key: cord-287036-swo90ji2
authors: Rauchbauer, Birgit; Grosbras, Marie-Hélène
title: Developmental trajectory of interpersonal motor alignment: positive social effects and link to social cognition
date: 2020-08-09
journal: Neurosci Biobehav Rev
DOI: 10.1016/j.neubiorev.2020.07.032
sha: 
doc_id: 287036
cord_uid: swo90ji2

Interpersonal motor alignment is a ubiquitous behavior in daily social life. It is a building block for higher social cognition, including empathy and mentalizing and promotes positive social effects. It can be observed as mimicry, synchrony and automatic imitation, to name a few. These phenomena rely on motor resonance processes, i.e., a direct link between the perception of an action and its execution. While a considerable literature debates its underlying mechanisms and measurement methods, the question of how motor alignment comes about and changes in ontogeny all the way until adulthood, is rarely discussed specifically. In this review we will focus on the link between interpersonal motor alignment, positive social effects and social cognition in infants, children, and adolescents demonstrating that this link is present early on in development. Yet, in reviewing the existing literature pertaining to social psychology and developmental social cognitive neuroscience, we identify a knowledge gap regarding the healthy developmental changes in interpersonal motor alignment especially in adolescence.

In our everyday social lives, we unconsciously and automatically align our behavior to the people around us. We define this interpersonal alignment as the reciprocal matching of behavior, posture, facial or vocal expression to the interaction partner. We propose to review the development of interpersonal motor alignment appearing as synchrony, mimicry or automatic imitation, the former two being ubiquitous in daily social life. We focus on interpersonal motor alignment that is automatic, spontaneous and mostly unconscious, as opposed to interpersonal coordination that is intentional and conscious, such as in cooperative tasks.

Synchrony refers to the temporally matched behaviors of interaction partners. Through temporal motor matching, it allows precise prediction of interactive behavior (Hove & Risen, 2009; Wiltermuth & Heath, 2009) . Synchrony may include verbal and non-verbal communicative, as well as emotional behaviors (Leclère et al., 2014) . In this review we focus on studies investigating the temporal and rhythmic matching of topographically isomorph and dynamic behaviors, such as during dance, but will also touch upon rhythmic vocal and musical synchrony, such as during singing and drumming. While synchrony refers to simultaneous motor behavior, mimicry refers to the matching of behavior occurring with a slight temporal delay, of the order of 3 -5 seconds (Chartrand & Bargh, 1999) . This phenomenon has also been coined the Chameleon-effect (Chartrand & Bargh, 1999) , alluding to the way chameleons change their colour to fit their environment. Automatic imitation on the other hand, is, with the exception of the kids' play "Simon says" that is prominent in different cultures and languages, mainly used as an experimental paradigm in cognitive psychology/neuroscience to assess underlying mechanisms of motor alignment (Brass, Bekkering, Wohlschläger, & Prinz, 2000) . Automatic imitation is most often instantiated in a stimulus-response compatibility (SRC) paradigm (e.g. (Brass et al., 2000; Kilner, Paulignan, & Blakemore, 2003) ), in which observing an irrelevant movement interferes with target movement execution. If the irrelevant action matches the target movement, response execution is facilitated, while a mismatch requires inhibition of the perceived action representation to execute the target response and thus impairs execution. This paradigm has the advantage of high experimental controllability, implementation as a within-subject, repeated measures design and suitability for neuroscientific investigations, with, for example functional magnetic resonance imaging (fMRI) J o u r n a l P r e -p r o o f own movement. It may also involve brain mechanisms related to temporal processing (see e.g., (Coull, Cheng, & Meck, 2011; Merchant, Harrington, & Meck, 2013) , for detecting timing of events and rhythmic properties. There is a tight relationship between these brain circuits and neural networks implicated in motor processes, as evidenced also by the fact that action facilitates time estimation in adults and children (Monier, Droit-Volet, & Coull, 2019) . In particular, the striatum and the supplementary motor area have been shown to be involved in perceiving time as well as in producing timed motor actions, rhythmic sequences and coordination of motor actions (Coull et al., 2011; Merchant et al., 2013) , and are thus likely to play a key role in interpersonal motor alignment. Yet, the interactions between motor resonance mechanisms and temporal processing may differ between synchrony, mimicry and automatic imitation given their specific reliance on precise timing.

From a developmental perspective, interpersonal motor alignment is tightly linked to the ongoing discussion of the origins of the human mirror mechanisms. Debated accounts range from completely innate mechanisms evolved through selection pressure (Meltzoff & Moore, 1997) , to a sole product of associative sensorimotor learning (Keysers & Perrett, 2004; Oostenbroek et al., 2016) , and experience (Catmur et al., 2007; Catmur, Walsh, & Heyes, 2009; Heyes, 2011; Heyes, Bird, Johnson, & Haggard, 2005; Ray & Heyes, 2011) with mixed-accounts in between (Farmer, Ciaunica, & Hamilton, 2018; Quadrelli & Turati, 2015) . While it is not our goal to contribute directly to this discussion, we argue that a critical review of the literature on the development of interpersonal motor alignment and its link to the construction of social abilities across the lifespan is currently lacking in this debate. Yet, including developmental aspects not just at the beginning of life, but at all ages until adulthood, may advance apprehension of the underlying mechanisms of interpersonal motor alignment. Considering the positive effects of interpersonal (motor) alignment, a deeper insight into its underlying processes at different stages of development across the lifespan, may offer a more nuanced, targeted insight.

Synchrony and mimicry both have been suggested to promote positive social behaviors. Joint singing, for instance, has positive, and fast, "ice-breaking" (Pearce, Launay, & Dunbar, 2015) effects on social bonding (Pearce et al., 2015; Weinstein, Launay, Pearce, Dunbar, & Stewart, J o u r n a l P r e -p r o o f 2016). The accelerated positive effects of joint singing may be product of multimodal motor synchronization of laryngeal muscles and respiration patterns and the production and perception of the same sounds (Good & Russo, 2016) . Relatedly, synchronized activation during joint laughter (Dunbar et al., 2011) , physical activity, such as dance and rowing (Cohen, Ejsmond-Frey, Knight, & Dunbar, 2010) , leads to similar cohesive group effects. Group synchrony may reinforce a group's cooperative tendency (Reddish, Fischer, & Bulbulia, 2013) . A group moving together may become a collective social unit (Good & Russo, 2016) through boundary loss and getting into a "we"-mode (McNeill, 1997) . But apart from (inter)active motor alignment, already the mere observation of interpersonal coordination enhances the perception of commitment to joint action (Michael, Sebanz, & Knoblich, 2016) , rapport (Miles, Nind, & Macrae, 2009 ), a feeling of unity (Lakens, 2010; Lakens & Stel, 2011) , and of a shared goal and cohesiveness (Ip, Chiu, & Wan, 2006) .

In line with this, mimicry has been suggested to enhance liking, social cohesion and prosocial behavior towards the interaction partner (Chartrand & Lakin, 2013; Chartrand, Maddux, & Lakin, 2005; Duffy & Chartrand, 2015; Van Baaren, Janssen, Chartrand, & Dijksterhuis, 2009) . Prosociality after being mimicked may be enhanced towards the mimicking confederate, but moreover extends to an unknown experimenter and to charities (Duffy & Chartrand, 2015; Van Baaren, Holland, Kawakami, & Van Knippenberg, 2004; Van Baaren et al., 2009) , as well as strangers on a street (Fischer-Lokou, Martin, Guéguen, & Lamy, 2011) .

Mimicry has thus been suggested to act as a "social glue" that may have an evolutionary function for establishing and maintaining social relations (Lakin, Jefferis, Cheng, & Chartrand, 2003) or to regain inclusion into a group (Lakin, Chartrand, & Arkin, 2008) .

While some reports suggest that interpersonal motor alignment is enhanced towards in-, as compared to out-group members (e.g. (Bourgeois & Hess, 2008; Yabar, Johnston, Miles, & Peace, 2006) , studies show that the positive effects of interpersonal motor alignment extend to intergroup relations. Enhanced synchrony with an out-group member may reduce intergroup boundaries and support closeness (Miles, Lumsden, Richardson, & Macrae, 2011) , which may be mediated by an increased perception of interpersonal similarity through the display of interpersonal synchrony (Rabinowitch & Knafo-Noam, 2015; Valdesolo & DeSteno, 2011; Valdesolo, Ouyang, & DeSteno, 2010) . Likewise, mimicry increases sympathy, closeness and the perception of harmonious interaction with an virtual avatar posing as an out-group member J o u r n a l P r e -p r o o f (Hasler, Hirschberger, Shani-Sherman, & Friedman, 2014) . Synchronous and mimicry behavior may enhance perceived self-other overlap between self-generated and other-produced movements. A constant matching between interactive movements may enforce an overlap in the shared representations of perception and execution of an action. This self-other overlap may lead to social cohesion (Lang, Bahna, Shaver, Reddish, & Xygalatas, 2017; Tarr, Launay, & Dunbar, 2014) , extending also to out-group members and contribute to overcoming group boundaries.

The positive intergroup effects of interpersonal motor alignment have also been demonstrated using the tightly controlled automatic imitation tasks. It has been suggested that automatic imitation measures covert (Cracco et al., 2018; Cracco & Brass, 2019; Heyes, 2011) , and to some extent overt, imitation (Cracco & Brass, 2019) , but this has recently been a matter of debate (cf. (Cracco & Brass, 2019; Ramsey, 2018) . While mimicry and automatic imitation may not be correlated (Genschow et al., 2017) , being mimicked may reduce inhibitory mechanisms of automatic imitation (Rauchbauer, Dunbar, & Lamm, 2020) , potentially indicating an interrelationship between the two phenomena.

In any case, automatic imitation tasks allow investigators to test a variety of modulatory factors in controlled settings using both behavioral and neuroscientific methods. Thereby they have provided evidence that interpersonal motor alignment is modulated by social context, like group membership (Gleibs, Wilson, Reddy, & Catmur, 2016; Marsh, Bird, & Catmur, 2016; Rauchbauer, Majdandžić, Hummer, Windischberger, & Lamm, 2015; Rauchbauer, Majdandžić, Stieger, & Lamm, 2016) , social contagion and group size , eyecontact (Marsh et al., 2016; Wang, Newport, & Hamilton, 2011) , and emotional facial stimuli (Butler, Ward, & Ramsey, 2016 ) (see also for meta analyses: (Cracco et al., 2018) ). They may also have the potential to contribute to identifying the factors that link interpersonal motor alignment and positive social behaviors. Divergent accounts suggest that increased social closeness and helping behavior may either rely on topographically isomorphic body movements , whereas other reports suggest a role of perceived contingency (i.e., predictive relationship) of one's own and the interaction partner's movements, rather than on their similarity (i.e., topographic isomorphism) (Catmur & Heyes, 2013 ). Yet again, other accounts suggest that social affiliation may not only be linked to high contingency, but also high contiguity (i.e., temporal proximity) of J o u r n a l P r e -p r o o f movements (Dignath, Lotze-Hermes, Farmer, & Pfister, 2018) and effector matching (Sparenberg, Topolinski, Springer, & Prinz, 2012) .

Moreover, both synchronous movements and exertion have been shown to induce beneficial social effects (Tarr, Launay, Cohen, & Dunbar, 2015) . Similarly, the choice of control condition in a mimicry or synchrony induction could potentially influence social-cognitive variables.

While prominent studies on mimicry have used control conditions without any movement (e.g., (Chartrand & Bargh, 1999; R. B. Van Baaren et al., 2004) , other studies have used Anti-Mimicry conditions (Dalton, Chartrand, & Finkel, 2010; Finkel et al., 2006; Kühn et al., 2010; Rauchbauer et al., 2020) . In an Anti-Mimicry control condition, the confederate's movements will be topographically misaligned to the participant's, as compared to the Mimicry condition in which they are aligned. A control condition without movement may itself evoke negative social effects due to perceiving the interaction partner uninterested and passive. Conversely, an Anti-Mimicry condition may also induce positive effects itself in case of perceived temporal contingency (Catmur & Heyes, 2013) or high exertion (Tarr et al., 2015) of movements. Thus, the mechanisms by which mimicry and synchrony may induce positive social-cognitive effects may not be entirely clear yet and lead to divergent results. As such, it has recently been suggested that being mimicked by topographically isomorph postures as compared to an Anti-Mimicry condition, does not influence experienced social cohesion with the confederate, measured with a rating scale (Rauchbauer et al., 2020) . Thus, there are certain contradictions in the literature of interpersonal motor alignment, which may require in-depth analysis. To date it remains unclear as to which conditions and mechanisms may give rise to which positive socialcognitive effects via synchrony or mimicry. For example, while mimicry seems to evoke a kind of generalized prosociality, synchronous behavior may rather evoke directed prosociality (Cirelli, 2018) . However, some studies have demonstrated that also synchrony induces generalized prosocial behavior (Reddish, Bulbulia, & Fischer, 2014; Reddish, Tong, Jong, Lanman, & Whitehouse, 2016) .

The aforementioned studies rely only on adult populations. Yet, interpersonal motor alignment is pertinent throughout ontogeny and may play an important part of a healthy social life, as a building block for higher social cognition, including empathy. Indeed inter-individual differences in experience of and ability for interpersonal motor alignment early in infancy is associated with empathic abilities in adolescence as well as with the ability to engage in intimate J o u r n a l P r e -p r o o f relationships across the lifespan (Feldman, 2007b (Feldman, , 2007c , suggesting a protective buffering role against psychosocial maladjustment. The "motor theory of empathy" suggests that human mirror mechanisms may mediate the understanding of others' intentions and feelings (Rizzolatti & Fabbri-Destro, 2008) and that empathy may originate from the perception-action link (Iacoboni, 2009 ; Leslie, Johnson-Frey, & Grafton, 2004) ). As such, action understanding may play a role in feeling emotions and empathizing with others (Carr, Iacoboni, Dubeau, Mazziotta, & Lenzi, 2003 ). Yet, this theory has also been criticized, arguing that contrary to previously mentioned findings, empathy does not necessarily involve the activation of the mirror neuron system (MNS) (De Vignemont & Singer, 2006) , or that its involvement may depend on the specific form of empathy (Baird, Scheffer, & Wilson, 2011) .

Regardless of this debate, studies show that right after birth, social contingencies in the form of multimodal responses by the mother to the infant's signals, and contingency detection on the side of the infant, aid to form reliable reciprocal interactions. Synchronous interactions, starting with caregiver-infant interactions, may provide a buffer against psychosocial maladjustment, shape empathic capacities in adolescents and lay the basis for engagement in intimate relationships across the lifespan (Feldman, 2007b (Feldman, , 2007c . Therefore, it is important to integrate the ontogenetic dimension while building accounts of these functions during typical and atypical developments. In the next sections we review existing behavioral and neuroimaging research on synchrony, mimicry and automatic imitation in infants, children and adolescents. For this we conducted a systematic search on Pubmed (https://pubmed.ncbi.nlm.nih.gov/; as of 15

May 2020), presented in Tables 1-3, and analysed the main findings. Keywords included (Infant OR Development) AND (Mimicry OR Synchrony OR Automatic Imitation OR Action Observation) AND (experiment OR EEG OR fMRI OR fNIRS OR EEG). Then results were filtered to select only the studies that focused on interpersonal motor alignment as we have circumscribed above, in particular excluding experiments on instructed or delayed imitation. We also excluded studies that concerned only atypical populations, or that did provide only qualitative data. This list, while probably not exhaustive due to limitations of the search algorithm, provides a panorama of the state of the research in this domain so far. We analyzed the main findings with regards to the nature of interpersonal motor alignment and its relationships with other social behaviors, on the one hand, and its neural mechanisms, on the other hand, at different ages.

Humans may be biologically prepared for coordinated interaction due to their responsiveness to rhythmical information and build-in tendency to detect contingencies (Feldman, 2007b) . A recent review suggests that the establishment of interpersonal synchrony in reciprocal interactions is supported by adults' rhythmical information to their infants (Markova, Nguyen, & Hoehl, 2019) . These rhythms are often spontaneously displayed by caregivers in the form of affective touch or singing to aid the infant's affect regulation (Provasi, Anderson, & Barbu-Roth, 2014 ). The caregiver's adaptive signals to the newborn's behavior form social contingencies, which the infant readily detects. These interpersonal mechanisms appearing in the first months of life may be multimodal precursors for synchronous interactions, including, apart from movements, vocalizations, gaze, touch, affect, position or proximity to each other. As the infant grows, the time lag between behavior and response diminishes and social contingencies develop into synchronous behaviors (Feldman, 2007b (Feldman, , 2007c .

A longitudinal study points towards crucial positive long-term effects of socially contingent and synchronous interactions. Affective synchrony in mother-infant interaction from three and nine months of age predicted self-regulatory capabilities in two, four-and six-year olds. The same study also found that synchronous mother-infant interactions at three months of age predicted empathic abilities in thirteen-year old adolescents (Feldman, 2007a) .

Apart from interactions with their primary caregiver, 12-and 14-month-old infants already use interpersonal movement information to guide social expectations. As such they attend to and interpret interpersonal synchronous and asynchronous behavior in a socially meaningful way (Cirelli, 2018; Fawcett & Tunçgenç, 2017) . They seem to prefer synchronously moving partners and show spontaneous helping behavior towards them and their affiliates (Cirelli, Einarson, & Trainor, 2014; Cirelli, Wan, & Trainor, 2016; Tunçgenç, Cohen, & Fawcett, 2015) . Even more so, when observing asynchronous dyadic interactions, infants believe the interaction partners to be nonaffiliates (Cirelli, Wan, Johanis, & Trainor, 2018) . This suggests that they can infer thirdparty affiliation on the basis of synchrony (Cirelli, 2018; Fawcett & Tunçgenç, 2017) .

Synchrony, as a social signal per se, may be a cue for self-similarity from the beginning of life on (Leclère et al., 2014; Xavier et al., 2016) , increasing prosocial behavior and encouraging empathy and affiliation (for review see Cirelli, 2018) . In four-year-old children, synchronous interactions of only three minutes may already lead to enhanced peer cooperation.

In children of eight to nine years of age they enhance closeness and feelings of similarity (Rabinowitch & Knafo-Noam, 2015; Rabinowitch & Meltzoff, 2017a) . These positive effects extend from synchrony in direct interaction to joint music making and rhythmic synchrony.

Children synchronize drumming with higher accuracy in a social condition, which elicits cooperative behavior and fairness (Kirschner & Tomasello, 2009 , 2010 Rabinowitch & Meltzoff, 2017a) . Also cooperative singing with peers, as reported for adults (Pearce et al., 2015) , increases group cooperation in children of seven to eight years more than art or competitive games (Good & Russo, 2016) . Furthermore, the positive effects of synchronous singing have been reported to contribute to feelings of social inclusion in refugee children (Marsh, 2017; Marsh & Dieckmann, 2017) . This suggests, that singing in synchrony, just as moving in synchrony (Tunçgenç & Cohen, 2016) or joint music making (Marsh, 2012 (Marsh, , 2017 , may support the establishment of a collective group membership by forging intergroup bonds.

In summary, synchronous behaviors can be observed, in different forms, throughout infancy and childhood (for a list of experimental studies on interpersonal motor alignment in developmental population see Tables 1-3) . First mainly restricted to caregivers, they extend to peer relations and can also be recognized in other dyads during observation. In all cases the ability to engage in synchronous behavior has positive prosocial outcomes especially with regards to interpersonal affiliation, similarly to what we have discussed in adults.

Interpersonal motor alignment in neonates has been evidenced by seminal studies showing imitative hand opening and mouth protrusion movements (Meltzoff & Moore, 1997) . This finding has led to an ongoing debate on whether spontaneous imitation is innate or not (see for example (Anisfeld, 1996; S. Jones, 2017; Kennedy-Costantini et al., 2017; Keven & Akins, 2017; Libertus, Libertus, Einspieler, & Marschik, 2017; Meltzoff, 2017; Meltzoff et al., 2018; Oostenbroek et al., 2016) ). Alternatively, it has been suggested that imitation is learned through an associative learning sequence (Catmur et al., 2009) , Hebbian learning (Keysers & Perrett, J o u r n a l P r e -p r o o f 2004) and correlated sensorimotor experiences, forming perception-action couplings through interaction with the primary caregiver (de Klerk, Lamy-Yang, & Southgate, 2019). As mentioned above, it is not our aim to contribute to this discussion in the present article, but rather to summarize facts about interpersonal motor alignment and their social correlates at different ages.

A large corpus of work shows that, during the first two years of life, infants faithfully imitate adults actions or just their goal (social or instrumental) in a variety of contexts (Yu & Kushnir, 2014) . Children of pre-school age imitate not only causally relevant, but also irrelevant actions (i.e. sequences of movements that are not necessary to achieve the desired goal). This "over-imitation" (e.g., Nielsen & Blank, 2011; Over & Carpenter, 2012 , may be an important learning mechanism to acquire cultural expertise. It seems to also have social reasons, such as the identification with a model and the social group in general. It may occur through social pressure (Over & Carpenter, 2012) , but can also be used to overcome in-group ostracism (Over & Carpenter, 2009; Watson-Jones, Whitehouse, & Legare, 2016) . The dual role of imitation, for social learning and for establishing group cohesion is supported by studies showing that 14-month-olds are more likely to imitate communicative gestures and familiar actions performed by same-age infants, than when they are performed by older children and adults (Zmyj, Aschersleben, Prinz, & Daum, 2012) . Nevertheless, in a context when they are presented with a novel object, they are more likely to reproduce the action that they have seen an adults perform on this object (Zmyj, Daum, Prinz, Nielsen, & Aschersleben, 2011) . This suggests that infants and toddlers may more likely align their behavior to that of an adult role-model in a learning context. In an affiliative context though it seems that children would preferentially imitate their peers, potentially to enhance group cohesion. Similarly, having been mimicked by an adult, 14 -18-month-olds learned better by observation (Somogyi & Esseily, 2014) , and were more prone to help this adult (Over & Carpenter, 2013) . This prosocial behavior can also extend to a stranger (Carpenter, Uebel, & Tomasello, 2013) . This suggests that being mimicked may evoke a generalized prosociality (Cirelli, 2018) .

The positive effects of interpersonal motor alignment throughout development are not only shown during imitation, mimicry and synchrony, but also in the more experimental environment of automatic imitation. In motor interference tasks, children are asked to perform straight lines in a vertical or horizontal movement on a tablet computer screen using a stylus, while an interaction partner performs a congruent or incongruent drawing movement (Marshall, J o u r n a l P r e -p r o o f Bouquet, Thomas, & Shipley, 2010; Saby, Marshall, Smythe, Bouquet, & Comalli, 2011; van Schaik, Endedijk, Stapel, & Hunnius, 2016) . Using this task, automatic imitation was increased in four-to six-year-olds when interacting with an out-group member .

This suggests, in line with adult studies , that even during automatic imitation the regulation of interpersonal motor alignment may be used to overcome intergroup differences. Also, 4-year-old children show a greater motor interference effect in interaction with peers, rather than adults (Marshall et al., 2010) , in line with the studies mentioned above on imitation (Zmyj et al., 2012 (Zmyj et al., , 2011 . Thus, already during childhood, group membership seems to influence the perception-action link. This is also demonstrated in a study with children of four and five years of age in which automatic imitation is modulated according to animacy beliefs of an interaction puppet (Saby et al., 2011 ). Yet, a recent meta-analyses suggested that automatic imitation may not be sensitive to animacy beliefs (Cracco et al., 2018) . Group membership may influence interpersonal motor alignment via top-down modulation. For instance, it has been suggested that during conscious imitation neural activity in a wide range of brain areas, and not only early visual areas, is modulated by the race of the model (Losin, Iacoboni, Martin, Cross, & Dapretto, 2012) . This may be driven by socially learned associations concerning race, rather than self-similarity (Losin, Cross, Iacoboni, & Dapretto, 2014) . This seems in line with motivational theories of automatic imitation, such as the social top-down response modulation account (STORM; Wang & Hamilton, 2012) . These accounts suggest that the motivation to affiliate (the wish to be liked by the interaction partner) may increase imitation. This has also been suggested to influence mimicry, via a strengthened the perception-action link (Chartrand et al., 2005; Lakin et al., 2008) . Group membership may be one salient social factor activating affiliative motivation. High motivation to affiliate with the in-group has been shown to moderate the influence of group membership during imitation (Genschow & Schindler, 2016) .

Other studies have found higher imitation for out-group members, which could suggest affiliative motivation for appeasement (Rauchbauer et al., 2015 . Contrary to these studies conducted in adult samples, social modulation of automatic imitation may be absent in adolescents. While automatic imitation has been observed to the same level as in adults in adolescents, no modulation by pro-social priming has been observed (Cook & Bird, 2011) . This would indicate that the adaptability and social function of interpersonal motor alignment is still fine tuning in adolescence.

Apart from Cook & Bird (2011) and as pointed in the previous sections, to this date, investigation of interpersonal motor alignment in adolescence is scarce (see Table 3 ). This may be due to the assumption of social cognitive maturity by mid-childhood (Blakemore & Mills, 2014) , leading to a wealth of studies of interpersonal motor alignment during child-and adulthood, but skipping the period of adolescence.

Yet, adolescence is an important social transition period, with ongoing brain development. This absence of studies warrants conclusions of social effects of adolescent interpersonal motor alignment and encourages further investigation of its potential positive effects. This is especially the case since adolescence is a period of enhanced importance of the peer group, sensitivity for social acceptance and rejection, as well as continuing structural and functional brain development (rev in Blakemore & Mills, 2014; Grosbras et al., 2007; Mills et al., 2016) . A more in-depth investigation of interpersonal motor alignment during adolescent interaction could inform on healthy adolescent social cognitive development. This could have implications with respect to psychopathology with a prevalent onset in adolescence like conduct disorder, social anxiety or schizophrenia. As these disorders have been associated with altered brain development, it is also paramount to link those observations to knowledge about subserving brain organisation and its development. In the next section we review studies using neuroscientific methods to investigate brain correlates of interpersonal motor alignment across development.

The neural correlates of interpersonal motor alignment have been investigated in adults in three different categories of studies. First, the regulation of automatic imitative tendencies, second, motor resonance phenomena investigated via action observation studies, and third, synchronized brain activity. Here we give only a brief overview of some adult studies that have used various neuroscientific methods, as our aim is to focus on the developmental aspect. This section illustrates that, although neural correlates of interpersonal motor alignment have been investigated broadly, both with respect to experimental paradigms and research methods, as a J o u r n a l P r e -p r o o f whole the corpus evidence converges to show that specific brain circuits and mechanisms are involved in the aligment of behaviors during interaction.

Automatic imitation tasks have been used to assess brain regions specifically involved in the control of imitative tendency. Functional magnetic resonance imaging (fMRI) studies indicate the engagement of prefrontal (inferior frontal and medial prefrontal) and parietotemporal regions when subjects imitate a movement (Carr et al., 2003) or perform a motor interference task (e.g., Mengotti et al., 2013; Rauchbauer et al., 2015) .

Patients with frontal lobe lesions tend to over-imitate, which suggests the existence of specific mechanisms to control this function (Brass, Derrfuss, Matthes-von Cramon, & von Cramon, 2003) . Non-invasive brain stimulation studies also show that disruption in inferior frontal regions interferes with imitation of simple finger movements (Heiser, Iacoboni, Maeda, Marcus, & Mazziotta, 2003) . Moreover, stimulating posterior regions in the parietal opercular region (Mengotti et al., 2013) or temporo-parietal junction (S Sowden & Catmur, 2013) impedes the ability to repress the automatic tendency to imitate.

Motor resonance phenomena, tightly linked to automatic imitation, can also be revealed in passive action observation tasks that do not require explicit control over imitation. Already observing other people's actions consistently engages a set of brain regions to a greater extent than watching other categories of visual movement. This "Action Observation Network" (AON) encompasses the human mirror neuron system, which, alongside fronto-parietal regions and temporal cortices, is also engaged during action execution (Caspers, Zilles, Laird, & Eickhoff, 2010; Grosbras, Beaton, & Eickhoff, 2012) . Thus, it contains an implicit "motor resonance" system, coupled with a mechanism to inhibit actual movement. This is also evidenced in indirect electrophysiological measures of brain activity using electro-or magneto-encephalography (EEG and MEG), which show the same signature of brain activity, namely a decrease in power in mu (8-13 Hz), and often also beta (15-20 Hz), frequency bands over the motor cortex (central electrodes) for both action execution and observation (rev. in (Marshall & Meltzoff, 2011) .

Experiments in non-human primates indicate that the mu rhythm would reflect activity of motor and mirror neurons (Bimbi et al., 2018) .

In addition, interpersonal synchrony has been associated to inter-brain synchrony, which has been measured directly during hyperscanning that is data recorded using neuroscientific methods, such as EEG or fMRI, simultaneously on multiple partners engaged in interpersonal J o u r n a l P r e -p r o o f motor alignment. Simultaneous EEG recordings in a dyad showed enhanced correlation in the theta and beta frequencies across brains during the execution of simple coordinated finger or hand movements (Dumas, Nadel, Soussignan, Martinerie, & Garnero, 2010; Yun, Watanabe, & Shimojo, 2012) . In cooperative settings, inter-brain activity coherence was also demonstarted using near-infrared spectroscopy (NIRS) (Cui, Bryant, & Reiss, 2012; Funane et al., 2011) .

Cooperative or affiliative behavior in economic games (Astolfi et al., 2014; De Vico Fallani et al., 2010) , as well as in more ecological situations like flight behavior cooperation in professional pilots (Toppi et al., 2016) and interaction in romantic partners (Kinreich, Djalovski, Kraus, Louzoun, & Feldman, 2017) was furthermore predicted by different patterns of brain-tobrain synchrony (most often in theta range and frontal cortex). In the same vein, it has been suggested that the greater the coupling between speakers' and listeners's brains, the greater the understanding between partners in communication (Stephens, Silbert, & Hasson, 2010) . A recent study showed a causal effect of neural synchrony on behavioral synchrony by entraining the motor activity of two individuals at the same time with transcranial alternating current stimulation: in-phase 20Hz stimulation facilitated the establishment interpersonal movement synchrony in a joint finger tapping task (Novembre, Knoblich, Dunne, & Keller, 2017) .

Describing inter-brain coupling throughout development is a key part of gaining full understanding of these meachnisms. Yet, developmental brain imaging studies have been mainly concerned with single subjects set ups, only a few implementing dual scanning paradigms.

Next, we review the main findings in infants and toddlers, primarily using electroencephalography (EEG) and functional near-infrared spectroscopy (fNIRS), and then turn to studies involving children and adolescents, which are mainly using fMRI. Table 1 -3 present a more complete list of developmental studies involving the different sorts of interpersonal motor alignment in these populations, as given by a systematic search on Pubmed.

Experiments using EEG or fNIRS have described components of the action observation network present very early on in infancy (see Table 1 ). Desynchronisation of the equivalent of the mu rhythm in infants (6-9 Hz over central cortex) (Marshall & Meltzoff, 2011) , has been reported in 4-11-month-olds when seeing adults walking (Virji-Babul, Rose, Moiseeva, & Makan, 2012).

Similarly, 9-month-old infants show mu rhythm desynchronization while they observe actors performing reach and grasp movements (Debnath, Salo, Buzzell, Yoo, & Fox, 2019; Southgate, Johnson, Osborne, & Csibra, 2009 ). This was also reported in 9-and 12-month-olds observing tool use (Yoo, Cannon, Thorpe, & Fox, 2015) , and in 18-36 month-olds watching actors pantomime actions without an actual object present . Moreover, mu rhythm desynchronization has been shown in 12-month olds observing unusual movements depending on the use of objects (e.g., bringing a phone to the mouth compared to a cup) (Stapel, Hunnius, van Elk, & Bekkering, 2010) and in 12-14 month olds depending on the unexpected use of effectors for the action (e.g., while having the hands free, using the head to turn on a lamp) (Langeloh et al., 2018) . Mu desynchronization in 30-month-old infants during observation of facial movements furthermore suggests an already functioning mirror mechanism of facial expressions during early stages of development (Rayson, Bonaiuto, Ferrari, & Murray, 2016) .

Desynchronization in the motor cortex in the frequency range of 8-12 Hz for both execution and observation of drawing actions has been confirmed in a 36-month-old child with pre-surgical cortical electrodes implantation using intracranial recording (Fecteau et al., 2004) . Using fNIRS, Shimada and Hiraki (2006) showed that 6-7 months infants who passively observed objected directed hand-arm actions engaged, although to a lesser extent, the same brain regions that when they manipulated the object themselves (Shimada & Hiraki, 2006) . Thus, these studies converge to indicate an overlap between action execution and observation very early on in development.

The relevance of automatic motor system engagement during passive action observation for studying interpersonal motor alignment is further supported by studies showing that motor resonance in 14-month-old infants is enhanced in an interaction context compared to the direct copying of adults' gestures. This points towards a stronger mirror resonance mechanism during interaction already in infants (Reid, Striano, & Iacoboni, 2011; Saby, Marshall, & Meltzoff, 2012 ).

Yet, the automatic engagement of neural mirror mechanism may depend on experience (rev. in (Marshall & Meltzoff, 2014) ). This is reflected in studies showing that 4-month-olds show the same response to movements performed by an artificial agent or by a human (Grossmann, Cross, Ticini, & Daum, 2013) . Furthermore, studies have reported that the degree of mu desynchronization during action observation is cumulatively correlated with motor skills according to experience ((e.g. grasping or crawling (van Elk, van Schie,

Hunnius, Vesper, & Bekkering, 2008)). As such, stronger mu-and beta-desynchronization have been observed in 14-to 16-month-olds watching videos of other infants crawling, as compared to seeing them walking, for which they hadn't developed rich experience yet (van Elk et al., 2008) .

Interestingly, similar findings have also been reported in very young macaques who exhibit signs of EEG desynchronization in sensorimotor cortices during grasping observation in the first two weeks of life, increasing as a function of rudimentary grasp development (Festante et al., 2018) .

These findings are compatible with a narrowing processing dependent on motor experience (Lloyd-Fox, Wu, Richards, Elwell, & Johnson, 2013) . However, these results are contradicted by a study, showing that sensorimotor activation in response to videos of someone walking is present even for infants (4-11 months old) who don't yet walk. This could suggest that visual familiarity with an action is sufficient to drive motor resonance, without direct implication for behavior . In the same vein, Shimada and Hiraki (2006) showed that motor engagement during action observation in 6-month-olds was larger for live than videotaped stimuli, the latter being supposedly less familiar at this age. Alternatively, some authors have suggested that the relationship between motor competencies and motor resonance might be more apparent in older than younger infants (12-month-old compared to 9 month olds) (Yoo et al., 2015) . In addition, experience beyond specific action execution skills may also be important.

As such, general early life history may have an impact on the degree of interpersonal brain resonance. Fourteen-month-old preterm infants show mu suppression during action observation only in the right parietal regions, whereas full term infants showed the effect in a bilateral frontoparietal network. Yet, no difference between groups was observed for action execution (Montirosso et al., 2019) .

Another line of research has looked at interpersonal neural synchrony during infant social interactions. For instance, Leong and colleagues (2017) showed that phase-locking of brain activity between infants and parents was related to communication features, like direct gaze or duration of vocalizations. This finding could be interpreted as a mechanism for aligning the periods of higher sensory receptivity between interpersonal partners (Giraud & Poeppel, 2012) and facilitate learning. Recently, some studies have used dual fNIRS scanning to look at correlation in brain activity between infant and parent. The strength of association between parent and child activity in prefrontal regions was increased when they were engaged in cooperative as compared to independent behavior (Miller et al., 2019) . Thus, some mirror like J o u r n a l P r e -p r o o f mechanisms seem to be present in the brain early in life and well established in the first two years potentially supporting learning, as well as sensorimotor and socio-emotional development.

More data is needed to draw stronger conclusions, however, in particular with respect to their link to the positive social effects described in the first section.

Most often data from infant populations are directly compared to adult studies, leading to discontinuity in the literature with respect to changes later in development. The reason for this may be that the methods and questions are often different when studying older children and adolescents. For these age groups most investigation has focused on action observation paradigms, which can give first insights into the perception-action link underlying interpersonal motor alignment during these years of development (see Tables 2 and 3) .

A handful of studies have investigated motor activity during observation of others in older children and young adolescents using the mu suppression index, confirming findings from infants and adult studies. Eismont and colleagues (2017) reported desynchronization of the mu rhythm (taking into account the differences in dominant mu frequencies at different age ranging from 6-13Hz) over central electrodes in children aged 4-14 years, during execution, observation and imitation of arm movements, with a stable effect across ages. Similar effects were also reported from other research groups, in 2-,4-, 5-, and 11-year-olds respectively (Cochin, Barthelemy, Roux, & Martineau, 2001; Lepage & Théoret, 2006; Martineau & Cochin, 2003) .

Extending these results, Bernier and colleagues, while showing similar effects in 5-7 year-olds, demonstrated that the strength of the interpersonal motor resonance effect was correlated with a measure of facial imitation abilities (Bernier, Aaronson, & McPartland, 2013) . Altogether these EEG studies demonstrate a signature of the engagement of children's own motor system when they observe actions from others that may be stable during childhood. Nonetheless pulling data from five different studies Oberman and colleagues (2013) concluded that mu suppression might indeed decrease with age (between 6 and 17). This is consistent with the report of Cheng and colleagues who observed higher mu suppression in 3-to 9-year-old children compared to adults (Cheng, Chen, & Decety, 2014 ). Yet, this is inconsistent with other reports of weaker suppression in 8-12 year old children compared to adults, especially when watching movements from an allocentric perspective (Nishimura, Ikeda, Suematsu, & Higuchi, 2018) , or of an J o u r n a l P r e -p r o o f increase from 10 to adulthood (Brunsdon, Bradford, Smith, & Ferguson, 2020) . More data from adolescents would be needed to ascertain this developmental trajectory. Other modulating factors such as attention and engagement with the stimulus, for example due to affiliation should also be investigated. Interestingly, Cheng and colleagues reported that contrary to adults, mu suppression in children was not modulated by the emotional content (painful or not) of the observed videos. This is in line with a study that used transcranial magnetic stimulation of motor cortex in combination with electromyography to measure motor resonance, and showed that contrary to adults, motor resonance was not modulated by emotion in 17 year olds (Salvia, Süß, Tivadar, Harkness, & Grosbras, 2016) . The presence of motor activity during action observation is confirmed using other methodologies. In a small sample of pre-adolescents (age 9-13), Kajume and colleagues used fNIRS and reported increased activity, compared to a baseline control, in the inferior-frontal / premotor regions when participants observed or imitated object-directed actions (Kajiume, Aoyama-Setoyama, Saito-Hori, Ishikawa, & Kobayashi, 2013) . A handful of fMRI studies also confirm that children and adolescents, similarly to adults, recruit the AON when observing object-directed hand actions (Biagi et al., 2016; Ohnishi et al., 2004; Shaw, Grosbras, Leonard, Pike, & Paus, 2011a , 2011b . When comparing the activity in the AON directly between children (7-15) and adults, Biagi and colleagues (2016) observed less lateralization to the left in children. This is partly consistent with a longitudinal study showing decreasing activity with age in the right parietal cortex during action observation (although mainly in males) (Shaw et al., 2011b) .

Assessing both action execution and observation suggests that the extent of shared activation, reflecting mirror activity, would increase from child-(age 7-10) to adulthood (Morales, Bowman, Velnoskey, Fox, & Redcay, 2019) . Structurally also, areas in the AON, as well as other regions of the social brain, undergo developmental changes throughout adolescence (Mills et al., 2016) .

Moreover, the activation of the mirror system or the AON is modulated by personal and social factors in development. For instance, children and adolescents (age 9-15) with autism showed higher precentral and middle temporal activity in an action simulation task (i.e., when simulation is necessary to solve a problem), than age-matched control participants (Wadsworth 2017) . For passive observation of emotional actions, adolescents with a higher resistance to peer pressure show more coordinated brain activity in the right dorsal premotor and the left J o u r n a l P r e -p r o o f dorsolateral prefrontal cortex, than adolescents with low resistance to peer pressure. Thus, the propensity to withhold social peer pressure seems related to neural interaction when observing emotional actions (Grosbras et al., 2007) . Indeed, the AON seems to be modulated by the emotional connotation of the observed action, with higher activity in fronto-parietal regions and additional supramarginal medial prefrontal and amygdala activity for angry as compared to emotionally neutral hand movements (Grosbras et al., 2007; Shaw et al., 2011b) . This emotional modulation is however expressed differently in boys and girls across development. At the age of 10 and 11.5 years, both girls and boys show a common level of activity in the AON during observation of angry hand movements, specifically in the posterior parietal cortex, extending into the parieto-occipital junction, the fusiform gyrus, cerebellum, right inferior parietal lobule (IPL), dorsal pre-motor cortex (PMC), intraparietal sulcus (IPS) and orbito-frontal cortex (OFC). Yet, by the age of 13 the same male participants showed higher engagement of a "socioemotional" network than the AON, as compared to their female counterparts. This network comprised brain regions of the so-called social brain, specific to the processing of emotional actions including the temporo-parietal junction, the orbitofrontal cortex and the insula. This suggests that male adolescents around the age of thirteen years do not recruit action observation, but rather socioemotional processes when observing angry hand actions (Shaw et al., 2011a) .These findings are paralleled by observations of structural brain development: adolescents with a higher degree of resistance to peer influence show higher interregional correlation of cortical thickness between nodes of the AON (Paus, Toro, et al., 2008) . This underlines that the importance of the peer group on behavior could be related to brain development of areas related to social cognition.

Another line of investigation looked directly at interindividual brain synchrony in social settings, using dual-functional fNIRS in naturalistic interactions between caregivers and their children of preschool (mean age of five years) (Nguyen et al., 2020) and school age (five to nine years of age) (Reindl, Gerloff, Scharke, & Konrad, 2018) . High neural synchrony between children and caregivers correlated positively with behavioral reciprocity, predicted problemsolving success (Nguyen et al., 2020) and cooperative performance (Reindl et al., 2018) . It has thus been suggested that neural synchrony may be a biomarker for interaction quality between a child and the caregiver, representing a neural mechanism for emotional connection linked to the development of adaptive emotion regulation. In a study on adolescents (17 and 18 years), Dikker and colleagues (2017) used portable EEG devices in a highschool classroom and observed that J o u r n a l P r e -p r o o f general measures of synchrony in the group (i.e., coherence between responses in multiple brain areas) were highly correlated with the level of engagement and enjoyment reported by students.

Furthermore, they showed that, as in adults, social priming through engagement in eye contact increased interbrain synchrony within student pairs. Nonetheless, in another study with the same protocol, Bevilacqua and colleagues (2019) reported that the level of "interbrain synchrony" was not related to memory retention. Research in this direction should be pursued to explore to what extent interpersonal neural synchrony relates to interpersonal alignment and to social facilitation at different stages of development.

In particular, the continuing development brain areas important for social processing during adolescence suggests interpersonal motor alignment as a potential connective element between the two networks for healthy adolescent development. This could furthermore inform maladaptive development, for example in autism spectrum disorder (ASD) and psychopathologies, such as schizophrenia, social anxiety and depression, whose onsets often fall into adolescence (Paus, Keshavan, & Giedd, 2008) . Adolescence, as a formative transition period from child-, to adulthood, may be the optimal period to intervene and prevent psychopathologies (Eldreth, Hardin, Pavletic, & Ernst, 2013) Implications: the importance of studying interpersonal motor alignment in healthy development, including adolescence

As we hope to have demonstrated, interpersonal motor alignment is an important social signal for the establishment and maintenance of relationships and group cohesion throughout development. Even more so, the lack of coordinated interactions starting already at birth may lead to persistent difficulties throughout life in domains of social and emotional development, as well as self-regulation and the capacity for intimate relationships (Feldman, 2007b (Feldman, , 2007c . We contend that this importance of interpersonal motor alignment continues throughout development, extending from early relations with primary caregivers to family and peer relationships. Especially during adolescence, given the reorientation of the social focus to peers and away from family members, social contingencies and interpersonal behavioral alignment with peers may come into focus.

Reliable reciprocated interpersonal motor alignment throughout childhood and adolescence could contribute to strengthen the link between functional and structural brain development in areas related to the social brain and the AON and thereby enhance social and emotional resilience during adolescence. This is underlined by findings showing that socially contingent and synchronous interactions between caregivers and infants predict adolescent empathic abilities (Feldman, 2007a) . But also, adolescents with higher resistance to peer influence show higher cortical thickness between nodes of the AON (Paus, Toro, et al., 2008) and highly coordinated brain activity in areas related to action perception and decision making (Grosbras et al., 2007) . Conversely, unreliable alignment with peers or the family could augment feelings of social rejection and stress, in a period already marked by heightened sensitivity to peer rejection (Sebastian, Viding, Williams, & Blakemore, 2010) .

Social stressors such as (cyber-) bullying (Crick et al., 2001) , social exclusion (Crone & Konijn, 2018) and enhanced risk-taking behavior (Chein, Albert, O'Brien, Uckert, & Steinberg, 2011; van Oosten & Vandenbosch, 2017) peak during adolescence. Moreover, adolescence, as a time of substantial neurobiological and behavioral changes, confers a vulnerability for certain types of psychopathologies (Paus, Keshavan, et al., 2008) . Continuing interpersonal motor alignment throughout adolescence may strongly support the establishment and maintenance of resilience and coping mechanisms. Yet research in this domain is critically lacking.

Indeed, as demonstrated throughout this article, reliable reciprocity through interpersonal motor alignment has positive social and emotional effects, which are, with the exception for action observation, underinvestigated during adolescence. Yet, especially its potentially strengthening effects of interpersonal motor alignment through mimicry and synchronous behavior on the connection between the AON and other brain regions implicated in social cognition, may have beneficial effects on adolescent resilience. This remains to be investigated, but interpersonal motor alignment programs to strengthen social resilience during adolescence and intervention programs targeting adolescents' psychological or psychiatric problems could be envisioned. This could be implemented, in (online) video games and social media to strengthen healthy adolescent development through interpersonal motoric reciprocity. Implementation on social media, via videoconferencing tools or on-or offline video games, may help reaching out to adolescents and making interventions more accessible to them. Yet, while this idea is attractive, technical limitations, such as delays of timing due to different internet connections, may hinder smooth interaction and impede mechanisms mediating positive effects. Aiming to overcome this problem, several software programs have been suggested for musicians and to sing together 1 . Social media and videoconferencing tools allow adolescents to stay connected, even in the face of isolation. This could be observed recently during social isolation in the wake of the COVID-19 pandemic, when two third of American teenagers reported using video chat to stay connected with their peers 2 . This makes this avenue worth pursuing.

This is the first integrative review of interpersonal motor alignment and its positive social functions from a developmental perspective. It highlights the importance of reliable interpersonal motor alignment for healthy social development and especially points towards the need to extend this research to adolescence, a period of enhanced social sensitivity. Indeed, although interpersonal motor alignment may link social cognition to cognitive control and may have an important role in healthy adolescent interactions, it is still underinvestigated during adolescence.

Moreover, since adolescence is a time of substantial neurobiological and behavioral changes, it may confer a psychopathological vulnerability (Paus, Keshavan, et al., 2008) . Studying interpersonal motor alignment as one of the links between social cognition and cognitive control may inform motor therapies for prevention and recovery and aid healthy adolescent development. Cracco, E., & Brass, M. (2018) . The role of sensorimotor processes in social group contagion. Table 1 : Experimental studies on interpersonal motor alignment on infants. Systematic search of studies on interpersonal motor alignment in infants (age <30 months) The table shows references with authors and year of publication, the type of interpersonal motor alignment (IMA), main findings and method used; * denotes studies mentioned in the text, EEG = electroencephalography, (f)NIRS = (functional) near-infrared spectroscopy, EMG = electromyography, n= number of participants (in the final sample of analyzed data). 

Only tongue protrusion modeling is matched by neonates

When age matters: differences in facial mimicry and autonomic responses to peers' emotions in teenagers and adults

Investigating the neural basis of cooperative joint action. An EEG hyperscanning study

Mirror neuron system involvement in empathy: a critical look at the evidence

Parieto-frontal circuits during observation of hidden and visible motor acts in children. A high-density EEG source imaging study

The role of imitation in the observed heterogeneity in EEG mu rhythm in autism and typical development

Brain-to-brain synchrony and learning outcomes vary by student-teacher dynamics: Evidence from a real-world classroom electroencephalography study

Action observation network in childhood: a comparative fMRI study with adults

Simultaneous scalp recorded EEG and local field potentials from monkey ventral premotor cortex during action observation and execution reveals the contribution of mirror and motor neurons to the mu-rhythm

Is adolescence a sensitive period for sociocultural processing?

The impact of social context on mimicry

Compatibility between observed and executed finger movements: comparing symbolic, spatial, and imitative cues

Imitative response tendencies in patients with frontal brain lesions

The inhibition of imitative and overlearned responses: a functional double dissociation

Imitation: is cognitive neuroscience solving the correspondence problem?

Inhibition of imitative behaviour and social cognition

Short-term physical training enhances mirror system activation to action observation

Effects of active and observational experience on EEG activity during early childhood

Neural circuits involved in the recognition of actions performed by nonconspecifics: An fMRI study

Making children laugh: Parent-child dyadic synchrony and preschool attachment

The influence of facial signals on the automatic imitation of hand actions

Action Observation and Acquired Motor Skills: An fMRI Study with Expert Dancers

Relations between infants' emerging reach-grasp competence and event-related desynchronization in EEG

Being Mimicked Increases Prosocial Behavior in 18-Month-Old Infants

Neural mechanisms of empathy in humans: a relay from neural systems for imitation to limbic areas

ALE meta-analysis of action observation and imitation in the human brain

Is It What You Do, or When You Do It? The Roles of Contingency and Similarity in Pro-Social Effects of Imitation

Making mirrors: premotor cortex stimulation enhances mirror and counter-mirror motor facilitation

Sensorimotor learning configures the human mirror system

Associative sequence learning: the role of experience in the development of imitation and the mirror system

The chameleon effect: The perception-behavior link and social interaction

The antecedents and consequences of human behavioral mimicry

Beyond the perception-behavior link

The ubiquitous utility and motivational moderators of nonconscious mimicry. The New Unconscious

Peers increase adolescent risk taking by enhancing activity in the brain's reward circuitry

An EEG/ERP investigation of the development of empathy in early and middle childhood

How interpersonal synchrony facilitates early prosocial behavior. Current Opinion in Psychology

Interpersonal synchrony increases prosocial behavior in infants

Infants' use of interpersonal asynchrony as a signal for third-party affiliation

Social effects of movement synchrony: increased infant helpfulness only transfers to affiliates of synchronously moving partners

Electroencephalographic activity during perception of motion in childhood

Rowers' high: behavioural synchrony is correlated with elevated pain thresholds

Social attitudes differentially modulate imitation in adolescents and adults

Neuroanatomical and neurochemical substrates of timing

Automatic imitation: A meta-analysis

Automatic imitation of multiple agents: Simultaneous or random representation? J o u r n a l P r e -p r o o f regulation

Mimicry and propagation of prosocial behavior in a natural setting

Impairments of social motor synchrony evident in autism spectrum disorder

Neural processing and production of gesture in children and adolescents with autism spectrum disorder

Synchronous activity of two people's prefrontal cortices during a cooperative task measured by simultaneous near-infrared spectroscopy

The influence of group membership on cross-contextual imitation

Mimicry and automatic imitation are not correlated

Cortical oscillations and speech processing: emerging computational principles and operations

Group dynamics in automatic imitation

Singing promotes cooperation in a diverse group of children

Not So Automatic Imitation: Expectation of Incongruence Reduces Interference in Both Autism Spectrum Disorder and Typical Development

Neural mechanisms of resistance to peer influence in early adolescence

Brain regions involved in human movement perception: A quantitative voxel-based meta-analysis

Action observation in the infant brain: the role of body form and motion

Virtual peacemakers: Mimicry increases empathy in simulated contact with virtual outgroup members

The essential role of Broca's area in imitation

Contagious yawning in autistic and typical development

Automatic Imitation

Experience modulates automatic imitation

Eight problems for the mirror neuron theory of action understanding in monkeys and humans

It's all in the timing: Interpersonal synchrony increases affiliation

Imitation, empathy, and mirror neurons

Birds of a feather and birds flocking together: Physical versus behavioral cues may lead to trait-versus goal-based group perception

Automatic facial mimicry in response to dynamic emotional stimuli in five-month-old infants

Can newborn infants imitate

Why do I like you when you behave like me? Neural mechanisms mediating positive consequences of observing someone being imitated

Movement synchrony and perceived entitativity

If they move in sync, they must feel in sync: Movement synchrony leads to attributions of rapport and entitativity

The chameleon effect as social glue: Evidence for the evolutionary significance of nonconscious mimicry

Using nonconscious behavioral mimicry to create affiliation and rapport

I am too just like you nonconscious mimicry as an automatic behavioral response to social exclusion

Sync to link: Endorphin-mediated synchrony effects on cooperation

Reduced mu power in response to unusual actions is context-dependent in 1-year-olds

Synchrony as an Adaptive Mechanism for Large-Scale Human Social Bonding

Why synchrony matters during mother-child interactions: a systematic review

Speaker gaze increases information coupling between infant and adult brains

system in children

EEG imaging of toddlers during dyadic turn-taking: Mu-rhythm modulation while producing or observing social actions

What" matters more than" Why"-Neonatal behaviors initiate social responses

Cortical activation to action perception is associated with action production abilities in young infants

Cortical activation to action perception is associated with action production abilities in young infants

Neural processing of race during imitation: Self-Similarity Versus Social Status

Race modulates neural activity during imitation

Activation of Mirror Neuron Regions is Altered in Developmental Coordination Disorder (DCD)-Neurophysiological Evidence using an Action Observation Paradigm

How imitation affects empathy and prosocial behavior: the role of movement congruency and temporal contingency

Neurobehavioral Interpersonal Synchrony in Early Development: The Role of Interactional Rhythms . Frontiers in Psychology

The beat will make you be courage": The role of a secondary school music program in supporting young refugees and newly arrived immigrants in Australia

Creating bridges: music, play and well-being in the lives of refugee and immigrant children and young people

Contributions of playground singing games to the social inclusion of refugee and newly arrived immigrant children in Australia

The imitation game: Effects of social cues on 'imitation'are domain-general in nature

Motor contagion in young children: Exploring social influences on perception-action coupling

Neural mirroring systems: Exploring the EEG mu rhythm in human infancy

Neural mirroring mechanisms and imitation in human infants

Visual perception in children: human, animal and virtual movement activates different cortical areas

Impaired cortical activation in autistic children: is the mirror neuron system involved?

Keeping together in time

Elements of a comprehensive theory of infant imitation

Explaining facial imitation: A theoretical model

Re-examination of Oostenbroek et al.(2016): Evidence for neonatal imitation of tongue protrusion

Imitating others' actions: transcranial magnetic stimulation of the parietal opercula reveals the processes underlying automatic imitation

Neural basis of the perception and estimation of time

Joint action modulates motor system involvement during action

Observing joint action: Coordination creates commitment

Do birds of a feather move together? Group membership and behavioral synchrony

The rhythm of rapport: Interpersonal synchrony and social perception

Inter-brain synchrony in mother-child dyads during cooperation: An fNIRS hyperscanning study

Structural brain development between childhood and adulthood: Convergence across four longitudinal samples

The beneficial effect of synchronized action on motor and perceptual timing in children

Exploring the EEG mu rhythm associated with observation and execution of a goaldirected action in 14-month-old preterm infants

An fMRI study of action observation and action execution in childhood

Single-neuron responses in humans during execution and observation of actions

The effects of interaction quality on neural synchrony during mother-child problem solving

Imitation in young children: When who gets copied is more important than what gets copied

Effect of visual orientation on mu suppression in children: a comparative EEG study with adults

Deficits in visuo-motor temporal integration impacts manual dexterity in probable developmental coordination disorder

Interpersonal synchrony enhanced through 20 Hz phase-coupled dual brain stimulation

Automatic imitation effects are influenced by experience of synchronous action in children

Developmental changes in mu suppression to observed and executed actions in autism spectrum disorders

Modulation of mu suppression in children with autism spectrum disorders in response to familiar or unfamiliar stimuli: the mirror neuron hypothesis

The neural network for the mirror system and mentalizing in normally developed children: an fMRI study

Comprehensive longitudinal study challenges the existence of neonatal imitation in humans

Priming third-party ostracism increases affiliative imitation in children

Putting the social into social learning: explaining both selectivity and fidelity in children's copying behavior

The social side of imitation

Why do many psychiatric disorders emerge during adolescence?

Morphological properties of the action-observation cortical network in adolescents with low and high resistance to peer influence

The ice-breaker effect: singing mediates fast social bonding

The action observation system when observing hand actions in autism and typical development

Infant negative reactivity defines the effects of parent-child synchrony on physiological and behavioral regulation of social stress

Rhythm perception, production, and synchronization during the perinatal period

Origins and development of mirroring mechanisms: A neuroconstructivist framework

Synchronous rhythmic interaction enhances children's perceived similarity and closeness towards each other

Joint rhythmic movement increases 4-year-old children's prosocial sharing and fairness toward peers

Synchronized movement experience enhances peer cooperation in preschool children

What are reaction time indices of automatic imitation measuring?

Being mimicked affects inhibitory mechanisms of imitation

Distinct neural processes are engaged in the modulation of mimicry by social group-membership and emotional expressions

The Modulation of Mimicry by Ethnic Group-Membership and Emotional Expressions

Event-related potentials of automatic imitation are modulated by ethnicity during stimulus processing, but not during motor execution

Imitation in infancy: the wealth of the stimulus

Mu desynchronization during observation and execution of facial expressions in 30-month-old children

Does synchrony promote generalized prosociality? Religion

Let's dance together: synchrony, shared intentionality and cooperation

Collective synchrony increases prosociality towards non-performers and outgroup members

Neural correlates of dyadic interaction during infancy

Brain-to-brain synchrony in parentchild dyads and the relationship with emotion regulation revealed by fNIRS-based hyperscanning

The mirror neuron system

The mirror system and its role in social cognition

Neurophysiological mechanisms underlying the understanding and imitation of action

Neural mirroring during the observation of live and video actions in infants

Exploring the role of neural mirroring in children with autism spectrum disorder

Neural correlates of being imitated: An EEG study in preverbal infants

An investigation of the determinants of motor contagion in preschool children

Infants' somatotopic neural responses to seeing human actions: I've got you under my skin

Mirror neurons system adults while viewing emotional gestures

Enhancing social ability by stimulating right temporoparietal junction

Social brain development and the affective consequences of ostracism in adolescence

Development of functional connectivity during adolescence: a longitudinal study using an actionobservation paradigm

Development of the action observation network during early adolescence: a longitudinal study

Infant's brain responses to live and televised action

What's so special about verbal imitation? Investigating the effect of modality on automaticity in children

Mimicry enhances observational learning in 16-Month-Old infants

Predictive motor activation during action observation in human infants

The Role of the Right Temporoparietal Junction in the Control of Imitation

The role of the right temporoparietal junction in the control of imitation

Minimal mimicry: Mere effector matching induces preference

Motor activation during observation of unusual versus ordinary actions in infancy

Speaker-listener neural coupling underlies successful communication

Developmental Differences in Cortical Activation During Action Observation, Action Execution and Interpersonal Synchrony: An fNIRS Study . Frontiers in Human Neuroscience

Synchrony and exertion during dance independently raise pain threshold and encourage social bonding

Music and social bonding: "self-other" merging and neurohormonal mechanisms

Silent disco: dancing in synchrony leads to elevated pain thresholds and social closeness

Investigating Cooperative Behavior in Ecological Settings: An EEG Hyperscanning Study

Movement synchrony forges social bonds across group divides

Interpersonal movement synchrony facilitates pro-social behavior in children's peer-play

Rock with me: The role of movement synchrony in infants' social and nonsocial choices

Infants' grip strength predicts mu rhythm attenuation during observation of lifting actions with weighted blocks

The modulation of facial mimicry by and their underlying affiliation motives in 3-year-olds: An EMG study

Synchrony and the social tuning of compassion

The rhythm of joint action: Synchrony promotes cooperative ability

Mimicry and prosocial behavior

Where is the love? The social aspects of mimicry

You'll never crawl alone: Neurophysiological evidence for experience-dependent motor resonance in infancy

Sexy online self-presentation on social network sites and the willingness to engage in sexting: A comparison of gender and age

Young children's motor interference is influenced by novel group membership

Little chameleons: The development of social mimicry during early childhood

Modulating mimicry: Exploring the roles of inhibitory control and social understanding in 5-year-olds' behavioral mimicry

Cooperative learning and interpersonal synchrony

Neural correlates of action understanding in infants: influence of motor experience

Neural correlates of action understanding in infants: influence of motor experience

The Action Imitation network and motor imitation in children and adolescents with autism

Social top-down response modulation (STORM): a model of the control of mimicry in social interaction

Eye contact enhances mimicry of intransitive hand movements

Infants' mu suppression during the observation of real and mimicked goal-directed actions

In-group ostracism increases high-fidelity imitation in early childhood

Singing and social bonding: changes in connectivity and pain threshold as a function of group size

Synchrony and cooperation

Interpersonal Synchronization, Motor Coordination, and Control Are Impaired During a Dynamic Imitation Task in Children With Autism Spectrum Disorder

A developmental and clinical perspective of rhythmic interpersonal coordination: from mimicry toward the interconnection of minds

Implicit behavioral mimicry: Investigating the impact of group membership

Desynchronization in EEG during perception of means-end actions and relations with infants' grasping skill

Social context effects in 2-and 4-year-olds' selective versus faithful imitation

Interpersonal body and neural synchronization as a marker of implicit social interaction

The Peer Model Advantage in Infants' Imitation of Familiar Gestures Performed by Differently Aged Models

Fourteen-Month-Olds' Imitation of Differently Aged Models. Infant and Child Development

This work was supported by the Fondation pour la Recherche Médicale (FRM, SPF20171039127) to BR, the grants from the Agence Nationale de la Recherche (ANR, France) ANR-14-ACHN-0023 and ANR-11-IDEX-0001-02) to MHG and benefited from the grants ANR-16-CONV-0002 (ILCB), ANR-11-LABX-0036 (BLRI) and the Excellence Initiative of Aix-Marseille University (A*MIDEX).

J o u r n a l P r e -p r o o f