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- mise à jour
du
- 21 mai
2026
- iScience
- 2026;29(3):115042
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- Action
units of facial expressions in emotional
contagion
- Celeghin
A, Castiblanco Jimenez IA,
- Froio M, Vezzetti E, Olivetti EC, Marcolin
F
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- Tous
les articles sur la contagion du
bâillement
- All
articles about contagious
yawning
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- Abstract
- While research on facial expressions has
traditionally focused on basic emotions, other
spontaneous reactions, especially those elicited
through emotional contagion, are far more common
in daily social interactions, with facial
movements playing a key role in conveying
emotions. Here, the authors explore spontaneous
facial expressions of laughter, yawning, and
mirror pain through a contagion experiment
involving 32 participants. They identified key
facial action units and used landmark-based
distances as morphometric features to capture
their dynamics. Their analysis revealed distinct
patterns that separate these expressions from
each other and from neutral faces, particularly
in the lower face. This work provides a curated
set of action units and features for the
assessment of laughter, yawning, and mirror pain
expressions, offering a valuable resource for
future spontaneous facial expression recognition
studies.
-
- Résumé
- Alors que la recherche sur les expressions
faciales s'est traditionnellement
concentrée sur les émotions
fondamentales, d'autres réactions
spontanées, notamment celles
provoquées par la contagion
émotionnelle, sont bien plus courantes
dans les interactions sociales quotidiennes, les
mouvements faciaux jouant un rôle
clé dans la transmission des
émotions. Dans cet article, les auteurs
explorent les expressions faciales
spontanées du rire, du bâillement
et de la douleur par contagion à travers
une expérience de contagion impliquant 32
participants. Ils ont identifié des
unités d'action faciale clés et
ont utilisé des distances basées
sur des repères comme
caractéristiques morphométriques
pour en saisir la dynamique. Leur analyse a
révélé des schémas
distincts qui différencient ces
expressions les unes des autres et des visages
neutres, en particulier dans la partie
inférieure du visage. Ce travail fournit
un ensemble sélectionné
d'unités d'action et de
caractéristiques pour l'évaluation
des expressions du rire, du bâillement et
de la douleur par contagion, offrant ainsi une
ressource précieuse pour de futures
études sur la reconnaissance des
expressions faciales spontanées.
- Introduction
- Emotional contagion, a fundamental mechanism
in human social dynamics, represents the
tendency of individuals to automatically mimic
and synchronize their emotional states with
others.1,2 Two reflex-like mechanisms are
involved in emotional contagion, the
unintentional imitation of the sender's
emotional expression, the so called emotional
mimicry, and some afferent feedback from such
mimicry eliciting the same emotional state in
the receiver.3 This phenomenon plays a crucial
role in shaping interpersonal relationships
facilitating empathy and social cohesion4 while
allowing a better coordination of emotional
responses and behaviors within the group.5 In
environments with rapidly changing elements, the
ability to learn from others' emotional
responses provides an evolutionary advantage for
survival. Indeed, groups can offer better
prospects through communication and cooperation
while rapidly sharing perceptions of potential
threats.6 The ability to share and understand
emotions has developed through several stages,
suggesting that emotional contagion developed
gradually during evolution.7,8 In its early
stages, animals within a group displayed similar
expressions in response to a shared stimulus,
like when all members became alert in the
presence of danger. Over time, this collective
reaction is transformed into something more
complex: the expression of a single individual
could trigger the same response in others, even
without the original stimulus.8 Social animals
can indeed learn to fear novel stimuli
indirectly by witnessing the conspecific
reactions (a phenomenon known as social fear
learning), while humans can likely understand
others' feelings and anticipate their actions
often without conscious awareness.9,10 As
emotional mimicry occurs through subtle,
non-verbal cues, facial expressions represent
one of the primary channels for transmitting and
triggering social or defensive responses.11 The
initial stages of social cue encoding, such as
the reaction to specific patterns in facial
expressions, may represent the initial phase of
emotional communication.
-
- The way behavioral mimicry contributes to
emotional contagion varies across different
species and expressions. Sometimes, it happens
almost instantly, like when we quickly return a
smile, other times, it develops more slowly, as
with yawning8,12 or mirror pain,13 with
contextual factors and social goals that
significantly influence facial expressions'
occurrence and intensity.14 In both humans and
other animals, mimicry appears first as a basic
copying of facial movements that could or could
not lead to experience the emotion itself, thus
creating a bridge between physical imitation and
emotional sharing.8 This process aligns with the
two-step model proposed by Dezecache et al.,15
where emotional contagion first requires the
observer to recognize changes in the
demonstrator's behavior, followed by the
imitation of these behaviors along with
experiencing the corresponding emotion. Most
importantly, this initial emotional transmission
occurs automatically, without necessarily
requiring conscious awareness of the other's
emotional state. This extensive range of facial
movements is allowed by the presence of shared
physical traits, influenced by different
evolutionary pressures and ecological niches
that each species occupies. The specific needs
highlight the diversity of life and the complex
ways in which different species adapt to their
surroundings. However, common anatomical
features such as facial muscles contribute to a
growing body of evidence suggesting that the
evolution of facial expressions was not driven
entirely by phylogenetic pressures, but that
other socio-ecological factors had a significant
influence. Among the facial expressions most
associated with this phenomenon, yawning,
laughter, and mirror pain are compelling
examples of how possible emotional states can
spread quickly within social groups.
-
-
- For example, while yawning serves several
physiological functions, such as brain cooling16
and increased alertness,17 its contagious nature
has gathered increased attention as studies
showed that observing or even thinking about
yawning can trigger the same response.18 The
characteristic physiological reflex typically
linked to boredom or drowsiness, according to
Kapitány and Nielsen19 may serve as a
basic form of emotional synchronization,
potentially enhancing vigilance and social
bonding, as well as the broader concept of
empathy.20 Indeed, the susceptibility to
contagious yawning appears to be linked to
empathic abilities: individuals with higher
levels of empathy are more likely to experience
contagious yawning, while those with conditions
characterized by reduced empathy, such as autism
spectrum disorders, show decreased
susceptibility.12 When studying how yawning
spreads between people, researchers have found
interesting patterns in facial responses.21,22
Even when people successfully stop themselves
from yawning, their facial muscles often show
subtle activity patterns similar to yawning.
These small muscle movements indicate that our
faces automatically prepare to replicate others'
expressions, even if we do not complete the
action.23 This automatic response helps explain
why yawning is so contagious, as our facial
muscles react before we are even aware of it.9
Additionally, studies show that people are more
likely to "catch" yawns from family members and
close friends than from strangers, implying that
social bonds strengthen this contagious
response.8
-
- Similarly, laughter plays a vital role in
social interactions and emotional regulation.
The way humans laugh shares important features
with the "play face" expressions seen in other
primates, particularly in the brain areas
involved and the way facial muscles move. This
similarity is not just coincidental, it means
that laughter started as a simple play signal
and evolved into the more complex social tool we
use today; in fact, when people laugh together,
they use the same facial muscles and brain
networks that other primates use during playful
social interactions.5 Moreover, laughter appears
very early in human development, starting around
3&endash;4 months of age, and typically happens
during playful interactions.24 As children grow,
they develop the ability to distinguish between
genuine and social laughter, showing how their
understanding of social and emotional signals
becomes more sophisticated.5 Spontaneous
laughter, opposed to voluntary laughter,25 has
specific properties and is more likely to elicit
contagious responses. Studies by Simonyan and
Horowitz26 show that spontaneous laughter is
preserved even in patients with bilateral damage
to speech motor areas who cannot speak or
vocalize voluntarily, requiring less conscious
control and showing shorter response
latencies.
-
- Also mirror pain, the phenomenon where
observers experience pain-like sensations when
witnessing others in pain, despite not
undergoing any physical trauma themselves,27
shows comparing facial expressions during actual
and observed pain. Indeed, when people see
others in pain, their facial muscles often
contract in patterns similar to those of the
person actually experiencing pain. These
responses happen quickly and automatically,
demonstrating they are part of our basic social
connection system.8 Brain imaging studies show
that watching others in pain activates some of
the same brain regions that process our own pain
experiences.28,29 This shared neural activity,
supposedly mediated by the mirror neuron
system,30 helps explain why we might grimace
when seeing someone hurt themselves.31,32 Shared
and distinct neural networks for
self-experienced and empathized pain, support
the idea that mirror pain extends beyond simple
motor imitation to include deeply emotional
processes. The intensity of these responses is
modulated by social context, indicating that
external factors can influence the degree of
mirror pain experienced.33 Interestingly, these
reactions become stronger when we have closer
relationships with the person in pain, similar
to how yawning and laughter spread more easily
between friends and family.8
-
- Traditional methods of studying basic
emotions have relied heavily on behavioral
observations and self-report measures. However,
the analysis of the action units (AUs) has
become a particularly promising approach for
measuring and understanding the relationship
between facial expressions and the underlying
emotion.34 The Facial Action Coding System
(FACS) and its associated AUs have provided
researchers with a standardized method for
describing facial movements.35,36 Each AU
corresponds to the movement of an individual
muscle (Figure 1) or specific muscle group of
the face, identified by a number (AU1, AU2,
etc.). AUs can manifest individually or in
combination with each other, and despite their
limited number, over 7,000 different
combinations have been observed.
-
- Discussion
- Emotional alignment and quantitative and
qualitative analysis of AUs
- The present study aimed to identify facial
AUs associated with laughter, yawning, and
mirror pain, which exhibit contagious
properties. To investigate mediated emotional
contagion, we designed an experimental paradigm
in which emotional states were first elicited in
a group of participants through audiovisual
stimuli and subsequently transmitted to a second
sample via the recorded facial expressions
alone. This design allowed us to test whether
emotional facial signals could propagate in the
absence of direct contextual cues, thus
assessing the transitive nature of emotional
transmission.
-
- Yawning emerged as the most consistently
reactive expression, with responses occurring
across variable temporal windows. Intra- and
inter-participant variability was evident in
both response modality and intensity. While some
individuals attempted to suppress their
reactions (e.g., covering their mouth), most
exhibited attenuated yet observable facial
responses. Whether yawning reflects a purely
automatic motor response or carries an emotional
component remains an open question.
-
- Yawning was the most frequently expressed
emotional state, primarily characterized by AU27
(mouth stretch) at varying intensities.
Suppressed yawns involved AU9 (nose wrinkler),
AU43 (eye closure) at multiple intensities
(AU43i-AU43iii), and AU45 (blink), with
behavioral indicators such as mouth covering
reflecting social inhibition.22 AU17 (chin
raiser), shared with mirror pain, was also
present (Figure S1). AU9, which shares
measurement parameters with AU4 (brow lowerer)
and AU10 (upper lip raiser), typically yielded
distance reductions; however, exceptions in
measurements 54-36 and 50-32 showed increases
due to co-activation with AU27 (Table S2; Figure
S3). Isolated AU27 activation produced
widespread distance increases characteristic of
traditional yawns, while combined AU27 and AU9
activation resulted in smaller values defining
suppressed yawns. Yawning exhibited greater
facial expressiveness than mirror pain, with
wider interquartile ranges and participant
variability. AU43, defined by the same distances
as AU7 (lid tightener), demonstrated marked
reductions consistent with full eye closure,
while AU27 produced substantial increases in
mouth-related distances, validating these
metrics for distinguishing yawning from laughter
and mirror pain.
-
- Yawning was the most distinctly
differentiated emotional state, as reflected in
the median and interquartile range analysis as
well as in the Conover test. This trend was
evident across discriminative facial features,
including distances 50-60, 54-56, 53-57, 51-59,
52-58, 63-67, 62-68, 64-66, and 55-46 (Figures
S8 and S9). Laughter also displayed unique
discriminative markers, with distances 54-36 and
55-46 and, also, 61-65 and 55-49 (Figure S11)
yielding statistically significant p values in
comparison to other emotional categories,
thereby reinforcing their relevance in
identifying laughter expressions. In the case of
mirror pain, inner mouth distances proved
particularly effective for differentiation,
especially when contrasted with yawning and
laughter. Conversely, outer mouth distances
showed stronger discriminative power when
compared to yawning alone. These findings align
with earlier analyses indicating that vertical
mouth metrics are good discriminators, not only
for yawning, but also for laughter and mirror
pain. Overall, the features identified through
the BWV analysis have confirmed these features
can distinguish better than other facial regions
between the targeted emotional states.
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