Oxytocin enhances pupil dilation and sensitivity
to ?hidden? emotional expressions
Siri Leknes,1,2Johan Wessberg,2Dan-Mikael Ellingsen,1,2Olga Chelnokova,1Ha ˚kan Olausson,2and Bruno Laeng1
1Department of Psychology, University of Oslo, Oslo, Norway and2Institute of Neuroscience and Physiology, University of Gothenburg,
Sensing others? emotions through subtle facial expressions is a highly important social skill. We investigated the effects of intranasal oxytocin treatment
on the evaluation of explicit and ?hidden? emotional expressions and related the results to individual differences in sensitivity to others? subtle expres-
sions of anger and happiness. Forty healthy volunteers participated in this double-blind, placebo-controlled crossover study, which shows that a single
dose of intranasal oxytocin (40IU) enhanced or ?sharpened? evaluative processing of others? positive and negative facial expression for both explicit and
hidden emotional information. Our results point to mechanisms that could underpin oxytocin?s prosocial effects in humans. Importantly, individual
differences in baseline emotional sensitivity predicted oxytocin?s effects on the ability to sense differences between faces with hidden emotional
information. Participants with low emotional sensitivity showed greater oxytocin-induced improvement. These participants also showed larger
task-related pupil dilation, suggesting that they also allocated the most attentional resources to the task. Overall, oxytocin treatment enhanced
stimulus-induced pupil dilation, consistent with oxytocin enhancement of attention towards socially relevant stimuli. Since pupil dilation can be
associated with increased attractiveness and approach behaviour, this effect could also represent a mechanism by which oxytocin increases human
Keywords: emotion; locus coeruleus; pupillometry; empathy; hormones; social
Recent excitement over oxytocin’s putative prosocial effects in humans
has been fuelled by repeated reports that intranasally administered
oxytocin enhances ‘mind-reading’, or the ability to assess others’
emotions. Related studies have demonstrated effects of centrally
enhanced oxytocin on social memory, behaviour in economic games,
social attention and the focus of eye gaze (e.g. Kosfeld et al., 2005;
Guastella et al., 2008; Unkelbach et al., 2008; Gamer et al., 2010;
Ellenbogen et al., 2012). However, oxytocin’s effects on emotional
processing have been variable and, in some cases, even contradictory
(for a comprehensive review, see Bartz et al., 2011). This is illustrated
by reports from three different research groups investigating oxytocin’s
effects on emotion recognition with morphed emotional faces of vary-
ing intensity. These found (i) enhanced recognition of positive but not
negative expressions (Marsh et al., 2010); (ii) enhanced recognition of
fear but not other emotions (Fischer-Shofty et al., 2010) and (iii) a
detrimental effect on fear perception (Di Simplicio et al., 2009).
A related set of studies have used images of eyes and more complex
emotional expressions such as amusement or scepticism. Oxytocin’s
enhancement of performance on this task has been reported for both
more difficult (Domes et al., 2007) and ‘easy’ items (Guastella et al.,
2010). Interestingly, since the study populations differed in social
competence, the ‘easy’ items in the study by Guastella et al. and the
‘difficult’ items used by Domes et al. may have represented a compar-
able challenge to their respective study populations (high-functioning
autists vs healthy volunteers). In other words, oxytocin’s prosocial
effects may be most pronounced for tasks that are challenging, but
not too difficult (see also Schultze et al., 2011).
Bartz et al. (2010) demonstrated that oxytocin’s effects on empathic
accuracy in healthy males were proportional to their level of autistic
traits, as assessed by the Autism Spectrum Quotient (AQ). Empathic
accuracy was defined by how closely participants’ assessment of
another’s emotion, during a videotaped speech, matched the speaker’s
self-reported emotion. Since a high AQ score is associated with low
sensitivity towards others’ expressions of emotion, this finding suggests
that oxytocin’s prosocial effects are modulated by individual differ-
ences in the ability to correctly judge others’ emotional states.
Specifically, it may be that oxytocin only improves empathic accuracy
for those participants who experience difficulty in evaluating others’
emotions. If so, this provides a possible explanation of the discrepan-
cies as well as the small effect sizes reported in the literature on
oxytocin and emotion recognition in healthy volunteers.
The basis for the recent increase in human studies using intranasally
administered oxytocin is an extensive body of research on oxytocin’s
often dramatic effects in non-human animals (for reviews, see e.g. Insel
and Young, 2001; Campbell, 2008). For instance, oxytocin enhances
nurturing and reduces maternal aggression towards rat pups.
Interestingly, it also enhances maternal aggression towards potential
threats (Campbell, 2008). Here, we investigated the role of oxytocin for
evaluation of two facial expressions related to prosocial and aggressive
behaviour in humans, happiness and anger.
The dataset of this study included faces showing happiness and anger
presented either explicitly or implicitly. Images containing implicitly
presented, ‘hidden’, emotional information were included to allow us
to investigate oxytocin’s effects of evaluative processing at a level of
greater task difficulty. Implicit images containing subtle or hidden emo-
tionswere ‘hybrids’created through
high-spectrum visual information from neutral facial expressions over
low-spectrum visual information from emotional expressions (see
Figure 1). Such ‘hybrid’ faces containing hidden anger or happiness,
sadness or fear have been shown to be judged as neutral, when observers
are asked to choose among various emotion labels, thus indicating that
Received 25 October 2011; Accepted 24 May 2012
Advance Access publication 29 May 2012
The authors are grateful to Karin Go ¨thner and Steven van der Pavert for technical assistance, to Martin Larsson,
Hannah Ellingsen, Gunnlaug Sæter Gitlestad, Gunn-Asbjørg Valen-Sendstad and Esther Wu for help with data
collection and to Guido Biele and Markus Handal Sneve for discussions of content and analyses. This work was
supported by the Swedish Research Council [grant number 2007-2912 to H.O.], the Marianne and Marcus
Wallenberg Foundation [grant number 2009.0080 to H.O.] and the Research Council of Norway [grant number
ES455867 to S.L.]
Correspondence should be addressed to Siri Leknes, Department of Psychology, University of Oslo, Postboks
1094, Blindern 0317, Oslo, Norway. E-mail: firstname.lastname@example.org
doi:10.1093/scan/nss062SCAN (2013) 8,741^749
? TheAuthor (2012).PublishedbyOxfordUniversityPress.For Permissions,pleaseemail:email@example.com
at University of Oslo Library on October 21, 2013
at University of Oslo Library on October 21, 2013
at University of Oslo Library on October 21, 2013
at University of Oslo Library on October 21, 2013
at University of Oslo Library on October 21, 2013
at University of Oslo Library on October 21, 2013
at University of Oslo Library on October 21, 2013
at University of Oslo Library on October 21, 2013
at University of Oslo Library on October 21, 2013
the underlying, ‘hidden’, expression cannot be consciously acknowl-
edged (Laeng et al., 2010). Despite the lack of awareness, the implicit
were rated in relation to a social trait (i.e. friendliness). These findings
are consistent with the idea that only a core emotional expression
(Berridge, 2003) can be processed when visual information is degraded
or data-limited, as in subliminal presentations using backward masking
procedures. However, in contrast to the backward masking technique,
hybrid stimuli avoid the interruption of visual processing of the emo-
tional information. The emotional stimulus (contained within a narrow
band of spatial frequency information) is neither ‘interrupted’ nor
‘erased’ and forms a constituent part of the stimulus that remains avail-
able in the visual input at all times. This property may be very advanta-
geous when studying physiological responses like changes in pupillary
dilation, which typically evolve slowly over time (e.g. Laeng et al., 2011,
2012). In addition, such low-passed emotional hybrids may provide a
privileged window into the functioning of the human amygdala. A sem-
inal functional magnetic resonance imaging study by Vuilleumier et al.
(2003) has revealed that this important brain structure is essentially
‘blind’ to all but the lowest visible spatial frequencies (>6 cycles/
image). Therefore, ‘hiding’ low-frequency information from emotional
expressions is not only a practical and useful method to present emo-
tional expressions implicitly but also to preferentially stimulate part of
the emotional subcortical network.
This study specifically investigated oxytocin’s effect on the evalu-
ation of happy and angry facial expressions presented both explicitly
and implicitly and related the results to the participants’ sensitivity to
others’ subtle emotions. Previous studies indicate that the level of
autistic traits moderates the effects of oxytocin on the evaluation of
others’ emotions when the task is relatively difficult (Bartz et al., 2010;
Guastella et al., 2010). As autism is marked by low sensitivity to others’
emotions, we hypothesised that oxytocin’s effects on the evaluation of
implicitly presented emotions would be moderated by emotional
sensitivity. Since the emotional content of the hybrid images is so
subtle that they cannot be consciously distinguished from neutral
expressions, differentiating between the images containing hidden
anger or happiness may require greater sensitivity towards others’
facial expressions of emotion. Therefore, we used each individual’s
ability to detect the difference between the implicitly presented
happy and angry expressions (after placebo treatment) to calculate
an ‘emotional sensitivity score’ for each individual. Specifically, high
scores were given to participants who perceived more anger in the
‘angry hybrids’ than in the ‘happy hybrids’ and similarly perceived
more happiness in the ‘happy hybrids’ than in the ‘angry hybrids’.
We also recorded pupil diameter changes during stimulus presenta-
tion as a measure of sympathetic nervous system activity. Phasic and
tonic pupil increases are tightly coupled to activity within the locus
coeruleus (LC), a brainstem nucleus, which is the seat of the brain’s
noradrenergic pathways (Aston-Jones and Cohen, 2005). Pupil dilation
can be used as a sensitive measure of cognitive load and task difficulty
(Kahneman, 1973; Laeng et al., 2012). We hypothesised that oxytocin
would improve the evaluation of others’ emotions and that this
prosocial effect would be mirrored by oxytocin’s effects on stimulus-
related pupil dilation, reflecting altered arousal or attentional effort.
Furthermore, we expected pupil dilation to reflect between-subject
Fig. 1 Overview of study design. After administration of nasal spray containing oxytocin or placebo, participants underwent a test protocol consisting of face stimuli presented simultaneously with either soft
stroking touch or vibration. The visual stimuli included five expression types: explicit anger, implicit (hybrid) anger, neutral, implicit (hybrid) happiness and explicit happiness. Implicit expressions contained
high-frequency visual information from a neutral expression and low-frequency visual information from the same person expressing either anger or happiness. These stimuli are perceived as neutral, but were
previously shown to evoke a core impression such that faces containing implicit happiness are perceived as more friendly than faces containing implicit anger, fear or sadness (Laeng et al., 2010). Participants
rated perceived emotion, social characteristics or tactile characteristics after each stimulus pair. Images were adapted from the Karolinska Directed Emotional Faces?KDEF (CD-ROM), by D. Lundqvist, A. Flykt, &
A. Ohman, 1998, Stockholm, Sweden: Department of Clinical Neuroscience, Psychology section, Karolinska Institutet. Reprinted with permission.
742 SCAN (2013) S.Leknesetal.
differences in task difficulty, as assessed by the sensitivity to differences
in implicitly presented happy and angry expressions.
Forty healthy right-handed volunteers were recruited for this study.
One participant participated in one session only and was excluded,
yielding a final study group size of 39 (20 females, mean age 26
years, range 20–39 years). All participants gave written informed
consent to participate in the study, which was approved by the local
ethics committee. Exclusion criteria were pregnancy and breastfeeding.
Fourteen of the female participants used oral contraceptives. Of the
remaining females, we estimated four to be in the luteal phase and two
in the follicular phase of the cycle during the two test sessions, based
on reported number of days since the last menses. Participants received
200 NOK (about 36 USD) per session.
An overview of the study design is presented in Figure 1. Each indi-
vidual participated in two sessions on separate days, once with 40IU
oxytocin (Syntocinon?, Novartis; 10 puffs alternating between the left
and the right nostril) and once with placebo (0.9% saline, Miwana; 10
puffs alternating as above). Since previous studies of oxytocin and
emotion perception using a smaller dose of 24IU have yielded incon-
sistent results, we chose to administer this higher dose used in some
earlier studies (e.g. Zak et al., 2007). The order of administration was
counterbalanced across participants, and neither experimenters nor
participants were aware of the contents of the spray (double-blind
design). Behavioural ratings and pupil diameter were recorded
during the experimental protocol, which was identical in the two
sessions, with exception of the identity and order of presentation of
stimuli. In both sessions, participants viewed black-and-white images
of faces displaying a range of emotional expressions on a computer
screen. Images were explicitly angry, implicitly angry, neutral, impli-
citly happy or explicitly happy. While viewing the images, participants
received concomitant tactile stimulation on the left forearm. After each
stimulus pair, participants rated qualities of the visual and tactile
stimuli. Only results from visual stimuli are presented here; results
relating to touch perception are presented elsewhere (manuscript in
preparation). Each session lasted for about 2h. The test phase
commenced on average 40min after administration of the nasal
spray and lasted ?25min. Before the test phase, participants were
seated alone in a room and were asked to refrain from any type of
social interaction. The experimental protocol consisted of 10 blocks: 5
touch blocks and 5 vibration blocks presented in alternating order. The
order of the first block type was counterbalanced across participants
and conditions. Each block consisted of 10 stimulus pairs (simultan-
eous visual and tactile stimulation) presented for 3s each. Before each
stimulus pair, participants viewed a fixation cross for 5s. Each stimulus
pair was followed by the presentation of two rating scales; each scale
was presented until the participant made a response.
In all, 120 images of faces (20 males and 20 females) displaying angry,
neutral and happy facial expressions were chosen from the Karolinska
Directed Emotional Faces (Lundqvist et al., 1998; Calvo and Lundqvist,
2008). Two implicitly emotional images of each face (happy–neutral
and angry–neutral) were created as described by Laeng et al. (2010). In
brief, images of happy and angry facial expressions were passed
through a spatial low-pass filter, keeping only frequencies of 1–6
high-frequency images of the same individual displaying a neutral ex-
pression (high-pass filtered to exclude spatial information below seven
A total of 200 images were used, depicting 20 males and 20 females
with the following five facial expressions: explicitly angry, implicitly
angry, neutral, implicitly happy and explicitly happy. One hundred
unique images were presented in each session. The order of presenta-
tion was pseudo-randomised according to the following rules: no more
than two consecutive images of the same expression; no more than two
consecutive images of the same person; all expressions were presented
within each 10-stimulus block; at least two images of each gender
within each block and finally, the same proportion of expressions
were displayed during the five touch blocks and during the five
vibration blocks within each session. Images of all 40 individuals
within the stimulus set were presented in each session, and no
images were repeated across sessions.
As pupil size is affected by ambient luminance, the background
section of each image was altered to obtain the same net luminance
for all images using Matlab (The Mathworks Inc., Natick, MA, USA).
The section of the images containing face or hair was unaltered. Each
image (11?11cm) was presented on a computer monitor situated
104cm in front of the participant, yielding a visual angle of 68, as used
by Laeng et al. (2010). Participants were tested in a windowless room
with constant artificial lighting. All visual stimuli and rating scales were
presented using e-Prime 2.0 (Psychology Software Tools, Inc).
Two types of innocuous tactile stimulation were presented during
viewing of facial expressions, stroking touch to the dorsal aspect of the
participant’s left forearm (5cm/s) and 70Hz vibration to the back of
the hand. The stimuli were matched for intensity.
Ratings of two aspects of the face stimuli were recorded during the ex-
perimental protocol: (i) perceived mood/facial expression and (ii) per-
ception of social characteristics. Each aspect was measured via two visual
analogue scales (VAS); one such rating scale pair was displayed after each
combined stimulus in pseudo-randomised order. The rating scales were
as follows?1A: How happy was the person? (anchors: Not Happy–Happy);
1B: How angry was the person? (anchors: Not angry–Angry); 2A: How
attractive was the person? (anchors: Unattractive–Attractive) and 2B:
How friendly was the person?(anchors: Not Friendly–Friendly).The inten-
sity and pleasantness of touch perception were also rated; these data are
presented elsewhere (Ellingsen, Wessberg, Olausson, Laeng, Chelnokova
and Leknes,manuscript in preparation).The orderof presentationof the
rating scale pairs was pseudo-randomised within each session and within
participants were unable to predict the occurrence of the rating scales.
Participants were informed of this before the experiment onset and were
instructed to pay attention to all aspects of the visual and the tactile
stimuli in every trial since the subsequent rating scales could revolve
Mood was measured at three time points during each session:
(i) before the nasal spray administration; (ii) immediately before the
experimental protocol and (iii) immediately after the experimental
protocol. Participants rated their current level of fear, sadness, irrit-
ability, happiness, calmness and anxiety using VAS scales with anchors
Not at all–Very much so.
The pupil diameter of the participant’s left eye was measured by using
a non-invasive infrared eye tracker (remote eye-tracking device,
Oxytocin, pupils andemotional sensitivity SCAN (2013)743
SMI-SensoMotoric Instruments?, Teltow, Germany) at a rate of
240Hz for the duration of each stimulus pair (3000ms).
Repeated-measures analyses of variance (ANOVAs) were conducted
using PASW Statistics version 18 (SPSS Inc.) for each of the rating
scales using the following within-subjects factors: treatment (oxytocin
or placebo); tactile stimulation (touch or vibration); facial expression
(explicit anger, implicit anger, neutral, implicit happiness and explicit
happiness) and face gender (male or female). We also investigated the
following between-subjects factors: participant gender (male or female)
and session order (oxytocin or placebo at session 1). A separate
repeated-measures ANOVA was conducted on the mood ratings treat-
ment, session and mood scale as factors. Additional ANOVAs assessing
oxytocin’s putative ‘sharpening’ effects on evaluative processing of
emotion were run for ratings of happiness or anger using the
within-subjects factors treatment (oxytocin or placebo) and facial
expression (mean anger and mean happiness). For statistically signifi-
cant effects, contrasts were performed to establish the exact nature of
the differences. Pearson’s correlation test and linear regression analyses
were used to assess the relationship between two or more normally
Assessing sensitivity to others? subtle emotions
We defined high emotional sensitivity as the successful differentiation
of implicitly presented anger and happiness in the placebo session. To
calculate an emotional sensitivity score for each participant, we
computed the average of (i) the difference in ratings of perceived
anger between the faces containing hidden anger and happiness and
(ii) the difference in ratings of perceived happiness between the faces
containing hidden anger and happiness. Those participants who rated
the implicitly angry expressions as angrier and less happy than the
implicitly happy expressions (and thus reported more happiness and
less anger in response to the implicitly happy expressions compared
with the implicitly angry images) during this baseline session received a
high emotional sensitivity score. In contrast, ratings of perceived
emotion in participants with low emotional sensitivity either did not
reliably differ between the two implicitly presented expressions or they
differed in the opposite direction. We entered this normally distributed
score in a linear regression analysis, where we also modelled treatment
order (oxytocin or placebo) as a covariate. This analysis assessed
whether emotional sensitivity at baseline predicted oxytocin-induced
improvement of emotion evaluation. To illustrate the relationship
between this baseline measure and the effects of oxytocin, we also
used this baseline emotional sensitivity score to divide the participants
into two groups of high and low sensitivity (median split).
Furthermore, a related analysis was run where we assessed the
relationship of task-induced pupil dilation to baseline emotional
sensitivity, as well as to oxytocin-induced improvement of emotion
Pupil diameter data for each participant and each session were
pre-processed in Matlab. Some datasets were lost due to technical con-
straints (malfunction of software or hardware). Good-quality recordings
from both sessions existed for 25 participants; only these data were
analysed (50 sessions). Eye blinks and artifacts were excluded, leaving
pupil sizes of 1–9mm. Average time series were created for each stimu-
lus type; these time series were smoothed using a 10Hz cutoff low-pass
filter (a five-pole Chebyshev Type II filter). The time series were normal-
ised to reflect the total dilation of the pupil for each stimulus type by
subtracting the average pupil size during the first 200ms from all points
in the time series. For statistical analysis, the trimmed mean pupil dila-
tion was computed for the 10 250ms- ‘bins’ between 500 and 3000ms
for each stimulus type, session and participant. The trimmed means
were entered into a linear mixed models analysis using PASW with
the following variables: drug treatment (oxytocin or placebo); tactile
stimulation (stroking touch or vibration) and visual facial expression
(explicit anger, implicit anger, neutral, implicit happiness and explicit
happiness). A subsequent analysis further included participant gender
and order of treatment presentation. In a third mixed models analysis,
we also included the between-subjects variable of emotional sensitivity
score, as defined from behavioural ratings.
Ratings of explicitly and implicitly presented angry and
happy facial expressions
Analysis of ratings of perceived anger, happiness, friendliness and
attractiveness confirmed the expected effects of the explicit and impli-
cit facial expressions. Specifically, we found a significant linear effect of
expression in ratings of anger [F(1,38)¼358, P<0.001], happiness
[F(1,38)¼441, P<0.001], friendliness [F(1,38)¼385, P<0.001] and
attractiveness (F(1,38)¼126, P<0.001), with explicitly angry faces
rated as the most angry, least happy, friendly and attractive and expli-
citly happy faces perceived as the least angry and most happy, friendly
and attractive. Planned contrasts confirmed that the implicitly angry
images were rated as significantly more angry (P<0.001), less happy
(P<0.001), less friendly (P<0.001) and less attractive (P¼0.043) than
the implicitly happy images.
Oxytocin enhanced evaluation of explicitly and implicitly
presented angry and happy facial expressions
We found a significant interaction between oxytocin treatment and
facial expression for ratings of perceived emotion [anger ratings,
P¼0.021]. As illustrated in Figure 2, this interaction was driven by a
stimulus-congruent ‘sharpening’ effect of oxytocin on perceived
emotion. Specifically, planned contrasts revealed that oxytocin
increased anger ratings for faces containing anger, but decreased
anger ratings of faces containing happy emotional information (expli-
cit expressions, P¼0.028, implicit expressions, P¼0.029). Similarly,
oxytocin enhanced the perception of happiness of happy expressions
and decreased happiness ratings of angry expressions (explicit expres-
sions, P¼0.24 and implicit expressions, P¼0.028). These effects of
oxytocin treatment were relatively small; however, the pattern of
stimulus-congruent rating changes was consistent across rating scales
(perceived anger and happiness) and across expression presentation
(explicit and implicit). Furthermore, separate ANOVAs set up to
specifically address the hypothesised ‘sharpening’ effect of oxytocin
on emotional evaluation, confirmed that oxytocin treatment increased
ratings of congruent and decreased ratings of incongruent emotional
expressions [see Figure 2A; facial expression?treatment interaction,
Sensitivity to differences in subtle expressions at baseline
predicted oxytocin-enhanced emotional sensitivity
Sensitivity to differences in emotional expression between faces con-
taining hidden anger or hidden happiness varied substantially between
participants. We computed a score for emotional sensitivity for each
participant, such that those who perceived implicitly angry faces as
angrier and less happy than implicitly happy faces received a high
emotional sensitivity score. In contrast, a low emotional sensitivity
744 SCAN (2013) S.Leknesetal.
score indicated a lack of sensitivity towards differences between the
implicitly presented happy and angry facial expressions. The emotional
sensitivity score significantly correlated with differences in perceived
friendliness of the implicitly angry and happy expressions, such that
those with high emotional sensitivity also reported the greatest
differences in friendliness (r¼0.46, P¼0.003).
We investigated the association between participants’ emotional
sensitivity score and oxytocin’s effects on task performance using
linear regression analyses. The results showed that the participants
who perceived implicitly angry faces as angrier and less happy than
implicitly happy faces without oxytocin pre-treatment showed little
benefit of intranasal oxytocin. In contrast, participants who were not
sensitive to the differences between the implicitly presented angry and
happy expressions at baseline showed greater improvement after oxy-
tocin treatment. Specifically, the emotional sensitivity score covaried
with how much oxytocin improved the sensitivity towards differences
between the implicitly presented expressions of anger and happiness
(perceived anger, t¼–2.3, P¼0.028; perceived happiness t¼–3.3,
P¼0.002). This relationship was not affected by the order of oxytocin
or placebo treatment (ts<0.7).
The moderating effect of baseline emotional sensitivity on oxytocin’s
effects on task performance is illustrated in Figure 3, which shows the
pattern of improvement when participants were divided into two
groups of high and low emotional sensitivity using a median split
based on the emotional sensitivity score. Paired two-tailed t-tests for
each group showed significant improvement with oxytocin only for the
low sensitivity group (anger ratings: high sensitivity P¼0.84, low sen-
sitivity P¼0.002; happiness ratings: high sensitivity P¼0.73, low
sensitivity P¼0.005). The median-split analysis is presented solely as
an illustration of the relationship between emotional sensitivity and
oxytocin’s effects and not as an independent analysis step.
Oxytocin enhances stimulus-induced pupil dilation
Average pupil size was 3.7mm, and the mean stimulus-induced pupil
dilation during the 3s stimulus presentation period was 0.3mm (8%).
The effects of drug administration, facial expression and other factors
on pupil dilation at 500–3000ms were assessed using a linear mixed
models approach. P values from type III F-test for fixed effects are
reported. For non-significant effects, example P values are reported
from 1700ms after stimulus onset. We found a significant main
effect of oxytocin on pupil dilation, such that stimulus-induced
pupil dilation was larger after oxytocin pre-treatment (see Figure 4,
P<0.001 in the interval 1000–3000ms). There was no significant main
effect of facial expression on pupil dilation at any time during stimulus
presentation (e.g. P¼0.45 at 1700ms), nor did we find evidence for a
treatment-by-expression interaction (P¼0.50 at 1700ms). There were
no significant effects of participants’ gender on pupil dilation (P¼0.46
at 1700ms), or the order of treatment presentation, oxytocin or
placebo first (P¼0.58 at 1700ms).
A correlation analysis of pupil dilation around peak (2000ms)
revealed a significant negative association between stimulus-induced
pupil dilation and emotional sensitivity score (r¼–0.37, P<0.05). In
other words, the greatest pupil dilation was found in the participants
who showed low sensitivity towards differences between the implicit
angry and the implicit happy facial expressions after placebo treatment.
Conversely, those who successfully distinguished between the implicitly
presented expressions of anger and happiness at baseline showed smaller
pupil dilation during stimulus presentation. A median-split analysis
illustrates this finding. The low emotional sensitivity group (pupillo-
metry data from 13 participants) showed a significantly greater dilation
of the pupil during stimulation than did the 12 participants with high
emotional sensitivity scores that were included in the pupillometry ana-
lysis (P<0.01 in the interval 1400–3000ms). As before, the median-split
analysis is provided for illustrative purposes only. This finding is
consistent with greater attention allocation to stimuli in those who
showed difficulty in evaluating the implicitly presented emotional
expressions at baseline. However, there was no evidence that oxytocin’s
beneficial effects on emotional sensitivity in this subgroup were due to
additional attention to the socially relevant stimuli. Instead we found a
trend towards the opposite effect, by which the high emotional
Fig. 2 (A) Oxytocin treatment caused a stimulus-congruent ‘sharpening’ of perceived anger and happiness, such that angry expressions were perceived as more angry and less happy, whereas happy
expressions were perceived as more happy and less angry. Oxytocin-induced ‘sharpening’ (i.e. oxytocin-induced changes in perceived mood, calculated as the between-session difference in mean ratings for each
participant and facial expression) is represented on the y axis. (B) This ‘sharpening’ effect of intranasal oxytocin treatment was evident across explicitly and implicitly presented expressions, as illustrated here
with a depiction of the raw scores. Error bars represent the standard error of the mean. **P<0.01, *P<0.05.
Oxytocin, pupils andemotional sensitivitySCAN (2013) 745
sensitivity group showed a greater oxytocin enhancement of pupil dila-
tion than did the low sensitivity group, with the effect being greatest in
the interval 1500–1800ms (P¼0.039 at 1700ms).
We also used the pupillometry data in a linear regression analysis to
support the above findings of how baseline emotional sensitivity mod-
erated the beneficial effects of oxytocin with task-induced pupil dila-
tion as an independent moderator. Since pupil dilation reflects task
demands and we expected those with low emotional sensitivity to find
the rating tasks more demanding than those with high emotional sen-
sitivity, we expected and found a trend towards a significant associ-
ation between task-induced pupil dilation and oxytocin-induced
P¼0.053). As before, a median-split analysis illustrates this relation-
ship. The participants with high task-induced pupil dilation (consist-
oxytocin-induced improvement in task performance for ratings of
perceived anger (high sensitivity P¼0.70, low sensitivity P¼0.014;
happiness ratings: high sensitivity P¼0.79, low sensitivity P¼0.021).
No effect of oxytocin on measures of mood
As expected, we found no main effect of oxytocin treatment on ratings
of fear,sadness, irritability,happiness, calmness oranxiety
(F(1,26)¼1.4, P¼0.246). There were also no significant interactions
between oxytocin treatment and session order (oxytocin or placebo
first) or time of rating (pre-treatment, pre-testing and post-testing)
on mood scores (all Ps>0.39).
The results from this study demonstrate that oxytocin consistently
enhances the perception of others’ emotional facial expressions,
‘sharpening’ the impression such that happy faces appear more
happy and less angry, whereas angry expressions appear more angry
and less happy. We also found that oxytocin significantly increased
stimulus-induced pupil dilation, consistent with oxytocin enhance-
ment of attention towards socially relevant stimuli. These effects of
treatment were present for both explicitly and implicitly presented
emotional facial expressions, across negative (anger) and positive
(happiness) emotions. Importantly, the degree to which oxytocin
improved the discrimination between faces showing implicitly pre-
sented anger or happiness depended on each participant’s ability to
perform this task at baseline. Participants with low emotional
sensitivity scores showed significant improvement in task performance
after oxytocin treatment. In contrast, when emotional sensitivity was
already high, oxytocin afforded little or no improvement. Baseline
Fig. 3 (A and B) A median-split analysis based on baseline emotional sensitivity scores illustrates the relationship between sensitivity to subtle differences in emotional expressions and oxytocin enhancement
of performance on this task. A high score on this measure indicates high sensitivity to differences between the two ‘hybrid’ images containing hidden anger or happiness in terms of perceived anger and
perceived happiness. Participants with a high emotional sensitivity score performed only marginally better in this task after oxytocin treatment (Ps>0.73). In contrast, oxytocin significantly improved the task
performance of those who did not reliably report more anger for the implicitly angry faces (relative to implicitly happy faces, chart A) or more happiness for the implicitly happy expressions (relative to implicitly
angry expressions, chart B) in the placebo condition (both Ps<0.01). (C) A similar relationship was found when task-induced pupil dilation was used as an independent moderator, such that those with greater
task-induced pupil dilation showed the largest improvement in emotional sensitivity after oxytocin treatment. Error bars represent the standard error of the mean.
Fig. 4 Oxytocin pre-treatment caused significantly larger stimulus-induced pupil dilation (A). Pupil responses in the oxytocin session were significantly larger than in the placebo session during the
1000–3000ms interval after stimulus onset. Task-induced pupil dilation correlated with individual variability in emotional sensitivity towards differences in subtle expressions (r¼–0.376, P<0.01). Those with
the lowest sensitivity showed larger pupil dilation during stimulus presentation, consistent with higher task difficulty for these participants. As illustrated by the median split analysis (C), oxytocin’s beneficial
effects on emotional sensitivity in the low sensitivity group were not underpinned by large increases in pupil dilation. In contrast, we found a trend towards a group-by-treatment interaction driven by a larger
oxytocin-induced increase in pupil dilation in the high emotional sensitivity group.
746 SCAN (2013)S.Leknesetal.
emotional sensitivity also covaried with stimulus-induced pupil dila-
tion. ‘Poor’ performers showed significantly greater pupil dilation,
likely reflecting increased attentional demands of the task in these
Our results highlight a neural mechanism potentially underpinning
oxytocin’s prosocial effects. Oxytocin significantly enhanced the pupil
dilation response for all facial expressions presented. Pupil dilation has
been successfully used as an index of interest, attention allocation or
cognitive load (Hess and Polt, 1960; Kahneman and Beatty, 1966;
Laeng et al., 2012). This measure has shown remarkable covariation
with the firing of neurons in the LC, the ‘hub’ of the noradrenergic
system in the brain (Aston-Jones and Cohen, 2005). LC signalling is
thought to be particularly important for event detection and closely
related to the ‘ventral attention network’ (Corbetta et al., 2008). The
LC also contains oxytocin receptors (Petersson et al., 1998). Thus, it is
possible that intranasal or endogenous increase of central oxytocin
levels causes pupil dilation via direct oxytocinergic actions on neurons
in the LC. Given the crucial role of oxytocin in pair bonding in
non-human mammals, this finding is highly interesting. Dilated
pupils are associated with increased attractiveness and they can influ-
ence approach behaviour (e.g. see Wiseman and Watt, 2010). Seeing
others’ dilated pupils also causes significantly higher amygdala activa-
tion than viewing undilated pupils, an effect that nonetheless appears
to be unrelated to subjective reports of attractiveness (Demos et al.,
2008; Amemiya and Ohtomo, 2012). A number of studies have also
shown that pupil dilation closely mirrors sexual interest or reward
value (Hess and Polt, 1960; Whipple et al., 1992; Laeng and
Falkenberg, 2007; Bijleveld et al., 2009).
It is unclear whether the oxytocin-induced increase in pupil dilation
demonstrated here similarly reflects enhanced attractiveness or interest
towards the face stimuli. Oxytocin enhancement of perceived attract-
iveness was reported in a previous study (Theodoridou et al., 2009).
However, ratings of face attractiveness were not significantly altered by
oxytocin treatment in this study. Nevertheless, we cannot exclude the
possibility that the pupil represents a more sensitive measure of
increased attraction or interest towards others than ratings based on
introspection or conscious acknowledgements. Since all stimuli in this
investigation were socially relevant, it is as yet unclear whether oxyto-
cin’s effects on stimulus-induced pupil dilation are specific for social
stimuli. Future pupillometry studies including both socially relevant
and non-social stimuli could clarify this question. Collecting larger
datasets would enable us to exploit this sensitive physiological signal
to a larger extent. This could for instance allow for expansion of the
study material to other emotions that are closer to each other in their
moderating effects, such as fear and sadness. Larger studies would also
enable the investigation of interactions between oxytocin and sex
hormones on pupil dilation and emotion perception. Oxytocin is
known to interact with sex hormones such as oestrogen, but few
studies in humans have so far had the necessary scope to address
such questions (Campbell, 2010). These results demonstrating
oxytocin-induced pupil dilation during the viewing of others’ faces
nevertheless point to mechanisms by which oxytocin facilitates
human affiliation, since oxytocin is thought to be released in situations
of trust and warm touch (Morhenn et al., 2008).
The findings reported here on perception of others’ emotions replicate
and extend upon previous reports on the prosocial effects of oxytocin.
Several avenues of research, employing a range of stimuli and tasks, have
reported that intranasal oxytocin treatment enhances the recognition
accuracy of one or more emotions displayed in others’ faces (Domes
et al., 2007; Di Simplicio et al., 2009; Bartz et al., 2010; Fischer-Shofty
et al., 2010; Guastella et al., 2010; Marsh et al., 2010; Schulze et al., 2011).
However, the nature of the reported improvement has varied consider-
ably. Some have reported enhanced empathic accuracy towards only
positive (Di Simplicio et al., 2009; Marsh et al., 2010) or negative emo-
tions (Fischer-Shofty et al., 2010), some have found that the effect is
strongest for difficult items (Domes et al., 2007) and others the opposite
(Guastella et al., 2010; Schulze et al., 2011).
Further discrepancies can be found in the literature on oxytocin’s
effects on evaluative processing. Petrovic et al. (2008) found that oxy-
tocin reduced the negative evaluation of a face predicting unpleasant
electric shocks. In contrast, Shamay-Tsoory et al. (2009) reported in-
creases in evaluation of both negative and positive feelings induced by
a monetary task. A third study on evaluative processing assessed posi-
tivity and negativity ratings of images on separate scales and reported
increased vector angle (i.e. a ‘sharpening’ of ratings through opposite,
stimulus-congruent effects on negativity and positivity ratings) for
pleasant, socially relevant images after oxytocin treatment (Norman
et al., 2010). This investigation replicated this evaluative ‘sharpening’
effect. Specifically, images of happiness were rated as happier and less
angry, and the opposite effect was found for images of anger.
Importantly, our findings extend upon the findings by Norman et al.
to include images of negative emotional value and also implicitly pre-
sented emotional information. This type of evaluative ‘sharpening’
effect could represent one mechanism by which oxytocin enhances
sensitivity to simple as well as more complex emotional expressions.
A recent study by Bartz et al. (2010) reported a compelling
interaction between oxytocin treatment and social competence, as
measured with the Autism Spectrum Quotient (AQ). The authors sug-
gested that oxytocin increases the salience of social cues and that treat-
ment with oxytocin should benefit individuals who are generally less
tuned to social information, but not socially adept individuals. Our
results are consistent with this notion. Here, we calculated an emo-
tional sensitivity score based on each individual’s ability to discrimin-
ate between faces containing hidden anger or happiness. Low scores
indicated participants who did not reliably rate the implicitly angry
faces as expressing more anger and less happiness than the implicitly
happy faces. The emotional sensitivity score significantly predicted the
prosocial effect of oxytocin on this task. A median-split analysis based
on this score illustrates this relationship clearly. Participants with high
emotional sensitivity were able to differentiate between the hidden
expressions of anger and happiness both with and without oxytocin
treatment. In contrast, the performance of those with low baseline
sensitivity to others’ subtle emotions was dramatically improved
after oxytocin pre-treatment. This finding was corroborated by a
related analysis demonstrating that the same pattern of results emerges
when task-induced pupil dilation, an independent moderator, is used
to index emotional sensitivity. Participants whose pupils dilated the
most during the evaluative task at baseline, suggesting greater atten-
tional allocation or cognitive load due to higher perceived task
difficulty, also showed the greatest improvement of oxytocin on
These findings support the suggestion by Bartz et al. that oxytocin’s
prosocial effects depend on how attuned an individual is towards social
information. Importantly, we replicate and extend their findings to
include static images and hidden emotional expressions. Our data
demonstrate that oxytocin’s effects can be predicted by baseline ability
to evaluate other’s emotions, a more basic measure of social skills than
the AQ in the sense that the present measure is behavioural and does
not rely on self-report about one’s own behavioural patterns. These
findings are consistent with the notion that oxytocin may prove a
useful treatment for other psychiatric populations than autism spec-
trum disorder. Impaired emotion recognition is a symptom of several
mental disorders, including schizophrenia and drug addiction (Penn
et al., 2008; Fernandez-Serrano et al., 2010). Interestingly, early inves-
tigations provide some encouragement that oxytocin could prove a
useful supplement to current treatment of both schizophrenia (Keri
Oxytocin, pupils andemotional sensitivitySCAN (2013)747
et al., 2008; Feifel et al., 2010) and drug addiction (You et al., 2001; Qi
et al., 2009; Carson et al., 2010). Future studies should address whether
the effects reported here for static facial expressions of happiness and
anger presented in a laboratory setting would generalise to other
emotions and more ecological interactions. Moreover, this study
used a single dose of intranasal oxytocin and repeated administration,
as used in some recent studies in clinical populations, could perhaps
enhance these effects (Feifel et al., 2010; Feifel, 2011).
The ability to ‘read’ the hidden expressions, as measured by the
emotional sensitivity score, also covaried with stimulus-induced
pupil dilation. The validity of the emotional sensitivity measure used
here is therefore supported by this physiological measure. The highest
task-related dilation was shown by participants with low emotional
sensitivity. We interpret this between-subjects finding as an indication
of task difficulty, whereby the task of rating aspects of the presented
stimuli demands higher attention allocation and represents increased
cognitive load in those with low sensitivity. This finding is consistent
with previous suggestions that oxytocin enhances performance on sub-
jectively difficult tasks (Domes et al., 2007; Guastella et al., 2010).
However, since oxytocin did not cause the largest pupil increases in
this subset of participants, our results do not provide direct evidence
that oxytocin’s prosocial effects are mediated by increased attention
allocation to social stimuli. Instead, oxytocin enhanced pupil dilation
relatively more in those participants whose emotional sensitivity was
already high. We speculate that this finding could reflect higher sen-
sitivity to oxytocin within brain structures related to arousal and/or
emotion perception in those with high emotional sensitivity. Oxytocin
treatment was recently shown to affect the autonomic nervous system
in proportion to self-reported loneliness (Norman et al., 2011), a
measure that has been related to reduced reactivity to pictures of
other people (Cacioppo et al., 2009). An alternative explanation
would be that pupil dilation during the placebo session was already
at ceiling level in the low emotional sensitivity group. We find this
unlikely, since maximum pupil dilation in this group after placebo
treatment was only 8.3% during the 3000ms of stimulus presentation.
In conclusion, we have shown that a single dose of intranasal
oxytocin (40IU) ‘sharpens’ evaluative processing of others’ positive
and negative facial expression for both explicit and hidden emotional
information. Our results point to mechanisms which could underpin
oxytocin’s prosocial effects in humans. Using a performance-based
measure of social sensitivity, we replicated and extended earlier find-
ings, showing that oxytocin specifically enhances the sensitivity of
others’ explicit and hidden emotions in individuals whose baseline
performance was poor. We contend that a putative explanation for
the numerous discrepancies in the literature on oxytocin’s prosocial
effects is the high emotional sensitivity in most healthy volunteer
populations, rendering effect sizes small or non-significant. Our find-
ings support early indications that oxytocin treatment holds promise
for not only for autism spectrum disorder but also for other psychiatric
populations including schizophrenia and drug addiction. Finally,
we report a significant increase in stimulus-related pupil dilation
after oxytocin treatment, consistent with a role for oxytocin in enhan-
cing the salience of socially relevant stimuli. Intriguingly, since
large pupil sizes are associated with increased attractiveness and ap-
proach behaviour in humans (Laeng and Falkenberg, 2007; Wiseman
and Watt, 2010), this physiological effect could also enhance others’
affiliative behaviours towards an individual with high central oxytocin
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