Chiara Fini¹
email: chiara.fini@uniroma1.it

Lara Bardi

Dimitris Bolis

Martina Fusaro

Matteo P Lisi

Arthur Henri Michalland

Vanessa Era

Building the physical and psychological self: from a bodily to a spoken interpersonal dialogue

Keywords: max 6) social touch, interpersonal interactions, implicit theory of mind, intersubjectivity, abstract concepts, bodily self

In this review, we propose that interpersonal bodily interactions represent a fertile ground in which the bodily and psychological self is developed, gradually allowing for forms of more abstract and disembodied interactions. We start by focusing on how early infant-caregiver bodily interactions, mediated by the sense of touch, play a crucial role in shaping the boundaries of the self but also in learning to predict others' behavior. We then explore the social function of the sense of touch in the entire life span, highlighting its role in promoting physical and psychological wellbeing by supporting positive interpersonal exchanges. We go on by introducing the concept of implicit theory of mind, as the very early ability to interpret others' intentions, possibly grounded in infant-caregiver bodily exchanges (embodied practices). In the second section, we embrace a more collective perspective about the role of sociality in human development, by describing the hypothesis of "dialectical attunement", reconsidering the self beyond the individual, where it really emerges, unfolds, and manifests itselfin social relationships. We finally consider the most evolved forms of social relationship: intellectual exchanges among individuals. In this regard, we defend the view that, beside the apparent private dimension of "thinking abstractly", using abstract concepts is an intrinsically social process, as it entails the re-enactment of the internalized dialogue through which we acquired the concepts in the first place.

Introduction

Self-awareness emerges from bodily exchanges

Bodily interactions among humans represent the necessary background allowing the progressive growth of the awareness of who we are. Through the interiorization of the epidermal dialogue with the mother, the child creates an implicit representation of her body and through the progressive knowledge of her internal states in conjunction with the social events, she becomes able to predict her own and other's intentions, emotions, and feelings. The epidermal dialogue will then leave the place to spoken dialogue, where physical proximity is not necessary anymore to be in relation with someone and to understand her internal state. Thanks to the abstraction/symbolization of interpersonal relations, two interacting bodies will thus be supplanted by two interacting minds. In this path of self-development, the other will contribute to the definition of the self especially through one sense among the five: the touch.

Touch more than other senses participates in defining the self-in-the-world (see [START_REF] Gallace | The science of interpersonal touch: an overview[END_REF][START_REF] Farmer | The bodily social self: a link between phenomenal and narrative selfhood[END_REF], a concept to which theorists and scientists refer with different terms: individuals' sensorimotor embodiment, [START_REF] Kyselo | More than our body: Minimal and enactive selfhood in global paralysis[END_REF], minimal self, (Blanke & Metzinger, 2009), or bodily self [START_REF] Apps | The free-energy self: a predictive coding account of self-recognition[END_REF].

Traditionally, the bodily self has been considered as composed of a motor component which is the sense of agency (the sense of being the one who generates an action) and of a perceptual component which is the sense of ownership (the feeling of owning our body, that is the source of our sensory experiences). Bodily self and peripersonal space representations [START_REF] Fanghella | Interpersonal Motor Interactions Shape Multisensory Representations of the Peripersonal Space[END_REF] are shaped by sensorimotor interactions with objects (tool use, [START_REF] Magosso | Neural bases of peri-hand space plasticity through tool-use: Insights from a combined computational-experimental approach[END_REF] or other individuals [START_REF] Soliman | Consequences of joint action: Entanglement with your partner[END_REF][START_REF] Bolt | Mutual coordination strengthens the sense of joint agency in cooperative joint action[END_REF][START_REF] Haggard | Sense of agency in the human brain[END_REF]. The link between sensorimotor interactions and the bodily self clearly appears since the earliest moments of life. During the first sensorimotor interactions with the environment, children get acquainted with the perceptual co-occurrence of multisensory signals and, by integrating and responding to them, they optimize the natural exchanges between the body and the environment. In order to maintain constant such homeostasis, anticipatory bodily signals are generated: on-line exteroceptive and/or interoceptive sensations are matched with patterns of learned or innate predisposition and, from this repeated matching, the organism strives to reduce the distance between what is experienced hic et nunc and its generative models of the world and of the body.

Crucially, through such constant optimization of the bodily state, the child might develop bodily self-consciousness (see [START_REF] Ciaunica | Minimal self-awareness: from within a developmental perspective[END_REF][START_REF] Fotopoulou | Mentalizing homeostasis: The social origins of interoceptive inference[END_REF]. Since the optimization of the child's bodily state depends on the caregiver, who is in charge of externally regulating the necessary exchanges between the child's body and the environment, the bodily self-consciousness appears as being clearly socially mediated (see [START_REF] Ciaunica | The touched self: Psychological and philosophical perspectives on proximal intersubjectivity and the self[END_REF].

From the perceptual integration and the sensorimotor routines (i.e., breastfeeding, washing, rubbing-cleaning, skin-toskin sleeping, body-to-body temperature regulation, and skin hydration) to which they are exposed (see [START_REF] Ciaunica | The touched self: Psychological and philosophical perspectives on proximal intersubjectivity and the self[END_REF], children learn to interpret social contexts formed by chains of events (their action and its consequences on the caregiver 's behavior and the caregiver's action and its consequences on their bodies). The caregiver, on the one hand, informs the child about the social impact this latter is exerting on the caregiver, while on the other, she informs the child about the social impact the caregiver herself is exerting on the child. By receiving such bidirectional perceptual and sensorimotor feedback, the child learns to predict and interpret the other's behavior. In other words, through the tactile exchanges with the caregiver's body, children acquire the awareness of their own bodies. Skin-to-skin contact or social touch has a crucial function not only during individuals' development but in the whole life span. Interpersonal touch can, in fact, be a powerful tool to convey prosocial and emotional communication by means of stroking, patting, holding, or hugging other individuals (Hertenstein & Weiss 2011). While social touch has a crucial role in shaping the bodily self and helping predict others' behaviour during the early stages of life, later it also assumes a communicative role and its saliency is dependent on the social identity of the interactor.

From bodily to mental intersubjectivity

Tactile behavior is also related to the development of Theory of Mind (the ability to impute mental states, such as beliefs, desires, or intentions, to oneself and to others (ToM;Happe & Frith, 2014;[START_REF] Wellman | Young children's understanding of perception, desire, and emotion[END_REF]; for a review, see [START_REF] Wellman | Theory of mind across the lifespan?[END_REF]). Although most children under four years old fail standard ToM tasks, their behaviour suggests that they understand the connection between others' mental states and their actions (see Gallangher et al., 2001;[START_REF] Teglasi | Explicit and implicit theory of mind and social competence: A social information processing framework[END_REF]. Accordingly with the embodied practice of intersubjectivity (Gallangher et al., 2001), infants plan their actions on the basis of the perceptual inputs received from the caregiver, and such intersubjective sensorimotor mapping might be the precursor of intersubjective mental mapping (see Gopnik and Meltzoff, 1997), on which the implicit ToM might lay down. In this regard, it has been proposed that implicit ToM might emerge in the first year of life, while explicit ToM would develop later (between the fourth and sixth year of life) [START_REF] Sodian | Theory of mind in infancy[END_REF](Sodian, , 2016)). Moreover, while explicit ToM is dependent on executive functions [START_REF] Devine | Relations between false belief understanding and executive function in early childhood: A meta-analysis[END_REF][START_REF] Ruffman | Age-related differences in deception[END_REF][START_REF] Setoh | Contrasting lexical biases in bilingual English-Mandarin speech: Verb-biased mothers, but noun-biased toddlers[END_REF], implicit ToM is already grounded in embodied practices, emotional, sensorimotor, perceptual, nonconceptual (see Gallangher, 2001) and it refers to infants' behaviour which clearly responds to others' goals, beliefs, intentions, knowledge, or feelings [START_REF] Baillargeon | False-belief understanding in infants[END_REF][START_REF] Buttelmann | Eighteen-month-olds understand false beliefs in an unexpected-contents task[END_REF][START_REF] Osterhaus | Scaling of advanced theory-of-mind tasks[END_REF][START_REF] Perner | From infants' to children's appreciation of belief[END_REF]. In [START_REF] Teglasi | Explicit and implicit theory of mind and social competence: A social information processing framework[END_REF], it has been shown that implicit ToM is correlated with social competence (the ability to process social information), but this is not the case for explicit ToM, after having controlled for the explicit language abilities and vocabulary. This result suggests that infants are early equipped with the ability to correctly interpret others' mental states and this predisposition does not require growth in cognitive abilities and language.

The early-emerging capability to spontaneously interpret others' intention aligns with the idea that our mind might be built on an implicit embodied dialogue with other human beings. In keeping with the Vygotskian perspective, the self would emerge within social relations as a space of intersubjectivity necessary to structure and cultivate it. Consistent with this view, neural evidence has shown that during introspection and social cognition there is a recruitment of similar brain circuits, including the dorso-medial prefrontal cortex and the precuneus [START_REF] Schilbach | Introspective minds: using ALE meta-analyses to study commonalities in the neural correlates of emotional processing, social & unconstrained cognition[END_REF][START_REF] Bolis | I interact therefore I am': The self as a historical product of dialectical attunement[END_REF].

As [START_REF] Bolis | I interact therefore I am': The self as a historical product of dialectical attunement[END_REF] proposed, such similarities in neural activations might indicate that social and individual thoughts are not separated by sharp boundaries. In this regard, the authors introduce the concept of "dialectical attunement" (composed of tightly interconnected internalization and externalization processes) to account for the intimate connection between the social and the individual dimensions. In more detail, humans co-construct bodily as well as interbodily anticipatory structures to predict the consequences of sensorimotor interactions (cf. internalization), while cotransforming their ecosocial niche (cf. externalization), continuously receiving and providing social feedback. This dynamic loop has been thought of as being realized across both collective (i.e., cultural practices and human evolution) and individual time scales (individual development and everyday sensorimotor functioning), enabling self-construction. Thus, within this theoretical framework, human mind development is inherently realized under a constant collective/interpersonal regulation. The role of interpersonal regulation is clearly manifested in the most sophisticated human ability: abstract thinking. The roots of abstract conceptualization are based in the dialogue underpinned among minds: through conversational practice the single individual finds the social validation of complex meanings, while the human community updates and filters the knowledge accumulated along the course of time.

Interbodily interactions anticipate symbolic/abstract communication

Bodily interactions among the infant and the mother begin before the birth and represent the main form of communication between them. These primary experiences of being in relation through the body with another individual will lay the ground for the development of more symbolic modalities of interaction. The leap to symbolic interactions (i.e., conversation, writing, chatting) allows being connected with the other without the bodily contact, which for the infant has represented the main channel at disposal to be in relation with the other. The progressive detachment from the spatio-temporal and bodily dimensions during social interactions entails moving towards highly abstract and at times "disembodied" modalities of being-with. The increased symbolic and abstract mode of social interactions (less physical, more linguistic) is flanked by an increased symbolic and abstract semantic content of the same (less bodily experienced topics i.e., "glass", "chair", "table", more complex sensorimotor detached topics i.e., "freedom", "peace", "justice"). In other words, how we socially interact and about what we interact as well, both conflate into a more abstract dimension. Humans, indeed, think abstractly. They form, update and negotiate abstract meanings across individual and collective scales in and through (linguistic) interactions. When processing abstract concepts, the linguistic experience through which they have been acquired is manifested in the inner speech (WAT theory, [START_REF] Borghi | The challenge of abstract concepts[END_REF][START_REF] Borghi | Varieties of abstract concepts: development, use and representation in the brain[END_REF][START_REF] Borghi | Words as social tools: Language, sociality and inner grounding in abstract concepts[END_REF][START_REF] Borghi | A future of words: Language and the challenge of abstract concepts[END_REF][START_REF] Borghi | Abstract Concepts and metacognition: searching for meaning in self and others[END_REF]. Inner speech represents the internalized dialogue through which the symbolic meaning has been learned during the linguistic exchanges and will serve the function of co-construing new meanings susceptible to be transformed under the contextual and historical dynamics. In other words, the individual masters abstract meanings by re-enacting the internalized intellectual interactions on which the concepts are grounded. Thus, mastering symbolic knowledge is a process with profoundly social roots.

Here, we offer a broad theoretical perspective on the relevance of social interactions to allow the maturation of all the human dimensions (bodily, psychological, cultural) across many scales (individual, collective). We put together studies suggesting how social interactions, especially in infancy, entail body-to-body experiences which represent the natural scaffolding of self-development. In this regard, the adult mind is shaped by the presence of others who are the necessary actors allowing the self to exist; indeed, firstly, through physical and then through psychological interactions, the human mind might be construed as "intrinsically relational". This perspective review is structured in two main sections: in the first one we review studies highlighting the impact of social interactions on the individual development; specifically we deal with the role of the sense of touch in the construction of the bodily self and in the prediction of other's behaviour early in life; we explore the social function of the sense of touch in the entire life span, highlighting its role in promoting wellbeing and positive as well as pleasurable interactions with close others during adulthood. We introduce the concept of implicit theory of mind and its fundamental role in supporting individuals' understanding of others' mental state since the very early stages of life. In the second section by stretching the role of social interactions to a collective perspective, we propose a theoretical framework structured around the concept of "dialectical attunement" in which the sense of self is considered as emerging within social interactions, finally, the last section proposes abstract thinking as an emblematic example of "dialectical attunement".

Sociality and human development across an individual scale: from the epidermal contact to mind reading

Interpersonal touch, prediction and bodily self

The bodily interactions with the caregiver is the relational terrain from which the infant starts to differentiate herself from the other and to build up the minimal self. Through the interpersonal affective touch expressed in many life routines, infants learn the concomitance of multimodal signals that act as external regulators of the infant's internal homeostasis.

The statistical regularities associated with the occurrence of interoceptive, tactile, visual, and auditory stimuli shared with the caregiver's body promote the development of the private experience of themselves, which has a necessary social origin. According to the Free Energy perspective [START_REF] Friston | A theory of cortical responses[END_REF], humans need to interpret many signals from noisy information produced by the organism and by the external world. They, indeed, necessitate to maintain specific homeostatic parameters (i.e., temperature, heartbeat frequency, oxygen and glucose intake, hydration) within certain ranges that are compatible with life, and thus they strive to functionally capture the association between the external and internal/biological events, which are the cause of their balance. Their aim is to predict future changes relevant for their body, and thus anticipate aversive or appetitive contexts. The Free Energy proposal [START_REF] Friston | A theory of cortical responses[END_REF] suggests that the human brain advances prediction about the association between noisy information and bodily events following a probabilistic approach, specifically by applying generative models. Such generative models are continuously updated to reduce the prediction error and to improve their fit with reality. The prediction represents infants' active disposition to identify abstract principles that undergo the noisy signals and thus, to order the reality. Such effort produces two consequences: i) the immediate satisfaction of the biological needs in order to maximise the probability of surviving; ii) the progressive integration of the multimodal signals to construe the bodily self. When the infant cries because she feels cold, and the caregiver approaches her to bring the infant to the chest, the latter learns the association between her sense of cold, the act of crying, the bodily contact with the mother and the re-establishment of the optimal temperature inside her body.

During the unfolding of the events, the infant not only has satisfied primary needs through the bodily interaction with the caregiver, but she has also learned a chain of probabilistic occurrences of the same multisensorial, external, internal events. Every time that the predictions will fit the reality, the infant will be more and more aware of her body, and she will identify herself as a separate individual from the mother. Crucially, the bodily contact between the infant and the mother is the interactive background where the infant can start to predict the statistical regularities that will be the basis of the "mentalization" (see [START_REF] Fotopoulou | The virtual bodily self: Mentalisation of the body as revealed in anosognosia for hemiplegia[END_REF] of the bodily self. As it has been already outlined by [START_REF] Ciaunica | The touched self: Psychological and philosophical perspectives on proximal intersubjectivity and the self[END_REF]: "the physical contact and interpersonal touch have a unique, primary role in the minimal self" and we do agree with the authors' position for the following reasons. Bodily contact not only serves the satisfaction of the primary needs (i.e. nourishment, temperature regulation, hygiene), but it is also the modality through which we create an implicit mental representation of our bodily self and through which our identity or narrative self is gradually shaped. Thus, the sense of touch, as the perimeter of our body, defines the beginning and the end of the self-construction processes.

Social and affective touch promotes wellbeing and positive interpersonal exchanges

The sense of touch defines our bodily-self by connecting ourselves with others and with the external world. Next to the function of tracing the perimeter of ourselves, it also represents the channel through which we express and regulate affective and social interpersonal relations. Around 8 weeks, fetuses already perform movements to touch parts of their environment, such as the womb, their own body, or the body of a twin if present [START_REF] Bradley | Fetal sensory receptors[END_REF]. Later in their development, during the third trimester, they start to actively respond to the tactile stimulation of the abdomen by the mother and they perform less self-touch [START_REF] Marx | Fetal behavioral responses to the touch of the mother's abdomen: A Frame-by-frame analysis[END_REF]. When the baby is born, touch keeps being a fundamental modality for the first interactions and for the general development of the brain (Björnsdotter et al., 2014). Most of the communication between the caregiver and the infant, in particular the one implicated in responding to basic needs, such as nursing and taking care of the baby's hygiene, is supported by touch. These tactile exchanges are critical not only to survival but are also involved in the relief of behavioural and physiological responses to baby's stress [START_REF] Feldman | Touch attenuates infants' physiological reactivity to stress[END_REF]. It has been demonstrated, for instance, that neonates show decreased heart rate (which is an index of reduced stress) and increased engagement with the caregiver when caressed at a CT-velocity [START_REF] Fairhurst | Physiological and behavioral responses reveal 9-month-old infants' sensitivity to pleasant touch[END_REF]. These effects are also relevant when the baby must undergo medical procedures: touch can have an analgesic effect [START_REF] Bellieni | Sensorial saturation for neonatal analgesia[END_REF] and can facilitate growth and reduce hospitalization [START_REF] Field | Tactile/Kinesthetic Stimulation Effects on Preterm Neonates[END_REF]. The skin-to-skin contact with the newborn child, i.e., the kangaroo care, is considered in many countries a fundamental practice in the maternal care unit that promotes successful breastfeeding and improvement in the development of preterm babies [START_REF] Moore | Early skin-to-skin contact for mothers and their healthy newborn infants[END_REF]. Moreover, these first dyadic interactions (not only their quantity but importantly their quality) are crucial for promoting the development of a positive attachment, the ability to recognize emotions and to self-regulate [START_REF] Crucianelli | The mindedness of maternal touch: An investigation of maternal mind-mindedness and mother-infant touch interactions[END_REF][START_REF] Crucianelli | Developmental perspectives on interpersonal affective touch[END_REF]. Throughout childhood, touch becomes an important factor in family dynamics; a study by Bai and colleagues in 2016 show how touch between children, parents and siblings is a strong predictor of the expression of positive emotions. Besides tactile interactions with the caregivers, few studies have investigated interpersonal touch between children and friends or non-familial adults [START_REF] Cascio | Social touch and human development[END_REF]. For instance, it has been demonstrated that friendly touches from a stranger [START_REF] Leonard | The effect of friendly touch on delay-of-gratification in preschool children[END_REF] or from a teacher [START_REF] Wheldall | A touch of reinforcement: The effects of contingent teacher touch on the classroom behaviour of young children[END_REF] can enhance children's self-control and attention and decrease disruptive behavior. Psychophysical, behavioral [START_REF] Fairhurst | Physiological and behavioral responses reveal 9-month-old infants' sensitivity to pleasant touch[END_REF] and neuroimaging studies [START_REF] Jönsson | Affective and non-affective touch evoke differential brain responses in 2-month-old infants[END_REF] have shown that children's responses to touch are similar to adults' responses, demonstrating the ability to differentiate CT touch from non-CT touch [START_REF] Croy | Gentle touch perception: From early childhood to adolescence[END_REF]. The intensity of the perceived touch seems to increase with age, suggesting that the maturation of circuits involved in tactile perception continues throughout the lifespan (Björnsdotter et al., 2014). A recent study by Haggarty and colleagues in 2021 demonstrated that children, differently from adults, rate CT-touches equally pleasant, regardless of the area being touched. This result might reflect the predominant sensory information that children use to evaluate touches, while in adults other top-down influences are at stake. Indeed, the transition to adulthood is characterized by increasing complexity of adaptive functions associated with touch, including the establishment of cooperative social bonds and romantic relationships [START_REF] Hertenstein | The communicative functions of touch in humans, nonhuman primates, and rats: a review and synthesis of the empirical research[END_REF], which seem to be embodied into different spatial touching patterns. The strength of the emotional bond with the toucher is directly associated with the number of bodily regions allowed to be touched and the judgment of touch pleasantness [START_REF] Suvilehto | Topography of social touching depends on emotional bonds between humans[END_REF]. This evidence is present across cultures [START_REF] Suvilehto | Cross-cultural similarity in relationship-specific social touching[END_REF], suggesting that the spatial-related allowance to be touched might be a biologically rooted, adaptive mechanism to regulate social relationships. While on the one hand, intimate forms of touch are the primary way to develop romantic relationships (Andersen et al., 2006), and they are highly associated with well-being (Von [START_REF] Mohr | Social touch deprivation during COVID-19: effects on psychological wellbeing and craving interpersonal touch[END_REF] and relationship satisfaction [START_REF] Nelson | Mutual grooming in human dyadic relationships: an ethological perspective[END_REF], on the other hand, unwanted forms of intimate touch can be an expression of inappropriateness and sexual harassment [START_REF] Lee | Types of touch in cross-sex relationships between coworkers: Perceptions of relational and emotional messages, inappropriateness, and sexual harassment[END_REF]. Brain imaging studies show that the information about the toucher is encoded at the early stages of sensory processing, in the primary somatosensory cortices (Gazzola et al., 2012) and the insular cortex [START_REF] Suvilehto | Relationship-specific encoding of social touch in somatosensory and insular cortices[END_REF]. Technological advances may allow detailed investigations on how toucher's social characteristics (such as relationship status and group membership) are integrated with the topographical features of touch to evaluate the touch (in)appropriateness, trigger positive or negative emotional states and behavioral outcomes of approach or avoidance (Lisi et al., 2021a, b). Recent studies show indeed that it is possible to induce vicarious sensations of intimate touch by observing an embodied virtual avatar being caressed on taboo zones (such as the genitalia; [START_REF] Fusaro | Heterosexual, gay, and lesbian people's reactivity to virtual caresses on their embodied avatars' taboo zones[END_REF]. Vicarious touches delivered by a male and a female avatar elicited different behavioral and physiological reactions according not only to the participants' sexual preference but also to their own virtual body appearance: when embodying an opposite-sex virtual body, participants responded as if they were in the shoes of their embodied avatar [START_REF] Mello | Wearing same-and opposite-sex virtual bodies and seeing them caressed in intimate areas[END_REF]. This result suggests that it is possible to enhance perspective-taking in people who may lack it and promote intervention on violence and sexual harassment.

Implicit theory of mind to spontaneously interpret others' behavior

Tactile behavior participates in defining our bodily self, in regulating emotional distress and interpersonal relationships, but there is something more: the very first reciprocal knowledge happens between two interacting bodies. Indeed, at the very beginning the main form of communication is embodied and later it will evolve in more abstract forms. In this regard, [START_REF] Gallagher | The practice of mind. Theory, simulation or primary interaction?[END_REF] introduces the concept of the embodied practice of mind, to point out the importance of primary and secondary intersubjectivity for social understanding. The primary intersubjectivity allows infants to grasp the intentions and emotions of other persons by detecting and tracking eye movements, by interpreting movements, gestures, postures, facial expressions and contextualized behaviors. The secondary intersubjectivity occurs in the first year of age and marks the acquisition of the capability to communicate with others about an object or an event. The infant's attention becomes shared, indeed by following gazes, pointing and communicating with others about objects, she attributes social meanings to events, routines and interactions (see [START_REF] Spaulding | Embodied cognition and mindreading[END_REF]. These embodied practices allow infants to grasp others' intentions without the need to cognitively infer others' desires or beliefs; in fact, the behaviour itself can be naturally decoded. Gopnik and Meltzoff (1997) have proposed that from the intersubjective mapping we might end up promoting abstract inferential processes on the other's mental state. Being already equipped with the abilities to interpret the coupled mapping of the caregiver's sensorimotor inputs onto our own sensorimotor system, means also being able to have access to others' intentions.

Theory of Mind (ToM, but also mentalizing or mind-reading) [START_REF] Premack | Does the chimpanzee have a theory of mind?[END_REF][START_REF] Wimmer | Beliefs about beliefs: Representation and constraining function of wrong beliefs in young children's understanding of deception[END_REF] has been the focus of intensive research in the last decades in both developmental psychology and neuroscience.

The dominant method to assess ToM abilities is the false-beliefs test. In this task, participants are asked to read stories about a person holding a false belief and then make predictions about his/her behavior. For example, in the Sally-Anne paradigm, Sally places a marble in a box (location 1) and then leaves the room. Anne then hides the marble in a different box (location 2). When Sally returns to the room, participants are required to identify the box where Sally will look for her marble. To pass the test, participants need to understand that Sally's behavior will be guided by her false belief that the marble is in location 1 and to inhibit the response related to the marble's actual location (the marble is now in location 2). Based on this kind of research, ToM has been initially defined as a complex process of inference about others' minds, which is acquired relatively slowly during development (children can pass the test around 3-4 years of age) and requires executive control and language abilities [START_REF] Baron-Cohen | Does the autistic child have a "theory of mind[END_REF][START_REF] Wellman | Meta-analysis of theory-of-mind development: the truth about false belief[END_REF].

Although this approach has been very fruitful in defining critical aspects of humans' social cognition, it doesn't tell us the full story. In our daily interaction with others, we are not always concerned with explicit reasoning about others' intentions and beliefs. In fact, rather than on deliberate reasoning, efficient social interaction is largely based on rapid and spontaneous/implicit interpretation of others' actions, facial expressions, gaze direction and touch (e.g., [START_REF] Kirsch | Reading the mind in the touch: neurophysiological specificity in the communication of emotions by touch[END_REF][START_REF] Becchio | Social grasping: From mirroring to mentalizing[END_REF].

In the last years, studies adopting non-verbal implicit measures, such as the analysis of eye movements and reaction times during simple detection tasks in a social context, suggested that both adults and infants before 3 years of age spontaneously or implicitly represent information from another person's perspective (e.g. Clements and Perner, 1994;[START_REF] Onishi | Do 15-month-old infants understand false beliefs? Science[END_REF][START_REF] Southgate | Action anticipation through attribution of false belief by 2-year-olds[END_REF][START_REF] Kovács | The social sense: susceptibility to others' beliefs in human infants and adults[END_REF][START_REF] Samson | Seeing it their way: Evidence for rapid and involuntary computation of what other people see[END_REF][START_REF] Schneider | Eye movements reveal sustained spontaneous processing of others' mental states[END_REF][START_REF] Nijhof | Measuring mentalizing ability: a within-subject comparison between an explicit and spontaneous version of a ball detection task[END_REF][START_REF] Surian | Attribution of beliefs by 13-month-old infants[END_REF][START_REF] Senju | Do 18-month-olds really attribute mental states to others? A critical test[END_REF]. For example, in the study of [START_REF] Kovács | The social sense: susceptibility to others' beliefs in human infants and adults[END_REF], participants observed some movie clips depicting an agent acquiring a true or false belief about the location of an object. At the end of the movie, participants had to press a key to the appearance of the object (expected or unexpected). Reaction times were fast not only when the participant expected the object to be present but also when the agent only (false belief condition) believed the object would be present. Critically, participants were never asked to consider the agent's belief (and remain unaware of the task manipulation as shown with variants of the same task, Bardi et al., 2017a). These results showed that adults track others' beliefs even when they are not required to do so and even in situations where the other's mental state is completely irrelevant to the current goals.

In developmental studies, anticipatory looking and looking time, as a measure of violation of expectation, have been used to test whether young children as early as 7 to 15 months of age show signs of false beliefs representation. In the same study by [START_REF] Kovács | The social sense: susceptibility to others' beliefs in human infants and adults[END_REF] for example, 7-month-old infants display surprise, as indexed by increased looking time, when they are presented with a scenario that is incongruent with an agent's previously acquired belief [START_REF] Kovács | The social sense: susceptibility to others' beliefs in human infants and adults[END_REF]. Evidence in support of the idea that implicit forms of mentalizing emerge early in life has been accumulated in the course of numerous studies (e.g., [START_REF] Southgate | Action anticipation through attribution of false belief by 2-year-olds[END_REF][START_REF] Surian | Attribution of beliefs by 13-month-old infants[END_REF][START_REF] Senju | Do 18-month-olds really attribute mental states to others? A critical test[END_REF]. However, possible limitations of some infant ToM tasks are currently a matter of intense debate (see non-replication studies, e.g., Grosse [START_REF] Grosse Wiesmann | Longitudinal evidence for 4-year-olds' but not 2-and 3-year-olds' false belief-related action anticipation[END_REF]Kulke et al., 2018).

Another open question concerns whether implicit and explicit ToM rely on the same mechanisms in the adult brain. Some authors have questioned whether implicit ToM tasks reflects mentalizing of any kind [START_REF] Heyes | Submentalizing: I'm not really reading your mind[END_REF], while others [START_REF] Apperly | Do humans have two systems to track beliefs and belief-like states?[END_REF] have proposed the existence of two mentalizing systems: (i) an implicit system that emerges early in life, is fast and efficient and operates unconsciously and (ii) an explicit form of mentalizing that develops later, is more deliberate and flexible. Finally, [START_REF] Carruthers | Two systems for mindreading? Review in[END_REF] postulates just a single mind reading system, which operates automatically or in a more controlled way depending on the environmental requirements. Interestingly, a direct comparison between adults' performance in the same task under implicit and explicit instructions showed that participants' performance always reflects the representation of the other's perspective or belief and is not affected by task requirements (Schneider et al., 2014a;[START_REF] Nijhof | Measuring mentalizing ability: a within-subject comparison between an explicit and spontaneous version of a ball detection task[END_REF][START_REF] Bardi | Brain activation for spontaneous and explicit false belief tasks overlaps: new fMRI evidence on beliefprocessing and violation of expectation[END_REF].

Neuroimaging studies have mostly focused on the neural correlates of explicit belief processing. For explicit ToM tasks, a pattern of brain regions has been identified when participants are asked to reason about somebody else's false belief.

What is referred to as the 'ToM network' includes most consistently the temporo-parietal junction (TPJ) and the medial prefrontal cortex (mPFC), but also the temporal cortex and the precuneus (e.g. [START_REF] Van Overwalle | Social cognition and the brain: a meta-analysis[END_REF][START_REF] Schurz | Fractionating theory of mind: a meta-analysis of functional brain imaging studies[END_REF][START_REF] Schurz | Toward a hierarchical model of social cognition: A neuroimaging meta-analysis and integrative review of empathy and theory of mind[END_REF].

Only a few studies have investigated so far, the neural bases of implicit ToM with inconsistent results (e.g., Kovács et al., 2014;[START_REF] Schneider | Spontaneous false-belief processing in the human brain[END_REF][START_REF] Hyde | Human temporal-parietal junction spontaneously tracks others' beliefs: a functional near-infrared spectroscopy study[END_REF]Bardi et al., 2017a;[START_REF] Bardi | Repetitive TMS of the temporo-parietal junction disrupts participant's expectations in a spontaneous Theory of Mind task[END_REF][START_REF] Naughtin | Do implicit and explicit belief processing share neural substrates?[END_REF][START_REF] Boccadoro | Defining the neural correlates of spontaneous theory of mind (ToM): an fMRI multi-study investigation[END_REF][START_REF] Grosse Wiesmann | Two systems for thinking about others' thoughts in the developing brain[END_REF]. Studies comparing implicit and explicit ToM, with the use of the same stimulus material and experimental procedure, found substantial overlap in brain activation within the right TPJ between the two forms of mentalizing (Bardi et al., 2017a;[START_REF] Nijhof | Brain activity for spontaneous and explicit mentalizing in adults with autism spectrum disorder: an fMRI study[END_REF]. A recent multi-study analysis on the same task found a number of distinct brain regions for implicit ToM but confirmed the existence of overlap with explicit ToM in the right TPJ [START_REF] Boccadoro | Defining the neural correlates of spontaneous theory of mind (ToM): an fMRI multi-study investigation[END_REF].

While neuroimaging studies point towards an (at least partial) overlap between implicit and explicit ToM in the adult brain, a recent study reports that maturation between distinct networks is responsible for different forms of mentalizing in the developing brain [START_REF] Grosse Wiesmann | Two systems for thinking about others' thoughts in the developing brain[END_REF]. More studies will be needed to better characterize belief representation in young children and the developmental trajectories and neural substrates of implicit and explicit ToM.

To sum-up, infants appear to be able to spontaneously interpret others' mental state (implicit ToM) since early in life, and this ability may be grounded in embodied practices (see Gallangher, 2001), not requiring the development of cognitive and linguistic abilities. Indeed, during the very first tactile interactions with the caregiver, infants functionally coordinate with her, by mapping their own sensorimotor signals onto the caregiver's ones [START_REF] Gallagher | The practice of mind. Theory, simulation or primary interaction?[END_REF], they also start to interpret others' behavior and intentions. In this sense, the human mind might be built on an implicit embodied dialogue with other human beings.

Sociality and human development across a collective scale: bodily and intellectual interpersonal regulation

The ability to predict the dynamics of the world on the basis of "internalized experience", while modifying them or the world in case of a misattunement of the two, is largely developed within social interactions. Thus, these mechanisms are not exempt from being affected by socio-cultural factors. For example, a child of this century can be afforded by a mobile phone as opposed to a child of the nineteenth century could not have been. The mobile phone itself represents a technological development, which has largely revolutionised gestures, behaviours and the modalities of being in relation altogether. The dialectic between what we have learned, inherited and what we actively transform by allowing cultural evolution, is oftentimes reflected in abstract thinking. This human skill is based on assimilation/accommodation processes of the interpretative models and the new contextual information. This dialectic itself is the basis of new interpretations of reality, in other words of new meanings, and it is pursuit by means of an intense internalized dialogue to structure our thoughts.

The sociogenesis of the human mind

Sociality permeates each single step of human development. The bodily [START_REF] Apps | The free-energy self: a predictive coding account of self-recognition[END_REF] and the narrative self [START_REF] Gallagher | Philosophical conceptions of the self: implications for cognitive science[END_REF] are the results of continuous interpersonal interactions unfolding over time. Having said that, human becoming is not only an inherently social product across an individual scale, but also across a collective/cultural and evolutionary scale (cf. [START_REF] Bolis | I interact therefore I am': human becoming in and through social interaction[END_REF][START_REF] Bolis | I interact therefore I am': The self as a historical product of dialectical attunement[END_REF][START_REF] De Jaegher | Participatory sense-making[END_REF][START_REF] Fotopoulou | Mentalizing homeostasis: The social origins of interoceptive inference[END_REF]Vygotsky, 1930Vygotsky, -1935[START_REF] Vygotsky | Mind in society: the development of higher psychological processes[END_REF]. People tend to 'fall in synchrony' in social interactions, e.g., through spontaneously aligning rhythmic behaviors, such as gait and clapping or adopting others' mannerisms, from shaking a foot or rubbing the nose, to lexical choices, accents, pauses, speech rates and tone of voice, all the way to actual emotions and moods (cf. [START_REF] Cappella | Talk and silence sequences in informal conversations III: Interspeaker influence[END_REF][START_REF] Chartrand | The chameleon effect: the perception-behavior link and social interaction[END_REF][START_REF] Garrod | Saying what you mean in dialogue: A study in conceptual and semantic co-ordination[END_REF][START_REF] Giles | Speech style and social evaluation[END_REF]Hatfield et al., 2011;[START_REF] Koban | Why do we fall into sync with others? Interpersonal synchronization and the brain's optimization principle[END_REF][START_REF] Longo | Behavior matching in multimodal communication is synchronized[END_REF]Neumann & Strack, 2000). Interestingly, people synchronize with each other in real-time even against their actual intention [START_REF] Yun | Interpersonal body and neural synchronization as a marker of implicit social interaction[END_REF], tending to spontaneously attune to others' behavior [START_REF] Kilner | An interference effect of observed biological movement on action[END_REF][START_REF] Forbes | Moving higher and higher: imitators' movements are sensitive to observed trajectories regardless of action rationality[END_REF][START_REF] Fini | Abstract concepts in interaction: The need of others when guessing abstract concepts smooths dyadic motor interactions[END_REF][START_REF] Boukarras | Competence-based social status and implicit preference modulate the ability to coordinate during a joint grasping task[END_REF]. In fact, such unconscious realtime mirroring, but also interpersonal similarity in longer scales, might facilitate human communication and social relationships by decreasing complexity in interpersonal predictive processing. For instance, it has been shown that the interpersonal similarity of autistic traits is related to friendship quality in the general population [START_REF] Bolis | I interact therefore I am': The self as a historical product of dialectical attunement[END_REF]. In fact, an increasing number of studies has been recently illustrating interpersonal synchrony in social interactions even at the level of brain function, across various time scales and contexts, ranging from motor synchrony [START_REF] Dumas | Inter-brain synchronization during social interaction[END_REF][START_REF] Era | Inhibition of left anterior intraparietal sulcus shows that mutual adjustment marks dyadic joint-actions in humans[END_REF] and shared psychological perspectives [START_REF] Lahnakoski | Synchronous brain activity across individuals underlies shared psychological perspectives[END_REF], to music experience [START_REF] Abrams | Inter-subject synchronization of brain responses during natural music listening[END_REF], educational processes (Dikker et al., 2017) and real-life social relationships [START_REF] Pan | Cooperation in lovers: an f NIRS-based hyperscanning study[END_REF]Parkinson et al., 2018).

In this light, the self has been reviewed as the dynamic interplay between internalization and externalization in and through social interaction (cf. [START_REF] Bolis | I interact therefore I am': human becoming in and through social interaction[END_REF][START_REF] Bolis | I interact therefore I am': The self as a historical product of dialectical attunement[END_REF]Vygotsky, 1930Vygotsky, -19355/1978). On one hand, internalization has been thought of as the co-construction of bodily hierarchical structures of the (social) world and the organism, while externalization as the collective transformation of the world. Such a notion of internalization is largely based on the Vygotskian take, defined as the internal reconstruction of an external operation (Vygotsky, 1930(Vygotsky, -1935[START_REF] Vygotsky | Mind in society: the development of higher psychological processes[END_REF], recently reviewed through predictive processing conceptualizations (cf. [START_REF] Bolis | I interact therefore I am': The self as a historical product of dialectical attunement[END_REF]. Predictive processing has been defined as a hierarchical bidirectional process through which an organism adjusts itself in order to 'optimally' predict environmental and bodily regularities, such as seasonal temperature fluctuations and one's own psychophysiological responses. Yet, organisms such as humans are not passive observers who internalize reality, trying merely to adapt to it (cf. [START_REF] Bolis | I interact therefore I am': human becoming in and through social interaction[END_REF]. On the contrary, humans continuously interact with their world (including their own body), adjusting it according to their prior expectations through processes of externalization. For instance, the body temperature tends to decrease beyond the expected value of ~37 °C after persistent exposure to the cold. Shivering, putting on a fire when outdoors or turning on the central heating of a building when indoors, are all typical strategies for reversing this tendency and by doing so increasing survival chances.

Importantly, the abovementioned hierarchical structures of predictive processing and active inference should be considered as inherently collective. First, we dynamically embody others in and through social interaction, shaping each other's bodily structure (cf. De Jaegher & Di Paolo, 2007[START_REF] Bolis | I interact therefore I am': human becoming in and through social interaction[END_REF][START_REF] Bolis | I interact therefore I am': The self as a historical product of dialectical attunement[END_REF], and, second, such structures might even be thought of unfolding within nested time and space scales, from biology and cognition all the way up to society (cf. Ramstead et al., 2018). In these lines, architecture and technology can be viewed as a collective strategy to optimize overall uncertainty via transforming the environment according to bodily and interpersonal expectations [START_REF] Bolis | I interact therefore I am': human becoming in and through social interaction[END_REF]. In a nutshell, humans actively co-regulate each other, as well as their ecosocial niches, facilitating survival (cf. [START_REF] Fogel | Developing through relationships: Origins of communication, self, and culture[END_REF].

Here, let us reconsider an illustrative example inspired by Lev Vygotsky (1930-1935/1978;cf. Bolis & Schilbach, 2020):

A baby, trying to regulate her hunger, tries to reach for food. However, she is unable to do so, even after overextending her body and specifically the index finger, as her action repertoire is still not sufficiently developed. A caregiver, predicting the baby's goal, easily completes the action by providing the food closer. After multiple repetitions, the extended index finger can potentially assume the symbolic function of pointing. In fact, the primary purpose of such early social interactions, which help build what we call the self (cf. [START_REF] Bolis | I interact therefore I am': The self as a historical product of dialectical attunement[END_REF], might be nothing more or less than homeostatic regulation (cf. [START_REF] Fotopoulou | Mentalizing homeostasis: The social origins of interoceptive inference[END_REF]. This is not surprising when one thinks of early social interactions as an extension of the interbodily interaction in the mother's womb (cf. Ciaunica et al., 2020). Stretching the above mentioned example to an intergenerational scale, we can envisage the social origins of cultural conventions and norms. Furthermore, considering possible interactions of these processes at the scale of phylogeny might allow us to review human anatomy itself as a social product. To give but an example, the human eye morphology has been thought of evolving in tandem with the enhanced gaze-based interaction in humans (cf. [START_REF] Bolis | I interact therefore I am': The self as a historical product of dialectical attunement[END_REF][START_REF] Dobson | Face to face with the social brain[END_REF]Kobayashi & Kohshima, 2001;[START_REF] Powell | Orbital prefrontal cortex volume correlates with social cognitive competence[END_REF]. These sorts of multiscale and inextricably linked processes of human becoming while attuning to one another and the environment, is what has been described as dialectical attunement [START_REF] Bolis | I interact therefore I am': human becoming in and through social interaction[END_REF][START_REF] Bolis | I interact therefore I am': The self as a historical product of dialectical attunement[END_REF]. In fact, Vygotsky had already suggested at the beginning of the previous century that all mental processes within an individual result from an internalization of prior social interactions between people; in a nutshell, "through others we become ourselves" (Vygotsky,1987).

3.2. Abstract concepts and their dialogical nature as a product of "dialectical attunement"

One emblematic example of dialectical attunement (see [START_REF] Bolis | I interact therefore I am': The self as a historical product of dialectical attunement[END_REF] and perhaps one of the most stimulating challenges for embodied psychology is the humans' highest level species-specific skill: talking and thinking abstractly.

Abstract concepts (i.e., "justice", "friendship", "democracy") typically lack an objectual referent: they are more detached from the sensorimotor experiences. They vehicle complex information that, through socio-cultural evolution, has become handly and efficiently shared among individuals along the time. As the content of abstract concepts cannot be experienced directly through the individual's senses, their acquisition is grounded in social interactions. Humans enrich their semantic knowledge of the world not only through interbodily social interaction serving immediate survival needs as we mentioned above, but also thanks to intellectual exchanges with other individuals: the dialogue becomes a source of negotiation and meaning validation. During the dialogical practice among humans, processes of assimilation/adaptation of complex meanings are generated. Two different levels can be distinguished behind the assimilation/adaptation processes: an internal/individual dimension of meaning-construction, and an external/social dimension of meaning-acquisition. These two processes are never definitive, but by virtue of a continuous intense intellectual activity realized in social contexts, abstract concepts are constantly objects of re-thinking.

The incessant update of human knowledge is possible through a mental function, which corresponds to the inner speech, which is defined as an initial outer speech, internalized during cognitive development (Vygotsky, 2012). Following the WAT theory [START_REF] Borghi | The challenge of abstract concepts[END_REF][START_REF] Borghi | Varieties of abstract concepts: development, use and representation in the brain[END_REF][START_REF] Borghi | Words as social tools: Language, sociality and inner grounding in abstract concepts[END_REF][START_REF] Borghi | A future of words: Language and the challenge of abstract concepts[END_REF], inner speech, might be more involved during the acquisition and processing of abstract as compared with concrete concepts (i.e., "cup", "table", "chair") [START_REF] Fini | Articulatory suppression delays processing of abstract words: The role of inner speech[END_REF][START_REF] Borghi | Abstract Concepts and metacognition: searching for meaning in self and others[END_REF]). Let's think, indeed, of the word "cup". When creating in our mind the physical features of the object, immediately the pre-reflective simulation of the sensorial consequences associated with the afforded action (i.e., grasping) is triggered.

The sensorimotor anticipation of the action's outcome permits regulating the interactive behavior with objects and individuals by guaranteeing the prompt unfolding of coherent/appropriate motor chains. However, when processing an abstract word, e.g., "justice", instead of the predicted sensorimotor expectations on the objectual referent, the reactualization of the linguistic experience through which we have learned the abstract concept will lead to the validation of a new fresh contextualised meaning. Put simply, we would internally "talk to ourselves" about a specific meaning to better master it. Thus, while for concrete concepts we anticipate/simulate the individual bodily experience with the objectual referent, with abstract concepts we anticipate/simulate the dialogical experience of the intangible knowledge.

In this regard, the social metacognition proposal [START_REF] Borghi | The challenge of abstract concepts[END_REF][START_REF] Borghi | Varieties of abstract concepts: development, use and representation in the brain[END_REF][START_REF] Borghi | Words as social tools: Language, sociality and inner grounding in abstract concepts[END_REF][START_REF] Borghi | A future of words: Language and the challenge of abstract concepts[END_REF] explains the role of inner speech during the processing of abstract concepts by invoking three different mechanisms: we might i) prepare ourselves to ask another individual information to fill the gap of the knowledge or to negotiate the word meaning with others (social metacognition), ii) to re-explain to ourselves the abstract concepts' meaning by collecting sparse informations, iii) simulate the dialogical experience through which we have learned in a first place the abstract concept. All these -not mutually exclusive-mechanisms clearly express how a dialogical practice can be interiorized (internalization process) and then re-enacted to flexibly provide a space of negotiation, re-calibration and social validation of a mental product, i.e. a meaning evoked by a concept, in order to better represent the reality (externalization process). The social validation of abstract concepts can be either vertical/asymmetric, such as in the case of a child who asks the teacher the meaning of a word; or horizontal/symmetric, as in the case of two peers discussing a concept [START_REF] Mazzuca | From Affordances to Abstract Words: The Flexibility of Sensorimotor Grounding[END_REF]Mazzuca & Santarelli, 2021). Both forms are important: in the first case the child is provided with the opportunity of efficiently acquiring cultural knowledge produced in larger time scales, while the second case can be seen as a prototypical exercise of exploring, negotiating, cooperating but also competing, co-constructing and communicating meaning between parties of equal knowledge and power. Our interpretative models of the world are thus, object of a constant update by means of the so called "internalization" and "externalisation" processes: within and through social interactions is constantly realized the categorization of new meanings based on the already owned knowledge (internalization) and these latter are renegotiated to fuel/ transform our representation of the reality (externalization). The "gnoseological dialogue" between the person and the world, however, is only apparently "individual", as the interpretative models we used to understand the reality arise from intellectual exchanges within communities. Hence, the human dimension of knowledge and cultural evolution stems from such dialectic of internalization and externalization at high levels of abstraction in and through social interactions (cf. [START_REF] Bolis | I interact therefore I am': human becoming in and through social interaction[END_REF]. While people interact, they mutually co-construct and co-regulate their bodily predictive hierarchies, by virtue of perceiving and acting upon each other. This means that humans are not only coconstructed in the 'here and now', but also beyond, via collectively generating abstract behavioural patterns and concepts, available in future interactions (cf. interpersonal dialogue), but also privately (cf. inner speech). Here, the dialectical attunement hypothesis [START_REF] Bolis | I interact therefore I am': The self as a historical product of dialectical attunement[END_REF] has suggested that a line of evolution, which has enabled such abstracting beyond the concreteness of face-to-face real-time social interactions, should have been toward the direction of deepened bodily and brain hierarchies, as higher levels of a hierarchy in this context corresponds to increased levels of abstraction (cf. [START_REF] Mathys | A bayesian foundation for individual learning under uncertainty[END_REF]. Crucially, such a structural deepening might have enabled major qualitative evolutionary leaps, such as the one from individual to shared intentionality (cf. [START_REF] Tomasello | Shared intentionality[END_REF]. Indeed, according to the shared intentionality hypothesis, certain ecological shifts in early human societies might have led to increased needs for cooperation (e.g. for foraging), which demanded the development of skills related to the coordination of roles and perspectives toward joint objectives. Such a progression, eventually, the hypothesis goes, might have led to novel forms of perspectival and symbolic representations, socially recursive inference and self-monitoring. Such qualitative leaps from individual to shared intentionality and language acquisition one could recognize in ontogeny scales as well (cf. [START_REF] Bolis | I interact therefore I am': The self as a historical product of dialectical attunement[END_REF][START_REF] Tomasello | What makes human cognition unique? From individual to shared to collective intentionality[END_REF]. In Vygotsky's words "the relation between higher psychological functions was at one time a physical relation between people" (Vygotsky, 1929[START_REF] Vygotsky | Concrete human psychology[END_REF].

Conclusions

Touching the mind without touching the body

The self-construction is a profoundly relational process. The progressive awareness of who we are becoming and what we can do, is indeed scaffolded on the proximal interactions between the new-born and the caregiver. They form a dyadic system where the new-born communicates his or her physiological needs to the caregiver who interprets and reacts to the new-born's bodily signals, by providing him/her with the necessary resources to survive (see Fotoloupou & Tsakiris, 2017). All the new-born's actions are oriented to express bodily perceptions (i.e., interoception, proprioception, nociception) and always have a structural social goal, that is asking for the caregiver's intervention to reach an ultimate goal: the regulation of the new-born's internal homeostasis. During dyadic routines, the new-born learns to causally associate her sensorimotor and internal states with the caregiver's sensorimotor states. She learns to predict their feelings as a consequence of the social events and to predict the social events as a consequence of her expressed internal feelings.

In fact, through the caregiver's body, the new-born acquires awareness of her own body and separates herself from the other. The multisensory experience of the new-born largely emerges from the epidermal contact with the caregiver, which is essential to fulfil basic physiological needs (temperature regulation, breast-feeding, washing, etc.) and to establish the very first affective bond with another human. The integration of sensorimotor information produced during the skin-toskin contact, as it is suggested by Fotoloupou & Ciaunica (2015), represents the most primitive channel through which we recognize ourselves as subjects-being in-the-world.

Indeed, the sense of touch can be thought of as perhaps the most privileged and primitive modality through which we experience being in a relation with another human and it is this social experience that allows the acquisition of bodily awareness. The very first awareness of ourselves pertains to the concept of minimal self which is the implicit representation of our body and its potentialities, including the sense of being the author of our own actions (sense of agency) and of being the owner of our own body parts (sense of ownership). Borrowing a term used by Fotopoulou 2015, the implicit awareness of ourselves is a basic form of "mentalization", intended as "the progressive integration and organization of sensory and motor signals that constitute the foundations of the minimal self". Crucially, the tactile contact not only is the modality through which, at very first, we build a bridge with another human and consequently the bodily self, but it is also the modality through which we define and characterize the nature of the affective relations. Social touch in fact, represents the primary tool to communicate, regulate emotions and express meanings since infancy, while in adulthood is affected by sexuality and romantic attractions (see [START_REF] Cascio | Social touch and human development[END_REF]. In a nutshell, social touch creates and keeps tight the bonds among people.

In accord with the Interaction Theory (IT) (De Bruin, van Elk, & Newman, 2012;[START_REF] Gallagher | The practice of mind. Theory, simulation or primary interaction?[END_REF], it is primarily through interpersonal sensorimotor interactions (i.e. through a second-person perspective, [START_REF] Reddy | Experiencing others: A second-person approach to other-awareness[END_REF][START_REF] Schilbach | Toward a secondperson neuroscience 1[END_REF]), that we can understand and experience our own and other's internal emotional states. During the embodied practice (i.e., gestures and facial expressions production), we experience internal states that we also predict and recognize in the other person.

Mutuality and reciprocity, not just the mere observation of others, constitute the foundations of social understanding. Thus, Theory of Mind (ToM) finds its roots in the primitive and very spontaneous embodied practices, where the child evolves in her "mentalization" of the sensorimotor signals exchanged during the interaction with the caregiver. At this regard, Fotoloupou & Tsakiris, 2017, embracing the Interaction Theory (IT) (De Bruin, van Elk, & Newman, 2012;[START_REF] Gallagher | The practice of mind. Theory, simulation or primary interaction?[END_REF], refer to the Theory of Mind as "an advanced form of more primitive inferential processes of embodied perception and action". Here, implicit Theory of Mind has to be distinguished from Explicit Theory of Mind: the first one is a very early ability to interpret the other's intentions and emotions during social interaction and does not require explicit reasoning about her beliefs, as in the case of the second one. While the embodied practice [START_REF] Gallagher | The practice of mind. Theory, simulation or primary interaction?[END_REF] during the new-born caregiver interaction will be the precursor of the Implicit Theory of Mind, this latter will lay the ground for the evolution of more structured cognitive processes based on mental inference.

The evolution of the self and the psychological potential to be connected with the others is a process that starts with bodily exchanges at an implicit level, where gradually we build up a minimal and then a narrative identity, we learn to understand other people, while becoming more and more capable of detaching by the "here and now" of social relationships and interactions. This level of maturation is reached through what [START_REF] Bolis | I interact therefore I am': The self as a historical product of dialectical attunement[END_REF] term "dialectical attunement": a dialectic of internalization and externalization, unfolded on both individual and collective levels. Put simply, according to this view, we bodily internalize the statistical regularities of our sensorimotor interactions continuously building up expectations about the world, others and ourselves, while we actively modify the environment in order to make it conform to these expectations (see also [START_REF] Clark | next? Predictive brains, situated agents, and the future of cognitive science[END_REF]. These processes of self-construction and self-regulation evolve from our interaction with others. From the embodied relations with humans, we understand who we are, then with the development of the language and cognitive functions, we culturally act in the world, and we build and negotiate new meanings, we create abstract concepts, norms and conventions, we move towards abstraction and symbolization. Thanks to the special human ability to abstract from reality, as an ultimate step, we can be connected with others without the need for constant tactile contact. We have indeed internalized the relation that has determined the development of ourselves, and such processes of self-development allow us to promote "more disembodied, that is abstracted, interactions", in other words: "to touch the minds without touching the body". LB is supported by Impulsion 2019, Idexlyon and by the Agence National de la Recherche (ANR JCJC, Grant titled "Implicit Theory of Mind: underlying mechanisms and functional role-IMPTOM" to Lara Bardi) MPL is supported by La Sapienza University of Rome (Avvio alla Ricerca, 2020, n°AR120172B17A70D1).

AHM is supported by the European Regional Development Fund (ERDF) and the Occitanie Region (convention number 182117SR) VE is supported by BE FOR ERC Grant from Sapienza University of Rome.

Compliance with Ethical Standards

The authors declare that they do not have any conflicts of interest. This article does not contain any studies with human participants performed by any of the authors.

Funding

  MF is supported by the Italian Ministry of Health(Ricerca Finalizzata, Giovani Ricercatori 2019, n°GR-2019-12369761) DB is supported by the Foundation for Science and Technology, Portugal (PTDC/FER-FIL/4802/2020) CF is supported by the funding of the project titled: " Abstract concepts: embodied and social aspects" (2022) at the Department of Dynamic and Clinical Psychology and Health Studies, Sapienza University of Rome and by the European Project Horizon 2020 titled:" "From Social Interaction to Abstract Concepts and Words: Towards Humancentered Technology Development", acronym: TRAINCREASE (CUP B89C21000410006).

References

Acknowledgements

We would like to thank Anna M. Borghi and Salvatore M. Aglioti for the fruitful comments on the manuscript.

Competing Interests

None


Avishek Das

Arthur Cesare

Anton A Popov

Andrei Pastukhov

Sergei M Klimentov

Andrei V Kabashin
email: andrei.kabashin@univ-amu.fr

Anderson S L Gomes
email: anderson.lgomes@ufpe.br

Plasmonically Enhanced Two-Photon Absorption Induced Photoacoustic Microscopy with Laser-Synthesized TiN Nanoparticles

Combining photonic excitation and acoustic detection, photoacoustic imaging (PAI) presents one of the most promising noninvasive biomedical diagnostic modalities, but this technique still lacks efficient nano-sized contrast agents absorbing light in the region of relative tissue transparency (630-900 nm). Here, we explore the use of titanium nitride (TiN) nanoparticles (NPs) fabricated by methods of pulsed laser ablation in liquids as a contrast agent in PAI. When prepared in acetone, the NPs are spherical, have an average size of 25 nm, and exhibit a broad plasmonic absorption peak around 700 nm. We show that solutions of these NPs render possible a strong nonlinear photoacoustic response and the generation of photoacoustic images with 67 μm resolution within the biological transparency window. The observed effect is explained by a plasmonically enhanced two-photon absorption process in TiN NPs. Combined with earlier demonstrated capability of generating photothermal therapeutic effect, relative chemical purity, and excellent biocompatibility, laser-synthesized TiN NPs promise attractive applications in biomedical theranostics involving imaging modalities based on photoacoustics microscopy or tomography.

A variety of novel optical imaging modalities beyond optical microscopes have been developed in the last decades for laboratory, preclinical and clinical diagnostic use, offering a series of unique characteristics, including non-invasiveness, high spatial resolution, controllable penetration depth, in-vivo real-time capabilities 1,[START_REF] Shi | Deep Imaging in Tissue and Biomedical Materials[END_REF] . Among modern optical imaging modalities, prominence can be given to fluorescence imaging [START_REF] Tang | [END_REF] , nonlinear optical microscopy 4 , optical coherence tomography 5 , hyperspectral imaging 6 , and photoacoustic imaging [7][8][9][10] . The impact of these optical imaging modalities has been unprecedented, although a lot of research is still being carried out aiming at further developments or exploiting multimodalities, which can benefit from the molecular specificity of each technique, as well-reviewed in ref. 11 .

Owing to its high efficiency, relatively deep probing depth, and the possibility of combining with phototherapy functionality, photoacoustic imaging (PAI) presents one of the most promising modalities for biomedical theranostics [7][8][9][10] . In this modality, photons, absorbed by an endogenous or exogenous medium, induce local heating, which in turn creates an expansion/contraction of the affected region with subsequent generation of acoustic waves. While the excitation is done by the light of different wavelengths, the detection is performed using the same acoustic detector. PAI can be broadly subdivided into photoacoustic microscopy (PAM) and photoacoustic tomography (PAT) 8,9 . Furthermore, nonlinear optical processes, particularly multiphoton absorption [START_REF] George | Nonlinear Optics: Phenomena, Materials and Devices[END_REF] can be employed to improve the resolution, leading to the so-called nonlinear photoacoustic imaging [START_REF] Gao | [END_REF] .

The efficiency of PAI methods can be much improved by the employment of nanomaterials, absorbing in the region of relative tissue transparency (630-900 nm), as agents to enhance the imaging contrast. Owing to excitations of free electron oscillations (plasmons) in metals, plasmonic nanomaterials can offer orders of magnitude stronger absorption compared to conventional absorbing dyes 14 , which makes them the most promising candidates for PAI and photothermal therapy modalities 15,16 . Gold (Au) nanostructures present an obvious candidate for photoacoustic imaging and other biomedical modalities, as they can combine chemical stability with strong plasmonic absorption, but spherical Au nanoparticles (NPs) have the plasmonic feature is outside the biotransparency window (520-560 nm) 14 . The plasmonic feature can be shifted toward this window by engineering complex structures such as SiO2@Au core-shells 17,18 or anisotropic nanorods 19 , but the core-structures are oversized for biological use (typically, larger than 100-150 nm), while the anisotropic shape of the nanorods should be stabilized by non-biocompatible surfactants (e.g., CTAB). Titanium nitride (TiN) NPs present an alternative plasmonic material, which is capable of providing a red-shifted peak around 650-800 nm 20,21 with a high photothermal conversion efficiency 21 . The possibility of using TiN NPs as the photothermal agent in cancer therapy and the contrast agent for bioimaging has already been justified 22,23 , but their biomedical applications have been very limited so far due to the difficulty of fabricating stable water-dispersible solutions of clean TiN NPs via conventional chemical or plasma synthesis routes. We recently demonstrated the solution to this problem by using methods of laser ablation in liquid ambiance 24 . In this approach, a TiN target is ablated by laser radiation in water or organics (ethanol, acetone), which leads to the formation of colloidal TiN NPs solutions stabilized by a negative charge of the NPs (NPs prepared in organic solutions can be easily transferred to water). Profiting from a strong and broad plasmonic absorption band with its peak around 700 nm and a long tail over 800 nm, lasersynthesized TiN NPs demonstrated a strong phototherapy effect excited by 800 nm laser radiation 24 . A series of tests in vitro and in vivo showed that these NPs have a very low toxicity profile to confirm their safety, while the coating of NPs with polyethylene glycol (PEG) makes possible their prolonged circulation in the bloodstream and an efficient passive accumulation in a tumor via EPR effect 25 . Furthermore, examining laser-synthesized TiN NPs by combined optical and acoustic Z-scan technique, we recently observed a strong plasmon-mediated nonlinear absorption (NLA) effect 26 , similar to what was earlier observed with Au NPs 27 , which promises the development of bioimaging modality based on this nanomaterial.

Here, we experimentally demonstrate the possibility of nonlinear imaging using laser-synthesized TiN NPs as an exogenous contrast agent for PAM in the first (NIR I) biological window. We also show that the observed contrast is due to twophoton absorption (TPA) of TiN NPs.

TiN nanoparticles were synthesized by methods of femtosecond (fs) laser ablation methodology from a solid TiN target (GoodFellow, hot pressed, 99.5% purity) in acetone, as earlier described in 24 . Briefly, a focused beam from a Yb:KGW laser (1030 nm, 270 fs pulse length, 100 μJ pulse energy, 10 kHz repetition rate, TETA 10 model, Avesta, Russia) was used to ablate the target for 20 min. The obtained nanoparticle concentration was measured to be 57 μg/ml by employing the gravimetric method. The morphological analysis of the obtained NPs was carried out using a transmission electron microscope (TEM, model JEOL JEM-2010, LaB6 filament, 200 kV, 0.23 nm point to point resolution). Prior to TEM imaging, 10 μl of TiN NPs in acetone solution was dropped onto the carbon-coated copper grid (200 mesh, Oxford Instruments) and dried. The optical absorption characterization of TiN NP/Acetone sample was carried out by UV-Vis-NIR spectrophotometer.

Photoacoustic characterization of TiN NPs was carried out using a customized lab-made PAM setup, as shown in Fig. 1. An optical parametric oscillator (OPO) (OPOTEK, Vibrant 355 LD, 410 nm -2.4 µm, 5 ns) operating at 20 Hz was employed as an optical source. The average diameter of the laser beam at its focus was measured to be ~ 50 μm. The sample was placed inside a distilled water cell, which was kept above the motorized twoaxis (X-Y) translation stage. The photoacoustic (PA) signal from the sample was detected using a 5 MHz water immersion ultrasonic transducer (UST) (Olympus V310-N-SU), whose bandwidth limited the axial resolution 9 ). For PA characterizations, a medical PVC (acoustic impedance of 3.2 x 10 6 rayls 28 ) tube (an IV Infusion set) was first chosen for filling TiN NPs, which was found to be rapidly corroded by the acetone media. Therefore, a non-reactive cylindrical glass (acoustic impedance of 13 x 10 6 rayls 29 ) capillary tube (PRECISION, China) having an internal (external) diameter of 1 mm (1.5 mm), and a length of 75 mm was selected to fill the TiN NPs solution. Prior to solution injection, the tube was cleaned via multiple acetone rinsing. The tube ends were sealed with a thermoplastic adhesive. The tube was attached to a plastic sample holder. Fig. 2(a) shows the morphology and size distribution of laser-synthesized TiN NPs. It is visible that the NPs were spherical with an averaged (mode) size of about 25 nm. An optical absorption spectrum of such NPs is shown by a solid blue line in Fig. 2(b). One can see that two remarkable absorption bands were present: a sharp transition below 400 nm associated with TiN band-to-band absorption, and a broad absorption band with a maximum at 700 nm, which is typically attributed to localized plasmon resonance (LPR) excited over TiN NPs 20,21 . In Fig. 2(b), a PA spectrum from TiN NPs (showed by red squares) is directly compared with its optical absorbance profile. Since the PA signal is directly proportional to the optical absorption of the sample, the PA spectrum follows its optical absorbance profile, with the peak FIG. 1. Schematic representation of the photoacoustic microscopy setup. Laser energy is controlled by two Glan polarizers and a small part of the beam is directed to Channel 2 (CH 2) of a digital storage oscilloscope (DSO) via a reference photodetector to trigger the data acquisition. After a size reduction down to 500 μm diameter, the remaining beam is focused on the sample via a 50 mm convex lens.

x-y translation of the sample is enabled by two 50 mm linear stages (MTS50-Z8 -50 mm, Thorlabs Inc.). PA signal is amplified by a low noise amplifier (ZFL-500LN-BNC) with a gain of 19.4 and is fed to Channel 1 (CH 1) of the DSO. intensity reached at the same wavelength. However, one can notice a deviation of the PA spectrum with respect to its absorbance profile. There are published reports where the PA spectrum hugely differed from the optical absorbance nature 30- 32 . The reports attribute such deviation to two important factors: (a) The involvement of different types of optical illumination sources (continuous/pulsed) and photophysical processes in the optical absorption and PA spectroscopy technique [START_REF] Jiang | [END_REF]32 , and (b) domination of the non-linear effects in a material at higher optical fluence 31 . In our case, the presence of plasmonically enhanced two-photon absorption (TPA) at 725 nm (confirmed from Fig. 3(b)) is considered to be the principal reason for a deviation in the PA spectrum for TiN NPs in Fig. 2(b).

To check the dependence of PA signal intensity on the concentration of TiN NPs in acetone, a sample having 57 μg/ml concentration was diluted to its 20 %, 25 %, 33.3 %, and 50 % value, and corresponding PA intensity was recorded under a constant beam fluence of 1.85 mJ/cm 2 at 725 nm excitation, as shown in Fig. 3 Theoretically, the lateral resolution of PAM is given by the spot size of the optical focus 9 , which was measured to be ~ 50 μm for our PAM setup. To determine the experimental value of resolution, a sharp-edged black colored pattern on a plastic sheet was imaged with 532 nm excitation as in Fig. 4(a). The measured PA profile was fitted to an edge spread function (ESF) and its first order derivative provided the line spread function (LSF) as shown in Fig. 4(a). The lateral resolution of the PAM was estimated from the full width at half-maximum (FWHM) of the corresponding LSF 33 , which was about 67 μm.

As a proof-of-concept for the use of TiN NPs as an exogenous agent contrast in a PAM imaging setup, images were generated for three different excitation wavelengths with the acetone TiN NPs suspension in a capillary tube, as shown in Figs. is ~ 50 μm, the scan step size or the pixel dimension is fixed to 50 μm on both the X and Y-axis. This step size eliminates any pixel overlapping which occurs due to beam size mismatch. Such scanning covers the whole 2D PAI frame with 40 x 40 pixels. The obtained images display, from their qualitative intensity behavior (see the false color scale on the right-hand side) that the highest intensity occurs at the plasmonic absorption peak wavelength, followed by a less intensity image at 880 nm and the lowest intensity for excitation at 532 nm. The dashed lines indicate the capillary tube lateral boundary, with a dimension of ~1 mm. It should also be noted that the highest intensity (yellowish) in Fig. 4(c) and (d) are not centered due to the position of the photoacoustic detector at a slight angle from the direct light beam incidence. For the lowest emitted PA signal at 532 nm excitation, it can also be seen that the spatial profile at its FWHM is ~ 453 μm. Similarly, for the 725 nm and 880 nm excitations, the spatial profile at its FWHM is ~ 518 μm and ~ 511 μm, respectively.

The generation of TPA-induced PA signal is well understood in terms of the two-photon excitation (TPE) and resultant non-radiative process upon photon absorption by the material. Since the TPE is a second-order process, the number of photons absorbed by a material per unit time Nabs depends on several factors including, the square of light fluence or energy density (EL), the TPA cross-section  [START_REF] Shi | Deep Imaging in Tissue and Biomedical Materials[END_REF] , the material (NPs in this work) concentration C and the effective volume V under twophoton illumination, and can be expressed as 34,35 :

𝑁 𝑎𝑏𝑠 (𝑡) = ∫ 𝑑𝑉𝜎 (2) 𝐶(𝒓, 𝑡) 𝑉 𝐸 𝐿 2 (𝒓, 𝑡) (1)
where r and t signify respective spatial and temporal components. Following the mathematical treatment of equation (1) 34,35 , one can arrive at the following equation for the spatial distribution of PA signal:

𝑃𝐴(𝒓) ≅  { 1 2 (1 - 2 )𝐶𝑆 2 𝐸 𝑡𝑟 𝜌(𝒓)𝜎 (2) } 𝐸 𝐿 2 (2)
In equation ( 2), the term (1 - 2 ) describes the nonradiative efficiency of the material,  is the Grüneisen parameter, which indicates the efficiency of conversion of absorbed optical energy to acoustic pressure, 𝑆 is the spatial distribution of the incident laser fluence, Etr is the transferred optical energy to heat energy, and 𝜌(𝒓) is the density, in which absorption occurs. This equation indicates a quadratic dependence of the PA signal on the incident laser fluence in a two-photon process, as expected.

To confirm the presence of the TPA effect for TiN NPs , or more precisely to prove that the PA signal in TiN NPs follows equation ( 1), the PA signal intensities were recorded with increasing laser fluence for different wavelengths. Since the LPR broadband covers ranges from visible to near-infrared regions (Fig. 2 b), three excitation wavelengths were selected: 532 nm, 725 nm, and 880 nm. Fig. 3(b) shows the ln-ln plot of the normalized PA intensity as a function of laser fluence for the three excitation wavelengths. Here, the PA amplitude normalization is carried out by dividing PA signal intensities (for each wavelength) by their respective maximal PA values. The linear fitting to such plots depicts the presence of plasmonically enhanced TPA with a slope of S = 2.0 for 725 nm excitation. However, the quadratic dependence is not evident for 532 nm and 880 nm excitation wavelengths, as the slope values are S = 1.15 and 0.98, respectively. This may be attributed to relatively low plasmonic absorption at these wavelengths, as confirmed by the spectroscopic results in Fig. 2(b).

Regarding the obtained images, it can be seen from Figs. 4 (b, c andd) that solutions of TiN NPs demonstrate significant PA signature for all three excitation wavelengths. The image contrast of PAI can be estimated using the Weber contrast W formula as 36 :

𝑊 = 𝐼 𝑆 -𝐼 𝐵 𝐼 𝐵 (3)
where IS is the intensity of the PA signal from the sample, whereas IB is the similar quantity from the background. The Weber contrast is calculated to be 31.95, 69.41, and 31.03 for the excitation wavelength of 532 nm, 725 nm, and 880 nm, respectively. Nearly two-fold enhancement in PA contrast for 725 nm wavelength is attributed to the plasmonically enhanced TPA in TiN NPs, as confirmed by data presented in Fig. 2 PAI images signify the lateral distribution of the PA intensities. It is to be noted that the TiN NPs are filled inside a cylindrical glass capillary tube. Naturally, the TiN volume will not be similar along the diameter. Moreover, equation (1) clearly states that the optical absorption by material and hence the PA signal is directly proportional to the volume of the nanoparticle. Therefore, the PA signal varies along the diameter of the tube for all the wavelengths as shown by the line profile in Fig. 4. The FWHM value for the 532 nm excitation is comparatively smaller than others which is attributed to the relatively lower PA intensity at this wavelength which is depicted in PA spectrum in Fig. 2(b).

The observation of a strong nonlinear photoacoustic response associated with plasmonically-enhanced two-photon absorption from laser-synthesized TiN NPs opens up an attractive avenue for the employment of these nanoparticles as contrast agents in photoacoustic biological imaging. Here, the demonstrated high PA contrast and high (67 μm) resolution of PA images promises the use of such NPs in different PA modalities, including photoacoustic microscopy (PAM) and photoacoustic tomography (PAT). It is important that owing to a broad plasmon-related absorption band (maximum at 700 nm) TiN nanoparticles provide the strongest PA signal in the region of relative tissue transparency (630-900 nm), which promises their efficient use in vivo. Furthermore, such absorption is generated by 20-30 nm TiN NPs, which are much smaller than Au-based nanostructures (core-shells, nanorods) used in similar tasks, which are typically larger than 100-150 nm [17][18][19] Such a small size of TiN NPs is critically important to ensure good transport in vivo and subsequent excretion of NPs 37 . As we showed in previous study 24 , the size of laser-synthesized TiN NPs can be further reduced down to 4-5 nm, but such small NPs are still capable of providing plasmonic features in the biological transparency window. It should be noted that in this study we demonstrated the feasibility of nonlinear photoacoustic imaging profiting on plasmonic absorption of TiN NPs placed in acetone solutions. As we showed in Ref 25 , TiN NPs can be easily transferred from acetone to water and then coated by polyethylene glycol (PEG) with a minimal loss plasmonic absorption intensity (less than 5%) and the conservation of spectral position of the plasmonic peak. Based on almost complete match of absorption spectra from these samples, one can expect the same efficiency of aqueous solutions of TiN NPs in photoacoustic imaging tasks. Photoacoustic imaging experiments in biological models are now in progress and will be published elsewhere. Another advantage of the used NPs consists in the absence of toxic impurities as a result of clean laser-ablative synthesis. It should be noted that the cleanness of nanomaterials is one of the main trademarks of laser-ablative technology. Such a cleanness makes possible high biocompatibility, which was previously demonstrated for a variety of nanomaterials, including Si 38,39 , Au 40 , Bi 41 , and B 42 NPs. As we showed in our recent study 25 , laser-synthesized TiN NPs are highly safe, which was demonstrated in a variety of tests in vitro and in vivo. We also showed that the coating of NPs by PEG leads to a substantial prolongation of circulation of lasersynthesized NPs and their enhanced accumulation in tumors. In this case, the proposed imaging modality based on non-linear PA response can be used to track the location of NPs in tissues, while an efficient heating effect under photoexcitation in the region of relative tissue transparency 24 can be applied to destroy cancer cells and achieve a therapeutic outcome. As another opportunity, we envision the use of TiN NPs as contrast agents for dental diagnostics in PAI and PAT modalities 43 . These experiments are now in progress.

In conclusion, we explored titanium nitride (TiN) NPs fabricated by methods of pulsed laser ablation in liquids as an exogenous contrast agent in PAI. The obtained results with PAM and PAS confirm the role of plasmonic enhancement in TPA in the nanoparticles. TiN NPs look advantageous over gold counterparts, since their plasmonic band occurs in the first biological window (NIR I), rendering possible a deeper optical penetration of exciting light for PAI generation. The improvement in axial and lateral resolution can be easily achieved by using appropriate focusing optics and a wider bandwidth acoustic detector. The proposed imaging technique can be combined with the phototherapy functionality of lasersynthesized TiN NPs and thus advance theranostic modality based on plasmonic nanomaterials.

  (a). As shown in the figure, PA intensity is linearly proportional to NPs concentration. A linear fitting to it provides an average slope of 8.38 without any nonlinearity or PA saturation effect.

  FIG. 2. (a) Typical TEM image of TiN NPs prepared by fs laser ablation in acetone (inset) and corresponding size distribution, (b) Comparative plot of absorbance and photoacoustic (PA) spectra of TiN NPs in acetone. PA spectroscopy is performed with 5 ns, 10 Hz tunable OPO laser. The amplitudes are normalized with respect to the plasmonic peak.

FIG. 3 .

 3 FIG. 3. (a) PA signal intensity as a function of TiN NPs concentration measured with 1.85 mJ/cm 2 , 725 nm, 5 ns, 20 Hz tunable OPO laser, (b) ln-ln plot of normalized PA intensity of TiN NPs vs. laser fluence for different plasmonic wavelengths.

  FIG. 4. (a) PAI of a sharp-edged black colored pattern on a plastic sheet and lateral resolutions of the PAI system. White dashed cut-line profile on PAI is used for obtaining ESF and LSF. PAI of TiN NPs in acetone placed in capillary tube under an excitation wavelength of (b) 532 nm, (c) 725 nm, and (d) 880 nm, performed with 1.85 mJ/cm 2 illumination, 20 Hz, 5 ns. False dashed lines indicate boundaries. The dotted line represents normalized horizontal cut-line spatial profile at y-axis = 1.0 mm. EXP: experimental PA values, ESF: edge spread function, and LSF: line spread function.

ACKNOWLEDGEMENTS

This work was funded by INCT of Photonics Program supported by CNPq, CAPES and FACEPE (Grant 465.763/2014-6) for optical characterization and the Russian Science Foundation (Project 19-72-30012) for fabrication and characterization of TiN NPs.

DATA AVAILABILITY

Data available on request from the authors

AUTHOR DECLARATIONS

Conflict of Interest

The authors confirm the absence of conflicts of interest associated with this publication.