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Our daily human life is filled with a myriad of joint action moments, be it children playing, adults working together (i.e., team sports), or strangers navigating through a crowd. Joint action brings individuals (and embodiment of their emotions) together, in space and in time. Yet little is known about how individual emotions propagate through embodied presence in a group, and how joint action changes individual emotion. In fact, the multi-agent component is largely missing from neuroscience-based approaches to emotion, and reversely joint action research has not found a way yet to include emotion as one of the key parameters to model socio-motor interaction. In this review, we first identify the gap and then stockpile evidence showing strong entanglement between emotion and acting together from various branches of sciences. We propose an integrative approach to bridge the gap, highlight five research avenues to do so in behavioral neuroscience and digital sciences, and address some of the key challenges in the area faced by modern societies.
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Neuroscience and Biobehavioral Reviews 131 (2021) 806–833
Available online 18 August 2021
0149-7634/© 2021 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license
Bridging the gap between emotion and joint action
Marta M.N. Bie´
*, Andrii P. Smykovskyi
, Temitayo Olugbade
, Stefan Janaqi
Antonio Camurri
, Nadia Bianchi-Berthouze
, Mårten Bj¨
, Benoît G. Bardy
EuroMov Digital Health in Motion, Univ. Montpellier IMT Mines Ales, Montpellier, France
UCLIC, University College London, UK
Casa Paganini-InfoMus, DIBRIS, Univ. Genoa, Genoa, Italy
KTH, KTH Royal Institute of Technology, Sweden
Socio-Motor interaction
Joint action
Affective computing
Machine learning
Articial intelligence
Multiple timescales
Multi-modal propagation
Models of human behavior
Our daily human life is lled with a myriad of joint action moments, be it children playing, adults working
together (i.e., team sports), or strangers navigating through a crowd. Joint action brings individuals (and
embodiment of their emotions) together, in space and in time. Yet little is known about how individual emotions
propagate through embodied presence in a group, and how joint action changes individual emotion. In fact, the
multi-agent component is largely missing from neuroscience-based approaches to emotion, and reversely joint
action research has not found a way yet to include emotion as one of the key parameters to model socio-motor
interaction. In this review, we rst identify the gap and then stockpile evidence showing strong entanglement
between emotion and acting together from various branches of sciences. We propose an integrative approach to
bridge the gap, highlight ve research avenues to do so in behavioral neuroscience and digital sciences, and
address some of the key challenges in the area faced by modern societies.
1. General introduction
We thrive on being surrounded by others; we wither when isolated
(Baumeister and Leary, 1995). The quantity and the quality of our social
interactions are one of the most robust predictors to both how well and
how long we live, beating the predictive power of exercise or obesity
(Holt-Lunstad et al., 2010). Our brains have been carved by evolution to
act together with others towards long-term mutual goals, by emergence of
‘self-transcendental emotions as opposed to immediate and egoistic
benets (Stellar et al., 2017). These emotions (i.e., compassion, gratitude,
or awe) exclusively promote coalitional behavior such as caretaking,
cooperation and coordination. For these reasons, moving in unison with
others is the rmest of social ties, a superglue, pushing us as a group to-
wards more ambitious goals and performance outcomes as opposed to
acting as individual units (Duranton and Gaunet, 2016; Hasson et al.,
2012; Marsh et al., 2009; Salmela and Nagatsu, 2017; von Zimmermann
and Richardson, 2016). Army drills based on marching together bring a
feeling of afliation and facilitate group performance (McNeill, 1995),
just like the Haka dance in a rugby match pumps up the morale before
meeting the enemy and perceptually diminishes the strength of the rival
ement, 2017; Fessler and Holbrook, 2014). We appraise being a
member of a larger group (‘tribe) by aligning our actions with others
(Tsai et al., 2011) to show we like them, we care about them and we are
ready to work with them (Mogan et al., 2017; Parkinson, 2020). Ac-
cording to anthropological and behavioral research, a group that chants
and dances well together also hunts well (von Zimmermann and
Richardson, 2016). Throughout centuries, political and religious power
holders have engaged crowds during rallies by using repetitive gestures or
vocal expressions (Heinskou and Liebst, 2016; Lukes, 1975). Such rituals
(for instance the Nazi salute during May Day rally, cf. Allert, 2009) put
normative pressure on individuals with the purpose of bringing up a
certain collective thought and feeling, captured by a classical, yet poorly
understood in neuroscience, sociological concept of ‘collective efferves-
cence(Liebst, 2019; Pickering, 2020). The social morphology of being in
a crowd is viewed as essential for motor and emotional synchrony
through entrainment of rituals (Borch, 2015; Collins, 2004) with the
emergence of contagion hot spots (‘transmitters) being the essence of
collective effervescence (Liebst, 2019; Zheng et al., 2020).
* Corresponding authors at: EuroMov, Universit´
e de Montpellier, 700 Avenue du Pic Saint Loup, 34090, Montpellier, France.
E-mail addresses: (M.M.N. Bie´
nkiewicz), (B.G. Bardy).
Contents lists available at ScienceDirect
Neuroscience and Biobehavioral Reviews
journal homepage:
Received 16 April 2021; Received in revised form 8 August 2021; Accepted 13 August 2021
Neuroscience and Biobehavioral Reviews 131 (2021) 806–833
Surprisingly, although our social interactions are highly intertwined
with emotions we convey or receive, emotions and joint actions are
primarily analyzed and modeled by different branches of science, and
are usually described separately from each other (Salmela and Nagatsu,
2017). Most models of emotion are individualistic and do not explicitly
consider the social context of interacting with others, recently identied
as the ‘dark matterof modern neuroscience (Schilbach et al., 2013).
Exceptions exist in research on empathy (e.g., the model of therapist -
patient relation by Koole and Tschacher, 2016), and in research dedi-
cated to movement expression and propagation in arts, such as musical
ensembles or dance performance (e.g., Alborno et al., 2015; Basso et al.,
2021; Camurri et al., 2011; Chabin et al., 2020; Jola et al., 2013).
However, the acting togethercomponent has not yet been tackled
(Butler, 2017; Goldenberg et al., 2016; Mayo and Gordon, 2020).
Reciprocally, human synchronization models, which dominate the
joint action research, are not usually inclusive of the emotional state of
agents and the manifestations of emotions or other affective components
in the way agents move to achieve an optimal outcome (Wallot et al.,
2016a; Wolpert et al., 2003). This urgency to address emotional social
interaction aspects was recently recognized by Shamay-Tsoory et al.
(2019) in their social alignment model, which incorporates the
emotional component of group behavior. The scarcity of research is
quite surprising, taking into consideration that both self-transcendent
emotions and cooperation evolved together as strong features of hu-
manity and contributed greatly to human dominance in the animal
kingdom (McNeill, 1995).
In this paper, we zoom into a large body of literature in order to give
a synthetic review of the current state of the art on emotion and on joint
action, currently two separate strands of research
. We present evidence
showing how these elds are intertwined, and in need of marriage to
create a more interdisciplinary and ecologically relevant branch of sci-
ence. Understanding how we feel impacts the way we act together, and
how we act together impacts our emotions, is a societal and scientic
challenge of the utmost importance (Barrett, 2017a; Feldman Barrett
and Finlay, 2018; Salmela and Nagatsu, 2017; Shamay-Tsoory et al.,
2019; Wallot et al., 2016b; Wolpert et al., 2003). As posited by Salmela
and Nagatsu (2017), we argue that the emotional component can reveal
a lot about the prediction of actions of others, but also about the
unfolding of joint acts and their outcomes. We propose a pathway for
updates of current models on joint action, such as synchronization, that
could be a start to a more integrative approach. We intend to encourage
the scientic community to join this conversation, by prompting po-
tential research questions, revise current models of emotion and joint
action across neuroscience, computer sciences and social sciences, and
to move towards more integrative, social approaches (e.g., Hasson et al.,
2012; Hoemann and Feldman Barrett, 2019).
Throughout our review, and in the future directions Section 5, we
show evidence that witnesses how the stem of this new research avenue
is now shaping the future of human machine interfaces (e.g., robotics,
interactive art systems, embodied social media). We believe this new
avenue will lend itself to informing occupational health; in promoting
efcient and human-friendly working environments and workows (be
it digital or physical) - on a micro-scale, but also crowd management
during sport, public and emergency events - on a macro-scale.
2. Joint action: humans act together
2.1. What is joint action?
Joint action can be regarded as any form of socio-motor interaction
whereby two or more individuals coordinate their actions in space and
time to bring about a change in the environment. Joint action depends
on the following mechanisms: joint attention, representations of others,
action prediction and coordination, as well as awareness of oneself and
the outcome of the actions of others (Sebanz et al., 2006). Acting
together can be emergent or planned and it encompasses the level of
intentions, action plans and movements (Knoblich et al., 2011). There is
a great variety of terms used by scientic communities in reference to
the phenomenon itself: joint action (Sebanz et al., 2006), interpersonal
coordination (Mayo and Gordon, 2020; Vicaria and Dickens, 2016),
interpersonal adaptation (Burgoon et al., 1995), nonverbal adaptation
(Bodie et al., 2016) or even social interactions (Hasson and Frith, 2016).
Furthermore, scholars have often offered narrow typologies for joint
action subclasses such as physiological or behavioral synchrony and
behavioral matching (Mayo and Gordon, 2020), interactional synchrony
and behavioral matching (Bernieri and Rosenthal, 1991), mimicry and
imitation (Chartrand and Bargh, 1999). Differences in the terminology
used by researchers usually stem from stressing and developing models
of a particular aspect of socio-motor interaction. For example, Jarrass´
et al. (2012) distinguished divisible, interactive and antagonistic tasks,
focusing on the objectives and roles each interactant follows (see
Table 1). Similarly, Clark (1996) and Toma (2014) both accounted for
the temporal relationship of interactions and concentrated on percep-
tuomotor aspects of interaction during communication. Burgoon et al.
(1995) proposed a more holistic framework of different dimensions (e.
g., directedness, timing, measure of behavioral change, intentionality,
etc.) to study dyadic adaptation (see Fig. 1).
What remains uncertain in this classication is the required number
of characteristics that need to be satised for interaction to be regarded
as joint action, and its sub-level type, and whether the individual change
occurs prior, during or after the interaction (Burgoon et al., 1995).
Importantly, joint action does not automatically imply cooperation; it
can also indicate competition in terms of individual performance within
the group, depending on whether action goal is driven by individual
gain (be better as a unit than others) versus collective gain (be better as a
group than others) (Tuomela, 2011), leading to multiple possible pat-
terns of coordinated dyadic and group joint action. Table 1 presents an
overview of the major sub-types of socio-motor interaction discussed
further in this review.
2.2. Models of human group synchronization
As a specic branch of joint action research, the synchronization of a
group of agents - such as humans and other animals or by robotic or
digital agents, all underlying the achievement of a common goal, is a
robust example of dismissing the emotion component during socio-
motor interaction. The state of the art in this domain spans over
several scientic disciplines including ethology (Couzin et al., 2005)
cognitive and movement neurosciences (e.g., Alderisio et al., 2016),
robotics (Iqbal and Riek, 2019) and various branches of mathematics
and physics (e.g., Ott and Antonsen, 2017; Strogatz, 2004). Synchroni-
zation phenomena have been investigated between individuals, ranging
from N =2 (e.g., Noy et al., 2011, to 710, e.g., Alderisio et al., 2017) to
N>10, such as in human crowds, (e.g., Gallup et al., 2012; Rio et al.,
2018). Simply stated, synchronization from a physical principle requires
two conditions to be met, (1) a certain behavioral proximity of the
systems to be synchronized, such as a common movement amplitude or
frequency, and (2) a coupling function between them, through for
instance informational exchanges. Typically, metrics for synchroniza-
tion built on frequency (e.g., the relation between individual and group
frequencies), phase (e.g., the order parameter of the synchronization),
and their stability characteristics (e.g., Pikovsky et al., 2002), allow us to
capture socio-motor coordination characterized by periods of synchro-
nization and desynchronization (Feniger-Schaal et al., 2018; Mayo and
Gordon, 2020; Noy et al., 2011).
The generalization of synchronization principles to situations
We ran a Google search and a PubMed search exploiting core keywords
linked to emotion and acting together (for the exact terminology used for the
search dated 18/05/2020 and updated 04/04/2021 please see Appendix A) and
followed up on cross-references.
nkiewicz et al.
Neuroscience and Biobehavioral Reviews 131 (2021) 806–833
involving more than two agents remains a very recent enterprise
. In a
nutshell, models of human synchronized behaviors can be categorized in
top-down estimation models and in bottom-up self-organized models. In
the rst category (e.g., Takagi et al., 2019), efcient synchronization
between connected participants sharing a common goal is ensured by
inference of the shared intention from perceived collective information
and consequent adaptation of each individual motion planning. The
second category concerns models proposing that synchronized motion
observed at the collective level emerges from local interactions between
nearby individuals. These models, such as the Kuramoto model (e.g.,
Strogatz, 2004) or the extended HKB model (for a review see Kelso,
2021) aim to decipher how local informational exchanges and motor
adaptations contribute to that emergence. For rhythmic biological
movements, coupled oscillator dynamic models have begun to explore
perceptuo-motor synchronization phenomena in situations where N>2
(e.g., Alderisio et al., 2017; Bardy et al., 2020; Zhang et al., 2019).
It is striking to observe that a single system of differential equations
(Kuramoto or extended HKB model) can capture the complexity of our
interactions in perceptuo-motor tasks, including leadership properties
(Alborno et al., 2015; Aucouturier and Canonne, 2017; Gnecco et al.,
2013; Hilt et al., 2019; Varni et al., 2010), paving the way for the
incorporation of yet missing emotional components (expressive qualities
of movement). Anticipating this trend, Varni et al. (2019) recently
proposed a Kuramoto-based model of entrainment in music performance
of an orchestra, with two components (cf. Phillips-Silver and Keller,
2012): temporal and affective. While the rst type relates to rigid tem-
poral alignments in synchronization, the second type, initially devel-
oped in childhood during socio-motor interactions with primary carers
(Barrett et al., 2007), allows for sharing emotional qualities. Thus, in
musical ensembles, entrainment is seen as a temporally exible orga-
nization between musicians, with successive periods of synchronization
stronger at the beginning and at the end of each musical phrase
compared to the middle portion (Yoshida, 2002) creating space for
the unfolding of expressive performance cues.
The models of perceptuo-motor social synchronization reviewed
above are only a sample of a wide state of the art across several elds of
research, and additional branches, for instance social contagion models
(Farkas et al., 2002; Mann et al., 2013; Ugander et al., 2012), have been
omitted. However, it is striking that none of these models have addressed
the various emotional qualities that are intrinsic to our efcient and
socially relevant cooperative actions. Emotional qualities have occa-
sionally been manipulated (Zhao et al., 2015) or measured (Zhang et al.,
2016) during dyadic interaction, although not to our knowledge when
N>2, and have not been considered in the various formalisms. In Sub-
section 5.2, we will propose new directions to incorporate these
emotional qualities in joint action models of social synchronization.
Fig. 1. Levels of joint action (after Burgoon et al., 1995); 1. (Bernieri and Rosenthal, 1991); 2. (Chartrand and Bargh, 1999); 3. (Argyle and Cook, 1976);
4. (Cappella and Greene, 1982); 5. (Altman et al., 1981); 6. (Gouldner, 1960); 7. (Giles et al., 1973); 8. (Roloff et al., 1988); 9. (Patterson, 1983);
10. (Hale and Burgoon, 1984); 11. (Bavelas et al., 1986).
In this article, we are not reviewing experiments and models of collective
motion in the animal kingdom such as bird ocks, sh schools, or reies
synchronized ashes, although some of them have inuenced emergent models
of human synchronization (see Clark, 1997; Frijda, 2007).
nkiewicz et al.
Neuroscience and Biobehavioral Reviews 131 (2021) 806–833
2.3. Neural origins of acting together
Moving together with others, as discussed in the previous section,
results from the coupling between dynamical systems sharing the same
space at the same time. Any stable and adaptive behavior emerges as the
resultant interaction of internal dynamics and the physical and infor-
mational structure of the environment in which the dynamical systems
evolve (Gibson and James, 1979; Warren, 2006). Agents learn how to
act in the most efcient manner, by searching, for instance, for the most
stable attractors (Thelen and Smith, 1994), or by exploiting optimal
control (Todorov and Jordan, 2002). This internal self-organization of
agent systems in response to external demands is fueled by previous
experiences and genetic make-up (e.g., Hainsworth, 1989). Perception
naturally plays a key role in delivering information to the brain about
the environment and enabling us to efciently maneuver in it. The in-
vestigations into further neural underpinnings of human joint action can
be divided into two main areas of research that we herein review in turn:
(i) the ability to imitate and understand movement intention in others,
and (ii) the synchronization of brain activities during multi-agent
2.3.1. Understanding movement intention in others
The cognitive ability to extract meaning from the perceived actions
of others shapes how we cooperate and communicate with them (Riz-
zolatti and Craighero, 2004). Primates and other social animals, such as
birds, are hard-wired to copy the behaviors of group mates (Heyes,
2021). Invasive brain recordings in macaque monkeys identied the
neural cornerstone of this ability to visuomotor neurons in the premotor
cortex V5. Commonly referred to as mirror neurons, they discharge both
when a monkey observes another monkey or another human performing
an action, as well as when the monkey performs the action itself (i.e.,
eating a peanut). Direct single cell recordings are rare in human
research, but scarce evidence suggests the existence of similar neurons in
the human brain. The supplementary motor area and hippocampus were
identied as active during both action observation and action execution,
such as reaching (Iacoboni et al., 2005). Interestingly, parts of this
network were shown to be active only during observation, indicating an
inhibitory role for releasing the imitation response, and other areas
Table 1
Summary of distinct features of different patterns (subtypes) of socio-motor interaction extracted on the basis of literature review
in Section 2.
Note. Selected features (rst column on the left) were identied as informative for understanding the subtle differences between
different modes (patterns) of socio-motor interaction. Icons at the bottom depict approximations of developmental stages: babies;
toddlers; preschool and school age; teens and young adults.
nkiewicz et al.
Neuroscience and Biobehavioral Reviews 131 (2021) 806–833
discharged only during execution, suggesting a dissection of this
network, referred to as the mirror neuron system (MNS) for action
observation (perception) and execution (action). Here, it needs to be
noted that the mirror neurons represent an undened percentage of all
neurons in the network for action observation and execution (di Pelle-
grino et al., 1992), and consequently the validity of using the term MNS
itself can be contested; Mirror neurons contribute to, but do not domi-
nate, complex neural networks involved in high-order understanding of
action (Heyes and Catmur, 2021). There is an emerging consensus that
mirror neurons play a key part only in the processing of low-level fea-
tures of action for recognition and discrimination, such as body move-
ment topography of another agent, but not of higher-level features such
as intention or goal reading (Thompson et al., 2019). Ramachandran
(2000) already suggested that the human leap in evolution the creation
of unique aspects of human culture such as language and arts is
endowed to the MNS, as it fosters knowledge transfer between in-
dividuals through imitation. Very recently, Heyes (2021) pointed out
that imitation is shaped by development and culture, as newborns do not
spontaneously imitate but learn to do so via sensori-motor learning,
which promotes transformation of neurons to mirror neurons. Emotion
and the reward-processing circuitry (the medial orbitofrontal cortex/-
ventromedial prefrontal cortex) were shown to be linked to the obser-
vation of being imitated by another (Kühn et al., 2010), an important
hint for the composition of the social glue that acting together promotes.
Some authors (Becchio et al., 2012; Centelles et al., 2011) attribute
to MNS, in conjugation with the mentalizing network, the ability to
distinguish individual movements from those that are socially con-
nected, and to understand the personal relevance of the movements of
others (Kourtis et al., 2010). Exaggeration or modulation of kinematics
in order to convey (socially relevant) intention is often referred to as
sensori-motor communication (Pezzulo et al., 2013). For example, ki-
nematic differences in the proximal and distal parts of body movements
provide pivotal information about whether someone is reaching for a
cup to take a sip or to pass it on, and allow us to make adequate pre-
dictions and react accordingly before the end of the movement (Ansuini
et al., 2014; Cavallo et al., 2016; Soriano et al., 2018). Regardless of the
not-yet-fully-understood contribution of mirror neurons themselves in
the network, the MNS acts like a bridge between rst-and-third-person
experiences, allowing for replay of cognitive representation, but
safe-guarding the correct placement of the self (Gallese et al., 2004;
Meltzoff and Decety, 2003).
2.3.2. Synchronization between brains in shared space
The moment one person interacts with another person in a shared
space, we can no longer analyze those entities separately, not only in
terms of motor output, but also in terms of brain activity (Balconi and
Vanutelli, 2017). Eye-to-eye contact engages language production and
reception areas inviting social expression and engagement (Hirsch et al.,
2017). Over the last two decades, the state of the art in social neuro-
sciences has indeed shown considerable evidence that our brain is
entrained by the structure of physical interaction in the same way it is
entrained by the activity of another brain when such interaction is of
social nature (Hasson et al., 2012). During social interactions our brain
activity is coupled with that of others (Hari and Kujala, 2009), by
viewing the same content (Hasson et al., 2004; Nummenmaa et al.,
2012), by movements (Basso et al., 2021; Dumas et al., 2010), and even
by speech (Hasson et al., 2012; Jasmin et al., 2016). Discussed tech-
niques, referred to as ‘hyperscanning (as more than one brain is being
recorded) were specically employed to look at dyads imitating mean-
ingless gestures, and they identied the alpha-mu band as critical for the
coordination of interpersonal dynamics, with asymmetrical patterns in
brain activity reecting the imitator-model roles (Dumas et al., 2010). In
another context, two particular indexes were identied within this band
in the centroparietal brain area - phi
, phi
; and were linked to the co-
ordination of individualistic versus cooperative behaviors in dyads,
translating into inhibition and enhancement of MNS activity (Tognoli
et al., 2007). Importantly, the multi-person framework of EEG research
has also started to address the niche of social affective interaction
(Acquadro et al., 2016). For instance, more pronounced brain-to-brain
synchrony (measured with EEG) was found in school classmates
(N>2) sharing attention and engaging in face-to-face interactions
(Dikker et al., 2017). Also, Babiloni et al. (2012) found that people with
high empathic disposition in saxophone quartets (N>2) had higher
alpha desynchronization in the BA 44/45 Broadmanns area during the
observation of video recordings of their performance as a musical
ensemble. We view this and other recent studies (Chabin et al., 2020;
Czeszumski et al., 2020) as promising attempts to investigate the
neurophysiology of embodied emotions during joint action.
2.4. Social benets of acting together
Acting together has been shown to have profound psychosocial
consequences, with evidence coming from studies looking primarily at
dyads and to lesser extent, groups. The story of the interplay of emotion
and synchrony starts in the most primal context of interpersonal re-
lationships the dyad composed of an infant and a mother. Human in-
fants have no capacity to survive on their own and need a primary carer
to regulate their physiological balance (allostasis) (Atzil and Gendron,
2017; McEwen, 2000; Van Der Veer, 1996). Emotionally receptive
parents cradle their infants on the left side of their body allowing the
ow of visual and auditory information to travel directly to the right
hemisphere empathy circuits (Malatesta et al., 2019). Multi-modal
channels of bidirectional, physiological concordance with caregivers
were identied in infants as early as three months old via heartbeat rate,
pupil size mimicry, vocal and affective non-verbal expression (Aktar
et al., 2020; Feldman et al., 2011; Palumbo et al., 2017). Further,
physiological and movement couplings were found to emerge only when
the infant is unsettled, resulting in the parent reducing their own arousal
to stabilize the infant (Wass et al., 2019). These rst experiences of
synchrony were identied to be a cornerstone for healthy emotional
development (Feldman, 2012). Parental mirroring allows children to
learn their own emotional responses, recognize and label them (Atzil
and Gendron, 2017; Pratt et al., 2017). Motor synchrony fosters the
building of rst trust relationships and prosocial behaviors in human
development as early as 14 months, when infants are more likely to pick
up a toy dropped by a stranger who bounced with them in synchrony a
moment before (Cirelli et al., 2014). Later in development, from
toddlerhood to teenage years, other aspects of social cognition and
self-regulation are carved by play experiences with caregivers and peers
using those very foundations (Nijhof et al., 2018; Viana et al., 2020;
Williams et al., 2020). Play promotes learning by imitation in the animal
kingdom, through discovering how to act with others, and understand-
ing what others feel (Feldman, 2007). This is a bedrock for empathy and
social interactions in humans (Donohue et al., 2020; Viana et al., 2020;
Xavier et al., 2016), and perhaps is one of the reasons why ability to
synchronize movements with others continues to develop until adult-
hood (Su et al., 2020b).
Moving in unison thus acts as a social glue and reinforces coopera-
tion (Hoehl et al., 2021; Wiltermuth and Heath, 2009), with a sense of
afliation between strangers (Cacioppo et al., 2014; Kragness and Cir-
elli, 2021) and boost in self-esteem (Lumsden et al., 2014), arising even
from very simple movements such as synchronous nger tapping (Hove
and Risen, 2009). The social cohesion instigated by this motor syn-
chronization (Lakens and Stel, 2011) is part of a virtuous cycle and
improves actual performance on subsequent joint action, by increasing
the perceptual sensitivity of agents towards changes in the environment,
those related for instance to the movements of others (Valdesolo et al.,
In that way, motor synchronization seems to be a currency for our
social likes and dislikes. Interpersonal attractiveness and likeability of
an interaction partner is linked to the magnitude of effort in coordi-
nating with them (in terms for instance of the relative phasing of our
nkiewicz et al.
Neuroscience and Biobehavioral Reviews 131 (2021) 806–833
body movements). When engaged in synchronization, we try harder
with the person who seems happy and non-threatening (Zhao et al.,
2020), or who we nd attractive (Zhao et al., 2015). If an interaction
partner makes a bad rst impression, the chances are we will not put in
our best to align our actions with them, as demonstrated by Miles et al.
(2009) in a study looking at dyadic synchronous stepping. This relation
is bidirectional, as moving in unison fosters interpersonal attractiveness.
For instance, Cheng et al. (2020) found that during paired walking tasks
a phase synchronization time ratio (how much time people walked in
phase with each other) was predicted by how much they liked a stranger
based on their initial impression. After a period of silent and ‘chatty
walking, strangers reported increased afliation with the person as a
consequence of synchronous walking. Atherton et al. (2019) reported
decrease in prejudice towards another ethnic group after physical and
imagined walking.
Even if people are not ‘on the moveacross space, their bodies show
gradual convergence towards postural alignment over the course of
almost any dyadic interaction (Chartrand and Lakin, 2013; Fujiwara
et al., 2019; Paxton and Dale, 2013). In human psychotherapy, which
aims at aiding any shortcomings in emotional regulation in adulthood,
therapists usually build a ‘trustrelation (therapeutic alliance) via the
above-mentioned elements of speech synchrony, as well as body
movements (Bar-Kalifa et al., 2019; Lutz et al., 2020), to access implicit
and explicit emotional regulation of the patient (Koole and Tschacher,
2016). For instance, head position synchrony between therapist and
patient has been found to be linked to the overall therapy outcome,
whereas body alignments were noted on shorter timescales to be pre-
dictive for each session outcome (Ramseyer and Tschacher, 2014) as
well as experience of pain with a therapist (Goldstein et al., 2020).
Weaker coordination patterns of head motion (angular displacement
and velocity of the heads yaw and pitch) were also reported during
conversations involving arguments between romantic partners
(Hammal et al., 2014) and were proposed to be predictors of the quality
of rapport with a therapist (Goldstein et al., 2020). Interestingly, people
who live together, and who are hence in permanent dyadic interaction,
such as roommates or couples, become alike, over time, in terms of their
emotional reactivity and emotional expressions (Anderson et al., 2003).
During conversations adults show convergence in use of certain speech
elements such as gures and grammar (Hasson et al., 2012). Joint
speech research revealed that merging ourselves with others is visible in
the neural activation patterns that are different from speech production
alone (Jasmin et al., 2016), putting joint speech in the realm of dynamic
interplay of socially shared cognition (Cummins, 2014).
In a study on large groups (N>5), von Zimmermann and Richardson
(2016) demonstrated that synchronous vocalization with others en-
hances memory performance and group effort, providing evidence for
hidden wisdom of group rituals such as dancing, or singing, or when
marching to ght a rival. Similarly, physiological concordance emerging
between newly met group members (i.e. intervals between heart beats)
explained one sixth of the variance of the performance in a drumming
task in another group study (Mayo and Gordon, 2020). Both physio-
logical and motor synchronization levels were predictive for the sub-
jective sensation of group cohesion. Also, a study by Mønster et al.
(2016) looked into the synchronization of bio-signals (heart rate and
electromyography of face muscles) during cooperation (line--
manufacturing of paper boats by triads). Participants were induced
emotionally by the researcher acting either warmly or coldly in the
interaction with the group. Exchange of smiles was linked to group
cohesion (primed with the warm behavior from the experimenter),
whereas synchronous increased skin conduction (interpreted as experi-
encing tension in a group caused by coldness in the demeanor of the
experimenter) correlated negatively with measures of cohesion.
In sum, the motor and physiological coupling between humans is
hardwired, and develops through childhood, to deploy group afliation
(we are members of the same tribe) and maximize collaborative efforts.
Developments in dyad research have provided ample evidence for the
social benets of interpersonal alignment, with group research still
being under-addressed. Therefore, the future of neuroscience of social
interactions needs to include not only a second-person perspective
(Schilbach et al., 2013) but also a multi-person or multi-agent
perspective, and it constitutes the raison dˆ
etre of our review.
3. Emotions
The previous section synthesized the current state of the art in the
main branches of sciences investigating how we act together, from
physics-based models of synchronization to social neuroscience in
humans and other animals, embodied cognition and developmental
psychology. While we demonstrated that emotion is not a dominant
focus to understand how people move in a group, we also mentioned
some burgeoning research incorporating emotional qualities and eval-
uating how they affect embodied social interactions. In the current
section, we reciprocate with a rst analysis of current models of
emotion, how they largely ignore joint action and the above models,
with some recent exceptions paving the way for a more integrated
3.1. What is an emotion?
Since Jamesattempt to answer the question What is an emotion?
(James, 1884), an unresolved yet debate started on the inherently
elusive nature of the phenomenon (Scherer, 2005), highlighted by Fehr
and Russells (1984) remark that everyone knows what an emotion is,
until they are asked to give a denition (p. 464). Four decades ago,
Kleinginna and Kleinginna (1981) found 92 different denitions of
emotion and stressed the need for consensus. Despite substantial ad-
vancements, there is not a unied theory of emotion that would
exhaustively address all the fundamental questions (Reisenzein, 2015).
Interestingly, the etymology of the word "emotion" contains in itself
"motion" or emovere, in Latin "to move", and the word "affect", more
general, which relates to "producing changes" (2020
). Arguably, a
straightforward example of emotion as a driving force of change comes
from the so-called ght-or-ight response (Cannon, 1953). When scared,
we prepare to run away in order to withdraw ourselves from the
perceived source of danger, and when angry, we prepare to stand up and
ght against the threat (Cannon, 1953; Jansen et al., 1995). Stemming
from this perspective, emotional arousal holds a motivational function
(Reisenzein, 2015), previously conceptualized as a mode of ‘action
readiness(‘Ur-affekte; Kafka, 1950). Emotions allow us to adapt to a
given set of circumstances with the aim of survival and enhancing
wellbeing (maintaining allostasis), thus they entrain different action
tendencies to satisfy different needs (Frijda, 2007; Frijda et al., 1989;
Frijda and Parrott, 2011; Ridderinkhof, 2017), weighed up by cognitive
processes against individual cost/gain and previous experience (Ferrari,
2014; Kiverstein and Miller, 2015; Padoa-Schioppa, 2011).
Classically, there were two main conceptual frameworks for studying
emotion: (i) discrete models, representative of individuals emotions (e.
g., anger, joy, fear, etc.), pioneered by Darwin (1872) and later devel-
oped, for instance, by Izard and Carroll (1971); and (ii) dimensional
models, with a specic positioning on the coordinate system for which
an emotion is an interplay between different dimensions (e.g., valence,
or arousal) acting as coordinates (Plutchik, 2001; Russell, 1980) or
categories linked by smooth gradients (Cowen and Keltner, 2017).
Recent models (e.g., Barrett, 2017a, b) propose that emotions are
emergent constructionsof the world based on interoception (signals
within milieu of the body), exteroception (signals from the environ-
ment) and previous experiences feeding in dynamic (visceromotor,
motor and sensory) predictions. In this context, emotions are not
(2020). Denition of emotion [online]. Oxford University Press. Available at:nition/wake (Accessed: 14 November 2020).
nkiewicz et al.
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demarcated events, but derivative of the constant interaction between
the complex dynamics of the nervous system, the body, and the sur-
rounding environment. We experience the world factually, but it is the
visceral reactions that validate the experience as ‘real(Duncan and
Barrett, 2007). In a Higher-Order Theory of Emotional Consciousness
proposed by LeDoux and Brown (2017), extending the high-order theory
of human consciousness to emotions, the sense of self is core to
emotional experiences. Although the sub-cortical circuitry, such as fear
or survival circuits, are crucial for providing inputs for adaptive
behavioral responses, they place emphasis on all conscious states (i.e.,
emotions) being instantiated in the general cortical network of cognition
(cortical circuits).
Recent neuroimaging studies have shown that the emotional
network is widespread in the human brain, and embraces cognitive
areas (such as anterior frontal areas) beyond typical affective ones, such
as amygdala, diencephalon and brainstem (Duncan and Barrett, 2007;
Kiverstein and Miller, 2015) and motor readiness circuitry - premotor
area (BA6) (Costa and Crini, 2011). Among other studies, Jastorff et al.
(2015) found that distinct categories of emotion emerged only when
looking at multi-voxel activity patterns in fMRI during discrimination of
visual and dynamic emotional stimuli of different saturation. During the
resting state in the MNS network, four hubs were identied as con-
necting points in the right hemisphere: anterior insula, right anterior
cingulate, right precentral sulcus and right fusiform gyrus. Four points of
connection with other structures were identied in the emotional
network: right amygdala, right insula, left putamen, and left middle STS.
Interestingly, Costa et al. (2014) demonstrated, using EEG recordings,
that emotions from different categories overlap spatially in activation
patterns with the emotional brain network, but also show distinct tem-
poral signatures (i.e. time to peak). This aligns with Barrett (2017a,
2017b) argumentation that there are no specic pathways for emotion
categories, but different neural activation can lead to the same emotion
(many-to-one) and reversely the same network can give rise to
different emotions (one-to-many) (i.e., notion of degeneracy, Edelman
and Gally, 2001).
3.2. Understanding others emotions
Despite the recent progress in emotion research, little is known about
the actual dynamic link between emotion and social interaction (Butler,
2017, 2015), and the models briey reviewed above remain largely si-
lent about the social nature of emotion. In this section, we focus on the
shareabilityof emotions, rst through the prism of empathy and
mechanism of mimicry, before addressing scarce evidence for the exis-
tence of group emotion.
3.2.1. Empathy
Broadly, empathy relates to understanding and ‘sharingwhat other
people feel, need or want to do (Bloom, 2017; Ferrari, 2014), but does
not infer action itself. If we refer to empathy as a parameter in prosocial
behavior, we mostly mean empathetic distress experiencing
discomfort caused by perception of distress in others which is
equivalent to emotional (‘hot) empathy (Bloom, 2017). However, other
possibilities of sharing an affective state include cognitive (‘cold)
empathy (conceptually understanding what another person experi-
ences), emotional contagion (i.e. ‘catchinganxiety because we share the
same physical space with someone who is anxious), and nally,
compassion leading to helping behavior and altruism (Bloom, 2017;
Preston and de Waal, 2002).
The well-known Perception-Action model (PAM) of empathy (Pres-
ton and de Waal, 2002) proposes that the perception of another agents
affective state activates the same neural representation of the observer
(without any particular ‘empathy center), leading to activation of the
same somatic and autonomic responses, an idea that was rst conceived
by Darwin (1872), later reinforced by Hommel (1998,1997). Research
on MNS has revealed that we do internally simulate the actions we
perceive (measured as shorter reaction times in consecutive execution of
action performed by another agent) (Craighero et al., 1998). For
example, whether we observe a facial expression of disgust, or imitate it,
the same parts of the brain activate as during actual experience of
disgust (Carr et al., 2003). In the same vein, observing fearful bodily
expressions in others activates the motor readiness circuitry (Borgo-
maneri et al., 2015). Human toddlers show distress when observing
others in distress (Zahn-Waxler et al., 1992), and they ‘try onemotional
reactions they observe in others to see how they feel, without particular
need of their own to be broadcasted (Einon and Potegal, 1994). This
behavior in adults, e.g., bursting out crying when someone else does,
would be considered as pathological or maladaptive, deprived of
emotional containment and dissociation between self and others.
The neural underpinnings of embodied empathy, i.e., the imitation of
emotional facial expressions, have so far been pinpointed to the right
ventral premotor cortex (Leslie et al., 2004), with the inferior parietal
lobule identied as a locus attributing a sense of agency for the self and
others (Meltzoff and Decety, 2003). The brain activation of visual in-
formation containing key action features ows from the superior tem-
poral cortex to the posterior parietal (to simulate action and code
kinematics), involves frontal MNS (to identify action goal), and ows
back to the superior temporal cortex to inform it about action prediction
and imitation plan if needed (Carr et al., 2003). In this connectivity
model the insula relays the action representation to the limbic system
from the MNS and motor areas. Although simulation is a key response
for understanding what others might feel, people are usually not an echo
chamber for other peoples feelings. The neural architecture of empathy
is complex and sophisticated, endowing a multitude of social interaction
scenarios, and is intertwined with cognition (Bernhardt and Singer,
2012; Ferrari, 2014). A double dissociation neurophysiological mecha-
nism was proposed based on lesion studies for (i) cognitive (cold)
empathy embedded in the ventromedial prefrontal areas, and for (ii)
emotional (hot) empathy rooted in the inferior frontal gyrus (Sha-
may-Tsoory et al., 2009).
3.2.2. The prism of mimicry
The motor phenomenon of mimicry, or ‘chameleon effect(Char-
trand and Bargh, 1999), overlapping in some ways but not to be
confounded with empathy (‘I felt what you feel), relates to involuntary
mirroring of expressions of our interaction partners (‘I saw and show
what you feel). Mimicry, unlike imitation and synchronization, is un-
, and at least partially independent of the performance ability
in the latter, despite clearly being nested in the same behavioral spec-
trum with strong functional interconnections (Genschow et al., 2017;
Rauchbauer and Grosbras, 2020). This comes with the caveat that
mimicry is usually recorded in a naturalistic observation, such as
matching up facial expression (rapid facial reaction) during face-to-face
contact with another person (Moody et al., 2007), whereas imitation and
synchronization are most often elicited and captured in less ecological
scenarios (with instruction). Unintentional mimicry of body and vocal
expressions unies emotional state by means of evoking the same in-
ternal responses in agents (Hateld et al., 2011, 1993), and is perhaps
the closest to the concept of emotional contagion. Neuroimaging studies
(hyperscanning mock setup for fMRI) show a similar time-locked pattern
of brain activity between people subjected to watching emotional ex-
cerpts from movies (Nummenmaa et al., 2012) and listening to auto-
biographical stories retold by interaction partner (Smirnov et al., 2019).
Adults also align their heart rate variability (Scarpa et al., 2018), pupil
diameter, respiration rate, body temperature and electrodermal activity
While for some scholars e.g., (Barrett et al., 2019b) mimicry is the process
of an unconscious copying of others postural, facial and other behaviors, for
other authors e.g., (Centelles et al., 2011; Clarke et al., 2005; Nackaerts et al.,
2012) to mimic is to intentionally imitate the behavior of the other, we us term
‘mimicryin this review in the former denition.
nkiewicz et al.
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by their mere physical presence in one space (see Palumbo et al., 2017)
for a comprehensive review on physiological synchrony). Emotional
response to others via mimicry is also mediated by oxytocin hormone
(Korb et al., 2016) which, in surplus, can increase mimicry of emotional
expressions such as demonstrated in a double-blind study on males
looking at adult and infant expressions. Similarly, Festante et al. (2020)
looked into EEG after intranasal oxytocin administration and found that
the enhanced alpha range of the mu rhythm desynchronization had an
impact on the sensorimotor circuits involved in social perception and
action understanding. Complimentary to this evidence, The Neuro-
cognitive Model of Emotional Contagion (Prochazkova and Kret, 2017)
proposed multi-modal connections between motor mimicry (facial and
body, inclusive of eye synchrony) and autonomic mimicry (physiolog-
ical synchrony). In this model, both mechanisms are considered sepa-
rately, but operate on the social interaction interface (cognition), with
MNS being an engine for shared emotional arousal and steering
empathic behavior. Also, the mimicry model by Wood et al. (2016)
proposes a decomposition of the perception-action loop into sensori-
motor layers, encompassing functions such as moderation of emotion,
prior beliefs, arousal and adaptive behavioral response. Mimicry, even
on a fast timescale under 1000 ms (as demonstrated in an EMG study of
facial muscles), is intermediated by the personsown affective state and
the environmental context (Hess and Fischer, 2013), as well as by
afliative goals (Rauchbauer et al., 2015). Interfering with facial mim-
icry (i.e., mouthguards) blocks out emotional recognition of the body
and the facial expressions of fear and happiness in others (Borgomaneri
et al., 2020; Rychlowska et al., 2014). This effect was reported to be
mitigated by individual levels of empathy; people with higher empathy
levels rely less on mimicry to recognize emotional states in others,
suggesting at least a partial functional independence of empathy from
mimicry (Borgomaneri et al., 2020). The social function of mimicry is
also associated with longer timescales. Hogeveen et al. (2015) identied
that mimicry might increase social attunement for a longer period than
initial interaction via increased mu-suppression activity in MNS.
Based on the evidence above, both empathy and mimicry are key
players in emotion propagation, interconnected at the functional level
and bridged on the neural level by MNS. However, the dynamics of
spreading motor, neural and physiological embodiment of emotions
between several agents, on short and transient timescales, as well as its
impact on joint action, short and long term, remains largely unexplored
today. One early exception is the model proposed by Kelly and Barsade
(2001), considering group emotion as resulting from individual states of
the agents (shared implicitly and explicitly), captured as an affective
tone that molds cohesion in the group and uctuates. For example,
adopting a group identity can inate saliency of negative emotions, such
as anger, through linking group-based appraisal to group emotion and
prompting pre-designated behavioral response (Kuppens et al., 2013).
One important consequence of this line of research is linked to educa-
tion, as group emotions in classrooms were reported to mediate atten-
tion sharing and learning, cornerstone of long-term academic
achievements (Eilam, 2019). To understand further the dynamics of
‘sharing emotion between multiple agents as they occupy or move in
the same space at the same time, we give an overview of the affective
embodiment research on the multi-modal signals that can carry infor-
mation about emotion qualities of the agents.
3.3. The embodiment and automatic recognition of emotions
The role played by the various layers of the moving body as both
receptacles and vehicles of emotional experiences has been largely
addressed. Here we briey synthesize research in psychology, neuro-
science and affective computing revealing how emotional qualities
emerge from multi-modal inputs (i.e., Arias et al., 2018), from face to
whole body and physiology, before showing the unanswered questions
at the heart of this review.
3.3.1. Face
As the most ancient and still most dominant area of emotion research
concerns faces (Ekman, 1992), it is not surprising that the majority of
research efforts in affective computing have been targeted to facial ex-
pressions (de Gelder, 2009). For instance, the Facial Action Coding
System (FACS) was developed to provide an objective, standardized, and
measurable coding system of emotional facial expressions (Ekman and
Friesen, 1978). Combination of facial muscle activations (e.g., raised
eyebrows, wrinkled nose, tightened lips) are differentiated as Action
Units (AU) of micro expressions i.e., instantaneous facial movements
hardly perceived by the naked eye and are subsequently identiable
as an experienced emotion (Ekman and Rosenberg, 2005). For instance,
lip-corner raising is identied as an AU12 that, among others, is asso-
ciated with joy. Thanks to progress in computational techniques and the
increase in size of facial expression datasets, raw data rather than FACS
or hybrid machine learning architectures are now used to let the
mathematical models identify the relevant muscle patterns. For a
detailed overview on the embodiment of emotions in facial expressions
please refer to Barrett et al. (2019a, 2019b). For an overview on auto-
matic recognition of facial expression see (Küntzler et al., 2021).
3.3.2. Whole body
A growing body of literature has shown that body expressions are at
least as powerful as facial expressions in conveying affect (Atkinson
et al., 2004; Hadjikhani and Thilenius, 2005; Wallbott, 1998). Studies
have shown that in certain situations or for certain emotional states, the
body is more informative than the face. For example, In the case of
incongruence between facial and body expressions, studies (Meeren
et al., 2005; Van den Stock et al., 2007) show that body posture has a
strong inuence on the perceived emotion. These ndings were also
supported by Aviezer et al. (2012). They show that evaluations made on
body expressions rather than on facial expressions lead to more accurate
assessment of the affective valence of the situation that triggered such
expressions. De Gelder (2009) added that the body does not only convey
a persons affective emotional state but also her actions and intention in
response to it. Further, it should be considered that at close distance
people may possibly rely on the face, but at a distance, where the facial
expressions are hardly perceived, the body becomes prevalent to un-
derstand and express emotions. For an overview of bodily manifestation
of emotions, please refer to Kleinsmith and Bianchi-Berthouze (2013),
Melzer et al. (2019) and Witkower and Tracy (2019). Unfortunately,
there is no equivalent to a FACs for the body. An initial equivalent
model, called Body Action Coding system (BACS), was proposed quite
recently by Huis In‘t Veld et al. (2014a, 2014b). They investigated covert
muscle activation across various body parts in the context of anger and
fear. Due to the lack of formal models from psychology and neuroscience
elds, researchers in affective computing have hence turned to other
elds for driving the design of automatic body expression recognition
models. The four factors of the Laban Notation System (effort, shape,
space, direction) (Laban and Ullmann, 1988) have provided the foun-
dation for most of the pioneering work in this direction. A multi-layered
approach inspired by Labans Effort Theory for the automated recogni-
tion of emotion in dance performance was proposed by Camurri et al.
(2003), through a computational model capturing how different dancers
perform the same choreography with different emotions. Speed and
energy showed to be correlated with the arousal dimension of the af-
fective states while an extended body was generally associated with
more positive states than a closed body posture. De Gelder and Poyo
Solanas (2021, p. 1) dened these features as middle level features, and
suggested that behaviorally relevant information from bodies and body
expressions is coded at the levels of mid-level features in the brain. A
computational framework to model non-verbal emotions was proposed
in the MEGA European project (Camurri et al., 2005) and a more recent
approach was proposed in Camurri et al. (2016). By taking advantage of
advanced machine learning architectures, there is today the tendency to
use a more agnostic approach based on the temporal sequence of row
nkiewicz et al.
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data (e.g., Wang et al., 2021) from movement (e.g., 3D position of each
joint, angles between body segments) or muscle activity sensors (e.g.,
intensity of muscle activity). Still, given the limited size of the datasets
and also the complexity of body expressions when embodied in everyday
activity, recognition performance may gain by a combination of low-
and mid-level features. By being not so directly connected to the specic
high-level semantic emotions, mid-level features provide a more func-
tional but still adaptive description of body expressions (de Gelder and
Poyo Solanas, 2021) possibly enabling computational recognition sys-
tems to be able to generalize across different contextual situations.
3.3.3. Physiology
Physiological changes in relation to affect have been for long
investigated. While pioneering work in the area of affective computing
had initially leveraged medical devices, technical advances in the low-
cost wearable sensing technology area has opened the possibility to
seamlessly set up and explore ubiquitous applications (for a survey see
Shu et al., 2018). Differently from facial and body expressions, physio-
logical changes are generally used to build systems that automatically
infer affective changes along the valence and arousal dimensions. This is
due to the lack of clear physiological patterns associated with discrete
emotions (for a review see Siegel et al., 2018). Applications for stress
and anxiety levels automatic detection are possibly the most investi-
gated areas in the computing domain (e.g., for a survey see Panicker and
Gayathri 2019). General approaches in building physiological-based
affect recognition models built on general statistical features (e.g.,
max, mean, std) extracted from the physiological responses to an
emotional event. To improve performances, more specic features are
extracted for each type of physiological signals. Heart-related physio-
logical activity is possibly the most explored metrics in affective
computing beyond galvanic skin conductance, as it appears related to
both valence and arousal. Features related to both sympathetic and
parasympathetic activities, in both time and frequency domains, have
been explored (e.g., Alberdi et al., 2016), for instance the ratio between
high and low frequencies. Beyond heart rate and skin conductance,
respiration (Cho et al., 2019), skin temperature (e.g., Goulart et al.,
2019; Wang et al., 2014), and brain signals (Alarc˜
ao and Fonseca, 2019;
Torres et al., 2020) have started to gain increasing attention, demon-
strating complementary performances. Similarly, research using elec-
tromyography (EMG) has shown evidence of muscle tension that is often
linked to anxiety (Pluess et al., 2009). In the work by Olugbade et al.
(2019), the use of EMG in concomitance with motion capture leads to
clear increase in automatic pain level recognition performances in
people with chronic pain. This is again thanks to a technology that is
more portable and acceptable for everyday use, enabling the extraction
of continuous signals, extending the possibility for measuring a large set
of statistical features, and in particular features that characterize the
variability of these signals (Cho et al., 2019). In a similar way to the
work on non-verbal modalities, there is also the tendency to use
advanced machine learning techniques that can work directly on raw
data or on low-level statistical features extracted continuously over
moving windows of the signals (Wang et al., 2021). However, this
approach is still challenged by the limited size of the available datasets.
While each modality carries emotion information, studies have
shown that multimodal recognition systems tend to lead to better per-
formances (Al Osman and Falk, 2017; DMello and Kory, 2015; Poria
et al., 2017). Since modalities work at different temporal scales as re-
sponses to emotional triggers, how to fuse such modalities has been, and
is still, a crucial question in the affective computing community. A va-
riety of fusion approaches have been considered. Solutions have
explored low-, mid- and high-level fusions, as well as more complex
architectures to fully capture the relationship between such modalities.
In particular, a typical issue in multimodal modeling is that some sensors
may only be available during the training phases of the model. This
could be due to sensor malfunctioning or sensor availability (e.g., pri-
vacy) during deployment. Some of the explored fusion approaches have
tackled such problems by learning the relationship between modalities
in order to infer the missing ones when the problem occurs (e.g., Cheng
et al., 2016; Rivas et al., 2021; Wagner et al., 2011). Transfer learning
approaches have also been used to this purpose together with addressing
the problem of limited dataset size (for a review, see Feng and Chaspari,
However, these are not the only critical questions that challenge the
affective computing community. Most of the work so far has focused on
mapping face, body and physiological features or their combination into
emotion semantic concepts. As we move into real-world applications,
such approaches are quite limited as affective experience, and its
perception, are subjective processes shaped by various factors such as
context (Barrett et al., 2019a) and personality (Komulainen et al., 2014).
Transfer learning approaches have been used to support the develop-
ment of models between for example datasets built in the lab and smaller
ecological dataset, or to compensate for the limited size of such datasets
(Feng and Chaspari, 2020). Other approaches have more specically
attempted to integrate the context directly in the model. For example,
the use of hierarchical architectures leveraging automatic human
automatic activity recognition as contextual information to body
expression recognition have shown to reach better recognition perfor-
mances and generalization capabilities across a variety of activities
(Wang et al., 2021). Such an approach was further supported by the use
of graphical algorithms that intrinsically capture body conguration
information critical to both the prediction of the activity performed and
the emotion expressed by the body. Similarly, Zhao et al. (2019) have
explored how personality can be leveraged to improve recognition
performances of personalized emotion recognition models. Using a
hypergraph learning framework, they captured the relationship between
individual personalities and physiological responses to stimuli, showing
a clear improvement in recognition rates, and suggested that the next
step would be to co-learn the personality scores of participants.
While the work above is supported by an increasing number of
multimodal and also multi-factors datasets, e.g., MAHNOBHCI (Sol-
eymani et al., 2012); DEAP (Koelstra et al., 2012); EMOPAIN (Aung
et al., 2016), ASCERTAIN (Subramanian et al., 2018), there is a real
need for larger and real-life datasets that are more inclusive, and that
can capture the variety of (social and activity) contexts and emotional
expressions. While personality surely contributes to the experience,
response and perception of emotions, there are many other personal
factors (e.g., cognitive and physical impairments) that are critical to
these processes. Existing datasets are still largely lacking the in-
vestigations of the above questions.
3.3.4. Multi-agent embodiment of emotion
One common characteristic of all the studies reviewed above, and of
the-state of the art of embodied emotion in general, is that they all,
almost exclusively, investigate the embodied manifestation of emotions
in one individual in space and time (see Niedenthal, 2007). However, as
said in our general introduction, humans are rarely withdrawn from
natural interaction with other conspecics. Particularly challenging is
the issue put forward by leaders in emotion research (e.g., Ekman, 1992)
that, arguably, the main job of emotions is to facilitate the engagement
in perceived appropriate behaviors, in situational encounters with
conspecics and others. Dyadic and group situations are not only natural
vectors of emotion diffusion, they are the instances where this diffusion
contributes to a successful communication and enhances prosocial be-
haviors. For instance, Mou et al. (2016) have shown that body behavior
nkiewicz et al.
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is a better predictor of emotion-group membership than facial expres-
sion, possibly because of the mirroring that may occur in group in-
teractions. Playing music together is one of the most signicant
examples of non-verbal human interactive, creative and social activities,
and, as music is widely regarded as the medium of emotional expression
in full body movement par excellence, it is not surprising to witness the
rst layer of research focusing on emotion transmission in embodied
joint action in this domain. For example, Glowinski et al. (2013)
compared the expressive movement of violinists when playing solo and
in ensemble conditions, and showed that when people perform a task as
part of a joint action, their behavior is not the same as it would be if they
were performing the same task alone. In the same vein, Varni et al.
(2010) showed, in a multi-modal interactive context of a violin duo and
a string quartet, that enacting pleasure while playing enhanced move-
ment synchrony, whereas enacting anger reduced it. In another study
with a triad of musicians, body sway was structured differently with
different levels of emotional expressivity during performance (Chang
et al., 2019). Higher Granger coupling within the triad of musicians
(piano, cello and violin) was linked to emotional expressions of happi-
ness when compared to sadness. Finally, the quality of dance perfor-
mance was found to benet from synchronized interpersonal
movements, a quality that was also enjoyed by the spectators (Vicary
et al., 2017). These examples illustrate the very recent move to under-
stand human emotion in the context of joint action. However, the full
picture remains obscure as we still have little to no understanding on
how emotion dynamically uctuates and propagates in multi-agent,
naturalistic scenarios (where emotion brews as a consequence of inter-
action between agents and environment, e.g., Dotov et al., 2021). Fig. 2
represents summary points from the Sections 3.13.3 showing how
emotion is intertwined with acting together.
3.4. Linkage between joint action and emotion in socio-motor interaction
Earlier (in Section 2.4), we presented evidence showing how moving
in synchrony with others can bring positive emotions such as afliation
and attractiveness. The impact of motor behavior and its shaping role for
the emotions experienced by individual agents has also been brought to
the spotlight by researchers interested in mental health and wellbeing
(Macpherson et al., 2020). Strong incentive for further exploration in
these domains comes from clinical research investigating psychiatric
conditions - neurodevelopmental ASD and severe long-term disorder of
schizophrenia (SCZ), which we now shortly review.
3.4.1. Clinical evidence from ASD studies
ASD is characterized by impaired development in terms of social
interaction in general, communication and motor behaviors (American
Psychiatric Association), with the underlying causes being still poorly
understood. The ability to share attention with others, as well as imitate
them, is pivotal for human development with the rst signs of sharing
experiences being recorded as early as in the rst year of age in typical
developing children (Kellerman et al., 2020; Tomasello, 2011). Those
two adaptive mental functions enable symbolic play later in toddlerhood
(Baron-Cohen and Cross, 1992), which lends itself to learning how to
cooperate (e.g., turn-take) and communicate with others (Nadel, 2015).
Hobson and Hobson (2007) proposed that ASD come from the difculty
to differentiate oneself from others (‘theory of mind). Fulceri et al.
(2018) reported that ASD children synchronize with others better and
imitate them more accurately (Jim´
enez et al., 2014) if the spatial goal
for their own movement is clearly demarcated. This helps to draw a
boundary with others. The evidence for the intact ability in ASD to
Fig. 2. Graphical summary of scientic ndings reviewed in Subsections 3.13.3 and referred to in further sections of the manuscript.
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Neuroscience and Biobehavioral Reviews 131 (2021) 806–833
imitate (Bird et al., 2007; Heyes and Catmur, 2021) is contradictory with
studies reporting spectrum of difculties with imitation and acting in
synchrony (Baillin et al., 2020; Brezis et al., 2017; Fitzpatrick et al.,
2017, 2016; Forbes et al., 2016; Koehne et al., 2016b; Marton-Alper
et al., 2020; Tunçgenç et al., 2021; Williams et al., 2004). Impact of ASD
on joint action during daily activities is also not clear from the scientic
literature (Cerullo et al., 2021), with reports of children with ASD
showing less interactional synchrony during naturalistic conversation
with their partners (Zampella et al., 2020). Another study looking at the
action of grasping a bottle, found that participants with ASD did not wait
for their action partner and showed prolonged movements (Curioni
et al., 2017). Trevisan et al. (2021) found that participants with ASD did
not perform as well as their typically developing peers (measured as task
performance and ability to sync steps with the interaction partner) in the
collaborative task of carrying a table. In both reports (Curioni et al.,
2017; Trevisan et al., 2021), difculties with joint action performance
were not linked to measures of other ASD-related motor decits. In
opposition to those reports, Scharoun and Bryden (2016) found no dif-
ferences in healthy controls and ASD in joint dyadic tasks involving daily
action (passing an empty glass of water to the researcher).
Complementary evidences exist in the brain neuroimaging literature.
Studies using fMRI for action imitation and observation revealed that
activity patterns for ASD in the MNS areas were altered compared to
healthy control, along with networks involved in the social cognition
and executive function (Chan and Han, 2020). Differences in neural
activation patterns with healthy controls were also found with fNIRS
during action observation and dyadic joint action in a block building
task (Su et al., 2020a). Also, a recent facial electromyography study
(Schulte-Rüther et al., 2017) demonstrated that although basic mirror
mechanisms in ASD are preserved, they do not link to the high-order
social cognition that allows emotion understanding and empathy. Dif-
culties reading facial expressions of negative emotions were reported
for young males with ASD using EEG recordings (Van der Donck et al.,
2020). Disrupted brain-to-brain coupling in ASD children was also found
in the hyperscanning fNIRS study looking at joint dyadic action of ASD
children with their parents during keypress tasks (Wang et al., 2020),
but not in the study by Kruppa et al. (2021). Linked to this topic, we
identied an fMRI study (Moriguchi et al., 2009) looking at alexithymia
in adults (self-awareness of emotions), demonstrating that people with
alexithymia have higher activation in the MNS area, therefore showing
similar difculties of differentiation between self and others (i.e., a
neural signature in the right superior parietal structure) to those found
in the ASD population. Dunsmore et al. (2019) investigated the physi-
ological linkage between interaction partners (heart inter-beat intervals)
and found that the patients suffering from ASD do not sync their heart
activity with a physical presence of another person in the room as
observed in healthy controls (Scarpa et al., 2018).
In sum, a prolic state of the art on ASD reveals differences at both
neural and behavioral levels to neurotypical peers, in the ability to share
attention during interaction with another person, to perform a joint task
together, and to read and recognize their emotions. There is a contra-
dictory evidence concerning the specic role of imitation ability in those
3.4.2. Clinical evidence from SCZ studies
Schizophrenia (SCZ) is usually diagnosed by the presence of negative
symptoms, understood as social withdrawal and emotional atness, and
positive symptoms, understood as change in behavior or thoughts due to
hallucinations or delusions. Green et al. (2015) reviewed the literature
describing the difculties with social interaction characteristics for SCZ
and summarized them as decits in empathy, reective social processing
- mentalizing, emotion regulation, face and voice perception. In parallel,
the decrease in synchronization performances in people with SCZ
compared to healthy controls was found in multiple studies, particularly
for intentional synchronization (Manschreck, 1981; Varlet et al., 2012),
predictive timing (Wilquin et al., 2018) and lack of sensitivity to social
cues facilitating the synchronous performance with others (Cohen et al.,
2017) or imitation (Sansonetti et al., 2020). In a pendulum-based syn-
chronization task, Del-Monte et al. (2013) also showed that non-affected
relatives of people with SCZ exhibit a decrease in synchronization
ability, due to compromised visual tracking, pointing toward genetic
SCZ phenotype for cognition. Interestingly, Raffard et al. (2018) found
compromised stability of synchronization in patients with SCZ, but
observed it to a lesser degree if the participants were positively primed
to improve their sense of ‘connectedness to their task partners before
performing the task. In a more ecologically set study by Kupper et al.
(2015), participants with a paranoid type of SCZ revealed broken body
synchrony patterns during dyadic role play conversation with healthy
people. Synchrony negatively correlated to the social competence and
severity of the negative symptoms, with participants not imitating the
movements of their interlocutors, regardless of SCZ medication. Positive
symptoms interacted with a lack of synchronous behavior by the healthy
interlocutor, which could display afliation, perhaps linked to the
erratic movements caused by psychotic behavior.
Neurophysiological investigations of social and emotional syn-
dromes in SCZ patients have also revealed interesting ndings in the
context of this review. For instance, searching for brain activation pat-
terns in a fMRI scanner when observing recordings of nger movement
and facial expressions, Horan et al. (2014) reported that people with SCZ
did not differ from healthy controls, but showed a disconnection be-
tween brain activation and self-reported empathy reported through the
Interpersonal Reactivity Index. In the same vein, Marosi et al. (2019)
used EEG to investigate face and facial affect recognition in people with
SCZ and revealed irregularities in the activity of the magnocellular
pathway for face and face emotion processing. In a large study
comparing MRI scans, Schilbach et al. (2016) reported differences be-
tween people with SCZ and healthy controls with regards to the con-
nectivity in the MNS network and mentalizing network (left dorsomedial
prefrontal cortex, left praecuneus, right and left temporo-parietal junc-
tion). Irregular connectivity in this area sheds light on the interpersonal
difculties that the patients with SCZ experience and on their ability to
act with others. Together, these clinical studies, along with Subsections
2.3 and 3.2, mount evidence that the MNS network might be a double
agent for emotion and acting together.
3.4.3. Interventions for ASD and SCZ
There is a great interest in using imitation as leverage for inter-
vention studies for both ASD and SCZ. In an early intervention report,
ASD children who received structured intervention focused on imita-
tion and joint attention improved their social interaction skills, such as
gaze following and requesting (Warreyn and Roeyers, 2014). Similarly,
Landa et al. (2011) ran a randomized control study looking at the
intervention targeted at imitation versus other therapy approaches
long term improvement of positive affect and joint attention. Koehne
et al. (2016a) reported benets of intervention for adults with ASD
using a dance therapy program focused on movement imitation and
synchronization which over three months improved emotion inter-
ference along with other abilities to imitate and synchronize with
others. For individuals with SCZ who participated in the therapy ses-
sions, involving imitation of others and other theory of mind compo-
nents (with a control group in a therapy focused on the problem
solving skills) improved emotion recognition from the social situation
and from the understanding of their intention of the movement (Mazza
et al., 2010).
In sum, research in ASD and SCZ (both socio-motor interaction def-
icits) shows complex relationship between multi-faceted difculties
(such as differentiation from others, poor synchrony and imitation) and
the ability to act together with others and understand their emotions
(see Fig. 3). Neuroimaging studies pinpoint differences in information
processing in those populations related to the MNS network along with
its linkages to higher social cognition and mentalizing networks.
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4. Emoting joint action with non-human agents
Consistently over the last years, research in Human-Robot Interac-
tion (HRI) and in Human-Computer Interaction (HCI) focused its efforts
on developing social articial agents that can initiate and maintain
efcient and pleasurable interaction with a human. Emotional conver-
gence benets coordination between partners (Butler, 2015), and design
studies in HCI have marshaled evidence that synchrony in movement
qualities for facial and body expressions is essential for uent human
communication with virtual agents (Castellano et al., 2010; Marin et al.,
2009; Numata et al., 2020).
Current developments in HRI are targeting real-time sensitivity to
human expressions and behavior to promote long-term relationships
(Castellano et al., 2008; Terada and Takeuchi, 2017). One of the main
challenges is the capacity of the agent to operate on fast timescales,
under one second, to be able to capture ‘social moments (Durantin
et al., 2017). For instance, the PEPPER robot can infer possible inter-
active scenarios with customers via algorithms analyzing facial move-
ment and voice signals in that time frame (Aaltonen et al., 2017). In
other studies, robots adjust the interpersonal distance as a function of
the estimated level of experienced emotions of the human in front of
them (Bajones et al., 2017). The affect control theory offers a guiding
principle used to create AI systems which are sensitive to affective states,
adjusting their operations as a function of the context and need of their
human interacting partners (Hoey et al., 2016). According to this theory,
humans engage in situations that evoke emotions and feelings corre-
sponding to ones culturally built affective span. In general, captured
data are used to infer human affective states to which social robots adapt
in various semi-autonomous ways.
Scholars in HRI have adapted and simplied the human emotional
repertoire to social robots. For instance, the ASIMO, JUSTIN and NAO
robots are programmed to express six basic emotions (and their various
combinations): anger, disgust, fear, happiness, sadness, and surprise. In
general, human participants correctly recognize all basic emotions from
the upper-body movements with a success rate of 75 %100 %, with
some exceptions however (van de Perre et al., 2015). Other robots, such
as the iCUB robot (Metta et al., 2008), crawl or semi-autonomously
manipulate objects in various dyadic contexts, and learn by doing and
imitating (Billard and Dautenhahn, 1998; Boucenna et al., 2014).
Core research activities in the eld of affective interaction with
articial agents have been established around two main populations
sensitive to affective interaction: people suffering from social disorders
(with a particular focus on children with ASD) and elderly people. Here
we briey summarize the state of the art in these two domains, and then
present recent trends in multi-agent and collaborative robotics.
4.1. Social HRI and HCI in ASD research
As it is unrealistic to expect children with ASD to continuously and
smoothly interact with affectively embodied robots, adjustments have
been made to simplify the child-robot interaction and discriminate be-
tween positive and negative emotions, particularly to launch social
interaction (e.g., Feil-Seifer and Mataric, 2011). The PROBO robot, for
instance, imitates animal movements which helps ASD children to
recognize basic emotions (Pop et al., 2013). While some robots are only
able to detect emotion displayed by humans, others, such as QRIO, also
depict facial expressions and include corresponding body manifestations
of some emotions, for instance happiness and fear, in a way that is
recognizable by humans (Tanaka et al., 2004). Another example of a
robot expressing emotions is MONARCH (Sequeira and Ferreira, 2016).
This is a companion robot deployed in childrens hospital facilities and
successfully integrated into a rich and complex clinical environment. In
the related eld of HCI, social robots are often replaced by virtual agents
designed to create a specic social relationship with their human
counterparts. Virtual and augmented realities are commonly used to
help ASD children to focus on and recognize facial nonverbal cues (Chen
et al., 2016), to learn to recognize and express emotions with their
full-body movement (Alborno et al., 2016), to learn the required social
skills (Lorenzo et al., 2019) or to promote verbal and nonverbal
communication skills via joint actions (Srinivasan et al., 2016, 2015).
Fig. 3. Graphical summary of Subsection 3.4 highlights. Images: Suspension Bridge by Pechristener; Brain, idea, mind icon from icon; Grabbing Hand by
Oleksandr Panasovskyi/Psychologist by Dirk-Pieter van Walsum from the Noun Project.
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4.2. Social robotics for the elderly
The ageing population is another major category targeted by
research on HRI, at home (e.g., Fischinger et al. (2016)) or in nursing
institutions (Moyle et al., 2013). Older people develop a wide variety of
age-related conditions that can cause their vulnerability to minor
stressor events and lead to loss of autonomy: this phenomenon is
commonly known as frailty. A number of interventions have been
developed to target negative symptoms such as loneliness, anxiety and
depression, which can also accompany dementia (Cifuentes et al., 2020;
Kachouie et al., 2014; Valentí Soler et al., 2015). One example is a NAO
robot-based rehabilitation program for people with dementia based in a
geriatric ward, which reported higher outcome scores than conventional
therapy on the immediate well-being and satisfaction (Rouaix et al.,
2017). Similarly, PARO is a robotic seal that elderly residents in nursing
homes benet from by verbally interacting with it (Moyle et al., 2013).
HOBBIT is another emotional assistive caregiving robot used at home to
prevent falling (Fischinger et al., 2016). Further, a Social Assistive Robot
exercise system was reported as more engaging the elderly in aerobic
physical activity than virtual coach (Fasola and Mataric, 2012). The
robot in Zhang et al. (2019) computes continuously the persons
movement trajectory, while assisting with their dressing, but is not
emotion aware. These are only ve examples that have been extracted
from a plethora of research and proof-of-concept studies and have
demonstrated how useful HRI and HCI approaches can be in the clinical
context, as well as at home, to accompany healthy aging.
4.3. Environments of multiple human and articial agents
When it comes to the environment being social, i.e., acting together
in a group, a modest number of studies overcome the limitation of a
robot(s) of working with more than one human, and only very few
robots are adapted to such interactions. For example, the interactive
robot KEEPON can engage in both dyadic and triadic interaction due to
emotional expressivity which aids to build joint attention with the
interaction partner, e.g., looking in the same direction as the human
(Kozima et al., 2005). Besides NAO, which is known to be able to work as
a guide in a museum for a group of visitors (Gehle et al., 2014) or with
school-aged children (Hood et al., 2015; Ros et al., 2014), and TIRO
which serves as a teaching assistant in musical classes (Han et al., 2009),
the literature on human-robot group interaction remains scarce and
almost exclusively in the form of one-to-many(a star graph as in the
guide situation) in contrast to a more generic form of many-to-many(a
complete graph, see Bardy et al., 2020). One of the unique endeavors
employed triads of BEAM robots in a semi-autonomous control mode
(Wizard of Oz) during game playing scenarios with human triads
(Fraune et al., 2019). Human participants reported changes in subjective
fear and motivation moderated by the perceived cohesion of the robot
group, in comparison to other typologies with one human versus three
robots and vice versa, and one-to-one interaction between a human and
a robot. This indicates a breadth of emotional component to be explored
in intergroup dynamics between human and articial multiple agents,
despite robots not being embodied with sensori-motor communication
abilities (embodiment of emotion). However, despite being ‘emotionally
neutral, in a study by Kochigami et al. (2018), robots NAO and PEPPER
played social roles by creating social ties between human group mem-
bers (children and adults), and successfully facilitated interaction be-
tween them. Examples of similar studies are limited in number (see
Fig. 4). Sebo et al. (2020) pinpointed key messages emerging from the
current state of the art: (i) behavior in one person to one robot does not
interpolate on the group behavior; (ii) verbal and non-verbal robot
behavior shapes the response within the group and can support cohe-
sion; (iii) people are more likely to engage with a robot when they are in
groups; (iv) similarity (anthropomorphism) to humans plays a role in the
Fig. 4. The 2D matrix of the number of agents (persons 2 and robots) in HRI interaction extracted from the literature review of Sebo et al. (2020). Examples of the
names of the robots used are placed on orange tiles with the count of overall studies identied by the researchers (refer to Table 1 in Sebo et al. (2020) for
further details).
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integration of robots in a group. The dynamics of how emotion can be
shared or propagated through the heterogeneous networks of several
humans interacting with several articial agents is unknown and has
profound implications for the future of collaborative robotics.
4.4. Collaborative robotics in the industry
Emotion sharing as a means to facilitate social interactions in HRI has so
far mostly been applied in therapeutic settings (see Subsections 4.1-4.2). In
industrial settings, however, despite the heavy reliance on industrial robots
for manufacturing, such examples are still rare. It is believed though that an
essential component of the next industrial revolution, often referred to as
Industry 5.0, will be that of the collaborative robot, a robot that can com-
plement human co-workers, performing tasks that are either tedious or
dangerous (Demir et al., 2019). The reason is because industrial sectors still
lacking in terms of automation are those that cannot be fully automated, as
they require human participation (Elprama et al., 2016). It has been
recognized, however, that introducing collaborative robots to workplaces
might have an adverse effect on social interactions in these workplaces.
Untrained personnel, in particular, tend to expect the same social signals
from robots as they would from human colleagues and expect the robots to
adhere to existing social practices (Fischer, 2019). If collaborative robots fail
to understand social signals and respond accordingly, they will be seen as
impolite, cold and uncooperative. It also represents a missed opportunity to
convey the robots capabilities, while making communication more
dependent on disruptive explicit signals, when more uent implicit signals
would have been preferable for seamless collaboration (Breazeal et al.,
2005). Fischer (2019) further argues that collaborative robots do not only
need to understand and produce social signals but that these signals need to
include emotional expression. The reason, as seen in the above sections, is
because sensori-motor communication of emotion and intention is an in-
tegral part of conventional social practice. A robot is simply expected to be
sad when delivering bad news or happy when successfully completing a
challenging task. Emotional expression may also be used to communicate
real needs, such as when the system is running out of power and needs to be
recharged. Recognition of human emotional expression under natural in-
dustrial conditions is difcult, as the technology needs to be both
non-intrusive and robust over time. Speech (Khalil et al., 2019), gaze
(Admoni and Scassellati, 2017) and facial expressions (Li and Deng, 2020)
become more convenient cues than gestures or full body movement (Liu and
Wang, 2018). However, with the introduction of cheaper wearable sensors,
emotion recognition from EEG has recently become a viable alternative
(Toichoa Eyam, 2019; Zheng et al., 2019). For expression of emotions,
collaborative robots are limited by their embodiment and interfaces have
more often been used for conveying information than for social signaling.
Most collaborative robots only rely on projections of faces on at screens to
express emotions (Kalegina et al., 2018), if such expressions are used at all.
There are recent examples, however, where the embodiment has in fact been
exploited for social signaling, even highlighting the importance of a
breathing motion (Maric et al., 2020; Terzioglu et al., 2020), opening new
emotion-based perspectives in collaborative robotics.
5. Avenues for future research
Sections 14 presented ample evidence for the interplay between
emotions and joint action. Humans are unequivocally attuned to each
other, with body movement being a powerful carrier of idiosyncratic
information (Cutting and Kozlowski, 1977; Loula et al., 2005; Troje,
2002) and socially meaningful qualities (e.g., Centelles et al., 2011;
Clarke et al., 2005; de Gelder and Poyo Solanas, 2021; Nackaerts et al.,
2012). Information about the arousal of a person encoded in movement
and their intention can be transferred to another person, for example, as
a forewarning of a threat. Being able to detect those non-verbal signals
from others, along with the ability of humans to couple their body and
brain activity, is the cornerstone of successful communication and
cooperation between people. We strongly believe that interaction is key
to understanding the human brain, as the human brain, through inter-
action with the environment, is of a physical, but also fundamentally of a
social nature (Section 4 recaps why this also applies to hybrid in-
teractions between humans and articial agents). We acknowledge, as
do other researchers, that social interaction should be at the forefront of
neuroscience research (Schilbach et al., 2013). Some recent attempts,
such as the social alignment theory, using herding modeling by Sha-
may-Tsoory et al. (2019), are providing other important milestones to-
wards this venture.
In this last Section (5) of our review, we put forward the idea that
emotional arousal should be considered as an integral part of the so-called
‘motor system, shaping and ne-tuning the real-time socio-motor interac-
tion with others. Emotions arise as responses to the stimuli in the environ-
ment (with a function to maintain/restore allostasis) and bear subsequent
impact on ones perception, affective state, ongoing and future movements
(e.g., Wood et al., 2016). Thus, we propose to incorporate emotion in a joint
action context as one entity, a ‘third eye that steers other mental and
physiological processes to navigate the rich, multi-modal layers of the
multi-agent social space. As emphasized before, the scientic evidence on
the emotional embodiment during socio-motor interaction is limited (see
Section 3.3 for overview), especially in terms of studies exploring real, not
enacted, emotional arousal with naturalistic scenarios as a backdrop. To
disentangle and decode the emotion propagation and socio-motor interac-
tion not only via movement, but also via physiological processes, we propose
to follow new research avenues (Subsections 5.15.5) to decipher the un-
knowns about interplay of emotion and joint action (see below the research
questions we have identied as a part of literature review process, continued
further on Fig. 6).
Research questions
We now know that some features of body posture and movement carry
emotional qualities (de Gelder and Poyo Solanas, 2021), but can we nd a
motor signature of emotional arousal and valence in body movements in
the context of socio-motor interaction (regardless of particular body parts)
specic to i.e., particular levels of valence or arousal? To what extent
context and culture shapes this signature during joint action? Is there a
group emotional signature emerging from individuals sharing space at the
same time (i.e., euphoria of football fans in the tribunes) (Subsection 5.1)?
How do emotions evolve and propagate through the network of agents
(humans or hybrid groups of human and articial agents) and if so, how
do they inuence the outcome of joint performance (Subsection 5.2)?
Further, we dive into the need for adoption of multiple timescales ap-
proaches, which emerged throughout this review (Subsections 2.4,
3.23.4, 4), showing that environmental function of emotion unfolds over
multiple time windows, throughout physiological and movement quali-
ties, as well as that some factors in socio-motor interaction are only
meaningful when looked through an appropriate temporal lens (i.e.,
expertise, culture, previous experience with an agent). What are then the
crucial timescales we need to integrate into the research agenda to
advance this enquiry (Subsection 5.3)? How can AI techniques assist in
this process, and unravel the patterns of information about emotio-
n/intentional qualities and its propagation in agents during joint action
(Subsection 5.4)? Finally, we dive into the world of digital, currently
disembodied interaction, and highlight the notions that recent pandemic
experience has left in terms of interplay of physical presence and emotion
embodiment during social interactions (Subsection 5.5).
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5.1. The notion of emotional motor signature in joint action
A large body of the work reviewed in Section 3.3 cements the
foundations of the embodied nature of emotions, according to which
expressions of emotional dimensions, rooted into common neurophysi-
ological structures between cognition and action, are diffusing into
movement physiology and are visible in facial, distal, as well as proximal
parts of the body (Barrett et al., 2019b; de Gelder and Poyo Solanas,
2021; Kleinsmith and Bianchi-Berthouze, 2013; Melzer et al., 2019;
Witkower and Tracy, 2019). Interestingly, this extensive literature does
not yet intersect with another parallel body of research developing
concepts and methods to assess Individual Motor Signatures (IMS), the
idiosyncratic way each individual moves (e.g., Słowi´
nski et al., 2016).
The pioneering work of Johansson (1973) showing that - trans-
formational invariants (Malcolm, 1953) persistence in some dimensions
(e.g., length, ratios) across the motion of others (e.g., global trans-
formation of the local optic ow) could help observers to quickly extract
person-related properties, is very relevant to the socio-motor interaction
context. IMS often relies on movement velocity as a key feature, as it is
both stable across time and repetitions for each individual (movement
similarity) and differential between individuals (inter-individual
movement difference). Differences in the way people move during the
performance of a motor task can be captured by using 95 % condence
interval ellipses in the similarity space (Słowi´
nski et al., 2016). This is an
abstract two-dimensional geometrical space minimizing distances be-
tween repetitions and individuals by using ad-hoc dimensional reduc-
tion techniques. Ellipses can be large or small depending on
intra-individual variability and can be close or distant from each other
depending on between-individual variability. The approach has proven
useful in identifying IMS in various populations, ranging from healthy
individuals to people suffering from schizophrenia (e.g., Słowi´
nski et al.,
2017). It has also proven useful in various tasks and contexts such as in
the mirror game (Słowi´
nski et al., 2016) or during improvisation
movement (e.g., Coste et al., 2019), and at different distal or proximal
(and more postural) parts of the body (e.g., Coste et al., 2021). Whether
IMS, {as well as Group Motion Signatures (GMS, inter-group movement
differences), i.e., the way IMS are assembled together in an ensemble of
individuals engaged in reaching a common goal during joint action}, are
emotionally neutral, and whether they are of different shapes and lo-
cations in the similarity space when produced in various emotional
contexts (intra-subject variability) remains open to investigation (see
Lozano-Goupil et al., 2021, for a rst evaluation of Emotional IMS).
Taking this road would not only answer the above questions, but would
also offer a way to reconcile existing theories of emotion and those of
embodied social interaction, inclusive of intra- and
inter-individual/group variability and concepts such as motor accents
(e.g., Ting et al., 2015), in a real-life context of a joint action.
5.2. Emotional group synchronization models
As emphasized in Section 2.2, models of perceptuo-motor social syn-
chronization when N>2 have not yet incorporated emotional qualities in
their constituents, i.e., they remain emotionally neutral, despite the evi-
dence gathered in this review that emotions are contagious, propagate
through the social network, and constitute the essence of joint action. One
urgent avenue of research requires a complementary approach incorpo-
rating emotional qualities in experimental and modeling scenarios. On the
experimental side, the manipulation of positive, negative, and mixed
emotional qualities, be they enacted or (ideally) induced, and the obser-
vation of how these qualities propagate from one node to the next across
the collective sensori-motor network, converge or conict, is requested.
On the modeling side, coloring coupled oscillator models of
synchronization with those emotional qualities would help to better un-
derstand, and generalize, the underlying propagation mechanisms. For
instance, the network of coupled Kuramoto oscillators presented in Sec-
tion 2.2, capturing group synchronization regimes when perception is
present (see Bardy et al., 2020, for details), needs to be adapted to
incorporate emotional qualities at individual levels, such as:
θl(t) =
aijsin(θjEM (t) − θiEM (t))
Where N is the number of agents, θ
the phase of the movement of
the i-th agent under emotion EM,
their natural frequency, and c the
strength of the coupling with the other agents when perceptual coupling
is established. Coefcients a
are set to a value between 0 and 1,
depending on the dyadic perceptual coupling between agents i and j, i.e.,
the spatial conguration of the group. Coloring such oscillatory models
with emotion-aware individual signatures (see Fig. 5, Example C),
nourished by experimental data, would therefore be an operational way
to close the gap between Sections 23 of the present review.
5.3. Embodied emotion across multiple timescales
In this review, we have hinted at the concept of multiple timescales
on a handful of occasions. In Greek pre-Socratic philosophy time was
represented by two notions; Chronos, sequential and linear time as we
currently understand and apply it in a metric system (chronological
time), and Kairos; which resembled the ‘righttime, especially in the
context of an action affordance (i.e., time for harvest). A myriad of
research studies has provided data-driven rationale in favor of the use of
multiple timescales to capture animal behavior and physiological pro-
cesses (be it signal duration, temporal resolution, units applied or tem-
poral dynamics). The evidence for multi-timescale behavioral
organization has been recently investigated in C. elegans; showing how
neural dynamics in this much simpler organism (slow - low frequency;
fast - high frequency) orchestrates different movements and allows for
exibility of behavior (Kaplan et al., 2019). In humans, communication
is regarded as a robust example of multi-modal behavior stretching
across multiple levels of temporal structures due to the variety of in-
terconnections between internal systems such as respiration and
movement (Bardy et al., 2015; Pouw et al., 2021).
5.3.1. What do we know about temporal aspects of emotions?
Since the works of Solomon and Corbit (1974) it has been widely
accepted that emotions unfold their dynamic over time, rather than
spike events, with complex temporal structure (De Gelder et al., 2004;
Frijda, 2007). Regardless if the stimulus is aversive or hedonic, the
response curve for high arousal physiological reactions (i.e., heartbeat)
unfurls as (i) rise to peak, (ii) adaptation period, (iii) recovery with
reversed peak to re-establish baseline within 3060 s. In a LORETA EEG
paradigm, Costa et al. (2014) found a precise pattern of neural signa-
tures of fear, disgust, happiness and sadness, with differences emerging
mostly in the temporal characteristics of neural activation, but not the
spatial spread. The differences that emerged are as follows: (i) Early
onset (around 200 ms post exposure) and shorter duration characterized
emotions - fear and disgust, which are associated with a need for quick
body reaction; (ii) Early onset (around 260 ms post stimuli) with a
second processing peak at around 400 ms in different areas - happiness;
and (iii) Late onset (around 400 ms post exposure) and longest duration
(90 ms) sadness. Personal diary study (Verduyn et al., 2015) reported
similar temporal patterns linking to the adaptive behavior evoked by
emotion, meaning that fear and disgust operate on fast timescales as
they require quick ght-or-ight reaction; whereas emotions like anger
nkiewicz et al.
Neuroscience and Biobehavioral Reviews 131 (2021) 806–833
or joy, on average, take longer to disperse through action (Costa and
Crini, 2011; De Gelder et al., 2004; Feldman Barrett and Finlay, 2018;
Frijda, 2007). Notably, sadness was earmarked as the longest in dura-
tion, perhaps because its presumed function is to pave a pathway to
rumination, motivation for change in personal circumstances or accep-
tance. Personal dispositions, from reactiveness and resilience on a
physiological level to higher cognitive functions such as emotion regu-
lation processes (reappraisal), were highlighted as subjected to
inter-subject differences (Solomon and Corbit, 1974)
5.3.2. How emotions inuence motor timing?
Distal movements in particular (i.e., object manipulation) are a
clear reservoir of emotions, a cornerstone assumption of forensic
criminology. Gao et al. (2012) analyzed movement in a touch-based
game and found (i) the length of the stroke to be indicative of the
dimensional quality of valence, (ii) speed and direction to be indicative
of arousal, while (iii) pressure specically discriminated anger from
other states (where increase in energy transmitted to movement has
functional signicance). Similarly, more frequent manipulation of the
computer mouse was found to be associated with higher stress (Her-
nandez et al., 2014). During paced synchronization, adults and chil-
dren tapped faster if they were primed with negatively valenced
pictorial stimuli before the trial (Monier and Droit-Volet, 2018). The
speeding up of motor response was interpreted as activation of the fear
circuitry evoked by negative emotional induction (LeDoux, 2014),
leading to the speeding up of the internal clock system (Cheng et al.,
2016) and shifting movement towards faster timescales. Further,
emotional arousal leads to subjective perception of time in some tasks
(Gil and Droit-Volet, 2012), making a point that time perception is tied
to the quality of stimuli (Grisey, 1987). This becomes particularly
relevant in the context of untangling the dynamics of joint action and
emotion (i.e., during synchronization).
5.3.3. What do we know about group temporal dynamics?
Vesper et al. (2011) has demonstrated that better coordination is
achieved in dyadic action when participants make themselves more
predictable (less temporally variable), in comparison to performing
identical (pointing) movement alone or next to another person without
intention to act together. Grammer et al. (1998) demonstrated that
opposite sex pairs show a complex temporal structure of interaction
patterns of body movement during conversation, which is repeated if
both sides show interest, and is unique for each dyad. In a previously
mentioned model of a psychotherapeutic alliance (Koole and Tschacher,
2016) three timescales were proposed for interpersonal synchrony,
namely; (i) a phasic time-scale, which runs from a few hundred milli-
seconds to 10 s, characteristic of motor synchrony, (ii) a tonic time-scale,
which runs from about 10 s to an hour, and involves more complex forms
of social cognition, such as language and reasoning, and (iii) a chronic
time-scale, stretching from weeks to years, and which involves the
development of complex emotion-regulatory abilities. Bardy et al.
(2020) reported that social memory (expertise in dance practice, related
to (iii)) can affect the ability to synchronize with others under different
perceptual strains. Similarly, experts in capoeira and tango had higher
ability (kinesthetic ability) to imitate and synchronize with others, in
comparison to athletes who also practice group sports, but without the
synchronization component (Koehne et al., 2016c). More broadly, Bur-
goon et al. (1995) suggested that behavioral norms can pass from one
generation to another as culture (i.e., think of the jovial behavior ex-
pected from a salesman versus the stoicism of a medical professional).
An unexplored territory is investigation of previous personal experience
with agents partaking in socio-motor interaction, which can trigger
certain emotions prior or during the execution, due to predictions of the
internal model (Barrett, 2017b).
Taken together, these ndings hint at a hidden hierarchy of socio-
motor interaction, from low-level, fast timescales, which are more
appropriate for immediate behavioral synchronization and responses, to
high-level, slower timescales, which involve complex cognition and
emotion regulation, linked to perception of emotional qualities and so-
cial memory. In this context, future research should apply a multi- (or
inter-) modal multiple timescale approach for studies set on the interface
of emotion and human joint action.
5.4. Leveraging articial intelligence (AI) methods to capture emotions in
socio-motor interactions
AI offers powerful analytical tools to deal with complex data, and
so it is a valuable method for investigating individual and group
motion signatures of emotions within the context of joint action. As
discussed in Section 5.3, a multiple timescale approach is required to
build a solid foundation of how various emotional qualities propagate
in joint action could provide a window into relative aspects of time
and how it is linked to the differentiation of emotional qualities in
movement and the opportunity to act collectively. However, AI
methods that address multiple timescales are largely limited to
encoding each temporal variable (e.g., each different modality or
modality dimension) separately. Such methods can only really create
models tuned to a single timescale per temporal variable. Perhaps the
more promising direction is methods that capture multiple timescales
within each variable itself.
The few studies in this direction (e.g., Gurcan and Nguyen, 2019;
Ma et al., 2019; Yamashita and Tani, 2008; Yang et al., 2020) have so
far been constrained to individual action modeling. Two of the few
exceptions are the multiple timescale recurrent neural network
(MTRNN) (Yamashita and Tani, 2008) and the Approach Group Graph
Convolutional Neural Network (AG-GCN) (Yang et al., 2020). Whereas
the AG-GCN was designed to model group behavior, the MTRNN was
originally developed for individual action scenario but has been
extended to dyadic (Hinoshita et al., 2009a) and group interaction
(Jaderberg et al., 2019), although only for robots in simple robot-robot
interactions and bots in a multiplayer computer game. There is not
much analysis of the behavior of the MTRNN in the multi-agent set-
tings, but a functional hierarchical structuring of events in the move-
ment sequences sampled was shown to emerge through modeling at
multiple timescales in its original use (Yamashita and Tani, 2008).
Similar ndings reported for multiple timescale AI architectures
explored in the context of natural language processing (Chung et al.,
2017) underline the value in pursuing multiple timescale modeling for
further understanding emotion in joint action. Moving forward, there
is the need for new architectures that simultaneously: 1) have path-
ways for both action and emotion recognition, drawing from neuro-
scientic ndings of multitask coding of observed action (behavior
identication and semantic interpretation) in humans (Gallese, 2007;
Iacoboni et al., 2005); 2) with multiple timescales processing for the
two, rather than for one or the other (such as in Hinoshita et al.,
2009b); and 3) capture individual as well as group signatures in group
settings. Such architectures have the potential to maximize the value of
AI for different aspects of emotion expression, perception, and prop-
agation in the context of joint action.
In the current age of deep learning where low-level layers are
entrusted with the extraction of signatures (i.e., features) from
continuous streams of sensor data, there is a shift away from focus on
hand crafting of computational features, with increasing signicance
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Neuroscience and Biobehavioral Reviews 131 (2021) 806–833
instead placed on the design of architectures that learn relevant fea-
tures organically and directly from sensor data. A more valuable
approach may be a blend of both methods. On one hand, automatic
feature extraction sub-architectures could perhaps lead to (deeper)
insight into what behaviors or other responses at individual and group
levels characterize emotions in joint action. Contemporary under-
standing of these individual and group signatures could, on the other
hand, be further explored by employing them in the form of hand-
crafted features. The large number of existing studies on affect
recognition (see Section 3.3 for a discussion of some of these) would
be valuable in guiding the choice of individual level features to
examine in the context of joint action. The minimal set of studies (Mou
et al., 2015; Ukita et al., 2016; Yang et al., 2020; Yücel et al., 2013),
that have used group relations features for affect recognition and
related AI areas highlight differences in distance, speed, and direction
(as well as displacement and/or velocity) between the individuals in a
group as additional features to consider. As anticipated by some
studies (e.g., Ukita et al., 2016; Yücel et al., 2013) the problem of
determining (the extent of) a group of interest is a challenge that may
need to rst be addressed, especially for autonomous AI to be inte-
grated into real world settings, before understanding how emotion
experience in joint action can be possible (for an overview of the
current state of the art on emotion recognition on groups please see
Veltmeijer et al., 2021). Such an AI system would need to be able to
determine if there is any joint action group present in a given location
or sequence of events, the number of such groups, and the member-
ship of each of them.
5.5. Emoted disembodied joint action in the digital world
The COVID-19 pandemic has speeded up our move into digital
encounters across all aspects of our social life, be it work, education,
leisure or even health. Virtual interactions have enabled millions of
people to continue working together remotely (video calls e.g., Zoom
or virtual spaces with people represented by avatars e.g., Sococo or
Virbela). However, these virtual interactions have been impoverished
in emotional context due to the lack of information, coming from
gestures, body postures and facial expressions, about emotional
arousal and agents intentions. These non-verbal cues are critical to
communication, understanding and bonding, recently captured by de
Gelder and Poyo Solanas (2021) as mid-level qualities. Musicians,
teachers and athletes across the globe have experienced how different
it feels to perform without an audience feeding back their reception.
After all, part of our identity comes from socio-motor interaction with
others, by which we can express our personal qualities, such as being
funny or having a preference to lead or to follow. Others reect our
qualities via sensorimotor communication, which is the foundation for
validation and updating our sense of self. The lack of such easily
accessible non-verbal cues in virtual spaces and the amplication of
facial and gaze cues over body cues in video calls (e.g., Zoom) have
been suggested to contribute to Zoom fatigue(Bailenson, 2021). In
sum, lack of socio-motor interaction with others deprives our brain
from the habitual process of predicting the unfolding of their actions in
order to efciently afliate and cooperate with them in real time.
Given that a digital environment will most probably last to a certain
extent post COVID-19, it opens the opportunity, but also calls for
embedding and facilitating joint emotional interaction to become
effective. How to enhance the communication of emotional expres-
sions in virtual spaces has been previously investigated. However,
these studies have been limited to simply manually expressing such
states (e.g., Pita and Pedro, 2011) showing that in such situations
people spend more time in carefully crafting verbal affective
expressions than they do in gestural ones, possibly because of the lack
of embodiment of the latter. Sensing technology, affective computing
and sensory interactions or substitution research can have a crucial
role in creating and sharing a sense of agency, a felt embodied affective
state and at the same time advancing our understanding of how emo-
tions become joint experiences. Leithinger et al. (2014) have shown
how our own hand gestures can be transferred to another physical
space as 3D objects for the others to experience in action. Remote
tactile interactions, through the use of wearable devices that stimulate
the other person skin in response to a remote tactile gesture (e.g.,
tactile exchanges in Huisman et al., 2013), such as skin stretching or
being pinched (Hamdan et al., 2019; Muthukumarana et al., 2020),
could help maintain the affective power of our non-verbal behavior
during remote communication. Unfortunately, none of these studies
have yet explored how such approaches are suitable to transfer the
emotion qualities of an action and denitely not how such emotion
qualities transfer across a group. Instead, transfer and group dynamics
have been explored through disembodied representations of
emotion-related signals (e.g., galvanic skin response or HRV) or
inferred emotions through computational algorithms (Ardizzi et al.,
2020; Gashi et al., 2019). Also, very little attention has been given to
the spatial and temporal aspects which characterize joint emotional
experiences, which are becoming even more critical than before.
Studies have also shown that the perception of self-location can be
altered through the right manipulation of sensory feedback, as in
Lenggenhager et al. (2007). As Nadler (2020) highlights, space takes
new meaning and creates new affordances in these virtual spaces that
alter the meaning of joint interaction. From a computational
perspective, modeling group emotion may require us to integrate in the
computational models the dynamic characteristics of such virtual
spaces that are affected by their properties and typology of information
ow (Bardy et al., 2020).
6. Summary
Emotions move us across multiple levels of qualities and timescales,
for our own survival, and for higher collective purposes. The sheer
physical presence of others in shared space and time fullls the most
primal of human needs, which is to belong to a group. The recent
pandemic experience (COVID-19) has demonstrated a devastating effect
of disruption of routine social interactions. Joint actions have been
obstructed by social distancing measures or by being moved entirely to
the digital world. Connement has had profound and not yet fully un-
derstood effects on mental wellbeing across all age and gender groups,
(Ammar et al., 2020), and has had impact on the development of social
skills in children deprived of contact with their peers (Gim´
et al., 2020). In this review we have highlighted a need to close the gap
in the research between emotion and socio-motor interaction across
different disciplines, and have prompted specic questions to the sci-
entic community to do so. Although various branches of science have
separately focused on joint action and on emotion, there is a growing
necessity to understand how emotions ow across our embodied social
interactions, and how they affect us as individuals, as a group and as a
Fig. 5 depicts dimensions that were identied in this review as
meaningful to obtain a full picture of emoted socio-motor interaction,
inspired by the research landscapeof second-person neuroscience,
proposed by (Schilbach et al., 2013). Sections 23 are represented in the
lateral panel as emotion depicted a shade gradient of agents engaged in
interaction (where color denotes interaction type), and organized the
possible consequences of socio-motor interaction, into three ‘working
categories that emerged during our literature search: performance,
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Neuroscience and Biobehavioral Reviews 131 (2021) 806–833
social and individual.
Recent models of emotion (i.e., Barrett, 2017b) have paved a
theoretical path to integrate aspects of the presence of others and
acting together in order to bridge a new, more informed, interdisci-
plinary avenue of research that is inclusive of dynamic relationships
between emotion and joint action performance, when more than one
agent is present, and of action context. The scientic evidence gath-
ered and synthesized in Sections 13 of this review provides a weighty
incentive to embrace more holistic and interdisciplinary approaches
that are built on the assumption that our brain is primarily predictive,
over reactive, and that our emotions are based of interoception and
exteroception, and play an important allostatic function. Human
abilities to understand emotions and to act together develop
simultaneously throughout the lifespan and show neural overlap in
brain activity suggesting that both have been shaped by evolution to be
interdependent. Therefore, the unraveling of the intrinsic relationship
between emotion and socio-motor interaction needs to be built on
modeling multi/inter-modal emotion propagation models, conceptu-
alizing what group emotion is, and whether some emotions are
exclusive to interaction (see Fig. 6).
This will nourish new modes of social interaction with non-human
agents, which can provide personalized care, entertainment and life-
long education for fragile populations. We argue that the deployment
of machine learning algorithms and models supporting multiple time-
scales might provide the apposite caliber of research machinery to
advance our comprehension of the dynamics of this ‘dark matter
Fig. 6. Questions we have not found answer to during our literature review and need to be addressed by inter-disciplinary research.
Fig. 5. Presentation of the future research
landscape for emotion and joint action. The
bottom horizontal axis represents multiple
timescales that can be extracted from data
(ranging from ms, i.e. brain and neural activity,
to hours and years, i.e. expertise). The vertical
axis denotes the number of agents engaged in
socio-motor interaction, being perceptually and
motorically active in the same physical or vir-
tual space. The colorful legend on the left
side represents possible types of socio-motor
interaction that emerged from the literature
review (delineating different spatio-temporal
relationships between agents - see Table 1 for
summary). The circle in the left corner rep-
resents models of emotions (gradients relate to
dimensions of valence and arousal) that need to
be adapted to multiagent scenarios, and here
are injected into a color scheme of interaction
types. Bottom panel lists multi-layered con-
sequences of the socio-motor interaction
across three main categories: performance
(quantitative and qualitative), social (i.e., afl-
iation and cohesion) and individual (impact on
personal motor characteristics and emotional
contagion from interaction with other agents),
identied from this review. Example A re-
cordings of brain activity (hyper scanning) and
heart activity during a naturalistic conversation
between three people; Example B
A group is
trying to follow and repeat the pattern from the
leader (Tai Chi class), captured with motion and respiration recordings; Example C Group of agents dancing in and out of synchrony with each other (music street
festival/parade), intertwined with periods of coordinated competition with each other, captured with motion recordings, following different typologies (Bardy et al.,
2020). Images: The Tai Chi and Old Man icons come from http://www. (author: Freepik).
nkiewicz et al.
Neuroscience and Biobehavioral Reviews 131 (2021) 806–833
(Schilbach et al., 2013), and dissect the multi-layered physiological,
socio-behavioral consequences of acting together, inclusive of the
emotion component and cohesion between agents. Progress in this
cross-disciplinary eld will feed future research and development in
many technological areas such as collaborative robotics for industry and
healthcare (i.e., incorporating the design of articial agent principles,
sensors and effectors for social-signaling and sensori-motor
communication), and will provide the tools for embodied digital in-
teractions (for virtual workspaces and education).