The Origins of Theory of Mind in Infant Social Cognition: Investigating Longitudinal Pathways from Intention Understanding and Joint Attention to Preschool Theory of Mind

A growing body of literature has established longitudinal associations between key social cognitive capacities emerging in infancy and children's subsequent theory of mind. However, existing work is limited by modest sample sizes, narrow infant measures, and theory of mind assessments with restricted variability and generalizability. The current study aimed to extend this literature by (a) recruiting a large sample of participants (n = 116; 53 boys; 63 girls; all U.S. residents; 88 White, 8 Hispanic or Latino, 2 Black or African American, 14 two or more races/ethnicities, 4 unknown; median family income: $74-122,000), (b) examining multiple measures of infant social cognition (intentional action understanding, responding to joint attention, initiating joint attention) at Time 1 (8-12 months), and (c) using an ecologically valid theory of mind assessment designed to capture individual differences in preschoolers' mental state understanding (the Children's Social Understanding Scale; Tahiroglu et al., 2014) at Time 2 (37-45 months). Measured variable path analysis revealed a significant longitudinal association between infants' initiating joint attention and later theory of mind: infants who engaged in more attempts to initiate joint attention with experimenters through gaze alternation or gestures went on to show better parent-reported mental state understanding as preschoolers. Notably, the paths from infants' responding to joint attention and intentional action understanding to later theory of mind did not emerge as significant. These findings bolster and clarify existing claims about how mental state reasoning is rooted in foundational social-cognitive capacities emerging in infancy.

A glimpse into social perception in light of vitality forms

The American psychoanalyst and developmental psychologist Daniel Stern's idea of vitality forms might suggest a new solution to explain how other minds are intensely expressed in their actions. Vitality forms characterize the expressive style of actions. The effective perception of vitality forms allows people to recognize the affective states and intentions of others in their actions, and could even open the possibility of properties of objects that are indicated by the given actions. Currently, neurophysiological studies present that there might be a neural mirror mechanism in the dorso-central insula (DCI), middle cingulate cortex (MCC), and other related cerebral areas, which serve to preferably perceive and deliver vitality forms of actions. In this article, possible types of vitality forms related to other minds, which have been brought to particular attention in recent years, have been collected and discussed in the following four areas: (1) Vitality forms on understanding non-verbal intention, (2) on understanding verbal intention, (3) vitality forms as grounding social cognition, and (4) as grounding social emotion. These four areas, however, might refer to an entirety of a binary actor-observer communicative landscape. In this review, we try to simplify the analysis by relying on two fundamental dimensions of criteria: first, the idea of vitality forms is conceived as the most basic way of observing subsequent higher-order dimensions of action, that is, understanding intention in the style of action. Thus, in the first two subsections, the relationships between vitality forms and their roles in understanding non-verbal and verbal intention have been discussed. Second, vitality forms could also be conceived as background conditions of all the other mental categories, that is, vitality forms can ground cognition and emotion in a social context. In the second dimension, the existence of social cognition or emotion depends on the existence of the stylistic kinematics of action. A grounding relation is used to distinguish a ground, that is, vitality forms, and its grounded mental categories. As relating with the domain of social perception, in this review, it has been discussed vitality forms possibly could ground social cognition and social emotion, respectively.

Adults Do Not Distinguish Action Intentions Based on Movement Kinematics Presented in Naturalistic Settings

Predicting others’ actions is an essential part of acting in the social world. Action kinematics have been proposed to be a cue about others’ intentions. It is still an open question as to whether adults can use kinematic information in naturalistic settings when presented as a part of a richer visual scene than previously examined. We investigated adults’ intention perceptions from kinematics using naturalistic stimuli in two experiments. In experiment 1, thirty participants watched grasp-to-drink and grasp-to-place movements and identified the movement intention (to drink or to place), whilst their mouth-opening muscle activity was measured with electromyography (EMG) to examine participants’ motor simulation of the observed actions. We found anecdotal evidence that participants could correctly identify the intentions from the action kinematics, although we found no evidence for increased activation of their mylohyoid muscle during the observation of grasp-to-drink compared to grasp-to-place actions. In pre-registered experiment 2, fifty participants completed the same task online. With the increased statistical power, we found strong evidence that participants were not able to discriminate intentions based on movement kinematics. Together, our findings suggest that the role of action kinematics in intention perception is more complex than previously assumed. Although previous research indicates that under certain circumstances observers can perceive and act upon intention-specific kinematic information, perceptual differences in everyday scenes or the observers’ ability to use kinematic information in more naturalistic scenes seems limited.

A longitudinal study of the differential social-cognitive foundations of early prosocial behaviors

A growing body of work has documented the emergence of instrumental helping and sharing in the second year of life; however, less is known about mechanisms that underlie development and production of prosocial behavior. The current study took a longitudinal approach to explore whether the origins of prosocial behaviors can be traced back to foundational social-cognitive capacities emerging in infancy. In a sample of 90 children, longitudinal relations were examined between intention understanding and joint attention measured in infancy (8-12 months) and later instrumental helping and sharing behavior assessed in the toddler years (18-25 months). We expected social-cognitive capacities supporting infants' understanding of others to be positively related to their prosocial behaviors as toddlers. Measured variable path analyses revealed two distinct developmental pathways from infant social cognition to later prosocial behavior: 1) Instrumental helping in the toddler years was positively predicted by intention understanding in infancy; 2) sharing in the toddler years was positively predicted by infants' initiating joint attention. These results lend support to proposals on the multidimensional nature of early prosocial behavior and offer the first longitudinal evidence that the origins of toddlers' prosocial behavior can be traced to social-cognitive capacities emerging in infancy.

Triadic interactions support infants' emerging understanding of intentional actions

Infants' understanding of the intentional nature of human action develops gradually across the first year of life. A key question is what mechanisms drive changes in this foundational social-cognitive ability. The current studies explored the hypothesis that triadic interactions in which infants coordinate attention between a social partner and an object of mutual interest promote infants' developing understanding of others as intentional agents. Infants' spontaneous tendency to participate in triadic engagement was assessed in a semi-structured play session with a researcher. Intentional action understanding was assessed by evaluating infants' ability to visually predict the goal of an intentional reaching action. Study 1 (N = 88) revealed that 8- to 9-month-olds who displayed more bouts of triadic engagement showed better concurrent reasoning about the goal of an intentional reaching action. Study 2 (N = 114) confirmed these findings using a longitudinal design and demonstrated that infants who displayed more bouts of triadic engagement at 6-7 months were better at prospectively reasoning about the goal of an intentional reaching action 3 months later. Cross-lagged path analyses revealed that intentional action understanding at 6-7 months did not predict later triadic engagement, suggesting that early triadic engagement supports later intentional action processing and not the other way around. Finally, evidence from both studies revealed the unique contribution of triadic over dyadic forms of engagement. These results highlight the importance of social interaction as a developmental mechanism and suggest that infants enrich their understanding of intentionality through triadic interactions with social partners.

Movement kinematics drive chain selection toward intention detection

Estimation of intentions from the observation of other people’s actions has been proposed to rely on the same motor chain organization supporting the execution of intentional actions. However, the nature of the mechanism by which a specific neuronal chain is selected among possible alternatives during action observation remains obscure. Our study shows that in absence of discriminative contextual cues, subtle changes in the kinematics of the observed action inform mapping to the most probable chain. These results shed light on the importance of kinematics for the attribution of intentions to actions.

The ability to understand intentions based on another’s movements is crucial for human interaction. This ability has been ascribed to the so-called motor chaining mechanism: anytime a motor chain is activated (e.g., grasp-to-drink), the observer attributes to the agent the corresponding intention (i.e., to drink) from the first motor act (i.e., the grasp). However, the mechanisms by which a specific chain is selected in the observer remain poorly understood. In the current study, we investigate the possibility that in the absence of discriminative contextual cues, slight kinematic variations in the observed grasp inform mapping to the most probable chain. Chaining of motor acts predicts that, in a sequential grasping task (e.g., grasp-to-drink), electromyographic (EMG) components that are required for the final act [e.g., the mouth-opening mylohyoid (MH) muscle] show anticipatory activation. To test this prediction, we used MH EMG, transcranial magnetic stimulation (TMS; MH motor-evoked potentials), and predictive models of movement kinematics to measure the level and timing of MH activation during the execution (Experiment 1) and the observation (Experiment 2) of reach-to-grasp actions. We found that MH-related corticobulbar excitability during grasping observation varied as a function of the goal (to drink or to pour) and the kinematics of the observed grasp. These results show that subtle changes in movement kinematics drive the selection of the most probable motor chain, allowing the observer to link an observed act to the agent’s intention.

Mirror neurons and intention understanding: Dissociating the contribution of object type and intention to mirror responses using electromyography

Since their discovery in the monkey and human brain, mirror neurons have been claimed to play a key role in understanding others' intentions. For example, "action-constrained" mirror neurons in inferior parietal lobule fire when the monkey observes a grasping movement that is followed by an eating action, but not when it is followed by a placing action. It is claimed these responses enable the monkey to predict the intentions of the actor. These findings have been replicated in human observers by recording electromyography responses of the mouth-opening mylohyoid muscle during action observation. Mylohyoid muscle activity was greater during the observation of actions performed with the intention to eat than of actions performed with the intention to place, again suggesting an ability to predict the actor's intentions. However, in previous studies, intention was confounded with object type (food for eating actions, nonfood for placing actions). We therefore used electromyography to measure mylohyoid activity in participants observing eating and placing actions. Unlike previous studies, we used a design in which each object (food, nonfood) could be both eaten and placed, and thus participants could not predict the actor's intention at the onset of the action. Greater mylohyoid activity was found for the observation of actions performed on food objects, irrespective of intention, indicating that the object type, not the actor's intention, drives the mirror response. This result suggests that observers' motor responses during action observation reflect the presence of a particular object, rather than the actor's underlying intentions.

Affective and cooperative social interactions modulate effective connectivity within and between the mirror and mentalizing systems

Decoding the meaning of others' actions, a crucial step for social cognition, involves different neural mechanisms. While the "mirror" and "mentalizing" systems have been associated with, respectively, the processing of biological actions versus more abstract information, their respective contribution to intention understanding is debated. Processing social interactions seems to recruit both neural systems, with a different weight depending on cues emphasizing either shared action goals or shared mental states. We have previously shown that observing cooperative and affective social interactions elicits stronger activity in key nodes of, respectively, the mirror (left posterior superior temporal sulcus (pSTS), superior parietal cortex (SPL), and ventral/dorsal premotor cortex (vPMC/dPMC)) and mentalizing (ventromedial prefrontal cortex (vmPFC)) systems. To unveil their causal organization, we investigated the effective connectivity underlying the observation of human social interactions expressing increasing cooperativity (involving left pSTS, SPL, and vPMC) versus affectivity (vmPFC) via dynamic causal modeling in 36 healthy human subjects. We found strong evidence for a model including the pSTS and vPMC as input nodes for the observed interactions. The extrinsic connectivity of this model undergoes oppositely valenced modulations, with cooperativity promoting positive modulations of connectivity between pSTS and both SPL (forward) and vPMC (mainly backward), and affectivity promoting reciprocal positive modulations of connectivity between pSTS and vmPFC (mainly backward). Alongside fMRI data, such divergent effective connectivity suggests that different dimensions underlying the processing of social interactions recruit distinct, although strongly interconnected, neural pathways associated with, respectively, the bottom-up visuomotor processing of motor intentions, and the top-down attribution of affective/mental states.

Intention understanding over T: a neuroimaging study on shared representations and tennis return predictions

Studying the way athletes predict actions of their peers during fast-ball sports, such as a tennis, has proved to be a valuable tool for increasing our knowledge of intention understanding. The working model in this area is that the anticipatory representations of others' behaviors require internal predictive models of actions formed from pre-established and shared representations between the observer and the actor. This model also predicts that observers would not be able to read accurately the intentions of a competitor if the competitor were to perform the action without prior knowledge of their intention until moments before the action. To test this hypothesis, we recorded brain activity from 25 male tennis players while they performed a novel behavioral tennis intention inference task, which included two conditions: (i) one condition in which they viewed video clips of a tennis athlete who knew in advance where he was about to act/serve (initially intended serves) and (ii) one condition in which they viewed video clips of that same athlete when he did not know where he was to act/serve until the target was specified after he had tossed the ball into the air to complete his serve (non-initially intended serves). Our results demonstrated that (i) tennis expertise is related to the accuracy in predicting where another server intends to serve when that server knows where he intends to serve before (but not after) he tosses the ball in the air; and (ii) accurate predictions are characterized by the recruitment of both cortical areas within the human mirror neuron system (that is known to be involved in higher-order (top-down) processes of embodied cognition and shared representation) and subcortical areas within brain regions involved in procedural memory (caudate nucleus). Interestingly, inaccurate predictions instead recruit areas known to be involved in low-level (bottom-up) computational processes associated with the sense of agency and self-other distinction.

Beyond human intentions and emotions

Although significant advances have been made in our understanding of the neural basis of action observation and intention understanding in the last few decades by studies demonstrating the involvement of a specific brain network (action observation network; AON), these have been largely based on experimental studies in which people have been considered as strictly isolated entities. However, we, as social species, spend much more of our time performing actions interacting with others. Research shows that a person's position along the continuum of perceived social isolation/bonding to others is associated with a variety of physical and mental health effects. Thus, there is a crucial need to better understand the neural basis of intention understanding performed in interpersonal and emotional contexts. To address this issue, we performed a meta-analysis using of functional magnetic resonance imaging (fMRI) studies over the past decade that examined brain and cortical network processing associated with understanding the intention of others actions vs. those associated with passionate love for others. Both overlapping and distinct cortical and subcortical regions were identified for intention and love, respectively. These findings provide scientists and clinicians with a set of brain regions that can be targeted for future neuroscientific studies on intention understanding, and help develop neurocognitive models of pair-bonding.

Grasping intentions: from thought experiments to empirical evidence

Skepticism has been expressed concerning the possibility to understand others' intentions by simply observing their movements: since a number of different intentions may have produced a particular action, motor information—it has been argued—might be sufficient to understand what an agent is doing, but not her remote goal in performing that action. Here we challenge this conclusion by showing that in the absence of contextual information, intentions can be inferred from body movement. Based on recent empirical findings, we shall contend that: (1) intentions translate into differential kinematic patterns; (2) observers are especially attuned to kinematic information and can use early differences in visual kinematics to anticipate the intention of an agent in performing a given action; (3) during interacting activities, predictions about the future course of others' actions tune online action planning; (4) motor activation during action observation subtends a complementary understanding of what the other is doing. These findings demonstrate that intention understanding is deeply rooted in social interaction: by simply observing others' movements, we might know what they have in mind to do and how we should act in response.