Since the beginning of child psychiatry with Itard’s description of the wild child (Constant 2014), the importance of parent–child interactions and the social environment have been widely acknowledged as playing a central role in early developmental processes. At the end of the nineteenth century, Archambaud and Parrot attributed even greater influence to psychological factors on children’s troubles growing up in institutions with no stable caregiver.

These first works focused on mother deprivation and its consequences. In this way, the works about the “hospitalism” phenomenon initially described by Pfaundler (1915) highlighted the somatic consequences of maternal deprivation apart from the hygienic and somatic care provided to these children in institution.

In the 1930s, as part of the same movement, psychiatrists and psychoanalysts around the world analyzed side effects of fostering children in institutions. David Levy, in the American Journal of Orthopsychiatry, published the first description of “affective indifference” in a young child following affective deprivation. He called this phenomenon “primary affect hunger” in children removed very early from their mothers and brought up in institutions and multiple foster homes. These children, though often pleasant on the surface, seemed indifferent underneath. He questioned whether there could be a “deficiency disease of the emotional life, comparable to a deficiency of vital nutritional elements within the developing organism.” A few psychiatrists, psychologists, and pediatricians were also concerned by the high mortality rate in hospitals and institutions obsessed with sterility to the detriment of any human or nurturing contact with babies.

During the years of World War II, evacuated and orphaned children were the subject of studies that outlined their reactions to separation, including the ability to cope by forming relationships with other children. The research of psychoanalyst R. Spitz (1946) on the effects of maternal deprivation and hospitalism focused on infants who had experienced abrupt, long-term separation from the familiar caregiver. Spitz adopted the term anaclitic depression to describe the child’s reaction of grief, anger, and apathy to partial emotional deprivation (the loss of a loved object) and proposed that when the object of love is returned to the child within 3–5 months, recovery is prompt, but that after 5 months, the child will show the symptoms of increasingly serious deterioration.

Then, following the observations and experiments with animals conducted by Lorenz and Harlow (Harlow and Zimmermann 1958; Lorenz 1935), John Bowlby developed his attachment theory (Bowlby 1969). He used his direct experience with deprived children through his work at the London Child Guidance Clinic to investigate children’s early lives. Bowlby proposed that two environmental factors were fundamental in early childhood. The first was death of the mother, or prolonged separation from her; the second was the mother’s emotional attitude toward her child. He developed his attachment theory in Maternal Care and Mental Health published in 1951. He described it to be essential that the infant and young child should experience a warm, intimate, and continuous relationship with his mother (or permanent mother substitute/caregiver) in which both found satisfaction and enjoyment. Given this kind of relationship, emotions of guilt and anxiety (characteristics of mental illness when in excess) would develop in an organized and moderate way.

At the same period, in a psychoanalytic perspective, Donald Winnicott observed that immediately after giving birth, the mother becomes fixated with the infant to the point where everything and everyone else plays a secondary role. This, he theorized, made her even more sensitive to the infant’s needs. He called this state “Primary Maternal Preoccupation” and described the “good enough” mother’s characteristics (Winnicott 1947, 1969).

The attachment theory included the development of the concept of the affectional bond, sometimes referred to as the emotional bond, which is based on the universal tendency of humans to attach, i.e., to seek closeness to another person and to feel secure when that person is present. Bowlby proposed that babies have an innate need from birth to form emotional attachments, i.e., bonds, because this increases the chances of survival by ensuring that they receive the care they need. Bowlby did not describe mutuality in attachment. Actually, the biological aspect of bonding is well studied, particularly the hormonal regulation within a continuum between the prenatal and postnatal periods.

The studies of Bowlby focused on infants’ behaviors and mothers’ attachment pattern, but it is with the emergence of the developmental movement that the baby will really be considered as a partner in the interaction.

For a long time, the baby was considered to be passive, being subjected to his environment and particularly to his/her mother. First, in the 1930s, Mélanie Klein proposed a concept of child development based on her psychoanalyses of young children, where she considered that the infant and the toddler had their self-psychic and relational life.

In the last 40 years, the developmental field as it relates to babies has made very significant advances. The abilities of both baby and fetus have been increasingly demonstrated.

We know now that young children can represent the acts of others and their own acts in multiple dimensions. In his experiments, Andrew Meltzoff (1995) found that 18-month-old infants could perform target manipulations that adult experimenters attempted and failed, suggesting that the infants could represent the object-manipulating behavior of adults as involving goals and intentions.

According to Meltzoff, the infant’s innate understanding that others are “like me” allows it to recognize the equivalence between the physical and mental states apparent in others and those felt by the self. For example, the infant uses his own experiences orienting his head/eyes toward an object of interest to understand the movements of others who turn toward an object, that is, that they will generally attend to objects of interest or significance.

Thus, Simon Baron-Cohen (1991) identified the infant’s understanding of attention in others, a social skill found by 7–9 months of age, as a “critical precursor” to the development of theory of mind. Understanding others people’s intentions is another critical precursor to understanding other minds because intentionality, or “aboutness,” is a fundamental feature of mental states and events.

These studies are closely connected to the discovery of neuron mirrors. At first studied in primate species and birds, researchers discovered in humans a small number of neurons that fired or reached peak activity both when an individual performed a task and observed a task. Other neurons seem to have anti-mirror properties, that is, they responded when the participant performed an action but were inhibited when the participant witnessed that action.

This phenomenon has been observed with respect to emotions, with the same cerebral areas being activated when a subject feels an emotion or observes someone feeling the same emotion. According to Decety’s experiments (Cheng et al. 2014), mirror neurons could be linked to the development of empathy. All children’s skills are linked to the interaction between the child and his caregiver.

With the advent of the “interactionist” movement, the model of the mother–baby relationship was modified. It was described not just as a relation on a linear causality mode or as addition of the mother’s and the child’s factors but as a transactional or interactional dynamic (Escalona 1968). Transactional theory considers that the environment (mother or father) and the newborn influenced each other in a continuous process of development and change. This model described the complex chain of bidirectional processes, which are described not in a closed circle but rather in a spiral.

Thus, the relationship between the baby and his environment is actually considered as a set of bidirectional processes, where the baby is not only influenced by his environment but is also at the origin of considerable modifications. This further supports the concept of interaction. The interaction defines itself as the mutual influence of two phenomena, two subjects with a notion of reciprocity and interdependence. Generally, the interactions are defined as all the dynamic phenomena that take place in the time between an infant and his different partners.

In analyzing synchrony as the unfolding dance between matched and mismatched states, (Tronick and Cohn 1989) pointed out that mother and child spend most of their playtime in mismatched rather than matched states, yet most of the mismatch is repaired in the next step. By highlighting the concepts of mismatch and repair, the theoretical focus shifts to the way dyads repair moments of miscoordination as the central component of intimate relationships and of the synchrony experience. A major function of the co-regulatory process, therefore, is the self-correcting capacities of the dyad and the infant’s growing appreciation that relationships are not always fully attuned to one’s needs. The interactive partnership between an infant and caregiver, usually called a dyad, is actually defined and explored as a single unit.

Because the relationship between an infant and his caregiver is bidirectional in nature, the dyad should be thought of as a dynamically interacting system. An infant can influence the care he receives from the caregiver in the ways he behaves. Given the dynamic relationship between an infant and his caregiver, a specific interest in the flow characterizing the exchange of information during infant–caregiver interactions has emerged, leading to the study of rhythm (balance between partners), reciprocity (the partners’ ability to show adaptation to each other), and synchrony (the dynamic and reciprocal adaptation of the temporal structure of behaviors between interactive partners). The recent discovery of both biological correlates of behaviorally synchronic phenomena and statistical learning validated the crucial value of studying synchrony during child development.

The dynamic and temporal component of the interaction is highlighted by most of the authors but difficult to assess. This dynamic is transversal, i.e., one moment and its interaction, but also longitudinal, i.e., an evolution during the development of the child and the adaptation of the parent.

A lot of words and concepts are used to describe this temporal component of interactions, underscoring its complexity. Some focus more on concordance of the act and affect concordance. Others focus on reciprocity and Contingence and even speak of Harmony (Lebovici and Stoleru 1999). Micro- and macroanalysis with videotaped sequences has itself allowed to best analyze the rhythmicity and synchrony components and allows access to the temporal and rhythmic structure of the interaction. Stern describes the repetitive–rhythmic structure of the interaction that he calls kinetic sentences and interactive dance. In her broad studies, Feldman assesses the synchrony component of the interaction (Feldman 2007).

Synchrony is defined as the dynamic and reciprocal adaptation of the temporal structure of behaviors between interactive partners and implies the following: (i) Behaviors include verbal and nonverbal communicative and emotional behaviors (e.g., gestures, postures, facial displays, vocalizations, and gazes) and (ii) synchronous interactions entail coordination between partners and intermodality. Caregivers and their children are able to respond to each other using different modalities starting from birth (Leclere et al. 2014)

Indeed, synchrony describes the intricate “dance” that occurs during short, intense, playful interactions; it builds on familiarity with the partner’s behavioral repertoire and interaction rhythms; and it depicts the underlying temporal structure of highly aroused moments of interpersonal exchange that are clearly separated from the stream of daily life.

Despite the similarities between synchrony and other established constructs in the mother–child relationship, synchrony is different in a number of meaningful ways. Synchrony encompasses both the mother’s and the child’s responsivity and their emotional capacity to respond to each other. During early development, synchrony involves a matching of behaviors, emotional states, and biological rhythms between parents and infants that together forms a single relational unit (dyad). Affiliative bonds, defined as selective and enduring attachments, are formed on the basis of multiple genetic, hormonal, brain, autonomic, epigenetic, behavioral, and mental processes that coordinate to establish the parent–infant bond. Oxytocin, considered to be the principal bonding hormone, appears to enhance physiological and behavioral readiness for social engagement in parent–infant interactions. Its biology is not fully elucidated but is, in part, related to epigenetic mechanisms.

We might thus consider a definition of interaction at three levels according to Bernieri, 1988: (1) the biological rhythms between mother and child with co-regulation and cycles, beginning during pregnancy, and even present after birth with other caregivers such as the father (Weisman et al. 2013); (2) imitation and simultaneous behaviors, well described as contingence, reciprocity, etc., through sequences of micro- and macro-rhythms (Harrist and Waugh 2002) that might be considered in parallel with micro- and macro-synchrony; and (3) unity of the dyad, as a system to study its globality, a systemic approach developed by Palo Alto Cybernétique (Codruţa and Hainic 2011).

Prenatal attachment and maternal representations during pregnancy are linked to the early mother–infant pattern and subsequently influence infant development and well-being (Feldman 2007; Field et al. 2003; Fonagy et al. 1991; Siddiqui and Hagglof 2000; Viaux-Savelon et al. 2012). These parental representations (Ammaniti 1991), such as a mother’s representations of herself as a mother and of the future baby, influence the early interactions and the child’s style of attachment. These representations are constructed even before conception, from the desire to have a child and intergenerational projections of the family construction. The construction of these representations is dynamic throughout the prenatal and postnatal periods. Numerous environmental factors might disrupt this dynamic. The representation of the child in the mother’s mind contains conscious and unconscious elements: fantasies, character traits, feelings toward the child, as well as her representation of herself as a mother, her representation of her own mother, the child’s father, and others (Viaux-Savelon 2014).

In the preconceptional period, difficulties getting pregnant might interfere with a woman’s confidence on becoming a mother. During pregnancy, the influence of prenatal diagnosis such as amniocentesis and routine ultrasound scans during the different trimesters of pregnancy has been analyzed in a growing corpus of research (Götzmann et al. 2002; Petersen and Jahn 2008; Viaux-Savelon et al. 2012). For example, ultrasound visualization of the fetus increases woman’s confidence in their childcare abilities, strengthens the relationship between mother/father and child, and intensifies positive emotions toward the child. The parents begin to view the child as a separate individual. However, when prenatal diagnosis signals possible fetal abnormalities, the situation becomes altogether different. Women followed up for prenatal fetus diagnosis more frequently suffered from negative emotions related to the risk of discovering pathological changes in the fetus. As a result, they were more likely to experience anxiety for the child’s health and their future parenthood. They identified themselves with the role of parent to a lesser degree, and a smaller number of them considered the child as a separate individual.

The first chapter of the new life begins during pregnancy. At the start of this period, the process of parenthood is engaged and the premise of mother–child interactions presents. Missonnier described it as “ROV,” virtual object relationship (Missonnier 2007).

Signs of this relationship are highlighted at the biological level, such as oxytocin mutual regulation (Feldman 2007) and reactions of the fetus to the mother’s voice, stress, etc., observable during fetal sonography through fetal movement and modulation of heartbeats.

Studies by DeCasper and Spence (1986) and Kisilevsky et al. (2009) showed that 32-week-old GA fetuses discriminate their mother’s voice from that of a female stranger, suggesting recognition/learning of some property of her voice. Motherese, this so particular vocal cue used by mothers, with high-pitched vocalizations specific to mother–child interaction, is observed around the fourth semester, when the mother perceives fetal movements. Moreover, prenatal stress has been shown to impair the quality of the mother–infant interaction in both animal (Kaiser et al. 2003) and human studies (Field, et al. 2003; Gerardin et al. 2011).

On the other hand, the emergence of the fetal biological clock (sleep–wake cyclicity) and sympathetic control over heart rhythms are dated to the last trimester of pregnancy, following structural and functional brain development, including the assembly of brain nuclei, rapid increase in synaptic growth, and maturation of neurochemical systems (Peirano et al. 2003).

The phase shift for the sleep–wake cycle occurs between 30 and 31 weeks of gestational age, while the phase shift for cardiac vagal tone, an index of parasympathetic control over heart rhythms, follows between 33 and 34 weeks of gestation. Infants with more mature biological rhythms at term age show higher levels of mother–infant synchrony at 3 months. The degree of maturity in each biological rhythm is uniquely predictive of mother–infant synchrony, indicating that each oscillator marks a specific pathway to the emergence of synchrony.

Concerning the endocrine system, the principal hormones studied were cortisol as the stress hormone and prolactin and oxytocin as the bonding hormones. Maternal behavior is supported by hormonal systems that undergo change during pregnancy and sensitize mothers to infant cues, such as oxytocin and prolactin (Grattan et al. 2001). Oxytocin, a neuropeptide released during uterine contraction and milk ejection, has been implicated in the initiation of maternal behavior in mammals and in close bonds throughout life (Kosfeld et al. 2005).

Average oxytocin levels across pregnancy and the postpartum period predicted not only the frequency of maternal bonding behaviors but also the degree of its coordination with the newborn’s alert state, pointing to the role of oxytocin in setting the stage for bonding in humans as well as in the development of coordinated interactions (Feldman 2007). Mothers who provided more coherent and emotionally rich narratives regarding their newborns and the maternal role were found to engage in more elaborate postpartum behaviors.