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What is the Social Brain and How Does it Develop?
The concept of the social brain has been widely studied in social psychology, anthropology and more recently neuroscience. The social brain hypothesis has developed on two tracks; from investigations focusing on its functional aspects related to our ability to assimilate the complexity of social stimuli, to later fields of inquiry on the structural loci in the human brain associated with social cognition. Both tracks consider the role of nature, our predisposition to process social stimuli, or nurture, how experience influences our ability to do the same. In both fields of study, emphasis has been on how the social brain develops over time, principally from infancy through adolescence.
The modern study of the social brain hypothesis traces its roots to non-human primate studies of the disproportionately large brain size in primates relative to body size and the increased complexity of social interaction present in primate groups (Brothers, 1990). From primate studies confirming the correlation between social group size and increased cortical size (Dunbar, 2003), inquiry turned to the functional development of the human brain represented by the role of social cognition characterised by responses to and understanding of social stimuli including but not limited to facial recognition, eye gaze, joint attention and emotion (de Haan, Johnson & Halit, 2003; Frith, 2008; Grossmann & Johnson, 2007). The functional social brain hypothesis emerging from this line of study has been described as “ability to use knowledge about other individual’s behaviour – and perhaps mind states – to predict and manipulate those individuals’ behaviour” (Dunbar, 2003, p. 167).
With advances in neuroscience and ability to investigate brain function through EEG, fMRI and other means, the structural theory of the social brain has emerged postulating a network of functional locations that respond to various aspects of social stimuli. The principal regions in the network have been identified to include the medial prefrontal cortex (mPFC), posterior superior temporal sulcus (pSTS), anterior temporal cortex (ATC) and the amygdala (Adolphs, 1999; Adolphs & Spezio, 2006; Dumontheil, 2016; Mills, Lalonde, Clasen, Giedd & Blakemore, 2014).
Today it is generally accepted that both nature and nurture play roles in the development of the social brain with a general predisposition to process social stimuli becoming more specialised over time due to accumulated exposure to stimuli (Deen, Richardson, Dilks, Takahashi, Keil, Wald, Kanwisher & Saxe, 2017). Some open questions persist regarding the relative importance of genetically predetermined brain development and the effects of social stimuli in that development (Mills et al, 2014) and on the effects of hormonal changes during puberty on structural brain development versus environmental influences (Bramen, Hranilovich, Dahl, Forbes, Chen, Tonga, Dinov, Worthman & Sowell, 2010; Mills et al, 2014). However, the literature suggests agreement that the functional as well as structural social brain continue to develop through adolescence before stabilising in the early twenties (Mills et al, 2014). As investigation into social brain development continues, further examination into these and other areas of inquiry would assist in refining our understanding of the social brain.
Development of the functional social brain starts in infancy. An important observation on development of the social brain is that infants have a broadly tuned ability to identify social stimuli which becomes more specialised as they mature. This ‘interactive specialisation’ theory (Johnson, 2001) is evident in multiple categories of early social stimuli including facial recognition, gaze processing, motion, joint attention and emotions (Grossmann & Johnson, 2007).
Responsiveness to faces is a well-studied aspect of social interaction occuring in early infancy. A leading theory on assimilation of facial stimuli holds that development of this area of social experience depends on both ‘Conspec,’ an inherent orientation towards attending to faces supporting the nature argument, and ‘Conlern,’ the effects of repeated experience supporting the nurture argument. The theory also postulates that through this Conspec/Conlern interaction the face processing system matures from non-specific recognition to a more finely tuned recognition of faces present in an individual’s social environment (deHaan et al, 2003). Supporting this theory, Deen et al (2017) found that infants between 4 to 6 months develop facial recognition abilities similar to adults albeit with less selective profile responses that become refined as they mature to adulthood. Consequently, both nature and nurture can be seen to play roles in functional social brain development. As well, the march toward development of adult-like specialised facial recognition supports the view that the role of repeated visual experience of faces is an important one in reaching the adult state of facial processing.
The study of infant eye gaze and associated attention to the direction of gazing also suggests functional social brain development during early life is dependent on the social stimuli received. Studies on sighted infants of blind parents (SIBPs) found that they attended less to adult directed gazes than did sighted infants of sighted parents notwithstanding other typical social skills development (Senju, Vernetti, Ganea, Tucker, Charman and Johnson, 2015). These findings imply that the variant from typical eye gaze behaviour is due to repeated exposure to post-natal social experience rather than a genetically predetermined developmental path (Sunju et al, 2015). These findings support the ‘interactive specialisation’ theory since the eye gaze divergence was minimal at ages 6 to 10 months but the divergence widens in the 12 to 16 month age category (Senju et al, 2015).
Infants in early months of life also use communicative and emotional methods like pointing to interact with others (Trevarthen & Aitken, 2001). As infants mature they display enhanced ability to respond to calls for attention (Jones, Gliga, Bedford, Charman and Johnson, 2014). By 8 to 10 months they have developed an ability to indicate shared attention and make requests through imitative learning (Bates & Dick, 2002; Begus & Southgate, 2012) further reinforcing the role of social experience in development of the functional social brain.
Further, Grossmann and Johnson (2007) recognise that infants early in development show differences in attention based on emotional valence and show adult-like biological motion processing through it refines over time.
The weight of studies on functional social brain development during infancy supports the important contribution of social experience in refining the processing of social stimuli as the brain matures. The literature further supports the argument that both nature and nurture play roles in the development of the functional social brain during infancy. It should be noted that an inherent limitation in these studies is the use of infant-friendly measurement methods which may not provide the richness of data found in adult experiments.
The development of the functional social brain does not stop after infancy but continues through adolescence with important advancements in social and affective capabilities (Crone & Dahl, 2012). Adolescence is commonly defined as the period from puberty to social independence (Blakemore & Mills, 2014). In contrast to the basic social cognitive processes influencing functional social mind development in infancy, adolescents experience more complex social stimuli such as understanding the mental states of others, emotional processing and multifaceted interpersonal relations (Blakemore & Milles, 2014).
During this period of exposure to more complex social stimuli adolescents display heightened sensitivity to peer pressure and social exclusion than do adults, which has been seen as an indication that the functional social brain continues to develop during adolescence (Dumontheil, 2016). In addition, both cognitive and affective theory of mind (ToM), inferences about the state of mind of others and the use of empathy to reach conclusions about others’ emotions, have been found to continue development through late adolescence to adulthood (Dumontheil, Apperly & Blakemore, 2010a; Sebastian, Fontaine, Bird, Blakemore, De Brito, McCrory & Viding, 2012). Other findings indicate that ToM and executive function interactions continue to develop in adolescence as well (Dumontheil et al, 2010a), further demonstrating the development of the functional social brain during that period of maturity.
On balance, it can be concluded that the functional social brain develops from infancy through adolescence by exposure to basic social stimuli in infancy to increasingly complex social stimuli in adolescence. The research also demonstrates that both nature and nurture have a role in functional social brain development. Nature through the frameworks enabling unrefined but evident predisposition to process social experience in infancy and nurture through ‘interactive specialisation’ and the later reaction to the complex social experiences of adolescence.
In parallel to functional social brain studies, advances in neuroscience investigative methods have promulgated studies on human social brain structure and development, building on earlier studies of non-human primates. Early studies of non-human primates investigated the relationship between social cognition and neural activity and were based on the theory that sophisticated social behaviour is inherent in primates and that they have the ability to understand the disposition of others and to act upon that understanding (Brothers, 1990). Investigations into correlations between primate brain volume and social complexity have centred on group size and associated social interactions as an indicator of complexity of social stimuli and have found a significant correlation (Dunbar, 2003). Alternative explanations for primate brain size holding that cognitive abilities are just by-products of biological evolution of larger brains have been discredited by the strong correlation between group size and primate brain volume (Dunbar, 2003).
Moving on from observations of non-human primates which focused on overall brain size to study the structural development of the social brain, later human studies investigated the localisation of regions in the brain as the centres for processing social information (Adolphs, 1999, 2001; Saxe, 2006). In 1999 Ralph Adolphs pointed to studies on autism that support the theory that discrete neural regions have domain specific functions related to social cognition. Adolphs also found evidence supporting the role of the amygdala in discerning the emotional import of information from faces enabled by the amygdala’s connections to the visual cortex (Adolphs, 1999), and the importance of the prefrontal cortex in the relationship between perception of stimuli and their social and emotional consequence (Adolphs, 2001).
Further research has identified the specific localised functions of the brain related to processing of social stimuli which can be viewed as a social brain network (Saxe, 2006). Research has also pointed to parallel changes in brain structure that occur along with functional social brain development indicating the path of structural social brain development (Vernetti, Ganea, Tucker, Charman, Johnson and Senji, 2018). In 2014 Mills et al summarised the elements of the structural social brain network that have been discovered by EEG and fMRI testing. They indicate that the pSTS is associated with motion and eye gaze, the TPJ shows activation when inferring the mind states of others, the mPFC appears related to the complex social stimuli of mind states and other aspects of ToM while the ACT is involved in the interpretation of social relationships and has connections to the mPFC and the amygdala (Mills et al, 2014).
Studies also show a trend in cortical development during the arc of structural social brain development from infancy to adolescence. They evidence a reduction in grey matter from infancy through adolescence, widely viewed as a result of neuronal pruning, while white matter increases over the same period attributed to growth in systemic connections between regions in the brain which may be due to processing of the more complex social stimuli experienced in adolescence (Dumontheil, Hassan, Gilbert & Blakemore, 2010b). It has been found that these changes in cortical matter cannot be explained solely by biological maturation and may therefore be due to the increasing complexity of neurocognitive stimulation. (Dumontheil, 2010b).
With these broad findings of a social network of brain loci associated with structural social brain development it remains to be seen which areas of the brain develop at which points along the continuum of time from infancy to adulthood. Research on infant brain development covers a wide range of specific social stimuli such as facial recognition, eye gaze, biological motion and emotion. The research also investigates multiple ages in infancy and the body of research is too broad to recount here in detail. However, findings converge on the theory that infant brain structures associated with processing of social stimuli, the social brain network, are similar to those in adults but are less specialised and continue to specialise and develop through infancy and beyond (Deen et al, 2017; deHaan et al, 2007; Grossmann & Johnson, 2007).
Beyond infancy, the structural social brain continues to develop during adolescence through to the late second decade of life (Mills et al, 2014). Studies have shown that mPFC, TPJ and pSTS reduce in size during adolescence and ATC increases which suggests that areas of the brain associated with complex social stimuli continue to mature during this period (Blakemore & Mills, 2014). Evidence also shows increases in limbic responses to affective social stimuli during adolescence (Crone & Dahl, 2012). Consequently, the body of research into development of the structural social brain supports the theory that its structures continue to develop through infancy and adolescence.
It is also interesting to note that although study of the social brain and how it develops has followed two broad paths, functional and structural inquiry, there is a thread of analysis that points to parallel paths in functional and structural development (Dumontheil et al, 2010b; Vernetti et al, 2018). This suggests that reaction to social stimuli is not only a key factor in structural development as well as functional social brain advancement but that the two may be linked in ways not currently understood. Further study on specific parallel development should be undertaken to examine the nature of this relationship.
As indicated in this analysis, the definition of the social brain throughout the body of research has been explained both in terms of the functional and structural social brain. The functional social brain hypothesis focuses on how we understand and use information about other individuals’ behaviour and mental states whereas the structural social brain definition relates to the network of brain regions associated with processing of social stimuli.
Both the functional and structural social brain have been shown to develop through infancy and adolescence. Functional developmental research indicates early predisposition to process basic social stimuli that become more refined in later infancy through repeated exposure. Further research shows that abilities to process and understand more complex social stimuli continue and continue to develop in adolescence. Adolescent variances in grey and white matter development, in the size of mPFC, pSTS, ACT and in limbic responses point to the continued development of the structural social brain.
Though the definition and developmental pathways of the social brain are much clearer as a result of the body of inquiry into the field, some open questions remain to be investigated. These include the relative influence of genetics and pubertal hormones versus social experience on brain development and the true nature of the relationship between functional and structural social brain development represented in evidence of their parallel development. The limitations of infant neural activity measurement methods in prior studies should also be considered and replication of the results should be sought as measurement advances progress.
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