Social interaction and social functioning involves a multitude of socially relevant cognitive processes including, to name a few, social perception, understanding others' actions, observational social learning, social decision-making, and empathy. Top-down influences of social information can directly drive how we process visual information. More evidence is emerging which suggests that a similar mechanism, or “shared neural representation” is used for understanding others' actions, whereby an internal model of others' actions allows us to make predictions about the consequence and outcome of an observed action, and consequently understand and interpret the goals and intentions of the action. Many authors have suggested how conceptualizations of fundamental cognitions such as learning, could be extended to explain mental processes required for social understanding, social interaction, and social learning (e.g., Rushworth et al.,
2009). There is also substantial work to indicate that there is top-down influence of social information and social interaction on fundamental error processing, learning, and decision-making processes.
We now present neurophysiological and behavioral findings, and computational principles that point to an essential relevance of predictive mechanisms in a broad variety of social cognitive processes that may be intrinsic to motor, perceptual, and learning processes, and permeate different levels of processing. Numerous parallels are also drawn to illustrate how the basic principles in prediction, inference, and simulation in non-social contexts can be applied to social aspects of cognition.
Social perception and seeing others' actions
Person perception can be described as the impressions or mental representations we form of others based on socially constructed information, for which the perception of actions and faces act as crucial cues, and which predictive mechanisms are likely to also play a central role. The superior temporal sulcus (STS) is involved in the perception of biological motion and in inferring the intentions or goals from biological motion (Perrett and Emery,
1994; Allison et al.,
2000; Jellema et al.,
2000), and has been implicated in the mirroring network (Molenberghs et al.,
2010). When we observe others' actions, we can see activity in the STS and it is, therefore, likely related to the mirror system and possibly in determining whether movements have social relevance. A recent study used an fMRI repetition suppression paradigm to measure activity in the action observation network while watching a robot, an android, and a human move (Saygin et al.,
2012). This study interestingly found neural activity that was distinctive for the mismatch between (human versus robotic) appearance and motion, which was proposed to reflect prediction error activity, possibly as an index of an expectancy violation. They also suggested that this mismatch prediction error signal could account for the “uncanny valley” in which androids are seen as strange and disconcerting if they are too human-like (Mori,
1970).
The STS also appears to have a role in face perception, and the perception of the dynamic features of a face (Haxby et al.,
2000; Gobbini and Haxby,
2007; Ishai,
2008). An MEG study from Furl et al. (
2007) found that evoked neuromagnetic fields, originating from the fusiform face area (FFA) and the STS, were modulated by adaptation to facial expressions, and that these predicted behavioral after-effects. They propose that this can be explained by experience-dependent coding, according to a predictive coding account, which consequently creates top-down biases in face perception. Another phenomenon of face perception, in which low level visual processes may be modulated by socially relevant factors, is the “other-race effect.” People have been shown to be better at recognizing faces of their own race as opposed to other races (O'Toole et al.,
1994; Meissner and Brigham,
2001), which appears to occur at the visual encoding stage of face processing (Walker and Tanaka,
2003). This effect could be accounted for by after-effects from visual adaptation to facial race categories (Webster et al.,
2004) that is likely to be based on long-term expertise and learning processes (Rhodes et al.,
1989; Stahl et al.,
2010), and has been represented by hierarchical generative models in a predictive coding framework (Furl et al.,
2007).
Social perception can refer broadly to high-level visual processing of socially relevant stimuli, though social factors can also influence low-level visual processing performance. One major challenge for theories of forward models of action is to demonstrate an inverse relationship in which motor behavior directly influences perception. It has been shown that synchronized and communicative interaction can influence visual discrimination performance (Neri et al.,
2006), and improve visual detection of biological motion (Manera et al.,
2011), respectively. Manera et al. (
2011) explain their finding in terms of predictive coding in that one's own communicative gestures can predict the other's expected action. Bortoletto et al. (
2011) found, with EEG event-related potentials (ERPs), that action plans and intentions of observed hand gestures can modulate ERPs associated with early visual processing of the observed actions. Motor training has been shown to directly modulate activity in the occipital lobe (Engel et al.,
2008), and TMS over the ventral premotor cortex, but not the primary cortex suppressed a visual after-effect when categorizing others' actions (Cattaneo et al.,
2011). These studies demonstrate an early effect of social interaction on low-level visual processing, at an early stage of processing before awareness, therefore confirming the inverse relationship. Research on the neural processing associated with observing others' actions has received widespread interest in a broad range of research areas, particularly in the last 15 years, though there still seems to be some divergence in theoretical standpoints, which could potentially be bridged with a common dialog of predictive mechanisms.
Prediction and the mirror system
The discovery of the activation of apparently functionally-specific “mirror neurons” in monkey premotor cortex during both action execution and action observation (Gallese et al.,
1996) has led to broad speculations about their role in social cognition through action understanding. This hypothesis is compatible with simulation theories of theory of mind (e.g., Davies and Stone,
1995), which in general argues that individuals utilize simulations of their own actions, and consequently their own thoughts, intentions, beliefs, and emotions to predict the mental state of others and therefore, ascertain knowledge of other minds. It is thought that this then ultimately provides the fundamental elements for the ability of an individual to understand, and empathize with, the social behavior of others. Naturally, this has also revealed a number of controversies questioning the functional specificity of the mirror system (e.g., Hickok,
2009), and the anatomical validity of a human mirror system as originally specified (e.g., Molenberghs et al.,
2009; Mukamel et al.,
2010).
Some alternative models of the mirror neuron system have been put forward to try to deal with some of these issues and controversies. One, which is most relevant here, is a predictive coding account of the mirror neuron system (Kilner et al.,
2007a,
b) that uses a Bayesian framework for its implementation. It argues that an internal model is generated during action observation, which in turn transfers an action prediction through backwards connections, from frontal areas implicated in the mirror system, to action representations in the STS and parietal mirror areas, which then produces an action prediction error. As with other predictive systems, the brain seeks to minimize the prediction error. This has been demonstrated with simulations of handwriting that artificially produce electrophysiological responses to movement expectation violations (Friston et al.,
2011). Another alternative account of the mirror system is based on associative learning (Heyes and Ray,
2000; Heyes,
2001), and argues that learned sensorimotor experiences, from self-observation and the observation of others, actually promote the formation and emergence of a mirror system, which is acquired and refined throughout development. The learned associations of action contingencies are thought to provide the basis for action understanding. This associative learning account is supported by findings related to expertise and familiarity of actions in motor cortex activity during action observation, and by studies showing neural activity outside of the mirror neuron system during observation of actions that are unfamiliar or difficult to understand (Brass et al.,
2007; Kilner and Frith,
2008). Greater expertise and familiarity of observed actions induces greater activity in the action observation/mirror-neuron network in the brain (Calvo-Merino et al.,
2005,
2006; Orgs et al.,
2008). This is evident from both practicing a particular motor sequence and from passively observing actions (Cross et al.,
2009). Automatic imitation and motor interference also appears to be influenced by previous sensorimotor experience (Capa et al.,
2011). These findings lend themselves to an associative learning account of imitation and the mirror system (Catmur et al.,
2009), whereby motor representations can be learned through observation (Hayes et al.,
2010). Although it is likely that the coding of motor sequences for observed and practiced actions differs (Gruetzmacher et al.,
2011), though this is still an elusive, but crucial issue in conceptualizations of imitation and action observation.
The only known single neuron recordings of the proposed mirror system in the human brain comes from Mukamel et al. (
2010), who intriguingly found activity in the hippocampus, an area never before included in the classical mirror system. The involvement of the hippocampus in a mirror neuron network could potentially be accounted for by Bar's (
2009) proposal of the predictive brain with memory “scripts” as predictions, and by Barsalou's (
2009) suggestion of the involvement of long-term memory in simulation and perceptual prediction, which may not have been detected previously with fMRI techniques. Bar presents an integrated framework of perception and cognition that argues that memory “scripts,” generated through learned associations from previously real and imagined experiences, form the basis for predictions of what is about to come next in our environment. It is also suggested that this association-based prediction framework can be applied to prediction in social interactions (Bar,
2007; Bar et al.,
2007). By taking an inference-based account of the mirror neuron system, this allows for the integration of Bar and Barsalou's frameworks into the realm of social cognition, action understanding, and the mirror neuron hypothesis.
These accounts of the mirror neuron system highlight the potential role of predictive mechanisms, particularly simulation, and inference with the predictive coding, and associative learning accounts, in social interaction. Consequently, these accounts could legitimately be extended to highlight the role of prediction, simulation and inference in other non-motor social cognitions associated with mirror neuron activity. Inference-based accounts of the mirror neuron system could potentially apply to some examples of work in social neuroscience showing that mirror neuron activity has been implicated in the distinction between self and other (Sinigaglia and Rizzolatti,
2011), mentalizing (De Lange et al.,
2008; Centelles et al.,
2011) and simulation of emotions (Bastiaansen et al.,
2009). Even though the mirror neuron hypothesis provides a very appealing explanation for the processing of others' actions, there are other theories also related to predictive mechanisms that propose integrative frameworks for sensorimotor control and social interaction.
Forward models of action and social interaction
Forward models of action and the corollary discharge are thought to be crucial in determining ownership of action, or sense of agency, and being able to distinguish between self and other by distinguishing between self-generated actions and movements generated by external forces (Fourneret et al.,
2001; Franck et al.,
2001; Knoblich et al.,
2004; Yomogida et al.,
2010). Numerous studies have shown that our sense of agency for our actions can be disturbed if there is a discrepancy in visuomotor perception between expected and intended actions (Daprati et al.,
1997; Franck et al.,
2001; Van den Bos and Jeannerod,
2002). One recent study demonstrating this found that a pre-reflective or implicit sense of agency can be influenced by the accuracy of sensorimotor predictions (Gentsch and Schutz-Bosbach,
2011). The ability to distinguish between self and other is a fundamental prerequisite for many social cognitive processes required for understanding others.
A corollary discharge has also been proposed to be present in the speech system, and is therefore, suggested to be responsible for attributing self-generated speech as one's own (Ford and Mathalon,
2004). Evidence mostly comes from ERP work on occasions when a disturbance in the corollary discharge occurs, which is relevant to symptoms seen in schizophrenia, particularly with auditory hallucinations. This auditory corollary discharge may also therefore, contribute to establishing the distinction between self and other in verbal communication. A recent study used MEG to compare valid and invalid predictions made between visual speech input and auditory speech signals (Arnal et al.,
2011). From their results, they inferred that top down predictions were coded by slower frequencies of neural activity, whereas prediction errors in audiovisual speech were reflected by high frequency ranges. In a social interactive setting, i.e., during natural verbal communication, Stephens et al. (
2010) found that spatiotemporal brain activity of the speaker and the listener became synchronized, and the greater this coupling, the greater the understanding. The findings also revealed anticipatory neural responses in the listener, particularly in the striatum, medial prefrontal cortex (MPFC) and dorsolateral prefrontal cortex (DLPFC), areas that also encode the reward prediction error and value representation.
An extension of one forward model framework of action, the MOSAIC model, has been applied to explain social interaction (Wolpert et al.,
2003). The model parallels the sensorimotor loop between the forward model and the incoming sensory information, with the social interactive loop being between self-generated and observed communicative actions. Communicative actions are thought to be generated from the motor commands observed by a confederate, which consequently causes changes in the observer's mental state, which in turn initiates communicative actions from the other person, which are perceived by the observer. This forward model of social interaction is proposed to allow us to make predictions and learn about the likely behavior of another person in response to our own communicative behavior. An inverse model of action in social interaction is proposed to be used to access the hidden mental states of others, and consequently predict their behavior. The internal models of other people are considered to be decoded and learned through the mappings between our own actions and our own mental states as
a priori information, thereby using our own motor system to compute the internal mental states of others, and are consequently suggested to form a basis for theory of mind.
There are crucial differences between the hypotheses of the mirror neuron system and forward models of action. Internal forward models of action are likely to be coded in the cerebellum (Wolpert et al.,
1998). Consequently, neuroimaging studies have suggested that some of the characteristics of internal models of action, seen from cerebellar activity, can be extended to understanding higher-level cognitions including optimization of behavior toward long-term goals and social interaction, particularly in predicting and understanding of others' actions, theory of mind and language processing (Imamizu and Kawato,
2009). Separate mechanisms in the cerebellum may underlie different processes for switching internal models, with predictive switching being based on changes in context, and postdictive switching being based on the sensorimotor prediction error (Haruno et al.,
2001). Though interestingly, activity associated with the prediction error, used for the postdictive switch, was found in the inferior parietal lobule (IPL) (Imamizu and Kawato,
2008), an area implicated in the human mirror neuron network (Chong et al.,
2008).
It is evident that predictive mechanisms of simulation and inference are likely to be central to both the mirror neuron system and social forward models of action, and may underlie fundamental processes recruited in social interaction. Predictive forward models of action generated from efference copies also may provide the basis for being able to dissociate ourselves from others, on different levels of processing and in different sensory and cognitive modalities. Novel comparisons can be established if forward models of action and the mirror neuron hypothesis are framed in a predictive coding scheme, and consequently stimulating more dialog between the mirror neuron work and work on forward models, while also having implications for social cognition. A crucial issue in making such comparisons is the degree to which neural activity associated with simulated/imagined actions or forward models of planned actions constitutes the same activity as that seen during the execution of a motor action.
Preparing, predicting, and imagining actions
The dynamic changes in neural activity during preparation, online control, and imagination of one's own movements are likely to correspond with, and be embedded in, the neural processes recruited in the prediction of action kinematics and action understanding, during observation of others during social interaction (Grezes and Decety,
2001). One crucial and unresolved issue when discussing the role of prediction in social cognition and motor actions is to what degree preparatory, imagined, predictive, and observational motor responses overlap in terms of neural activity and cognitive function. For example, it may be the case that preparing for an action recruits a forward model, and therefore, the associated neural activity could in part reflect the generation of the forward model and the corollary discharge. It is also not clear as to whether imagined actions also recruit a forward model, but without the matching process of incoming sensory feedback, which could also apply to the observation of others' actions in social interaction. To further clarify the role for motor-related neural activity in social interaction and social cognition, these issues need to be first resolved.
An ERP that has been found to be associated with motor preparation is the contingent negative variation (CNV) (Walter et al.,
1964). The CNV partly overlaps with the lateralized readiness potential (LRP), another similar motor preparatory response. Kilner et al. (
2004) have found that a CNV is also evoked for observed actions, reflecting a preparatory or predictive response to others' actions. The LRP is thought to reflect choice response (Coles,
1989), whereby lateralized motor cortex activity is seen according to the hand used for response, before the response is made. The LRP could be another ERP to use for future explorations of how these preparatory motor responses interact with social cognitions and social contexts, such as task-sharing and action co-representation (Hollander et al.,
2011). If forward models are involved in motor preparation, then such ERPs most likely reflect the neural processing of the efference copy or corollary discharge for both one's own and for others' actions.
It is quite possible that the neural activity seen in preparatory motor responses substantially overlaps with the neural activity during the prediction of one's own and of others' forthcoming actions. Predictable stimuli lead to faster reaction times, for which the temporoparietal junction (TPJ) has been implicated in terms of predictive motor coding (Jakobs et al.,
2009), an area also crucial to the mentalizing network. Prediction and simulation of an observed action in real-time is most relevant to everyday action observation and action understanding in social interaction. Graf et al. (
2007) showed subjects actions where part of the movement sequence was occluded, demonstrating better predictive performance when the timing of the occluder duration fit with the predicted movement, therefore suggesting that predictive mechanisms involved in the observation of others' actions uses real-time simulations. An intriguing study from Miles et al. (
2010) found that mental time-travel, i.e., imagining the past and the future, correlated with the direction of subjects' movements, with subjects swaying forward when thinking about the future and swaying backwards when thinking of the past, suggesting an embodied representation of time and space. Interestingly, Mitchell (
2009) highlights overlapping brain areas responsible for mental state inference and remembering the past, imagining the future, and spatial navigation to argue that internal self-projections are central to theory of mind processes.
The difference between the underlying neural processing involved in imagining and observing actions has relevance to the ideomotor theory of action. Recently, numerous confirmations of ideomotor principles have been revealed with neuroimaging techniques, particularly with studies demonstrating motor cortex activation for imagined actions (e.g., Decety,
1996). The ideomotor principle has also been used to explain imitation in an attempt to overcome the correspondence problem of imitation, in that movement specifics are not directly observable by the observer, and therefore, there is no direct way to match sensory input of another's actions onto our own sensorimotor system (Iacoboni,
2009; Massen and Prinz,
2009). Imagined actions and events have also been found to influence self-monitoring (De Lange et al.,
2007; Turner et al.,
2008), inferring a possible role in self-referential processing and consequently also in dissociating between self and other. An intriguing fMRI study has revealed that the prediction of sequential patterns can evoke activity in areas of the premotor cortex that are related to motor properties of the context of the prediction (Schubotz and von Cramon,
2002), without the execution of an action. This suggests that there may be a somatotopic mapping during the prediction of upcoming sequential events on corresponding motor cortex. The specificity of neural activity and dynamic changes involved in action execution, observation, and imagination are yet to be fully clarified. Paradigms investigating neural activity and behavior in more ecologically-valid social interactive scenarios, such as those using cooperative actions, are likely to shed more light on these questions.
Predicting and moving together
Studies investigating coordinated and cooperative actions are particularly relevant to social interaction and everyday social scenarios, in addition to passively observing actions. Joint action can be defined as a social interaction whereby two people coordinate their actions, often with a shared goal in mind (Sebanz et al.,
2006), in other words, a co-representation of the action and its goal (Wenke et al.,
2011). Given the implied role of the mirror system in imitation, co-representation, and coordinated actions, similar predictive mechanisms of prediction and simulation recruited during action observation in the mirror system could also be extended to apply to joint action, imitative, and synchronous behavior.
Imitation and synchronization of action with another person may reflect preparatory or anticipatory offline mechanisms during action observation and online real-time prediction of action (Konvalinka et al.,
2010). Both may rely on similar processes of motor simulation in the brain that directly relate to inferential and predictive processes, in terms of prediction of forthcoming action and forward models of action, whereby an internal representation may guide imitation and synchronization facilitating matching of the other's actions. Individual differences in the ability to make temporal predictions for forthcoming events have been found during interpersonal sensorimotor synchronization (Pecenka and Keller,
2011), suggesting that temporal predictions could be trained through observation (Scully and Newell,
1985), and are also a necessary precursor to causal predictions, and action-effect contingencies. Therefore, the ability to make temporal action predictions may also be directly related to the ability to make more high-level, non-motor causal associations, inferences, and interpretations in social scenarios, such as during the process of mentalizing.
Automatic imitation and mimicry are thought to reflect underlying shared neural representations of action and mirror system related activity (Brass and Heyes,
2005). Imitative performance can be modulated by the social context of the action such as whether the performer is a human or not, the degree to which the observer relates to the performer of the action (Kühn et al.,
2011), the level of self-focus (Spengler et al.,
2010), the strategic context (Cook and Bird,
2011) and social attitudes (Cook and Bird,
2011). Synchronized movement promotes cooperative behavior (Wiltermuth and Heath,
2009) and the degree to which we perceive others as similar to ourselves (Valdesolo and Desteno,
2011) and the ability to pursue mutual goals together (Valdesolo et al.,
2010), thereby also likely encouraging social cohesion. Joint action and interpersonal synchrony can also be influenced by social context, including perceived group membership (Chartrand and Bargh,
1999; Miles et al.,
2011). Muller et al. (
2011) found that ethnically white participants only showed a joint compatibility effect when observing a white hand, but not for a black hand, though this was eliminated when subjects were asked to take the perspective of the performer. Differences in group relations were also found to influence the tendency to co-represent remembered items of the co-actor (He et al.,
2011). In addition to this, Humphreys and Bedford (
2011) used neurological patients to infer that theory of mind and joint action may have some common neural substrate.
It is clear that much work has already been done to investigate the interdependency between high-level social cognitive processing and low-level motor processes. The top-down influence of social information on bottom-up neural motor activity and the apparent embededness of social cognitive processing in the processing of both one's own and others' motor actions demonstrates the potential coupling of movement to social cognition. It is also evident that predictive mechanisms of simulation and inference, and predictive coding frameworks, provide a fruitful foundation on which to build further common dialogs between currently disparate research disciplines and theoretical viewpoints. However, it is not only the motor response associated with the observation of others' actions that is represented in the observer's brain, but also includes the consequence of the outcome and the implications of the observed action in terms of error, feedback, and reward, and therefore consequently influencing decision-making and learning. Predictive mechanisms also lie at the core of the processes of evaluation of the outcomes of others' actions, and can be applied to both non-social and social contexts.
Cognitive control and error monitoring in a social context
The ability to accurately detect and process errors is crucial for learning. Certain EEG ERPs are thought to be indices of error-processing and the reward prediction error. The feedback-related negativity (FRN) is evoked when negative or positive feedback is given following response choice and is considered to be an index of reward prediction and expectancy violation (Holroyd and Coles,
2002). An error-related negativity (ERN) is seen following the onset of muscle activation during an erroneous response in a forced choice reaction time task (Falkenstein et al.,
1990). The ERN is an index of error-processing and response monitoring, when the intended response is different from the executed response (Baker and Holroyd,
2011), and has been found to originate from the anterior cingulate cortex (ACC) (Dehaene et al.,
1994). Both the ERN and FRN are intrinsically linked to each other and are mediated by the mesencephalic dopamine system and projections to the ACC (Holroyd and Coles,
2002).
Some studies have recently shown that corresponding brain activity involved in error and feedback processing can also be evoked by the observation of others' performance. An ERN and FRN is evoked when watching other people's mistakes (observational ERN or oERN) (Van Schie et al.,
2004) and when observing feedback from other people's response choices (observational FRN or oFRN), respectively, with the oERN and oFRN both also thought to originate from the ACC (Yu and Zhou,
2006). Shane et al. (
2008) have confirmed the activation in the ACC, in the dorsal region, during one's own and observation of a confederate's errors, with additional activity also being found in orbitofrontal areas and premotor cortex. Though, interestingly, a dedicated network appears to be active only when observing others' errors, which includes the inferior parietal cortex (IPC) and the rostral and ventral parts of the ACC (r/vACC), with the IPC correlating with measures of perspective-taking and the r/vACC correlating with self-reported empathetic concern (Shane et al.,
2008,
2009). Another recent study found activity in the MPFC, an area associated with the mentalizing network, specifically activated for errors that affected others (Radke et al.,
2011).
Observational error and feedback processing also seems to be influenced by the degree of self-relatedness and the interpersonal relationship between the observer and the performer, i.e., if the performer is a friend or a stranger, with differences seen in activity in error-related brain areas (Newman-Norlund et al.,
2009), and in error-related (Carp et al.,
2009) and feedback-related ERPs (Kang et al.,
2010; Ma et al.,
2011). Competition and cooperation appear to modulate processing of observed errors to the degree that they influence performance monitoring and even modify performance adjustments. For example, when observing someone else's errors, it appears that a post-error slowing occurs for one's own errors in a cooperative scenario, although there is a post-error speeding in the competitive scenario (De Bruijn et al.,
2012; Nunez Castellar et al.,
2011). An ERN has also been found to be evoked by observed errors performed by cooperators, whereas observed correct responses of competitors evoked a later ERN (Koban et al.,
2010). It has been confirmed that this activity is likely to be not just associated with self-reward, but is a reflection of performance monitoring and updating of expected outcomes based on others' actions (De Bruijn et al.,
2009). The FRN and oFRN have also been shown to be modulated by competition and cooperation (Itagaki and Katayama,
2008; Rigoni et al.,
2010; Van Meel and Van Heijningen,
2010), suggesting that this neural response is influenced by both the benefit or loss to oneself, and the benefit or loss of others (Marco-Pallares et al.,
2010).
These studies all demonstrate how the neural processing of both one's own and others' errors and feedback can be directly influenced by social context and by differences in the social relationships between confederates involved in a social scenario. Therefore, the central role of error and feedback in predictive mechanisms of inference and learning provides a fundamental link between prediction and social cognition. However, an important note to make here is that it is not clear as to how others' gains interact with our own processing and valuation of reward, i.e., from the observed choices of others. This is a crucial issue, as it addresses the degree to which others' gains can be rewarding for us. Differences in neural activity may be wholly reflecting some form of “empathetic” response to others' experience, or, though not mutually exclusive, may be an index of the relevance of the reward to oneself, as the outcome of others' choices may be indirectly associated with a reward for us.
Social learning and social rewards
Observational learning is acquired through making associations between actions and their outcomes, and the value associated with that action and the predicted outcome. It is becoming more apparent that there are some common cognitive and neural processes driving both active experiential learning and observational social learning. In particular, social learning has been proposed to be based on the same simple processes recruited in associative learning. Heyes (
2011) compares learning across different species suggesting that learning only becomes social through adaptation to interactions with conspecifics, and “tuning in” of perceptual, attentional, and motivational information channels to other social agents. She convincingly argues that social learning does not involve mechanisms that are different from those used in non-social learning, and therefore do not have special “social” properties. In support of this, Jones et al. (
2011) found neural activity in areas associated with basic reinforcement learning during a task involving acceptance from peers. One fMRI study also revealed similar underlying neural mechanisms during social valuation and non-social associative reward-based learning, finding a “social prediction error” (Behrens et al.,
2008). Computational modeling has also been used to relate the brain network responsible for reward-related processing with the theory of mind network (Behrens et al.,
2009). Therefore, the principles underlying associative learning can also be extrapolated to explore the role of predictive mechanisms in observational social learning.
It is already known that the processing of reward is dependent on the context in which the reward is presented (Nieuwenhuis et al.,
2005). Although, there is much evidence to suggest that there is something special about social contexts (e.g., cooperation versus competition) and social relations (e.g., ingroup versus outgroup) that modulate the computation of value. Differences in activity can be seen in brain areas associated with motivated behavior and reward evaluation when a social betting task is compared to non-social betting (Nawa et al.,
2008), namely the amygdala, the right DLPFC and the ventral striatum. The processing of feedback and one's own experience of reward, for others' gains or losses interacts with how the observer views the other person, in terms of the opinion or social evaluation of them. Our own valuation of objects can be influenced by others' opinions, as Campbell-Meiklejohn et al. (
2010) demonstrated differences in activity in the ventral striatum, an area thought to code prediction error-related activity, depending upon the opinion of an “expert” reviewer. Ratings of subjective value and the associated neural activity have also been shown to be affected by the valuations made by one's peers, particularly in the nucleus accumbens and the orbitofrontal cortex (OFC) (Zaki et al.,
2011). In addition, the dorsal striatum has been found to encode reward prediction errors in both one's own experiential instrumental conditioning and the observation of others' (Cooper et al.,
2012). There is some evidence to suggest that there may be a common underlying neural network related to one's own valuation of rewards and the valuation of others' action outcomes during observational learning, which may culminate in the ventromedial prefrontal cortex (VMPFC) (Behrens et al.,
2008; Hare et al.,
2010). A review of social preferences collates numerous fMRI studies to find common activation in the dorsal and ventral striatum for the processing of social rewards, with these areas substantially overlapping with areas related to reinforcement learning and anticipation of monetary reward (Fehr and Camerer,
2007), further adding to the argument for shared neural representations for one's own and others' rewards.
Sescousse et al. (
2010) found prediction error-related activity in the ventral striatum, anterior insula, and the ACC for monetary reward, and from the presentation of erotic stimuli, suggesting some common coding of prediction errors regardless of the type of reward, social or not. However, a more recent study found distinctions between brain areas activated during the processing of financial reward feedback, and the valuation of social stimuli, suggesting some separability between the brain's classical reward circuit and the network responsible for the valuation of social stimuli (Evans et al.,
2011). Furthermore, a distinction between action prediction errors and outcome prediction errors have been made in neural areas associated with observational learning (Burke et al.,
2010). The action prediction error is proposed to reflect the discrepancy between expected and observed action choices, coded in the DLPFC, and the outcome prediction error is thought to represent the discrepancy between the expected and observed outcome received by others, coded in the VMPFC.
In social learning, it may be the case that social context and our opinion of others induces different motivational states that correspond to different utility functions, in terms of reinforcement learning theory and expected utility theory, which consequently dictate social decisions and future social judgments. The motivational states in learning theory (Niv et al.,
2006) are mappings of the utility onto the outcome, whereby valuation is driven by both external factors (i.e., the probability of the occurrence) and the internal context (the motivational, emotional, and cognitive state). This could be paralleled in social contexts in which the internal state is driven by predetermined judgments and opinions of the other person and our intrinsic social needs (the internal context), that is weighed up against a statistical probability calculation based on prior experience, learned through socially relevant stimuli and cues (the external factor).
There is conflicting evidence to argue for and against a distinction between social observational and non-social/active learning. However, it appears that more weight is given to the side that proposes a lack of distinction, in that both largely share some common neural substrates, with both also utilizing a form of prediction error, associated with both the valuation of one's own and of others' outcomes in non-social and social scenarios, respectively. Although it is clear that social learning involves an additional dimension in which the social context can directly influence the valuation of an outcome. The social context created by the external environmental situation (e.g., competition or cooperation) or by internal motivational states (including that created from prejudice or through the social relationship with a confederate) can determine the valuation of rewards from others' outcomes. Consequently, social contextual factors will contribute to the formation of social judgments and as a result could also drive decision-making in social situations.
Social decision-making and economic games
Social decision-making deals with high-level computations based on complex socially relevant information such as fairness, trust, social norms, and social preference. Economic decision-making in social contexts, though apparently recruiting additional processes, is still rooted in reward processing and cognitive control, and can also be framed in terms of probabilistic predictive computations, and consequently has been shown to involve similar neural structures in both social and non-social decision-making. This has been largely explored with economic games that include a social component, often with some form of social interaction.
Feedback indicating a violation of a social norm and social expectation has been shown to evoke an FRN, suggesting that the brain treats social deviance in a similar way to a prediction error (Harris and Fiske,
2010; Kim et al.,
2011). Klucharev et al. (
2009) also confirmed this with activity seen in the ACC and supplementary motor area when there is conflict with a social norm. It is likely that cooperative behavior and biding by social norms is based on observational learning and inference-based processes (Boyd and Richerson,
1988; Seymour et al.,
2009; Yoshida et al.,
2010). These findings relate closely to studies showing the effect of others' opinions on our own valuation of objects, as previously mentioned. Popularity ratings have been shown to influence the valuation of adolescents' ratings of music, and interestingly, the tendency to change one's opinion of a song positively correlated with activity in the anterior insula and the ACC (Berns et al.,
2010). Activity in the DLPFC and ACC, both crucially involved in cognitive control and error processing, have consistently been found to be activated when making moral decisions, in particular, when making decisions between fair and unfair offers (Sanfey et al.,
2003; Greene et al.,
2004). A study using a social comparison scenario that induced self-reported envy found that activity in the dorsal ACC was positively correlated with levels of envy (Takahashi et al.,
2009). The ACC and DLPFC have also been shown to be activated when one breaks a promise, as compared to fulfilling that promise (Baumgartner et al.,
2009).
The overlap between reward prediction error-related neural activity and activity utilized in social judgments implies an underlying role of prediction in more complex, higher-level socially relevant psychological processes, such as empathy, trust, judgments of fairness, envy, shame, and guilt. Activity in the ventral striatum has been found during the experience of mutual human cooperation, as opposed to cooperating with a computer (Rilling et al.,
2002,
2004), with two other reward-related areas implicated in reciprocated cooperation, namely the caudate nucleus (Rilling et al.,
2002,
2004; Delgado et al.,
2005) and the OFC (Rilling et al.,
2002,
2004). It is likely that people experience some hedonic pleasure when acting altruistically (Thibaut and Kelley,
1959), which outweighs the potential financial cost. This is confirmed in studies showing activity in the reward circuit when giving charitable donations (Moll et al.,
2005; Harbaugh et al.,
2007). A proposed computational model of decision-making demonstrates that the application of reinforcement learning theory in game-theoretic social interactions and imitative or inference based observational learning can be used to generate altruistic behavior (Seymour et al.,
2009). The evaluation of fairness and the social comparison of monetary rewards have been associated with activity in the ventral striatum (Fliessbach et al.,
2007), with fairer offers also inducing greater activation in the VMPFC and higher subjective ratings of happiness (Tabibnia et al.,
2008). The VMPFC has also been found to be implicated in judgments of trust (Krajbich et al.,
2009) and being trusted by others (Li et al.,
2009). The DMPFC, caudate nucleus, and the striatum have been shown to be activated when learning the trustworthiness of another person (King-Casas et al.,
2005). Findings of activity in the VMPFC, medial OFC, and DLPFC in emotional synchrony of another person also relate to this (Kühn et al.,
2011). Implicit judgments of trustworthiness from facial cues influence social decision-making (Van 'T Wout and Sanfey,
2008; Schlicht et al.,
2010), and the use of reinforcement learning models of trustworthiness also suggests that the evaluation of trust is based on probabilistic beliefs that are dynamically updated according to the proceeding experience and prediction error (Chang et al.,
2010).
The substantial overlap between areas encoding prediction errors, error monitoring, and those implicated in social decision-making tasks implies a common neural basis for social and non-social decision-making processes. This therefore, also highlights the central role of predictive mechanisms of inference in social decision-making and the formation of complex social judgments. In addition to shared neural representations of others' motor actions, outcomes of others' actions, and the implications of others' actions on the observer, there can also be shared sensory and emotional experiences when watching others. This then brings us closer to a conceptualization of empathy in which we not only experience the cold cognitive processes of others, but also experience others' emotional state during observation.
Predicting others' feelings
An interesting study has recently shown synchronized arousal, reflected in heart rate data, among spectators and fire-walkers during observation of a collective ritual (Konvalinka et al.,
2011). Just as sensorimotor matching or motor resonance can occur during action observation, it appears that other people's sensations and emotions can also be contagious and therefore, has consequently been linked with the mirror system. A recent fMRI meta-analysis of areas implicated in the human mirror system found significant overlap with areas involved in emotional processing (Molenberghs et al.,
2012). Observed tactile stimulation can induce shared experiential and neural representations of the others' somatosensation, including another's pain. Threat detection is involved in evaluations of trustworthiness and social decision-making. This clearly has adaptive advantages for survival and has evolved from the ability to efficiently perceive fear-related stimuli, and can also be transmitted through social interaction. Fear-conditioning is likely to be based on similar predictive mechanisms as in reinforcement and associative learning, and these principles could legitimately be extended to explain the social transmission of fear by observational learning processes. Emotional contagion forms the basis for affective empathetic responses (Decety and Ickes,
2009), which may be founded on internal predictive or anticipatory emotional representations (Gilbert and Wilson,
2007). Simulations of emotions, or “pre-feelings,” may not only be used to imagine future emotional states, but may also be used to simulate others' emotional states in social interactive scenarios.
Observing actions, tactile, and painful stimulation in others all invoke activity in the brain of the observer in secondary somatosensory cortex (SII) and BA2 (Brodmann Area 2—posterior primary somatosensory cortex), which is adjacent to SII (Keysers et al.,
2010). Some authors have pointed out a lack of distinction between motor and somatosensory activation during the observation of others' actions, and consequently argue for a lack of distinction between somatosensation and motor processes in the hypothesized mirror neuron system (De Vignemont and Haggard,
2008). Numerous fMRI studies have confirmed this overlap showing activation of SII, but more significant activation of BA2 during action observation (Grezes et al.,
2003; Dinstein et al.,
2007; Gazzola et al.,
2007; Gazzola and Keysers,
2009; Turella et al.,
2009). Empathy and self-versus-other-related processing can also influence somatosensory perception (Jackson et al.,
2006; Lawrence et al.,
2006). Serino et al. (
2009) found that tactile somatosensation on one's own face, while observing another person's face being touched, was enhanced when the observed face was of the same ethnic or political group. Serino et al. (
2008) also previously discovered that viewing one's own face can enhance tactile sensitivity, which is also reflected by enhanced neural activity in a ventral parietal area in a later fMRI study (Cardini et al.,
2011). This also appears to work in the other causal direction, in which somatosensory stimulation of one's own face can improve self-face recognition (Tsakiris,
2008).
The prediction error signal has a crucial role in fear-conditioning and avoidance behavior, achieved by learning relationships between harmful events and environmental stimuli (Delgado et al.,
2008; McNally et al.,
2011; Spoormaker et al.,
2011). Many different animals can evidently learn fear from the observed fear-related behaviors of a conspecific (e.g., John et al.,
1968; Kavaliers et al.,
2001; Munksgaard et al.,
2001; Knapska et al.,
2010). Aversive learning can be communicated by primates through fearful face expressions, with some studies providing support for the suggestion of common processes in fear conditioning and observational fear learning (Mineka et al.,
1984; Mineka and Cook,
1993). Facial expressions are also one of the main ways for socially transmitting fear in humans, with the expression of another's response to stimuli serving as the Pavlovian aversive US (unconditioned stimulus) for the observer (Gerull and Rapee,
2002). Other physical cues can also lead to learned fear responses through observation (Berber,
1962), and even just abstracted information about a fearful response can lead to social transmission of fear (Hygge and Ohman,
1978), though this is also determined by context (Lanzetta and Englis,
1989; Singer et al.,
2006). Neuroimaging studies reveal similar networks involved in both fear conditioning and observational fear learning. Primarily, the amygdala is central to the processing of fear, from both one's own experience and from others', though additional areas have been implicated exclusively in observational fear learning, including the anterior insula and ACC, possibly reflecting anticipation, and parts of the MPFC likely to be involved in some mental state inferential processing of the observed person (Olsson et al.,
2007).
Fear and pain are directly related to one another, and in a social context, observing someone else's pain can induce a representation of that pain in the observer, with activations seen in the observer's somatosensory cortex (Cheng et al.,
2008). Observation of others' pain is also directly related to the social transmission of fear. Neural responses induced by empathy for others' pain have also been shown to be modulated by perceived fairness (Singer et al.,
2006), group membership (Forgiarini et al.,
2011), emotional closeness (Beeney et al.,
2011), emotional context (Han et al.,
2009), self-relatedness (Perry et al.,
2010), the identity of the person being observed and personality differences of the observer (Mazzola et al.,
2010; Goubert et al.,
2011). Goubert et al. (
2011) have presented an intriguing account of the observation of pain and pain-related fear from an observational learning perspective, with added recent experimental evidence (Helsen et al.,
2011). In support of this, Meulders et al. (
2011) have demonstrated that pain-relevant fear conditioning is driven by associative learning mechanisms. It may also be the case that learned aversive behavior is directly linked to reward processing, in that it has been shown to be modulated by monetary reward (Guo et al.,
2011), and the avoidance of aversive outcomes may in itself be rewarding, and therefore reinforcing aversion avoiding behavior (Kim et al.,
2006).
In sum, it is evident that anticipatory neural responses, and predictive coding in the context of learning, are crucial to empathetic somatosensory representations of others' experiences and consequently have a central role in observational learning of fear and pain and emotional contagion. The learning of aversive behavior, transmitted socially by others, will have substantial mediating effects on social decision-making and social behavior. Both fundamental predictive inferential mechanisms and high-level expectations are likely to be at the root of such processes, with interaction and interdependence between processing levels forming the basis for fear-related social decision-making. Predictive mechanisms of simulation and inference are likely to form the underlying processes that allow us to have empathy for others' pain and to learn about aversive stimuli through observation. The ultimate function of shared representations of others' actions, errors, rewards, sensations, and emotions is likely to be the basis for understanding others' minds in social interaction. Arguably, at the highest level of understanding others' minds is the ability to make inferences about others' mental states, which may be founded upon many of the principles already discussed.
Predicting others' minds
In social neuroscience, “mentalizing” or “theory of mind” refers to the ability to infer the mental states of others, ultimately to predict another person's behaviors, and is a central topic of discussion. Daunizeau et al. (
2010) present a meta-Bayesian model for solving the Inverse Bayesian Decision Theory problem, which is the problem of inferring the hidden causes of sensory signals under a prior assumption about the causes. These signals are hidden both in our own experience of sensory input and also hidden from the observer when observing others' behavior, as we do not have perceptual access to these sensory signals. When observing others, the problem is that we are required to determine someone else's prior beliefs and goals with only their behavior as the information available to infer this. This meta-Bayesian solution has been suggested to explain processes such as metacognition, mental state inference, and theory of mind. A recent predictive model of theory of mind has been proposed by Baker et al. (
2011), which relies on Bayesian inferential statistics to model belief and goal-dependent action, which is mediated by the state of the environment and perceptual access to the belief state, and by general knowledge of the world and by general preferences. Bayes' rule is used to model mental state inference, in which action understanding is acquired from integrating “bottom-up information from observed actions and top-down constraints from the prior to infer the goal, given observed actions and the environment.” This inverse planning model is described to account for goal-based predictions of future actions in new situations, according to predictions formed from similar previous experiences. Comparable to this, the mental state inference model (MSI), another computational model of mental state inference, uses forward models of action in a prediction circuit to incorporate visual feedback as the control mechanism for inferring the goals and intentions of others through mental simulation or motor imagery (Oztop et al.,
2005). Mental state inference and theory of mind may represent, and be achieved by, the culmination of many fundamental predictive inferential, and simulation processes related to the processing of others' actions, errors, rewards, and emotional cues. It is also evident that the models of mental state inference that incorporate some predictive principles, such as Bayesian inferential statistics and forward models of action, can accurately simulate behavior.