)
, which were more complex and
naturalistic than stimuli used in the majority of explicit tasks. The
stimuli used in the majority of explicit mentalizing tasks
were passages of text, still images or cartoon strips which
provide very simplistic representations of social interactions
and a number of these studies found no mentalizing deficits
in adults with ASD
(e.g. Kana et al. 2009; Kirkovski et al.
2015; Ponnet et al. 2004; Roeyers et al. 2001; Spek et al.
. In support of this argument, two previous studies
(Ponnet et al. 2004; Roeyers et al. 2001)
abilities of adults with Pervasive Development Disorders
(PDD; including ASD) to explicitly infer the mental states
of others using both simple stimuli (images of people’s eyes
and short passages of text) and naturalistic videos of social
interactions. The adults with PDD were not impaired on the
explicit mentalizing tasks that used the simple stimuli but
did show impairments with the more complex naturalistic
(Ponnet et al. 2004; Roeyers et al. 2001)
Additionally, the only previous study that has investigated both
implicit and explicit mentalizing abilities using complex,
naturalistic stimuli found that adults with ASD displayed
equivalent impairments on both implicit and explicit tasks
(Rosenblau et al. 2015)
Differences in the way mentalizing performances have
been measured may have also contributed to existence of
consistent implicit mentalizing deficits but inconsistent
data regarding explicit mentalizing abilities in the previous
adult literature. Some studies have measured implicit
mentalizing abilities using eye-tracking data alone
et al. 2014; Senju et al. 2009)
. In these studies, participants
watched animations in which a character wrongly believed
an object was in a certain location. Adults with ASD spent
shorter periods fixating on the place in which the character
wrongly believed the object was located. This was
interpreted as impaired implicit mentalizing. However, a number
of studies have reported that adults with ASD have unusual
patterns of eye gaze when processing social stimuli
(Kliemann et al. 2010; Pelphrey et al. 2002)
and unusual fixation
patterns have been found during face processing tasks in the
absence of behavioural differences
(Rutherford and Towns
2008; Spezio et al. 2007)
. Therefore, adults with ASD may
be able to deduce the internal states of others despite
atypical eye movements. In contrast, explicit mentalizing studies
have always used measurable behavioural outcomes to assess
The term ‘mentalizing’ covers a variety of internal state
inferences which may involve different processes
Tomasello 2008; Pineda and Hecht 2009)
; it is possible that
the different internal state inferences required across studies
may have also contributed to the heterogeneity in the
literature. Previous studies have reliably found that adults with
ASD are impaired at inferring others’ intentions
(Kana et al.
2014; Murdaugh et al. 2014; Ponnet et al. 2004; Roeyers
et al. 2001)
and others’ emotions
(Atkinson 2009; Cassidy
et al. 2013; Enticott et al. 2013; Hubert et al. 2007;
Nackaerts et al. 2012)
. However, the existing literature is more
inconsistent regarding abilities to infer others’ mental states
(Baron-Cohen et al. 1997; Kana et al. 2009; Kirkovski et al.
2015; Kleinman et al. 2001; Roeyers et al. 2001; Spek et al.
or false beliefs
(Frith and Happé 1994; Schuwerk et al.
2014; Senju et al. 2009)
. The neuroimaging and
developmental literature also support the argument that the different
subcomponents of mentalizing reflect different processes;
the results of a meta-analysis suggest that children develop
the ability to infer others’ desires before they are able to infer
others’ beliefs and can detect others’ emotions before they
can deduce false beliefs
(Wellman and Liu 2004)
Additionally, neuroimaging studies have shown that different brain
areas are active during mentalizing tasks depending on the
inferences being made
(Pineda and Hecht 2009; Schurz et al.
. Collectively, these data suggest that the
subcomponents of mentalizing are distinct processes associated with
different brain areas and developmental trajectories.
In summary, although ASD is associated with
mentalizing deficits, the nature of these deficits is unclear. The
existing literature suggests that adults with ASD are more
likely to show impaired performances on implicit
mentalizing tasks using complex naturalistic stimuli that probe
understanding of intentions or emotions. To our
knowledge, only one study to date has assessed both implicit
and explicit mentalizing abilities in adults with ASD using
measurable behavioural outcomes
(Rosenblau et al. 2015)
In this study, a comparison between adults with and
without ASD found that participants with ASD showed reduced
abilities to both implicitly and explicitly infer the mental
states of actors from short movies but there was no
difference in the degree of impairment between tasks. However,
this study did not use a non-mentalizing control task so
it is unclear whether the poorer performances observed
in adults with ASD were specifically due to mentalizing
deficits or whether poorer performances reflect reduced
abilities to perform the experimental tasks in general. Thus
the current study aimed to test the abilities of adults with
ASD to both implicitly and explicitly mentalize, using
complex stimuli, measurable behavioural outcomes and a
non-mentalizing control task.
This study specifically investigated the abilities of adults
with ASD to both implicitly and explicitly infer the
intentions of others from the kinematics of their hand actions
using the same naturalistic stimuli. Previous studies have
shown that hand actions with different intentions display
subtle differences in action kinematics and adults without
ASD are able to infer others’ intentions from these
differences in action kinematics
(Ansuini et al. 2015; Sartori et al.
. In the first experiment, participants watched videos
of actors playing a poker chip game and had to decide which
actor, from a choice of two, they would prefer to play the
poker chip game with. Participants were shown one video
depicting an actor deliberately not passing a poker chip to
another player (‘spiteful’ action) and a video of another
actor accidentally not passing a poker chip to another player
(‘clumsy’ action). In this task, participants were not
explicitly asked to infer actors’ intentions; rather participants’
choice of actor was dependent upon ‘covert’ mentalizing
(implicit mentalizing task). In contrast, during the second
experiment, participants watched the same movies and
were explicitly asked to infer the intentions of the actors.
In addition to contrasting the performance of the ASD and
typically developing groups, due to the spectral nature of
ASD, we then examined the relationship between the level
of autistic traits displayed and abilities to infer others’
intentions across all participants. We also tracked participants’
eye movements during both experiments in order to test
whether any potential behavioural differences associated
with autistic traits could be explained by atypical fixation
(cf. Schuwerk et al. 2014; Senju et al. 2009)
. It was
predicted that adults with ASD would display reduced
abilities to infer the intentions of others compared to matched
control participants and across all participants higher levels
of autistic traits would predict poorer performances. We also
hypothesised that mentalizing deficits associated with ASD
would be more evident in the implicit task compared to the
Twenty-one adults with Autism Spectrum Disorder (ASD;
14 male) were recruited for this study. The majority of the
ASD sample were students from the University of York
(n = 13) and the remaining ASD participants were recruited
from a local support group. Four participants were excluded
for having scores that were not significantly higher than
chance on the control task (see below). This resulted in a
final participant sample of 17 adults with ASD (10 male ages
18–56, mean age = 23.71, SD = 9.24) and 17 individually
age, sex and IQ matched control participants
(TD—Typically Developing; ages 18–55, mean age = 23.71, SD = 9.07).
See Table 1 for participant demographics.
All participants in the ASD group had a clinical diagnosis
of Asperger’s (n = 14) or Autism Spectrum Disorder. All
diagnoses were issued by qualified clinicians external to this
study. None of the ASD participants had a history of delayed
language development or existing learning difficulties. All
participants had IQ scores above 100. All neurotypical
participants reported that they had no neurological disorders
and adults diagnosed with ASD reported no other
Experiments were approved by the ethics committee of
the Department of Psychology, University of York, and were
performed in accordance with the ethical standards outlined
in the 1990 Declaration of Helsinki.
The Autism Diagnostic Observation Schedule
Lord et al. 2000)
, Social Responsiveness Scale
Constantino et al. 2003)
, The Awareness of Social Inference
(TASIT; McDonald et al. 2006)
, Autism Quotient
Baron-Cohen et al. 2001)
and Wechsler Abbreviated Scale
of Intelligence (WASI; Wechsler 1999) were administered
to all participants. The ADOS-2 assessments were filmed
and then scored by both the experimenter and an
independent rater who was blind as to whether participants had a
diagnosis or not. Both the experimenter and independent
rater were trained to the level of research reliability on the
ADOS-2 assessment. If the ADOS-2 scores differed between
the experimenter and independent rater, the assessment
movies were re-watched and a final score was agreed on. The
independent ADOS-2 scores never differed by more than 2
points between the raters. The SRS and TASIT are designed
to detect social impairment. The SRS is a self-report
measure and TASIT measures abilities to detect sarcasm and lies
from movies showing social interactions. The AQ is a
selfreport measure of autistic traits. The two subtest version of
the WASI was used to measure the IQ of participants. All
these psychological tests have been shown to have good
(Allison et al. 2011; Constantino et al.
2003; Hurst et al. 2007; McDonald et al. 2006; Oosterling
et al. 2010)
The movie stimuli were designed to show different actors
playing a poker chip exchange game. The poker chip game
involved passing poker chips to another player through slots
in a white wooden board (see Fig. 1). Ten different types of
hand actions were filmed (Panasonic TM900 HD-DV
camera; 1920 × 1080 pixels at 50 Hz progressive scan). Five of
the hand actions involved pushing poker chips with the index
finger of the right hand through a slot in the board which
was level with the surface of the table. The other five hand
actions involved grasping poker chips with the index finger
and thumb of the right hand and passing them through a slot
in the board at head height. Two different types of actions
Fig. 1 Example screenshots
from the hand action movies
depicting positive and negative
intentions. a The actor pushes
five poker chips with a positive
intention (clumsy 5 pushing
action). b The actor deliberately
pushes a poker chip away from
the slot (spiteful 1 pushing
action). c The actor accidentally
drops a poker chip (clumsy 1
grasping action). d The actor
deliberately drops the poker
chip (spiteful 1 grasping action).
The squares overlaid onto
action a illustrate the regions
of interest (ROIs) used for the
were used to generalise results across different action types.
Both pushing and grasping actions were executed by the
actor in five different ways: (1) clumsily failing to pass one
poker chip—here the actor had a positive intention to pass
the chip to the other player, but the outcome of the action
was unsuccessful (clumsy 1); (2) Clumsily failing to pass
five pokers chips; positive intention to pass the chips, but
the outcome of the action was unsuccessful (clumsy 5); (3)
Spitefully (deliberately) failing to pass one poker chip; no
intention to pass the chip to the other player and the outcome
of the action was unsuccessful (spiteful 1); (4) Successfully
passing one poker chip; the actor intended to pass the poker
chip and the action was successful (successful 1); (5)
Successfully passing five poker chips; the actor intended to pass
the poker chips and the action was successful (successful 5).
Twenty-eight different actors (14 female) were filmed
performing all ten actions, from a three-quarters view from
behind at an angle that allowed their right hand to be seen
in front of them for the entire duration of the hand action
but only showed a limited side profile of their face. This
prevented participants from using facial information to infer
the intentions of actors and required intentions to be inferred
from the action kinematics alone
(cf. Sartori et al. 2009;
Ansuini et al. 2015)
. This was done in order to investigate
whether adults with ASD are impaired at inferring others’
intentions irrespective of reduced fixation on the eyes, which
has been well reported
(Bird et al. 2011; Kliemann et al.
2010; Papagiannopoulou et al. 2014; Tottenham et al. 2014)
The actors sat in front of a white wooden board
measuring 84 × 61 cm with two slots (4 × 17 cm) cut out of it
(see Fig. 1). Actions started with the actor’s right hand
resting on a small marker for 3 s. In order to ensure all hand
actions lasted approximately 2 s, a buzzer indicated to the
actors when to move their hand towards the poker chips and
signalled again to indicate when the actors should let go
of the poker chips. Actors performed each of the 10
different actions at least 3 times; for each actor the action with
the best timing and that best depicted the particular
intention was selected for the final movie. Movies were edited
(Sony Vegas Pro 10) to finish 0.4 s after the poker chips
left the actors’ hands; for grasping actions, this was always
before the poker chips hit the table. In addition, the starts of
all movies were trimmed such that they lasted exactly 4 s.
Editing the movies in this way meant that movement onset
occurred at slightly different times in each movie (frames
The chosen movies were rated by 30 independent
observers who were students at the University of York.
Observers classified each action as either ‘clumsy’, ‘spiteful’ or
‘neither’ by pressing one of three keys on the computer
keyboard. Clumsy responses were coded as − 1, spiteful
responses were coded as 1 and neither responses were coded
as 0. For each action, scores were averaged across
participants to generate an index of the degree of ‘spitefulness’
conveyed by each movie where − 1 indicates a strong
evaluation of the action as clumsy, + 1 indicates a strong evaluation
of the action as spiteful, and 0 indicates an evaluation of the
action as neither clumsy or spiteful. Spiteful videos were
required to have spitefulness indexes higher than 0.4 and
clumsy videos were required to have indexes below − 0.4
to be included in the stimuli set. Three clumsy movies had
spitefulness indexes that were higher than − 0.4 and
therefore were deemed to not clearly portray the desired intention
(0.16, 0.03 and − 0.03 spitefulness indexes). These movies
were replaced with new stimuli which were rated by another
30 independent observers and these stimuli all obtained
ratings lower than − 0.4. The final stimuli used fell into three
significantly (F(2,165) = 1644.94, p < .001, ηp2 = .95) distinct
groups; clumsy (M = − .68, SD = 0.15), spiteful (M = 0.80,
SD = 0.13) and successful (M = 0.01, SD = 0.03) actions.
Experiment 1 (Implicit mentalizing): Design and Procedure
Experiment 1 tested the participants’ abilities to implicitly
infer the intentions of others from their hand actions. The
task was adapted from one previously used with children
(Behne and Carpenter 2005)
(Call et al.
. In these studies, experimenters either deliberately
or ‘accidentally’ did not give the chimpanzees or children
rewards (in the form of food or a toy respectively). Both
the chimpanzees and the children attempted to interact with
the experimenters for longer when experimenters
accidentally dropped the reward rather than when they deliberately
did not give the reward. This implied the experimenters’
intentions had been implicitly inferred and consequently the
appropriate social decisions were made.
In our experiment, each participant took part in a poker
chip exchange game with the experimenter prior to the main
experiment in order to familiarise them with the actions
shown during the experiment, and to demonstrate the value
of receiving poker chips from a partner. Participants were
told that the experimenter would start with 8 poker chips
that were each worth one pound. However, in order for the
experimenter to receive money for their poker chips at the
end of the game, they had to give at least one poker chip to
the participant. If the experimenter had all the poker chips
on their side of the board at the end of the game, neither the
experimenter nor the participant would receive any money.
The experimenter then had three chances to make a deal with
the participant; they would pass some poker chips through
the slots in the wooden board to the participant on the other
side. The participant had to accept or reject the number of
poker chips that were offered each turn. If the participant
accepted then they would receive a pound coin for every
chip on their side, if they rejected the number of poker chips
offered, then the experimenter would have to offer a different
number of chips. If no agreement was reached after three
rounds then neither the participant nor the experimenter
received any money. The aim of the game for the participant
was to end up with as many chips as possible on their side
of the board. Every participant played the poker chip game
four times to gain a good understanding of the purpose of
passing the chips and the value of the chips (three times as
the participant and once in the experimenter role). Over the
three games in the participant role, each participant
experienced (i) a round in which the experimenter acted
spitefully (experimenter offered no chips to the participant and
explained they were doing so in order to reduce the number
of chances to make a deal and increase the chances of the
participant accepting a lower offer); and (ii) a round in which
the experimenter acted clumsily (experimenter accidentally
dropped the poker chips and thus failed to make an offer) so
that all participants had practical experience of both clumsy
and spiteful actions. Participants also played one game in
which they switched roles with the experimenter to ensure
they understood the game fully.
A PC running MATLAB R2015a controlled the
experiment and recorded participant responses. Participants sat
approximately 60 cm from an Acer GD245HQ 24″ HD
monitor on which all stimuli were presented. Participants’
eye movements were recorded during the experiment using
an EyeTribe eye tracker (The EyeTribe Abs, Copenhagen).
Participants rested their heads in a chin rest and fixation
data from both eyes was recorded at 30 Hz. A 9-point
calibration procedure was carried out before conducting each
experiment. Participants for which the eye-tracker could not
reach a satisfactory level of accuracy on the calibration (3/5
star rating; indicating < 1° accuracy) were excluded from
subsequent eye-tracking analysis. Eye tracker data recording
was controlled using the EyeTribe MATLAB toolbox
(Dalmaijer; available on GitHub: https://github.com/esdalmaijer/
Participants were told that they would watch movies of
individuals playing the poker chip game they had just played
themselves. Each movie would show a player’s first attempt
to offer poker chips to someone on the other side of the
board. The participants watched pairs of movies and had
to decide subsequently whether they would rather continue
playing the poker chip game with the actor in the first or
the second movie. Each trial consisted of two actions of the
same type (either both grasping or both pushing) presented
sequentially with an inter-stimulus interval (ISI) of 1000 ms,
during which the screen was black except for a white
fixation cross. Following the second movie a response screen
was displayed and participants had to indicate whether they
would rather interact with the actor in the first or second
movie by pressing either 1 or 2 on the keyboard (see Fig. 2).
Different forms of decision making were required to
make a choice between the first and second actors in three
different conditions; we refer to them as ‘Mentalizing’,
‘Action’ or ‘Either’ conditions. (1) Mentalizing
condition: correct decisions could be based upon inferences of
intention only and consisted of movies of an action with a
positive intention (clumsy 1) and an action with a negative
intention (spiteful 1). Here, in order to decide between
the actors, participants needed to infer the intentions of
the actors from the kinematics of their actions. The
outcomes of the actions were the same (both actors failed
to pass a poker chip to another player) but the intentions
of the actors were different. (2) Action condition: correct
decisions could be based upon action recognition only,
consisted of movies of actors successfully passing poker
chips (successful 1 and successful 5). Here, in order to
decide between the actors, participants needed to
recognise whether the actor was passing 1 or 5 poker chips, and
did not require participants to mentalize in order to
complete the task. (3) Either condition: decisions were based
upon recognition of the action, or possibly inferences of
intention, and consisted of movies of actors attempting to
pass poke chips (clumsy 1 and clumsy 5). Here, in order
to decide between the actors, participants were expected to
focus on the number of chips being offered and choose the
actor trying to pass the higher number of poker chips, but
participants may have automatically processed the actors’
intentions and recognized that both actors have the same
positive intention. This condition was included in order to
test for the differences in success of the actions between
the Mentalizing and Action conditions, given that
Mentalizing trials always showed unsuccessful actions and
Fig. 2 Sequence of stages
during a Mentalizing trial in the
implicit task. Action 1 shows a
female actor deliberately
dropping a poker chip (spiteful 1)
and action 2 shows a male actor
accidentally dropping a poker
chip (clumsy 1). In order to
decide whether to interact with
actor 1 or actor 2 the participant
must infer the intentions of
the actors from the
kinematics of their actions because the
outcomes of the two actions are
Action trials always showed successful actions; Either
trials always showed unsuccessful actions but did not require
At the start of testing, participants completed six practice
trials (two of each condition) in order to familiarise them
with the experimental procedure. The stimuli used in the
practice trials were not included in the main experiments
and the actors compared against each other in the implicit
practice trials were not compared against each other in the
main implicit experiment.
Participants completed 72 trials in total, viewing 144
actions (12 repeats of each action except clumsy 1 which was
shown 24 times); trials lasted approximately 12 s depending
on response times, and testing took approximately 15 min.
The same actor never performed the same action (e.g.
spiteful1 pushing action) twice, such that participants did not
learn to associate certain behaviours with specific
individuals. Every actor was seen the same number of times and
each actor performed a preferable action 50% of the time;
actor gender was also counterbalanced. Condition order was
randomised and action order was counterbalanced so that the
preferred action would occur first in 50% of the trials, e.g. on
50% of the Mentalizing trials the clumsy movies were shown
before spiteful movies.
Experiment 2 (Explicit mentalizing): Design and Procedure
In the second experiment, participants were asked explicitly
to report the intentions of actors presented in movies.
Participants returned approximately 3 months (average 112 days)
after they completed Experiment 1 to complete Experiment
2. This helped minimalize the possibility of participants’
previous implicit judgements influencing their explicit
judgements of the actions. Two of the ASD participants
were unable to return to complete the explicit experiment,
leaving a sample of thirty participants (15 matched pairs) in
the explicit experiment.
As with experiment 1, participants first completed six
practice trials (two of each stimulus type), in order to
familiarise them with the experimental procedure. Participants
then viewed all 144 of the movies seen in the Experiment 1.
After each movie, participants had to indicate whether they
thought the movie showed a ‘spiteful’ (deliberate), clumsy
(accidental) or successful action by pressing 1, 2 or 3
respectively on the computer keyboard. The experiment consisted
of two blocks of 10 min (72 movies shown in each). Each
block contained 36 clumsy actions, 12 spiteful actions and
24 successful actions, the order of movies was randomised
within each block and no movies were repeated. A response
screen was shown after each movie until the participant
responded. The PC, display and eye-tracker were all
identical to Experiment 1.
Behavioural Performance Analysis
For Experiment 1, the numbers of correct responses each
participant gave in each condition (Mentalizing, Either,
Action) were calculated. All 34 participants included in
the analyses had scores significantly higher than chance
in the Action condition (Binomial test (0.5), p < .05,
scores > 17/24), indicating that all individuals
understood the task. We then subtracted the number of correct
responses on the Action condition from the number of
correct responses on both the Mentalizing and Either condition
for each participant. This allowed us to identify any task
specific deficits rather than generalised poorer performances
on experimental tasks.
For Experiment 2, we calculated the proportion of
correct responses for the mentalizing conditions (clumsy and
spiteful actions) and non-mentalizing condition (successful
actions) for each participant. Similar to Experiment 1,
differences between mentalizing and non-mentalizing
conditions were calculated to provide a specific measure of the
ability of participants to explicitly infer the intentions of
others, whilst controlling for ability to do a simple action
Task-specific scores were not normally distributed even
after log transformations had been applied. Therefore,
nonparametric analyses (Mann–Whitney U tests) were used to
investigate group differences in mentalizing abilities.
Further, due to the spectral nature of ASD, linear regressions
were used to examine the influence of autistic traits
(continuous independent variable) on task-specific performances
(continuous dependent variables). These linear regressions
were conducted in order to identify whether any significant
group differences that were found also showed a significant
relationship with the continuum of autistic traits across all
participants. In order to obtain a single score for each
participant that reflected the level of autistic traits that they
displayed, we performed a principal components analysis
(PCA) on all the psychological test scores (ADOS-2, AQ,
SRS and TASIT). The only factor with an eigenvalue higher
than Kaiser’s criteria of 1 was extracted and used as a
measure of autistic traits. Data analysis was carried out using
R i386 3.2.3 (The R Foundation for Statistical Computing,
Vienna, Austria, http://www.r-project.org).
Eye tracking data was analysed using the EyeMMV
(Krassanakis et al. 2014)
. Data from the
implicit and explicit experiments were analysed in the same
way. First, heatmaps were created using the data from all
participants in order to identify regions of interest (ROIs);
these were: the head of the actor, the initial start position of
the hand with the poker chips, and the grasp release point.
Three rectangular ROIs were drawn for each movie outlining
these areas of interest. Due to the similarity in the spatial
extent of the actions on the screen it was then possible to
combine the co-ordinates of the ROIs from all 144 movies to
make a single set of ROIs that encompassed the ROIs from
all movies (see Fig. 1a). We then calculated the number and
duration of fixations within each ROI during each condition
for each participant. We defined the minimum duration for
fixation detection as 150 ms.
The duration of fixations in each ROI as a percentage
of the total number of fixations were calculated for each
participant in each condition. As for the behavioural data,
for Experiment 1 the duration each participant fixated in
each ROI during the Action condition was subtracted from
the time spent fixating in each ROI during the Mentalizing
and Either conditions. For Experiment 2, the durations of
fixation in each ROI during the non-mentalizing condition
were subtracted from the durations of the fixation in each
ROI during the Mentalizing condition. For Experiment 1,
group differences in fixation patterns were tested using
separate mixed-model ANOVAs for each ROI (with condition
[Mentalizing-Action, Either-Action] as the within subjects
variable and diagnosis as the between subjects variable). For
Experiment 2, the eye-tracking data were found to violate
the assumption of normality even after a log transformation
had been applied so non-parametric Mann–Whitney U tests
were conducted to examine potential group differences in
mentalizing-specific fixation patterns. For both experiments,
linear regressions were used to examine the influence of
autistic traits on changes in the duration of fixations in each
ROI across conditions. The data from different ROIs were
treated separately because the data were not independent
(participants could only fixate in one ROI at a time).
All psychological assessment scores were highly
correlated with each other except for IQ which did not correlate
with the scores on any other psychological tests (Bivariate
Pearson correlations; see Table 2). Three female
participants with an ASD diagnosis obtained ADOS scores below
the clinical cut-off. However, all of these participants had
AQ scores above the clinical cut-off as well as SRS scores
that indicated either moderate or severe social impairments
(see Table 3 for group means scores on all psychological
Given that the psychological test scores assessing autistic
traits were highly correlated with each other (all rs > 0.54)
they were suitable for principal component analysis, the
Kasier-Meyer-Olkin measure of sampling accuracy was 0.81
(above 0.6) and Barlett’s test of sphericity was significant
χ2(6) = 108.82 p < .001. Additionally, the communalities
were all above 0.7 supporting the inclusion of all the
psychological tests in the principle components analysis (PCA).
PCA with varimax rotation was used. The initial
eigenvalues from the PCA analysis showed that one factor (with an
eigenvalue of 3.23) explained 80.81% of the variance in
psychological test scores. No other factors had eigenvalues
higher than Kaiser’s criteria of 1 and therefore only one
factor was extracted. This factor was labelled ‘autistic traits’
(see Table 3 for group mean values).
ASD participants displayed poorer performances on
the implicit task than matched controls (see Table 4) but
group differences were not significant (Mentalizing-Action
scores: U = 112.50, p = .27, r = .19; Either-Action scores:
U = 90.00, p = .06, r = .33). Linear regression analyses also
showed that higher levels of autistic traits were associated
with poorer performances on the implicit task but this trend
was not significant (see Fig. 3; Mentalizing-Action scores:
Fig. 3 The relationship between the levels of autistic traits displayed
and performances on the implicit task in the Mentalizing condition
(a) and Either condition (b). Although there was a trend of poorer
performances with high levels of autistic traits, linear regression
analysis found that the level of autistic traits displayed was not a
significant predictor of performance in the Mentalizing (F(1,32)= 3.11,
p = .09, R2 = 0.09) or the Either condition (F(1,32) = 3.54, p = .07,
R2 = 0.10). The curved lines represent 95% confidence intervals
F(1,32) = 3.11, p = .09, R2 = 0.09, 95% CI [− 5.91, − 2.33];
Either-Action scores: F(1,32) = 3.54, p = .07, R2 = 0.10, 95%
CI [− 3.14, 0.124]).
In addition, adults with ASD did not show atypical
changes in fixation patterns between conditions in the
implicit experiment and changes in fixation patterns were
not significantly different across Mentalizing and Either
conditions for any of the ROIs [head ROI: task [F(1,26) = 0.45,
p = .51, ηp2 = .02], diagnosis [F(1,26) = 0.77, p = .39,
ηp2 = .03], task*diagnosis interaction [F(1,26) = 0.23,
p = .63, ηp2 = .01; Poker chip ROI: task [F(1,26) = 2.41,
p = .13, ηp2 = .09], diagnosis [F(1,26) = .32, p = .57,
ηp2 = .01], task*diagnosis interaction [F(1,26) = 0.70, p = .41,
ηp2 = 0.03]; Release point ROI: task [F(1,26) = 3.27, p = .08,
ηp2 = .11], diagnosis [F(1,26) = 2.99, p = .10, ηp2 = .10],
task*diagnosis interaction [F(1,26) = 0.55, p = .47, ηp2 = .02].
Group average values for the percentage of time spent
fixating in each ROI can be seen in Table 5. The level of autistic
traits that participants displayed also did not significantly
predict changes in the duration of fixation within any ROI
between conditions (see Table 6).
Participants in the ASD group displayed significant explicit
mentalizing deficits (Median = − 1.88; IQR = 3.50)
compared to matched controls (Median = − .74; IQR = 1.13;
U = 61.50, p = .03, r = .39). The participant in the ASD group
with the highest level of autistic traits was identified as an
outlier in the linear regression analysis for the explicit task
(Cook’s distance > 1 and leverage value > n/4), therefore this
participant was removed from the linear regression analysis.
Table 5 Percentage duration of fixation in each ROI; Mean (SD)
Poker chip ROI
Release point ROI
Participants with higher levels of autistic traits displayed
poorer performances on the explicit mentalizing condition
but this was a non-significant trend
(mentalizing-non-mentalizing scores; F(1,27) = 3.42, p = .08, R2 = 0.11, 95% CI
[− 1.15, 0.06] see Fig. 4).
Participants with ASD displayed typical changes in the
duration of fixation between mentalizing and
non-mentalizing conditions for all ROIs (Head ROI: U = 75.00, p = .32,
r = .19: Poker chip ROI: U = 77.00, p = .36, r = .17: Release
point ROI: U = 74.00, p = .29, r = .20). The level of autistic
traits that participants displayed did not significantly predict
changes in the duration of fixation between mentalizing and
Table 6 Results of the
linear regression analyses
between the eye-tracking data
and the level of autistic traits
Poker Chip ROI
Release Point ROI
This study aimed to investigate the abilities of adults with
ASD to both implicitly and explicitly infer the intentions
of others. In the first experiment, participants completed a
task where mentalizing was implicit. Here participants were
asked to make decisions about who they wanted to interact
with between pairs of actors and in some cases these social
decisions required the intentions of the actors to be inferred
in order to make the appropriate choice. In contrast, during
the second experiment, participants were explicitly asked to
report the intentions of actors. Our results showed that adults
with ASD displayed explicit mentalizing deficits compared
to matched controls. Adults with ASD did not display
significant implicit mentalizing abilities. Furthermore, ASD
participants did not display atypical fixation patterns
during both the explicit and implicit experiments. Therefore, the
explicit mentalizing deficits exhibited by adults with ASD
cannot be explained by differences in fixation.
The explicit mentalizing deficit found with adults with
ASD in this study supports a number of previous studies
which found adults with ASD were impaired at
explicitly inferring others’ intentions (
; Kana et al.
2014; Moran et al. 2011). Our data are also consistent with
reported difficulties for adults with ASD in everyday life
(O’Neal 2013; The National Autistic Society 2014)
However, some previous studies have not found a connection
between ASD and impairments in explicitly inferring the
intentions of others
(McAleer et al. 2011; Ponnet et al. 2004;
Roeyers et al. 2001; Schuwerk et al. 2014)
. This may be
due to the simplicity of the stimuli used in these studies,
e.g. passages of text and still images
(Ponnet et al. 2004;
Roeyers et al. 2001; Schuwerk et al. 2014)
. In contrast, our
study used a task with complex, naturalistic stimuli more
akin to social environments in which individuals are required
to make judgements. The use of more simplistic stimuli in
previous studies may have allowed some adults with ASD
to explicitly infer the intentions of others, perhaps with the
help of learned strategies, which are of less help in more
complex and natural settings. In support of this argument,
two previous studies have investigated the ability of adults
with Pervasive Development Disorders (PDDs; including
ASD) to infer mental states both using simple stimuli and
complex, naturalistic stimuli
(Ponnet et al. 2004; Roeyers
et al. 2001)
. Their results showed that adults with PDDs
were only impaired when complex stimuli were used.
In addition to the group analysis, we also investigated the
relationship between autistic traits and performance across
all participants. This additional analysis was conducted as
ASD is a spectrum disorder rather than a dichotomous
classification and our results clearly show that participants
displayed a range of autistic traits (see Figs. 3, 4). The linear
regression analysis showed that across all participants the
wide range of autistic traits shown was negatively associated
with performance on both implicit and explicit mentalizing
tasks, but these remained non-significant trends. A
previous study found no relationship between autistic traits and
both explicit and implicit mentalizing abilities
(Nijhof et al.
. However, this study did not recruit adults with an
ASD diagnosis and therefore may have not had the range of
autistic traits required to find a relationship between autistic
traits and mentalizing performance.
Although a trend was found in our study for poorer
implicit mentalizing abilities associated with higher levels
of autistic traits, there was not a significant group difference
in performance between those with ASD and their matched
controls. This lack of clear evidence for a significant implicit
mentalizing deficit in adults with ASD was unexpected. We
had more participants in this study than in the explicit study,
which revealed clear significant results, so it is unlikely the
null result is simply due to insufficient statistical power. It
is possible that if the data had been normally distributed,
therefore allowing parametric analyses to be carried out,
the interaction between task and participant group would
not have been significant, reflecting comparable deficits
on both tasks. However, the effect size was much larger for
the explicit experiment (r = .39) compared to the implicit
experiment (r = .19), supporting the presence of a significant
explicit deficit but no clear implicit mentalizing deficit in
these adults with ASD. The existing literature shows
consistent implicit mentalizing deficits in adults with ASD
et al. 2002; Rosenblau et al. 2015; Schuwerk et al. 2014;
Senju et al. 2009)
. The methods we used to measure
mentalizing abilities may have contributed to the discrepancy
between our findings and the previous literature. Our study
measured implicit mentalizing abilities using a measurable
behavioural outcome and performances were assessed
relative to a control task. Previous implicit mentalizing studies
in adults with ASD that used complex stimuli have either
used eye-tracking data alone as a measure of mentalizing
(Schuwerk et al. 2014; Senju et al. 2009)
included a control task
(Rosenblau et al. 2015)
. Without the
inclusion of a control task, it cannot be determined whether
poorer performances linked to ASD are mentalizing-specific
or more generalised deficits. Additionally, this study was the
first to investigate abilities to implicitly infer intentions in
adults with ASD; in contrast previous implicit mentalizing
studies in adults have assessed abilities to infer others’
mental states and false beliefs
(Castelli et al. 2002; Rosenblau
et al. 2015; Schuwerk et al. 2014; Senju et al. 2009)
Neuroimaging studies have shown that different brain areas are
active during different types of mentalizing tasks
et al. 2007; Pineda and Hecht 2009; Saxe and Powell 2006;
Schurz et al. 2014)
, suggesting that the systems used depend
on the specific mentalizing task being performed. Therefore,
it is possible that ASD is related to more pronounced deficits
on certain subcomponents of mentalizing than others.
The lack of clear evidence for a significant implicit
mentalizing deficit in adults with ASD in the current study may
also be due to the use of action stimuli; implicitly inferring
others’ intentions from their actions may involve different
processes than implicit mentalizing in the absence of action
information. Actions with different intentions have been
shown to display different kinematic profiles
(Manera et al.
2011; Sartori et al. 2011)
. The dual-process model
suggests that when intentions are inferred from others’ actions,
these differences in action kinematics allow automatic,
subconscious processing of intentional information in the
observer’s own motor system before intentions are actively
interpreted in a higher-level cortical system
(de Lange et al.
2008; Keysers and Gazzola 2007; Spunt and Lieberman
2012; Uddin et al. 2007)
. Neuroimaging data suggest that
in the absence of action information, others’ intentions aren’t
subconsciously processed in the motor system
(see a review
and meta-analysis; Gallagher et al. 2000; Schurz et al. 2014)
Therefore, because intentional information in our study was
provided by differences in action kinematics, it is possible
that subconscious processing of intentional information in
the motor system allowed adults with ASD to select
preferable kinematic profiles (required in our implicit task).
Whereas, if intentional information was provided by other
cues, not solely by differences in action kinematics, then a
significant implicit mentalizing deficit may have been found.
A larger number of implicit mentalizing studies have been
carried out in young children than adults and a number of
studies have shown that children with ASD can implicitly
infer others’ intentions when intention is portrayed using
(Aldridge et al. 2000; Berger and Ingersoll 2014;
Carpenter et al. 2001; Colombi et al. 2009; Liebal et al. 2008;
Schietecatte et al. 2012)
but not when intentions are
portrayed by social-emotional cues such as eye gaze or facial
(Berger and Ingersoll 2014; Vivanti et al. 2016)
These data support the theory that inferring intentions from
action kinematics involves different processes than
inferring intentions using different cues and that implicitly
inferring intentions from action kinematics is not significantly
impaired in ASD.
Individuals with ASD and high levels of autistic traits
also showed relatively poor performance on Either trials. It
seems likely that mentalizing may have influenced the social
judgments participants made during the Either condition
even though, in principal, mentalizing was not required. The
Either condition was included in this study in attempt to
control for differences in the success of actions across
mentalizing and non-mentalizing (Action) conditions. In the
Mentalizing condition, unsuccessful actions were always seen
and in the Action condition only successful actions were
seen. The Either condition showed unsuccessful actions but
did not require mentalizing in order to complete the task, if
participants made their decisions based purely on the
number of poker chips involved in the hand actions then they
would make correct choices. However, previous evidence
suggests that the intentionality of observed hand actions
is automatically processed
(Liepelt et al. 2008)
, and given
participants were blind to the condition, from the
participant’s perspective, the relevant feature of the action (number
of chips/intention of the actor) only became clear after the
second movie had been viewed. Thus, it may have been an
effective strategy to pay attention to the intention of the actor
in all trials. This may have affected performance in several
ways. First, participants with higher levels of autistic traits
may have wrongly attributed negative intent to the preferable
actions (the actor attempting to pass more poker chips) in
the Either condition resulting in incorrect choices. Second,
reading actor intentions may have distracted participants
from focussing on the number of chips being passed and
thus the inclusion of both actor intentions and differential
number of chips, may have placed a higher cognitive load
on participants, compared to other conditions and this may
have contributed to the relatively poor performances in this
Despite the poorer explicit mentalizing abilities found in
adults with ASD compared to matched controls in our study,
fixation patterns were not different in the ASD group. The
typical fixation patterns exhibited by adults with ASD in
this study may also be due to the use of action stimuli. The
majority of the literature reporting atypical fixation patterns
in adults with ASD have found atypical fixation patterns
during face processing, in particular, showing reduced fixation
on the eyes
(Dalton et al. 2005; Klin et al. 2002; Pelphrey
et al. 2002; Sterling et al. 2008)
. In the current study, the
actors’ faces were not shown and intentional information
was portrayed by the kinematics of the actions alone. Adults
with ASD may alter their eye movements appropriately
according to differences in the mentalizing demand of the
task when intentional information is portrayed by action
kinematics but not when internal state inferences require face
processing. This theory is supported by data from a previous
study that showed that when adults with ASD
naturalistically viewed videos and pictures of social scenes they
displayed reduced fixation on people’s faces but showed
equivalent fixation on bodies to control participants (Rigby et al.
2016). The typical eye-tracking data in conjunction with the
explicit mentalizing deficit in the current study suggest that
despite receiving the visual cues they needed, adults with
ASD could not accurately interpret the social cues
embedded within the action kinematics in order to explicitly infer
the actors’ intentions. This dissociation between the
behavioural data and the eye-tracking data has implications for
future research assessing mentalizing abilities. Some
previous studies have investigated mentalizing abilities using
eye-tracking data alone
(Schuwerk et al. 2014; Senju et al.
. However, our results demonstrate that poorer
mentalizing abilities associated with high levels of autistic traits
are not always accompanied by atypical visual fixation
patterns. This is supported by previous research which showed
that although adults with ASD spent less time fixating on
the eyes of others, these atypical fixation patterns did not
correlate with poorer mentalizing performances (Cassidy
et al. 2013).
In conclusion, we found that adults with ASD were
significantly impaired at explicitly but not implicitly inferring
the intentions of others from their hand actions. Although
there was a trend for adults with high levels of autistic traits
to display poorer implicit mentalizing performances, this
relationship did not reach significance. The lack of a
significant implicit mentalizing deficit may be due to subconscious
processing of intentional information when intentions are
portrayed by action kinematics. Adults with ASD displayed
typical fixation patterns when both implicitly and
explicitly inferring the intentions of others. The inconsistency we
observed between impaired explicit mentalizing but typical
fixation patterns suggests that reduced abilities to explicitly
infer intentions from hand actions cannot be attributed to
dissimilarities in fixation patterns. Our findings suggest that
future research should consider the stimuli used and assess
mentalizing abilities with both behavioural and eye-tracking
Acknowledgments Eleanor Cole was funded by an Economic Social
Research Council (ESRC) 1 + 3 PhD studentship. We thank Greta
Vilidaitè for her assistance with the ADOS coding, and all the participants
for their contributions.
Author Contributions EJC designed the experiment, collected and
analysed the data, wrote the first draft of the manuscript, edited the
manuscript, and approved the final version. KES designed the experiment,
helped data analysis, edited the manuscript and approved the final
version. NEB designed the experiment, helped data analysis, edited the
manuscript and approved the final version.
Compliance with Ethical Standards
Conflict of interest The authors declare that they have no conflict of
Open Access This article is distributed under the terms of the Creative
Commons Attribution 4.0 International License
(http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use,
distribution, and reproduction in any medium, provided you give appropriate
credit to the original author(s) and the source, provide a link to the
Creative Commons license, and indicate if changes were made.
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