Curr Psychol
DOI 10.1007/s12144-016-9448-9
Spontaneous Facial Expressions Are Different from Posed Facial
Expressions: Morphological Properties and Dynamic Sequences
Shushi Namba 1 & Shoko Makihara 1 & Russell S. Kabir 1 & Makoto Miyatani 2 &
Takashi Nakao 2
# Springer Science+Business Media New York 2016
Abstract The correspondence between facial expressions
and emotions has been widely examined in psychology.
However, studies have yet to record spontaneous facial expressions under well-controlled circumstances, thus the characteristics of these expressions remain unclear. Therefore, we
compared the morphological and dynamic properties of spontaneous and posed facial expressions related to four different
emotions: surprise, amusement, disgust, and sadness. First, we
secretly recorded participants’ spontaneous facial expressions
as they watched films chosen to elicit these four target emotions. We then recorded posed facial expressions of participants when asked to intentionally express each emotion.
Subsequently, we conducted detailed analysis of both the
spontaneous and posed expressions by using the Facial
Action Coding System (FACS). We found different dynamic
sequences between spontaneous and posed expressions for
surprise and amusement. Moreover, we confirmed specific
morphological aspects for disgust (the prevailing expressions
of which encompassed other emotions) and posed negative
emotions. This study provides new evidence of the characteristics for genuinely spontaneous and posed facial expressions corresponding to these emotions.
Keywords Emotions . Facial expressions . Spontaneous .
Posed . Dynamic aspects
* Shushi Namba
sushishushi760@gmail.com
1
Graduate School of Education, Hiroshima University, 1-1-1,
Kagamiyama, Higashi-Hiroshima 739-0046, Japan
2
Department of Psychology, Hiroshima University, Hiroshima, Japan
Introduction
The correspondence between facial expression and experienced emotion is one of the most researched themes in psychology (e.g., Ekman 1980; Izard 1977). The best-known theory about this correspondence is basic emotion theory (BET;
Ekman 1994), which assumes universal coherence between
specific combinations of facial movement and basic emotions
such as anger, surprise, disgust, fear, happiness, and sadness
(Ekman 1993, 2003; Ekman et al. 2002). This theory has been
supported by many previous studies (Ekman et al. 1980;
Levenson et al. 1992; Mauss et al. 2005; Rosenberg and
Ekman 1994). Ekman et al. (2002) probed these relationships
using the Facial Action Coding System (FACS; Ekman and
Friesen 1978). FACS is an objective, comprehensive system
based on anatomy that has been often used to describe visible
facial movements (e.g., Ekman and Rosenberg 2005; Sato
and Yoshikawa 2007). Facial movements are identified in
FACS by Action Units (AUs). For example, movement of
the zygomatic major muscle to pull the corners of the lips
(AU12) and contraction of the outer orbicularis oculi muscle,
which raises the cheeks (AU6), imply happiness in BET
(Ekman et al. 2002).
Although BET has been widely accepted, several studies
have emphasized two problems of previous BET studies
(e.g., Tcherkassof et al. 2007; Krumhuber and Scherer
2011). First, the dynamic aspects of facial expressions for
various emotions have been ignored. The importance of
dynamic aspects of facial expression has been underscored
in previous studies of facial mimicry, facial recognition,
and neuroimaging (e.g., Sato and Yoshikawa 2007; Wehrle
et al. 2000; Mühlberger et al. 2011). For example, in a neuroimaging study, Mühlberger et al. (2011) compared how areas
of the brain responded to the starting and stopping of the same
emotional expression. It was indicated that the onset and
Curr Psychol
endpoint of emotional facial expressions involved different
activity in the brain networks, highlighting the importance of
temporal characteristics of facial expressions for social communication. Thus, in order to comprehensively understand the
characteristics of facial expressions, it is important to examine
their dynamic aspects.
Second, although there are significant differences between
spontaneous and posed facial expressions (Buck and VanLear
2002), most studies based in BET have relied on posed facial
movements (e.g., Kaiser and Wehrle 2001; Matsumoto 1992),
while few studies have investigated spontaneous movements
(e.g., Dimberg et al. 2000; van der Schalk et al. 2011). The
movements inherent to posed facial expressions display an
emotion an expresser ostensibly intends to convey, whereas
spontaneous facial expressions correspond to an expresser’s
actual, unmitigated emotional experiences. Johnston et al.
(2010) found that static spontaneous smiles were evaluated
more positively than posed smiles. This result indicates that
the evaluation of morphological aspects of posed and spontaneous facial expressions can differ significantly. Although
several studies have found actors’ expressions to be relatively
similar to spontaneous expressions (e.g., Carroll and Russell
1997; Gosselin et al. 1995; Krumhuber and Scherer 2011;
Scherer and Ellgring 2007), actors’ expressions are designed
to emphasize a message, and should be regarded as pseudospontaneous displays that involve intentional and strategic
manipulation (Buck and VanLear 2002). That is, the artificial
facial expressions generated by professional actors are different from the spontaneous expressions made in our daily lives.
Two studies have examined the morphological aspects of
spontaneous facial expressions of athletes (Matsumoto and
Willingham 2009) and those of blind and sighted children
aged 8 to 11 (Galati et al. 2003). However, because the facial
expressions in these studies were recorded with another person present, these study designs may have inhibited participants’ natural facial expressions. Social intention is related to
emotion itself in these facial expressions (Hess and Kleck
1997; Kunzmann et al. 2005). Moreover, these previous studies
did not examine the participants’ actual experiences of emotion
and the dynamic aspects of facial expressions. To understand
the correspondence between facial expressions and experienced emotions, we need to record spontaneous facial expressions under well-controlled circumstances (i.e., in the absence
of other persons), so as to obtain spontaneous facial expressions
that reflect only emotional aspects. We need to confirm
participants’ experiences of the target emotions and the
dynamic aspects of the face’s movements.
There have been several studies on the dynamic aspects of
spontaneous facial expressions (e.g., Hess and Kleck 1997;
Schmidt et al. 2006; Weiss et al. 1987), however, these studies
had some methodological issues and experimental limitations.
The findings of Weiss et al. (1987) were limited in scope as
they employed a hypnotically induced affect approach in a
restricted sample of only three female participants. Schmidt
et al. (2006) only explored movement differences in posed and
spontaneous smiling using tracking points. Hess and Kleck
(1997) attempted to distinguish dynamic aspects of facial
expressions, but only measured duration and did not report
specific facial actions. In summary, several studies have
examined the dynamic aspects of facial expressions, but
the characteristics of spontaneous facial expressions of
emotion remain unclear.
This study investigates the morphological and dynamic
aspects of spontaneous facial expressions that reflect emotional experiences and excludes the confounding effects of social
factors. We compared spontaneous and posed facial expressions of emotion in terms of the types of AUs displayed and
the sequence of AUs using FACS. We defined the emotional
facial expressions displayed intentionally by participants as
posed, and those demonstrated by participants as they
watched films that tended to elicit particular emotions as
spontaneous. Many previous studies have used films as
stimuli to examine the correspondence between emotions
and facial expressions (e.g., Ekman et al. 1980; FernándezDols et al. 1997; Rosenberg and Ekman 1994). Films are
recognized as appropriate material for experiments because
of their predictable arousal of emotion, ecological validity,
and ease of use (Gross and Levenson 1995). Following the
example of these previous studies, we used films as stimuli to
elicit specific emotions. We asked participants to watch the
films alone to rule out any confounding effect of social factors
as well as to ensure that well-controlled, spontaneous facial
expressions are elicited. Furthermore, to prevent the conscious
awareness of being on camera that has contributed to confounding factors in past studies, we made the recordings without the awareness of the participants.
We first recorded spontaneous facial expressions of emotion: surprise, amusement, disgust, and sadness. Next, we recorded posed facial expressions of emotion, instructing participants to express the target emotions. Then, we coded the facial
expressions recorded in this study and analyzed each sequence.
Finally, we compared the morphological and dynamic properties of spontaneous and posed facial expressions of emotion.
Method
Participants
Thirty-one undergraduate students (13 males; Mage = 20.19,
SD = 1.37, range = 18–24) at Hiroshima University participated in this experiment on a voluntary basis, and were given a
monetary compensation of ¥500 ($4.56). All participants
were native Japanese speakers with normal or corrected-tonormal vision. There was no evidence of the presence of neurological and psychiatric disorders. Written informed consent
Curr Psychol
was obtained from each participant before the investigation, in
line with protocols set and approved by the Ethical Committee
of the Graduate School of Education, Hiroshima University.
Stimuli
From the films developed by Gross and Levenson (1995), we
selected one film for each emotion: When Harry Met Sally,
Pink Flamingos, The Champ, and Capricorn to represent
amusement, disgust, sadness, and surprise, respectively.
These films had been confirmed as eliciting the desired emotional experiences in Japanese participants (Sato et al. 2007).
As a neutral film, we used a sample video (Wildlife in HD)
preloaded onto the Windows 7 operating system. After we had
collected data from 10 participants, we noticed that When
Harry Met Sally elicited unintended negative emotions (e.g.,
confusion and embarrassment) more than the target emotion
of amusement. Therefore, we replaced this film with another
one, Trololo Cat. The clip lengths were as follows: 30 s for
neutral, 51 s for amusement, 30 s for disgust, 171 s for sadness, and 49 s for surprise. All of the films had sound.
Japanese subtitles were added in the films intended to elicit
amusement and sadness.
For the emotional assessment of each film, we implemented the following methods used by Sato et al. (2007).
Discrete Emotions For discrete emotional assessment, we
used a 16-item emotion self-report inventory. The items included amusement, anger, arousal, confusion, contempt, contentment, disgust, embarrassment, fear, happiness, interest,
pain, relief, sadness, surprise, and tension on a 9-point scale,
ranging from 0 (not at all) to 8 (the strongest in my life). These
discrete terms of emotion were presented in a randomized
sequence for each participant.
Valence and Arousal For broad emotional assessment, we
used an affect grid to estimate emotional state in terms of
valence and arousal (Russell et al. 1989). For valence, the
scale ranged from 1 (unpleasant) to 9 (pleasant); for arousal,
the scale ranged from 1 (sleepiness) to 9 (high arousal).
Procedure
The participants were tested individually. Participants were
seated facing a personal computer screen (VPCF14AFJ,
SONY). They were instructed to watch the films presented
on the PC screen and to evaluate the emotions elicited by the
films. Participants were also instructed to clear their mind of all
thoughts, emotions, and memories between film presentations.
After the instructions, participants were left alone in the room
and initiated the presentation of each film stimulus by clicking
the mouse button. After the neutral film was presented as a
practice, the other four films followed. The order of the films
was varied using incomplete counterbalancing. After viewing
each film, participants assessed their emotional state using the
16-item emotion self-report inventory and the affect grid for
rating valence and arousal. Half of the subjects completed the
16-item emotion self-report inventory before completing the
affect grid, whereas the other did so in the reverse order. As
they were watching each film, we secretly recorded their facial
expressions via the camera embedded on the PC screen. The
recordings were then used to analyze their spontaneous emotional facial expressions.
In debriefing sessions, participants were informed about
the previously undisclosed recording of their facial expressions by embedded camera and were given the choice to either
sign a second consent form permitting analysis of the recorded
facial expressions or have us delete the data. If we did not
obtain consent, the experiment was terminated at that point.
If we obtained consent, we then continued with the recording
of posed facial expressions. We instructed the participants “to
express the target emotions” while gazing at the center of the
PC screen until they felt they gave their best take. We recorded
the resulting posed facial expressions according to the procedure of Takahashi and Daibo (2008). The order of the emotion
categories was randomized across participants. We confirmed
that none of the participants noticed that they were being recorded by camera during sessions.
Emotional Experiences of Participants
To confirm the validity of the stimuli intended to induce spontaneous emotional facial expressions, we examined the emotional experiences of participants when they were viewing the
films. Figure 1 shows the mean intensity of the discrete emotion ratings. To investigate whether each film elicited the target emotion more strongly than the other emotions, planned
comparisons were carried out according to the procedure of
Sato et al. (2007) using Steel’s one-tailed t-test for each target
emotion (e.g., the surprise ratings on the surprise film vs. the
other ratings on the surprise film). For clarity, we have not
shown the complete affect grid. More detailed results are
available upon request.
In amusement film 2, the emotion of amusement was
rated significantly higher than that of the other emotions
(ts(40) > 2.29, ps < 0.05, gs > 0.69) except for happiness
and interest (ts(40) > 1.82, ps < 0.10, gs > 0.55). As
noted by a previous study (Sato et al. 2007), amusement,
interest, and happiness can be synonymous; therefore, we
considered the significantly high rating for these three
emotions reasonable. Consequently, amusement film 2 was
considered effective in eliciting the target emotion. For the
surprise film, the surprise rating was significantly higher than
that of the other emotions (ts(60) > 2.80, ps < 0.01, gs > 0.70)
except for arousal (t(60) = 1.56, p = 0.11, g = 0.39). The
co-occurrence of surprise with arousal was considered
Curr Psychol
Fig. 1 Mean emotion ratings for
target emotions as a function of
the type of eliciting stimulus. The
columns indicate the stimulus and
the rows indicate the ratings
reasonable. Consequently, the surprise film was perceived
as successful in eliciting the target emotion of surprise. In
the disgust film, the target emotion of disgust was rated
significantly higher than in the other emotions (ts(60) > 2.27, ps < 0.05, gs > 0.57). Among the nontarget
emotions, contempt and embarrassment were marginally
significant (ts(60) > 1.83, ps < 0.10, gs > 0.46), while
surprise was not significantly different (ts(60) = − 0.35,
ps = 0.37, gs = 0.09). This film elicited not only disgust
but also surprise. With regard to the high-activation state,
the content of the film, which depicts a woman eating dog
feces, elicited strong negative emotions and feelings of
surprise from participants, in accord with the previous
study (Sato et al. 2007). Therefore, the co-occurrence of
disgust and surprise was reasonable. In the sadness film, the
target for sadness was rated significantly higher than the other
emotions (ts(60) > 2.85, ps < .001, gs > 0.71). The sadness film
is appropriate for the intended purpose.
In summary, some films elicited nontarget emotions as well
as the target emotion, but the particular nontarget emotions
experienced in this study seemed reasonable and consistent
with the target emotion. Therefore, we assume that three of
the stimuli (i.e., amusement film 2, the surprise film, and the
sadness film) used in the present study could elicit the desired
target emotion as well as those used in the previous study (Sato
et al. 2007), whereas the disgust film as stimulus elicited both
the desired target emotion and unrelated nontarget emotions.
Selection of Apex for Spontaneous Emotional Expressions
To detect the onset and apex of each spontaneous facial expression, eight people unaffiliated with the experiment were
designated as evaluators (four males; Mage = 21.75, SD = 0.71,
range = 21–23; Fig. 2). The recorded facial expressions to be
evaluated were presented without sound so that the audio accompanying the film would not affect the ratings. Each evaluator viewed and rated all 106 videos of spontaneous facial
expressions, viewing each one only once. The length of each
video was the same as that of the emotion elicitation films
(30 fps). The order of the videos was randomized. The onset
point was defined as the moment when any individual facial
actions appeared, whereas the apex point was according to
evaluators’ ratings. These apex points can be described as the
perceptual apex. However, no facial movements of sadness
Curr Psychol
Fig. 2 The measurement of apex
for spontaneous emotional
expressions. a Evaluators view
and rate the recorded spontaneous
facial expressions in real-time
using the Visual Analogue Scale
(VAS). b Individual ratings for
each target emotion. c Mean
ratings are compiled to determine
the perceptual apex
were found, and hence we could not detect the onset and apex
for spontaneous facial expressions of sadness. Therefore, the
following analysis does not include expressions of sadness.
Facial Coding
We coded the facial expressions recorded in the spontaneous
and posed sessions using FACS. We used facial expressions
only from the neutral status to the apex of the target expression
and did not examine the period after the apex because
overt endpoints were vague and hard to identify. During
the debriefing session, one participant refused to permit
the use of her facial expressions. In addition, the spontaneous facial expressions of one participant and the posed
facial expressions of two participants were not available
due to problems with the camera. Furthermore, as discussed
above, we replaced the amusement film after initiating the
study. For these reasons, 19 participants’ spontaneous facial
expressions of amusement, 29 participants’ spontaneous
expressions of surprise and disgust, and 28 participants’
posed facial expressions of all four emotions were available
for the following analyses.
A trained FACS coder scored all expressions frame by
frame (30 fps) and recorded all AUs until each apex was
reached. We excluded blinking (AU45), head movement,
and gaze direction (AUs 50 to 66) because we were focusing
on emotional facial movements.
To check reliability, a second trained FACS coder coded
the same dataset frame by frame. Mean agreement regarding
the presence of AUs (Cohen’s κ = 0.83) and reliability of
Curr Psychol
FACS (.84; Ekman and Friesen 1978) were high enough to
suggest intercoder reliability.
Statistical Analysis
To examine the differences in morphological aspects between
spontaneous and posed facial expressions, we tested the frequency of appearance of each AU individually by following
the method used in previous studies (Carroll and Russell
1997; Galati et al. 2003; Gosselin et al. 1995).
In order to determine the dynamic aspects of the expressions, there must be at least two kinds of movement in the face
to permit inspection of sequences (Krumhuber and Scherer
2011). Moreover, if we used the most frequent occurrences
of four or more types of AUs, the proportion of participants in
which these AUs co-occurred decreased, and hence we were
not able to examine dynamic aspects substantially. Therefore,
we used the most frequent occurrences of three kinds of AUs,
as observed at the apex of each emotional portrayal, to compare the differences in the sequential occurrence of AUs between spontaneous and posed expressions.
Results
Comparison between Spontaneous and Posed
Expressions: Morphological Aspects
To compare the AUs between the posed and spontaneous expressions, we conducted McNemar’s test. Figure 3 shows the
results of comparisons between two expressions for each emotion. Table 1 shows the frequency of occurrence of each AU at
apex for surprise. Five AUs of posed expressions were more
Fig. 3 Results from comparing
the frequencies of spontaneous
and posed facial expressions. We
obtained direct permission from
the participants to report their
spontaneous expressions. The red
outer frame indicates the
characteristics of spontaneous
expressions, and the green outer
frame indicates the characteristics
of posed expressions
frequent than in spontaneous expressions: the Eyebrow Raiser
(AU1 and AU2), the Upper Lid Raiser (AU5), the Lips Part
(AU25), and the Jaw Drop (AU26), (χ2s (1, N = 28) > 4,
ps < 0.05, ORs > 4.55).
With regard to amusement, each AU analyzed yielded no
significant differences between spontaneous and posed expressions (Table 2).
With regard to disgust, the Upper Lip Raiser (AU10) and
the Lip Corner Puller (AU12) occurred more often in spontaneous than in posed expressions (χ2s (1, N = 28) > 4.17,
ps < 0.04, ORs > 11.5), whereas the Chin Raiser (AU17)
appeared significantly more often in posed than in spontaneous expressions (χ2 (1, N = 28) = 5.14, p = 0.02,
OR =13.5; Table 3).
Sequences of Spontaneous and Posed Expressions:
Dynamic Aspects
Figure 4 shows the three most frequent types of AUs at apex.
In the following analysis, an extended Fisher’s exact test was
performed on the sequential model of each emotional portrayal
of surprise and amusement. An exact binomial test was performed on the sequential model of disgust since the AUs at
apex differed between spontaneous and posed expressions.
For surprise, the most co-occurred facial actions were composed of raising the eyebrow and opening eyes (AU1, AU2
and AU5; Fig. 4). AU5 occurred earlier in sequence in spontaneous surprise than in posed surprise, whereas AUs 1 + 2 or
AUs 1 + 2 + 5 occurred earlier in sequence in posed than in
spontaneous surprise (spontaneous n = 6, posed n = 15,
p = 0.02; Fig. 5).
For amusement, smiling with opening the mouth and raising the cheeks were most commonly observed AUs (AU6,
Curr Psychol
Table 1 Occurrence of AUs for spontaneous surprise and posed
surprise at apex (n = 28)
Table 3 Occurrence of AUs for spontaneous disgust and posed disgust
at apex (n = 28)
AU
AU
Spontaneous
Posed
χ2
n
%
n
%
AU1:Inner Brow Raiser
6
23
16
57
7.69**
AU2:Outer Brow Raiser
AU4:Brow Lowerer
6
2
23
7
16
2
57
7
7.69**
0
AU5:Upper Lid Raiser
AU7:Lid Tightener
10
1
36
4
21
0
75
0
AU10:Upper Lid Raiser
AU11:Nasolabial Furrow Deepener
1
0
4
0
0
1
AU12:Lip Corner Puller
1
4
AU14:Dimpler
AU17:Chin Raiser
0
0
AU18:Lip Pucker
0
Spontaneous
disgust
Posed disgust
χ2
n
%
n
%
AU1:Inner Brow Raiser
AU2:Outer Brow Raiser
5
3
19
11
0
0
0
0
3.20
1.33
5.88*
0
AU4:Brow Lowerer
AU5:Upper Lid Raiser
12
2
43
7
19
0
69
0
3.27
0.50
0
4
0
0
AU7:Lid Tightener
AU9:Nose Wrinkler
14
1
50
4
11
3
39
11
0.31
0.25
1
4
0
AU10:Upper Lid Raiser
14
50
1
4
9.60**
0
0
1
1
4
4
0
0
AU12:Lip Corner Puller
AU14:Dimpler
6
0
21
0
0
3
0
11
4.17*
1.33
0
1
4
0
AU15:Lip Corner Depressor
3
11
0
0
1.33
1
4
0
0
0
AU21:Neck Tightner
3
11
0
0
1.33
AU16:Lower Lip Depressor
AU24:Lip Presser
AU25:Lips Part
AU26:Jaw Drop
0
5
2
0
19
7
1
16
9
4
57
32
0
7.69**
4*
AU17:Chin Raiser
AU24:Lip Presser
AU25:Lips Part
0
0
13
0
0
46
7
1
8
25
4
29
5.14*
0
1.07
AU27:Mouth Stretch
1
4
4
14
0.8
AU26:Jaw Drop
AU27:Mouth Stretch
3
1
11
4
2
0
7
0
0
0
Bold letters indicate the three most observed AUs that were used for the
analysis of the dynamic aspect
*
p < .05. ** p < .01
Bold letters indicate the three most observed AUs that were used for the
analysis of the dynamic aspect
*
AU12, and AU25; Fig. 4). In spontaneous amusement, AU12
and AUs 6 + 12 occurred earlier in sequence than AU25,
whereas AU25 occurred earlier than AU6 and AU12 in
sequences of posed expressions. There was a significant difference in the sequential model of AUs between spontaneous
Table 2 Occurrence of AUs for spontaneous amusement and posed
amusement at apex (n = 19)
AU
AU1:Inner Brow Raiser
AU6:Cheek Raiser
AU7:Lid Tightener
AU9:Nose Wrinkler
AU12:Lip Corner Puller
AU14:Dimpler
AU17:Chin Raiser
AU25:Lips Part
AU26:Jaw Drop
Spontaneous
amusement
Posed amusement
n
%
n
%
1
15
7
0
15
0
0
12
3
5
79
37
0
79
0
0
63
16
0
9
6
1
15
3
3
10
3
0
47
32
5
79
16
16
53
16
χ2
0
2.5
0
0
0
0
1.33
0.1
0
Bold letters indicate the three most observed AUs that were used for the
analysis of the dynamic aspect
*
p < .05. ** p < .01
p < .05. ** p < .01
and posed amusement (spontaneous n = 12, posed n = 13,
p = 0.02; Fig. 5).
For disgust, the three most frequently observed AUs for
spontaneous disgust were composed of squinting eyes and
raising the upper lip (AU6, AU7 and AU10; Fig. 4).
Although AU6 and AU10 occurred earlier than AU7,
there was no significant sequential difference (n = 8,
p = 0.18). For posed disgust, the most co-occurred facial
actions were a glare and raising the chin (AU4, AU7, and
AU17; Table 3). We could not identify a consistent sequential model of posed disgust, because only four participants displayed all three of these AUs. However, a general tendency for AU4 or AU 4 + 7 to occur earlier than
the other AUs was observed. Although AU25 tied for third
place among spontaneous expressions (13 of 29, 45 %) and
was third among posed expressions (8 of 28, 29 %), we excluded it because this AU offered less dynamic information
(e.g., in the posed sessions, four participants left their mouths
open from the beginning of the recording).
For posed sadness, AU4 (6 of 28, 21 %), AU7 (3 of 28,
11 %), and AU17 (10 of 28, 36 %) were the three most frequently observed AUs (Fig. 4). However, as only three participants displayed more than two AUs, there was minimal information from which to construct a sequence. Thus, we did
not test any statistical hypothesis.
Curr Psychol
Fig. 4 The three most observed
AUs in this study. The red outer
frame indicates the characteristics
of spontaneous expressions, and
the green outer frame indicates
the characteristics of posed
expressions
Discussion
Surprise
The present study investigated the characteristics of spontaneous facial expressions of emotion, focusing on both
morphological and dynamic properties. First, we compared
the frequency of each AU between spontaneous and posed
expressions. Then, to examine the sequences, we identified
the three most frequent AUs at apex for each emotion.
Finally, to examine the dynamic aspects of the expressions, we compared the sequences between spontaneous
and posed expressions or confirmed the specific sequences
of individual emotions.
We observed significant morphological differences in the frequency of each AU between spontaneous and posed expressions at apex. The Eyebrow Raiser (AU1 and AU2), the Upper
Lid Raiser (AU5), the Lips Part (AU25), and the Jaw Drop
(AU26) were less frequently observed in spontaneous than in
posed expressions (Fig. 3). These AUs are regarded as main
components in BET (Ekman et al. 2002). One might suspect
that this difference in frequency could be the consequence of a
relative failure to observe spontaneous expressions of surprise. However, the percentage (11 %) of co-occurrences of
Curr Psychol
Fig. 5 Observed sequences for
each emotional portrayal in this
study. The red outer frame
indicates the characteristics of
spontaneous expressions, and the
green outer frame indicates the
characteristics of posed
expressions
AU1, AU2, AU5, and AU25 to express spontaneous surprise
in this study is greater than that in previous studies that
examined the co-occurrence of these AUs, to which the
percentage of participants exhibiting the same set of AUs
in response to surprise events ranged from 0 % to 7 %
(e.g., Reisenzein et al. 2006; Vanhamme 2003; Wang
et al. 2008). The results of this study thus identify the
existence of a spontaneous surprise expression more clearly
than previous studies, and they also support the claim that posed
facial expressions are more exaggerated than spontaneous facial
expressions (Hess and Kleck 1994).
There was a difference between spontaneous and posed
expressions for the dynamic aspect of surprise (Fig. 5). In
spontaneous surprise, AU5 was displayed first, followed by
Curr Psychol
AUs 1 + 2. This spontaneous sequence accords with sequences rooted in perceptual expectations of observers (Jack
et al. 2014). However, in posed surprise, AUs 1 + 2 preceded
AU5, or AUs 1 + 2 + 5 were simultaneously expressed. We
thus determined that the sequential pattern of spontaneous
surprise among participants was different from that in posed
expressions. Considering that someone who is surprised wants
to search for information to understand an unexpected situation, the opening of the eyes may be an adaptive response for
gathering more visual information (Darwin 1872). Susskind
et al. (2008) also provided evidence of enhanced visual-field
size in situations of fear and explained that faster eye movements are important for locating the source of a surprising
event. Our study supports these claims that adaptive functions
in eye movements occur because of spontaneous surprise.
Overall, the results for the morphological aspects of
surprise were in accord with previous studies (Ekman
et al. 2002; Reisenzein et al. 2006), but with regard to
the dynamic aspects, the sequential pattern of spontaneous
surprise differed from that of posed surprise. This result
shows the importance of the appearance order of AUs
when judging facial expressions.
Amusement
We did not find any significant differences in the frequency of
each AU between spontaneous and posed expressions of
amusement. Frank et al. (1993) pointed out that the presence
of the Cheek Raiser (AU6) in spontaneous expressions of
amusement was considered the distinguishing factor from
posed expressions. However, we did not observe this distinction. As AU6 in posed expressions of amusement has been
observed in previous studies (Carroll and Russell 1997;
Krumhuber and Scherer 2011), it is possible that AU6 is, in
fact, not a consistent distinguishing factor.
As Fig. 4 and Table 2 show, although the three most commonly observed AUs at apex were the same in both spontaneous and posed expressions: AU6, the Lip Corner Puller
(AU12), and the Lips Part (AU25), there was a difference in
the dynamic sequences (Fig. 5). In the sequential model for
amusement, sequences of spontaneous expressions were characterized by AU12 and AU6, followed by AU25. On the other
hand, in posed expressions AU25 usually preceded AU12,
followed by AU6. In posed expressions, the preceding activity
of opening the mouth might represent the preparatory state of
a more exaggerated smile and thus might reflect a strategic
motivation, whereas a spontaneous sequence displays smiles
that indicate authentic pleasure (Ekman 2003).
These results indicate that differences between spontaneous and posed expressions of amusement were observed not
in terms of the frequency of facial actions but in the order of
facial actions.
Disgust
We found significant differences in the frequency of each AU
between spontaneous and posed expressions of disgust. The
Upper Lip Raiser (AU10) and the Lip Corner Puller (AU12)
occurred more often in spontaneous than in posed expressions
(Fig. 3). AU10 has been regarded as the central component of
disgust in BET (Ekman et al. 2002). The hypothesized origin
of disgust as a rejection impulse to avoid disease through toxic
or contaminated food suggests that AU10 has evolved as a
response to something revolting (Darwin 1872; Rozin et al.
2008). Thus, AU10 might represent disgust in negative experiences (Galati et al. 2003). AU12 in spontaneous disgust
showed the possibility that elicited emotions might be not only
restricted to disgust but also other negative emotions. For
example, in our film depicting a woman consuming fecal matter, AU12 might play a role in the emotional elicitation of
contempt or embarrassment. Alternatively, to decrease stress
by producing strong negative emotions including surprise, this
action might have been developed for adaptation (Kraft and
Pressman 2012). Meanwhile, the Chin Raiser (AU17) appeared
significantly more often in posed than in spontaneous expressions (Fig. 3). Kaiser and Wehrle (2001) indicated that AU17
was observed in posed negative emotions. Thus, AU17 might
reflect more of a posed negative expression than other AUs.
As for the AUs of disgust expressions at apex (Fig. 4 &
Table 3), in spontaneous expressions, the Cheek Raiser
(AU6), the Lid Tightener (AU7), and AU10 were the three
most frequently expressed AUs. As already noted, AU10 is
anticipated in expressions of disgust according to BET
(Ekman et al. 2002). AU7 has been frequently observed in
fear situations (Galati et al. 2003), and the eye constriction
associated with AU6 and AU7 has been found in infant cryfaces to convey distress (Messinger et al. 2012). AU6 and
AU7 are consistent with visual sensory rejection, and disgust
has been shown to dampen perception (Susskind et al. 2008).
Therefore, this study would provide a different view of the
typical facial expressions used to represent disgust. In the case
of posed expressions, the Brow Lowerer (AU4), AU7, and
AU17 were the three most frequently observed AUs (Fig. 4).
Kaiser and Wehrle (2001) proposed that the combination of
AUs 4 + 7 + 17 represents posed sadness or hot anger.
Nevertheless, these AUs were observed in posed expressions
of disgust in this study, suggesting that posed expressions of
disgust, sadness, and hot anger might have similar morphological aspects.
For the sequential model of the emotional portrayal of disgust, we could not establish a consistent sequence model of
both expressions. It is possible that the processing of each
facial action is carried out without regard to order in these
cases.
Given this inconsistent evidence, no specific sequence
could be observed with regard to dynamic aspects of disgust.
Curr Psychol
However, the results present the possibility that posed expressions might differ substantially from spontaneous expressions
across various negative emotions in terms of their morphological aspects.
Sadness
For morphological properties of sadness, spontaneous facial
expressions were not observed. One possible reason for this
finding is that the film used in this study is too short or too
unconnected to the viewer to elicit visible expressions of sadness; it could be argued that the nature of sadness requires a
longer-term experience or one to which the individual feels
personally linked (Bonanno and Keltner 1997; Ekman 2003).
In posed expressions, AU4, AU7, and AU17 were most frequently observed (Fig. 4 & Table 4). As stated above, the
combination of AUs 4 + 7 + 17 was observed in posed sadness, hot anger (Kaiser and Wehrle 2001), and posed disgust
in this study. This result suggests the possibility that these
AUs especially AU17, are involved in the display of negative
emotional content with intention.
As for the dynamic aspects of posed sadness, only three
participants expressed the co-occurrence of more than two
AUs; thus, no sequence could be observed.
As a whole, the fact that no visible facial movements were
identified in spontaneous facial expressions of sadness could
simply indicate that the film used was not suitable to elicit
strong feelings of sadness. Similarly, the inconsistencies in
the properties of posed expressions of sadness may demonstrate that this expression is difficult for nonprofessional actors
to express intentionally.
Limitations and Future Studies
Although the present study has revealed new morphological
characteristics and dynamic aspects that may help to identify
the spontaneous facial expressions corresponding to surprise,
amusement, and disgust, several limitations must be noted.
Table 4
Occurrence of AUs for posed sadness at apex (n = 28)
AU
AU1:Inner Brow Raiser
AU4:Brow Lowerer
AU6:Cheek Raiser
AU7:Lid Tightener
AU14:Dimpler
AU15:Lip Corner Depressor
AU17:Chin Raiser
Posed sadness
n
%
1
6
1
3
1
1
10
4
21
4
11
4
4
36
Bold letters indicate the three most observed AUs
First, some methodological issues were present. We reasoned
that generating posed expressions invokes some degree of
conscious awareness because of the fact that cameras and
observers in prior experiments were shown to cause a change
in participants’ facial expressions (Barr and Kleck 1995;
Ekman et al. 1980; Bainum et al. 1984; Matsumoto and
Willingham 2006). Therefore, the order of generation was
constant in this study. As a result, choosing to begin with the
spontaneous generation of expressions might have affected
the posed generation of them in undetermined ways.
Moreover, the present films elicited not only the desired target
emotions but also nontarget emotions. Thus, it might be difficult to conclude that a specific facial expression is related to a
single emotion because of its associations with several emotions. The comparison between spontaneous facial expressions and posed facial expressions may be problematic because of these differences. In order to investigate the characteristics of facial expression that correspond more closely to
the pure experience of a single emotion, future studies should
incorporate a method that more effectively separates target
emotions from nontarget emotions.
Generalizing our observed sequences of various portrayals
of emotions to daily life is difficult because we analyzed only
the non-social sequences of participants who expressed the
three most observed AUs for each emotion. Thus, future studies will be necessary to carefully distinguish social spontaneous facial expressions from non-social ones. In addition, as the
results of this study are based on small samples of young
Japanese participants, generalization to other groups is
uncertain.
Although we wished to investigate the dynamic properties
of facial expressions, the duration and endpoint of each facial
movement were not measured in this study. We considered
only the one apex after onset. For example, a few expressions
returned to a neutral state until the end of the video clip.
Therefore, this study focused on facial expressions from no
facial actions to apex based on perceptual ratings. However,
several studies have shown that suddenness of onset and ending of facial motions, along with overall duration, can be important in distinguishing between spontaneous and posed expressions (e.g., Hess and Kleck 1997; Schmidt et al. 2006). To
obtain better information about the differences between spontaneous and posed facial expressions, further examination of
these cues will be necessary. Furthermore, our limitations in
dynamic analysis underscore the need for a more systematic
means of interfacing with data in this domain. It could be
useful for future research to employ automated methods for
detecting and recognizing facial expressions (Corneanu et al.
2016). Such methods might entail 3D face trackers that create
wireframe models and extract appearance feature geometry
(Cohen et al. 2003; Zeng et al. 2009). Multimodal approaches
may enable more precise ways to represent and capture the
elusive dynamic dimensions of facial expressions.
Curr Psychol
Summary and Conclusion
This study provides evidence of differences between spontaneous and posed facial expressions of emotions. With regard
to surprise, there were differences in both morphological and
dynamic aspects; with regard to amusement, there were differences in dynamic aspects; with regard to disgust, there were
differences in morphological aspects. Moreover, we confirmed specific morphological aspects for posed negative
emotions. Research on emotions has depended heavily on
the use of posed and static facial expressions that have been
based on BET. However, the dynamic sequences of facial
movements associated with actual emotional experiences
have been ignored (e.g., Ekman and Friesen 1976; Elfenbein
and Ambady 2002; Russell 1994). Through well-controlled
investigation of the characteristics of spontaneous facial expressions, research in this area can be more productive in
capturing how emotions affect facial expressions.
Compliance with Ethical Standards
Funding This research was supported by the Center of Innovation
Program of the Japan Science and Technology Agency (JST) and by
the Japan Society for the Promotion of Science (JSPS) KAKENHI
Grants 26285168 and 25870467.
Conflict of Interest The authors declare that they have no conflict of
interest.
Ethical Approval All procedure performed in studies involving human
participants were in accordance with the Ethical Committee of the
Graduate School of Education, Hiroshima University.
Informed Consent Written informed consent was obtained from all
individual participants before and after the investigation.
References
Bainum, C. K., Lounsbury, K. R., & Pollio, H. R. (1984). The development of laughing and smiling in nursery school children. Child
Development, 1946–1957. doi:10.2307/1129941.
Barr, C. L., & Kleck, R. E. (1995). Self-other perception of the intensity
of facial expressions of emotion: do we know what we show?
Journal of Personality and Social Psychology, 68(4), 608–618.
doi:10.1037/0022-3514.68.4.608.
Bonanno, G. A., & Keltner, D. (1997). Facial expressions of emotion and
the course of conjugal bereavement. Journal of Abnormal
Psychology, 106(1), 126–137. doi:10.1037/0021-843X.106.1.126.
Buck, R., & VanLear, C. A. (2002). Verbal and nonverbal communication:
distinguishing symbolic, spontaneous, and pseudo-spontaneous nonverbal behavior. Journal of Communication, 52(3), 522–541. doi:10.
1111/j.1460-2466.2002.tb02560.x.
Carroll, J. M., & Russell, J. A. (1997). Facial expressions in Hollywood’s
portrayal of emotion. Journal of Personality and Social Psychology,
72(1), 164–176. doi:10.1037/0022-3514.72.1.164.
Cohen, I., Sebe, N., Garg, A., Chen, L. S., & Huang, T. S. (2003). Facial
expression recognition from video sequences: temporal and static
modeling. Computer Vision and Image Understanding, 91(1), 160–
187. doi:10.1016/S1077-3142(03)00081-X.
Corneanu, C. A., Oliu, M., Cohn, J. F., & Escalera, S. (2016). Survey on
RGB, 3D, thermal, and multimodal approaches for facial expression
recognition: history, trends, and affect-related applications. IEEE
Transactions on Pattern Analysis and Machine Intelligence, 99.
doi:10.1109/TPAMI.2016.2515606.
Darwin, C. (1872). The expression of emotion in man and animals. New
York: Oxford University Press.
Dimberg, U., Thunberg, M., & Elmehed, K. (2000). Unconscious facial
reactions to emotional facial expressions. Psychological Science,
11(1), 86–89. doi:10.1111/1467-9280.00221.
Ekman, P. (1980). The face of man. New York: Garland Publishing, Inc..
Ekman, P. (1993). Facial expression and emotion. American Psychologist,
48(4), 384–392. doi:10.1037/0003-066X.48.4.384.
Ekman, P. (1994). All emotions are basic. In P. Ekman & R. J. Davidson
(Eds.), The nature of emotion: Fundamental questions (pp. 15–19).
New York: Oxford University Press.
Ekman, P. (2003). Emotions revealed. New York: Times Books.
Ekman, P., & Friesen, W. V. (1976). Pictures of facial affect. Palo Alto:
Consulting Psychologists Press.
Ekman, P., & Friesen, W. V. (1978). Facial action coding system. Palo
Alto: Consulting Psychologists Press.
Ekman, P., & Rosenberg, E. (2005). What the face reveals (2nd ed.). New
York: Oxford University Press.
Ekman, P., Freisen, W. V., & Ancoli, S. (1980). Facial signs of emotional
experience. Journal of Personality and Social Psychology, 39(6),
1125–1134. doi:10.1037/h0077722.
Ekman, P., Friesen, W. V., & Hager, J. C. (2002). Facial action coding
system (2nd ed.). Salt Lake City: Research Nexus eBook.
Elfenbein, H. A., & Ambady, N. (2002). On the universality and cultural
specificity of emotion recognition: a meta-analysis. Psychological
Bulletin, 128(2), 203–235. doi:10.1037/0033-2909.128.2.203.
Fernández-Dols, J. M., Sanchez, F., Carrera, P., & Ruiz-Belda, M. A.
(1997). Are spontaneous expressions and emotions linked? An experimental test of coherence. Journal of Nonverbal Behavior, 21(3),
163–177. doi:10.1023/A:1024917530100.
Frank, M. G., Ekman, P., & Friesen, W. V. (1993). Behavioral markers and
recognizability of the smile of enjoyment. Journal of Personality and
Social Psychology, 64(1), 83–93. doi:10.1037/0022-3514.64.1.83.
Galati, D., Sini, B., Schmidt, S., & Tinti, C. (2003). Spontaneous facial
expressions in congenitally blind and sighted children aged 8-11.
Journal of Visual Impairment and Blindness, 97(7), 418–428.
Gosselin, P., Kirouac, G., & Doré, F. Y. (1995). Components and recognition of facial expression in the communication of emotion by
actors. Journal of Personality and Social Psychology, 68(1), 83–
96. doi:10.1037/0022-3514.68.1.83.
Gross, J. J., & Levenson, R. W. (1995). Emotion elicitation using
films. Cognition & Emotion, 9(1), 87–108. doi:10.1080/
02699939508408966.
Hess, U., & Kleck, R. E. (1994). The cues decoders use in
attempting to differentiate emotion-elicited and posed facial
expressions. European Journal of Social Psychology, 24(3),
367–381. doi:10.1002/ejsp.2420240306.
Hess, U., & Kleck, R. (1997). Differentiating emotion elicited and deliberate emotional facial expressions. In P. Ekman & E. L. Rosenberg
(Eds.), What the face reveals (2nd ed., pp. 271–288). New York:
Oxford University Press.
Izard, C. E. (1977). Human emotions. New York: Plenum Press.
Jack, R. E., Garrod, O. G., & Schyns, P. G. (2014). Dynamic facial
expressions of emotion transmit an evolving hierarchy of signals
over time. Current Biology, 24(2), 187–192. doi:10.1016/j.cub.
2013.11.064.
Curr Psychol
Johnston, L., Miles, L., & Macrae, C. N. (2010). Why are you smiling at
me? Social functions of enjoyment and non-enjoyment smiles.
British Journal of Social Psychology, 49(1), 107–127. doi:10.
1348/014466609X412476.
Kaiser, S., & Wehrle, T. (2001). Facial expressions as indicators of appraisal processes. In K. R. Scherer, A. Schorr, & T. Johnstone (Eds.),
Appraisal processes in emotion: Theory, methods, research(285–
300). New York: Oxford University Press.
Kraft, T. L., & Pressman, S. D. (2012). Grin and bear it the influence of
manipulated facial expression on the stress response. Psychological
Science, 23(11), 1372–1378. doi:10.1177/0956797612445312.
Krumhuber, E. G., & Scherer, K. R. (2011). Affect bursts: dynamic patterns of facial expression. Emotion, 11(4), 825–841. doi:10.1037/
a0023856.
Kunzmann, U., Kupperbusch, C. S., & Levenson, R. W. (2005).
Behavioral inhibition and amplification during emotional arousal:
a comparison of two age groups. Psychology and Aging, 20(1),
144–158. doi:10.1037/0882-7974.20.1.144.
Levenson, R. W., Ekman, P., Heider, K., & Friesen, W. V. (1992). Emotion
and autonomic nervous system activity in the Minangkabau of
west Sumatra. Journal of Personality and Social Psychology,
62(6), 972–988. doi:10.1037/0022-3514.62.6.972.
Matsumoto, D. (1992). American-Japanese cultural differences in the
recognition of universal facial expressions. Journal of CrossCultural Psychology, 23(1), 72–84. doi:10.1177/0022022192231005.
Matsumoto, D., & Willingham, B. (2006). The thrill of victory and the
agony of defeat: spontaneous expressions of medal winners of the
2004 Athens Olympic games. Journal of Personality and Social
Psychology, 91(3), 568–581. doi:10.1037/0022-3514.91.3.568.
Matsumoto, D., & Willingham, B. (2009). Spontaneous facial expressions of emotion of congenitally and noncongenitally blind individuals. Journal of Personality and Social Psychology, 96(1), 1–10.
doi:10.1037/a0014037.
Mauss, I. B., Levenson, R. W., McCarter, L., Wilhelm, F. H., & Gross, J.
J. (2005). The tie that binds? Coherence among emotion experience,
behavior, and physiology. Emotion, 5(2), 175–190. doi:10.1037/
1528-3542.5.2.175.
Messinger, D. S., Mattson, W. I., Mahoor, M. H., & Cohn, J. F. (2012).
The eyes have it: making positive expressions more positive and
negative expressions more negative. Emotion, 12(3), 430–436.
doi:10.1037/a0026498.
Mühlberger, A., Wieser, M. J., Gerdes, A. B., Frey, M. C., Weyers, P., &
Pauli, P. (2011). Stop looking angry and smile, please: start and stop
of the very same facial expression differentially activate threat- and
reward-related brain networks. Social Cognitive and Affective
Neuroscience, 6(3), 321–329. doi:10.1093/scan/nsq039.
Reisenzein, R., Bördgen, S., Holtbernd, T., & Matz, D. (2006). Evidence
for strong dissociation between emotion and facial displays: the case
of surprise. Journal of Personality and Social Psychology, 91(2),
295–315. doi:10.1037/0022-3514.91.2.295.
Rosenberg, E. L., & Ekman, P. (1994). Coherence between expressive
and experiential systems in emotion. Cognition & Emotion, 8(3),
201–229. doi:10.1080/02699939408408938.
Rozin, P., Haidt, J., & McCauley, C. R. (2008). Disgust. In M. Lewis, J.
M. Haviland-Jones, & L. F. Barrett (Eds.), Handbook of emotions
(3rd ed., pp. 757–776). New York: Guilford Press.
Russell, J. A. (1994). Is there universal recognition of emotion from facial
expressions? A review of the cross-cultural studies. Psychological
Bulletin, 115(1), 102–141. doi:10.1037/0033-2909.115.1.102.
Russell, J. A., Weiss, A., & Mendelsohn, G. A. (1989). Affect grid: a singleitem scale of pleasure and arousal. Journal of Personality and Social
Psychology, 57(3), 493–502. doi:10.1037/0022-3514.57.3.493.
Sato, W., & Yoshikawa, S. (2007). Spontaneous facial mimicry in response to dynamic facial expressions. Cognition, 104(1), 1–18.
doi:10.1016/j.cognition.2006.05.001.
Sato, W., Noguchi, M., & Yoshikawa, S. (2007). Emotion elicitation effect
of films in a Japanese sample. Social Behavior and Personality:
An International Journal, 35(7), 863–874. doi:10.2224/sbp.2007.
35.7.863.
Scherer, K. R., & Ellgring, H. (2007). Are facial expressions of emotion
produced by categorical affect programs or dynamically driven by
appraisal? Emotion, 7(1), 113–130. doi:10.1037/1528-3542.7.1.113.
Schmidt, K. L., Ambadar, Z., Cohn, J. F., & Reed, L. I. (2006). Movement
differences between deliberate and spontaneous facial expressions:
Zygomaticus major action in smiling. Journal of Nonverbal
Behavior, 30(1), 37–52. doi:10.1007/s10919-005-0003-x.
Susskind, J. M., Lee, D. H., Cusi, A., Feiman, R., Grabski, W., &
Anderson, A. K. (2008). Expressing fear enhances sensory acquisition. Nature Neuroscience, 11(7), 843–850. doi:10.1038/nn.2138.
Takahashi, N., & Daibo, I. (2008). The results of researches on facial
expressions of the Japanese: its issues and prospects. Institute of
Electronics, Information, and Communication Engineers,
108(317), 9–10.
Tcherkassof, A., Bollon, T., Dubois, M., Pansu, P., & Adam, J. M. (2007).
Facial expressions of emotions: a methodological contribution to
the study of spontaneous and dynamic emotional faces.
European Journal of Social Psychology, 37(6), 1325–1345.
doi:10.1002/ejsp.427.
Van Der Schalk, J., Fischer, A., Doosje, B., Wigboldus, D., Hawk, S.,
Rotteveel, M., et al. (2011). Convergent and divergent responses to
emotional displays of in-group and outgroup. Emotion, 11(2), 286–
298. doi:10.1037/a0022582.
Vanhamme, J. (2003). Surprise … surprise. An empirical investigation on
how surprise is connected to customer satisfaction (No. ERS-2003005-MKT). ERIM Report Series Research in Management,
Erasmus Research Institute of Management (ERIM). Retrieved from
http://hdl.handle.net/1765/273.
Wang, N., Marsella, S., & Hawkins, T. (2008). Individual differences in
expressive response: a challenge for ECA design. Proceedings of the
7th International Joint Conference on Autonomous Agents and
Multiagent Systems, Portugal, 3, 1289–1292.
Wehrle, T., Kaiser, S., Schmidt, S., & Scherer, K. R. (2000). Studying the
dynamics of emotional expression using synthesized facial muscle
movements. Journal of Personality and Social Psychology, 78(1),
105–119. doi:10.1037/0022-3514.78.1.105.
Weiss, F., Blum, G. S., & Gleberman, L. (1987). Anatomically based
measurement of facial expressions in simulated versus hypnotically
induced affect. Motivation and Emotion, 11(1), 67–81. doi:10.1007/
BF00992214.
Zeng, Z., Pantic, M., Roisman, G. I., & Huang, T. S. (2009). A survey of
affect recognition methods: audio, visual, and spontaneous expressions. IEEE Transactions on Pattern Analysis and Machine
Intelligence, 31(1), 39–58. doi:10.1109/TPAMI.2008.52.