Introduction

Humans are inherently social. A large proportion of the human brain is involved in social interaction and understanding other people. The brain regions that are involved in social cognition are collectively referred to as the 'social brain'. Certain regions in the social brain undergo structural and functional changes during development. Recently, neuroimaging studies have focused on adolescence — usually defined as the period of physical, psychological and social transition between childhood and adulthood — as a time of significant functional development of the social brain. This research points to continued development throughout adolescence of social processes such as the recognition of conspecifics and the understanding of others' emotions, intentions and beliefs. This fits with evidence from social-psychology studies that suggests that adolescence is characterized by social change, including heightened self-consciousness, increased importance and complexity of peer relationships and an improved understanding of others. Social-brain development during adolescence is probably influenced by multiple factors, including changes in hormone levels and changes in the social environment. In addition, significant neuroanatomical changes occur in parts of the social brain that are likely to affect cognition and behaviour.

Regions of the social brain: Regions that are involved in social cognition include the medial prefrontal cortex (mPFC) and the temporoparietal junction (TPJ), which are involved in thinking about mental states, and the posterior superior temporal sulcus (pSTS), which is activated by observing faces and biological motion. Other regions of the social brain on the lateral surface are the inferior frontal gyrus (IFG) and the interparietal sulcus (IPS). Regions on the medial surface that are involved in social cognition include the amygdala, the anterior cingulate cortex (ACC) and the anterior insula (AI).

In this Review I describe the social brain and its functional development during adolescence, and attempt to explain how these functional changes relate to the structural changes that occur.

The social brain

The social brain is defined as the complex network of areas that enable us to recognize others and evaluate their mental states (intentions, desires and beliefs), feelings, enduring dispositions and actions. Brain areas that are involved in social cognitive processes include the medial prefrontal cortex (mPFC), the anterior cingulate cortex (ACC), the inferior frontal gyrus, the superior temporal sulcus (STS), the amygdala and the anterior insula. Over the past two decades, research has begun to shed light on how the brain enables the diverse set of functions that allow humans to understand and interact with each other. These functions include recognition of faces and bodily gestures, evaluation of what another person is thinking or feeling, prediction of what that person is about to do next and communication with the person. In this Review I focus on a subset of these functions, namely those that are involved in understanding others, from the recognition of conspecifics to the understanding of emotions and mental states — because these processes have been investigated in adolescence.

Recognition of conspecifics. A fundamental component of social interaction is the ability to recognize conspecifics. Newborn babies seem to be equipped with the ability to detect human faces: at birth, babies prefer to look at photographs and cartoons of faces than at other objects or inverted faces. This early face recognition probably relies on subcortical structures, but in adults face recognition relies on additional cortical areas. Single-cell studies in monkeys have identified neurons in the STS that respond selectively to faces. Evidence from various sources, including electroencephalograms, functional MRI (fMRI) and brain lesions, indicates that the posterior STS (pSTS) is one of the regions that is specialized for the detection of faces and eye gaze in humans.

Another aspect of recognizing other individuals is the detection of motion of conspecifics. Research on biological motion often uses point-light displays (recordings of a person with light sources attached to the joints of their body moving in a dark room), which result in a schematic representation of biological motion that consists of a moving set of dots. The ability to detect biological motion from point-light displays is present from an early age: infants as young as three months of age show a preference for upright point-light displays compared with both inverted point-light displays and displays in which the absolute motion is normal but the spatial relationship between the points of light is changed. In both monkeys11 and humans, the pSTS is involved in the perception of biological motion.

As well as recognizing a stimulus as a conspecific, we automatically evaluate its emotional state. A complex network of regions is involved in the recognition of basic emotions, such as disgust and fear, and in the recognition of more complex emotions, such as trustworthiness16, guilt and embarrassment. This network includes the amygdala, the anterior insula, the STS and the PFC. The mPFC is involved in understanding social emotions, such as guilt and embarrassment. Posterior regions of the inferior frontal gyrus (IFG) are involved in emotional judgement and might have a role in top-down aspects of emotion recognition, such as deciding what action to take based on someone's emotion or predicting what someone is about to do.

Attribution of mental states. Another aspect of social cognition, which enables us to predict the future actions of others, is the ability to work out a conspecific's mental state, including their intentions, desires and beliefs — this ability is known as 'theory of mind' or 'mentalizing'. Using functional imaging and a wide range of stimuli, several independent studies have shown remarkable consistency in identifying the brain regions that are involved in mentalizing. These studies used stimuli such as stories, sentence, words, cartoons and animations that were designed to elicit the attribution of mental states. In each case the mentalizing task resulted in the activation of a network of regions that included the pSTS at the temporoparietal junction (TPJ), the temporal poles and the mPFC. Lesion studies have consistently demonstrated that the superior temporal lobes and the PFC are involved in mentalizing, as damage to these brain areas impairs mentalizing abilities. It is interesting to note that one study reported a patient with extensive PFC damage whose mentalizing abilities were intact, suggesting that this region is not necessary for mentalizing. However, there are other potential explanations for this surprising finding. It is possible that, owing to plasticity, this patient used a different neural strategy in mentalizing tasks. Alternatively, it might be that in some cases damage to this area during adulthood spares mentalizing abilities, whereas damage that occurs earlier in life might always be detrimental. This kind of pattern has also been reported in relation to orbitofrontal cortex (OFC) damage: whereas patients with adult-onset OFC lesions showed no impairment on social–moral reasoning tasks, two patients whose OFC lesions occurred before 16 months of age showed significant impairment.

Recent meta-analyses of mPFC activation by different mentalizing tasks indicate that the peak activation lies in the dorsal mPFC. This region is activated when one thinks about psychological states, regardless of whether these psychological states are applied to oneself, one's mother, imagined people or animals. Competitive games that involve surmising an opponent's mental state also activate the mPFC; thus, it has been proposed that the mPFC is involved in the necessary decoupling of mental states from physical reality. Although the mPFC is activated during tasks that require mental-state attribution relative to matched control tasks, activity in the same region is often highest during low-demand baseline conditions, such as simply looking at a fixation cross. In other words, the mPFC is deactivated during mentalizing tasks relative to low-demand baseline conditions. However, this is probably an artefact of using rest or low-demand tasks as a baseline. Such baseline conditions allow participants to indulge in a maximum degree of spontaneous mentalizing — that is, they might naturally start thinking about mental states (what they want to eat for lunch, whether they enjoy the experience of lying in a noisy brain scanner, et cetera).

Given the role of the pSTS in the perception of faces and biological motion (described above), Frith has suggested that the nearby region of the pSTS/TPJ, which is implicated in mentalizing, is involved in predicting what movement a conspecific is about to make. Saxe, on the other hand, makes the case that the pSTS/TPJ is specifically involved in understanding other people's mental states. However, a recent fMRI study showed that the specific region of the pSTS/TPJ that is activated by mental-state attribution is also activated by a non-social attentional reorienting task. Based on this finding, Mitchell has proposed that the pSTS/TPJ might have a more general role in representing beliefs about, or attention to, stimuli (social or otherwise).

The agreement between neuroimaging studies in terms of the localization of activity in both the pSTS/TPJ and the mPFC during mentalizing tasks is remarkable because subtracting a control condition from a mentalizing condition isolates a high-level cognitive process rather than a low-level sensory one. For example, in the task described in Box 1, participants viewed animations that depict triangles moving in such a way that they seem to possess mental states and emotions. Participants' brain activity during this condition was compared with the activity that was elicited when they observed animations of the same triangles moving in patterns that did not elicit the attribution of mental states or emotions. Brain activity that was associated with processes common to viewing both types of animation (processing visual motion, paying attention, following instructions, et cetera) was subtracted in the comparison between these two conditions to leave only the brain activity that was associated with the attribution of mental states and emotions. Such a high-level cognitive process might not be expected to have a modular functional architecture, as it presumably involves multiple component processes that might not be domain-specific. The consistent localization of activity to a network of regions that included the pSTS/TPJ, the mPFC and the temporal poles suggests that these regions are key to the process of mentalizing.

In the next section I describe the functional development during adolescence of brain regions that are involved in mentalizing and other aspects of social cognition.

Functional changes in the adolescent social brain

There is a large body of literature on the development of social cognition in infancy and childhood that points to step-wise changes in social cognitive abilities during the first five years of life. However, there has been surprisingly little empirical research on social cognitive development that takes place beyond childhood. Only recently have studies focused on the development of the social brain beyond early childhood, and these studies support evidence from social psychology that adolescence represents a period of significant social development.

Most researchers in the field use the onset of puberty as the starting point for adolescence. The end of adolescence is harder to define and there are significant cultural variations. However, the end of the teenage years represents a working consensus in Western countries. Adolescence is characterized by psychological changes that affect an individual's sense of identity, their self-consciousness and their relationships with others. Compared with children, adolescents are more sociable, form more complex and hierarchical peer relationships and are more sensitive to acceptance and rejection by their peers. Although the factors that underlie these social changes are most likely to be multi-faceted, one possible cause is development of the social brain. Some neuroimaging experiments have investigated the development of the social brain during adolescence, focusing on face processing and mentalizing.

Development of recognition of conspecifics during adolescence. Some of the earliest empirical studies on cognitive development during adolescence investigated the effect of puberty on face recognition in girls and produced a surprising result. Performance on face-recognition tasks improved steadily during the first decade of life, but this was followed by a decline at approximately age 12. Puberty (rather than age per se) was implicated in this decline, as a later study showed that mid-pubertal girls performed worse than prepubertal or postpubertal girls matched for age. A more recent study also found evidence of a pubertal 'dip' on a match-to-sample task in which participants ranging from 10 to 17 years of age had to match emotional faces to emotion words. A 10–20% increase in reaction time on the match-to-sample task was found in children of pubertal age (10–11-year-old girls and 11–12-year-old boys) compared with younger children. Performance then improved, regaining its earlier level at approximately 16–17 years of age. Whether this dip can be replicated, whether it is specific to face processing and what causes it are questions that remain to be answered.

A number of functional neuroimaging studies have investigated the neural correlates of facial-expression recognition from childhood to adulthood. An fMRI study reported increased activity in a number of lateral and superior prefrontal regions (bilaterally for girls, right sided for boys) in response to fearful faces in individuals between the ages of 8 and 15 years. Thus, frontal activity increased between childhood and adolescence in this study. In a different study, adolescents (aged 9–17 years) showed activation of the ACC and left OFC during passive viewing of fearful faces (relative to neutral faces), whereas adults (aged 25–36) did not. When attention was directed to a non-emotional aspect of fearful faces, activity in the ACC was higher in adolescents than in adults. Therefore, in this study, frontal activity tended to decrease between adolescence and adulthood. In addition the findings indicate that, whereas adults modulate brain activity based on attention demands, adolescents modulate activity based on the emotional nature of a stimulus. This suggests that the neural basis of the ability to pay attention to a non-salient stimulus (in this case, the nose of a fearful face) in the presence of emotionally evocative, attention-grabbing stimuli (the eyes of a fearful face) is still undergoing maturation between adolescence and adulthood.

To summarize, there is some indication that, notwithstanding potential sex differences, activity in parts of the frontal cortex during face-processing tasks increases between childhood and adolescence and then decreases between adolescence and adulthood.

So far, few studies have investigated the development of the neural substrates of biological-motion processing. However, a recent study indicated that activity in the STS, which is associated with biological-motion perception, increased with age in children aged 7–10 years.

Development of mentalizing ability during adolescence. Although there is no strong evidence that performance in mentalizing tasks changes during adolescence, fMRI studies of mental-state attribution have shown that frontal-cortex activity decreases between adolescence and adulthood. A recent fMRI study investigated the development of our ability to perceive communicative intent, using a task in which participants had to decide whether a speaker was being sincere or ironic. Understanding irony requires the ability to separate the literal meaning of a comment from its intended meaning. In children/young adolescents (aged 9–14 years), the mPFC and left inferior frontal gyrus were more active during this task than they were in adults (aged 23–33). The authors interpreted the increased mPFC activity in young adolescents as a reflection of the need to resolve the discrepancy between the literal and the intended meaning of an ironic remark. The region of the mPFC that was more active in young adolescents than in adults (Montreal Neurological Institute (MNI) coordinates −8, 58, 20), as well as the region in which activity was significantly negatively correlated with age over the whole group of participants (MNI coordinates −2, 44, 32), lies in the dorsal mPFC, an area that is consistently activated by mentalizing tasks in adults.

A similar region of the dorsal mPFC was found to be more active in adolescents than in adults in an fMRI study that involved thinking about one's own intentions. Adolescents (aged 12–18 years) and adults (aged 22–38 years) were presented with scenarios about intentional causality (involving intentions and consequential actions) or physical causality (involving natural events and their consequences). In both groups, intentional causality (relative to physical causality) activated the classic mentalizing network that includes the mPFC, the temporal poles and the pSTS/TPJ. However, intentional causality activated the dorsal mPFC (MNI coordinates 12, 42, 21) more in adolescents than in adults, relative to physical causality. A different activity cluster in the same region (MNI coordinates 15, 45, 18) was negatively correlated with age over the whole group of participants. Conversely, a region in the right STS was more activated by intentional causality in adults than in adolescents, relative to physical causality. These results suggest that the neural strategy for thinking about intentions changes from adolescence to adulthood. Although the same neural network is active, the relative roles of the different areas change with age, with activity moving from anterior (mPFC) regions to posterior (STS) regions.

In the intentional-causality study described above, the scenarios pertained to the self insomuch as they asked about participants' own hypothetical intentions. In another developmental study that focused on the processing of self-related sentences, children (aged 9.5–10.8 years) and adults (aged 23–31.7 years) read phrases about academic skills and social competence. In the 'self' condition, participants were asked to indicate whether the phrases accurately described themselves. In the 'other' condition, they were asked to indicate whether the phrases accurately described a fictional, familiar other person (Harry Potter). The mPFC (MNI coordinates −16, 54, 24 and −10, 54, 14) and the ACC (MNI coordinates −12, 42, 2) were more active in children than in adults during self-knowledge retrieval relative to other-knowledge retrieval. The authors suggested that, compared with adults, adolescents might rely more on 'on-line' self-reflective processing that is performed by the mPFC.

In another study of mentalizing, participants aged 9 to 16 years were scanned during participation in an animation-based mentalizing task. Age correlated positively with activity in the dorsal mPFC (Talairach coordinates −6, 57, 14) and negatively with activity in the ventral mPFC (Talairach coordinates 10, 43, 0). The researchers suggested that this might reflect a change in strategy, from simulation in childhood (based on the self, and involving the ventral mPFC) to a more objective strategy in adults (involving the dorsal mPFC). The developmental change in dorsal mPFC activation reported in this study seems to contradict those of the mentalizing studies described above. However, this task is different from other mentalizing tasks in that it is non-verbal, and the attribution of mental states to the shapes in the animations is illusory and subjective. In addition, the oldest participants in this study were 16 years of age, and it is unknown how activity in the dorsal mPFC during participation in the animations task changes after this age. Because no adult group was included for comparison, it cannot be ruled out that activity in this region decreases between 16 years of age and adulthood.

To summarize, as of yet there have been only a handful of developmental-neuroimaging studies of social cognition, but there does seem to be some consistency with respect to the direction of change in frontal activity. Overall, the studies reviewed here have found that activity in various frontal regions decreases between adolescence and adulthood, despite performance being equated across groups. Equating performance between groups is critical for the interpretation of the functional neuroimaging data: if performance between groups were significantly different, it would be impossible to know whether a group difference in neural activity was the cause, or simply a consequence of, the difference in performance. On the other hand, matching performance between groups negates important differences between adolescents and adults in terms of social cognition, as has been reported in a large number of social psychology studies. If the neural substrates of social cognition change during adolescence, what are the consequences for social cognitive behaviour? Most developmental studies of social cognition focus on early childhood, possibly because children perform adequately in even quite complex mentalizing tasks by five years of age. It is a challenge therefore to design a task on which older children and adolescents do not perform at ceiling level (note that there is one recently developed ingenious mentalizing task on which even healthy adults make mistakes).

One possible explanation for the decrease in prefrontal activity on social-cognition tasks is that adolescence represents a time of synaptic reorganization in the PFC. This is discussed in the following section.

Structural brain development during adolescence

Cellular development. Research in the 1970s and 1980s carried out on post-mortem brain tissue revealed that the human PFC undergoes a protracted period of synaptic development that continues well into adolescence. Huttenlocher's research contrasted with earlier studies on the development of sensory and motor regions in animal brains (in particular those of cats and monkeys), which demonstrated that synaptogenesis and synaptic pruning occur early in an animal's life. In the primary motor cortex of macaque monkeys, for example, synaptogenesis starts early in fetal development and results in a greater synaptic density in early infancy than in adulthood. The excess synapses (that is, synapses that are not included in functional neuronal circuits) are then pruned back, and synaptic density declines to adult levels by approximately three years of age (the age of sexual maturity in monkeys). Huttenlocher's research, by comparison, demonstrated that in the human brain synaptic density reaches a maximum before one year of age in the primary auditory and visual cortices and at approximately three and a half years of age in the PFC (middle frontal gyrus). Interestingly, whereas in the human auditory cortex synaptic elimination is complete by 12 years of age, pruning continues until mid-adolescence in the PFC.

Developmental MRI studies of the human brain. Huttenlocher's research relied on post-mortem human brains. In the past decade or so, structural MRI has enabled researchers to investigate the development of the living human brain. Since the first developmental MRI studies there have been numerous large-scale studies. Some were longitudinal, and scanned the same children more than once over time to see how brain regions change with age; others were cross-sectional, and scanned participants of different ages to see how their neuroanatomy compared. MRI studies have mostly focused on developmental changes in grey matter (which corresponds to cell bodies, synapses and neuropil) and white matter (which corresponds to myelinated axons). The results of these studies are remarkably consistent: several cortical regions, in particular parts of the PFC, the temporal cortex and the parietal cortex, as well as a number of subcortical structures, undergo substantial changes in white- and grey-matter volume during the first two (and in some studies even three) decades of life. White-matter volume seems to increase linearly during the first two decades. This increase in white-matter volume has been suggested to reflect protracted axonal myelination in some cortical regions throughout the first two decades of life.

By contrast, grey-matter volume in several cortical areas decreases between childhood and adulthood. A recent longitudinal study of anatomical brain development in participants aged between 4 and 21 years showed that grey-matter loss occurs initially in the primary sensorimotor areas and then spreads over the PFC, the parietal and occipital cortices and finally the temporal cortex. This finding has been replicated a number of times, with other studies showing that decreases in grey-matter volume continue throughout adolescence, in particular in the lateral and superior PFC. The decrease in grey-matter volume in the PFC during adolescence is proposed to reflect, at least in part, synaptic elimination.

Other studies report a nonlinear, 'inverted-U'-shaped pattern of grey-matter change with age in frontal, parietal and temporal regions. In an early study by Giedd and colleagues, the region in which grey-matter volume peaked latest (at approximately 16 years of age) was the temporal cortex, and this was followed by a decline during late adolescence and the early twenties. Sowell and colleagues observed a similar inverted-U-shaped pattern of grey-matter volume in the STS. It is unknown why some studies report an inverted-U-shaped pattern whereas others report a steady decline in grey-matter volume between childhood and adulthood. However, it is possible that the increase in grey-matter volume during childhood that is reported in some studies reflects prolonged synaptogenesis.

Relationship between neuroanatomical, cognitive and functional brain development. One possible consequence of the relatively late elimination of excess synapses in the human PFC and other cortical regions is that it renders information processing in the relevant brain regions less efficient. The excess, 'untuned' synapses are thought to result in a low signal-to-noise ratio. Input-dependent synaptic pruning eliminates those excess synapses, thereby effectively fine-tuning the remaining connections into specialized functional networks. After pruning, it is possible that it takes fewer synapses to do same amount of work, because the remaining synapses are more efficient. This would engender a system with a higher signal-to-noise ratio, which might result in more efficient cognitive processing and improved performance with age. The dip in performance at puberty on face-processing tasks might be related to the increase in grey-matter volume at approximately this age in frontal and temporal regions. On the other hand, there are other potential causes of a pubertal dip in performance, including changes in hormones and changes in the social environment — puberty is the age at which most children enter new schools and are exposed to many new faces. Rendering the picture even more complex, hormonal and environmental changes might trigger, or at least influence, alterations in grey- and white-matter volume in the social brain, which might then influence social cognition and behaviour. It is currently difficult to disentangle genetically pre-programmed developmental changes from those that are triggered by changes in the environment.

The developmental neuroimaging studies of social cognition reviewed above tend to show a decrease in frontal activity between adolescence and adulthood. In face-processing tasks, activity in the lateral and superior PFC seems to increase between childhood and adolescence and then decrease between adolescence and adulthood. This nonlinear pattern of activity might be related to synaptic reorganization. Excess synaptogenesis during childhood might result in increasing levels of activity in the relevant brain region, owing to a low signal-to-noise ratio for the neuronal networks involved. Synaptic pruning during adolescence could then lead to a higher signal-to-noise ratio for the neuronal networks, possibly resulting in decreased levels of activity. This might account for the decrease in activity in the lateral and superior PFC in face-processing tasks and in the mPFC in mentalizing tasks between late childhood/early adolescence and adulthood.

This is a purely speculative idea that makes several assumptions that are yet to be tested. First, it assumes that a larger number of synapses in a given unit of brain tissue results in an increased blood-oxygen-level-dependent (BOLD) signal if those synapses are active. This notion assumes that there is a more-or-less linear relationship between synaptic density and the BOLD signal. Exactly how linear the coupling between synaptogenesis and the BOLD response is remains to be determined. It is likely that the coupling is different in different brain regions and that there is at least some degree of nonlinearity. Second, it makes the assumption that vascular changes correlate with synaptic changes; whether this is the case is as yet unknown. Furthermore, it would be useful to know whether there are correlations between structural changes (in grey matter) and functional changes (to the BOLD signal) in the same individuals. Although this has been investigated in one neuroimaging study on executive control, none has yet attempted to correlate structural and functional development of the social brain.

Conclusion and questions for further research

Here I have reviewed evidence that certain areas of the social brain, namely the pSTS and the mPFC, undergo substantial functional and structural development during adolescence. Recent functional neuroimaging studies of social cognitive development suggest that activity in a number of prefrontal areas increases between childhood and adolescence and then decreases between early adolescence and adulthood. The decrease in prefrontal activity during adolescence might be related to structural development in this area, namely the elimination of unused synapses.

There are many outstanding questions. A fundamental question is how synaptic reorganization affects neural activity and cognitive function and what triggers its reorganization at puberty. Virtually nothing is known about this relationship, but it seems likely that hormonal changes at puberty trigger neuroanatomical changes.

How the environment influences brain development during adolescence is another empirical question. It has been proposed that synaptic pruning in early development fine-tunes neuronal circuitry in an input-dependent manner. Rats that are brought up in an enriched environment — that is, together with other rats in a cage with toys and exercise wheels — have higher synaptic density in the visual cortex than rats that are brought up individually with little stimulation. The 'enriched' rats also show better performance on spatial-navigation tasks, but at present the relationship between enrichment, synaptic density and memory is purely correlational and no inference about causality can be drawn. In humans, synaptic pruning in early childhood has been proposed to underlie sound categorization. The ability to distinguish certain speech sounds depends on being exposed to those sounds in early development. For example, before approximately 12 months of age, babies that have been brought up in the United States can detect the difference between certain sounds that are common in the Hindi language; after 12 months they can no longer distinguish these sounds because the English language does not contain them79. Although there is no direct evidence, this fine-tuning of sound categorization is thought to rely on the pruning of synapses in sensory areas that are involved in processing sound. It is unknown whether the pruning of synapses in the PFC during adolescence might be similarly influenced by environmental input. It has recently been suggested that cannabis use might influence the development of the brain during adolescence, although the precise mechanisms by which cannabis might affect synaptogenesis or synaptic pruning are not known.

A number of neuroimaging studies have reported lower levels of activity during face-processing, emotion-recognition and mentalizing tasks in the PFC in adults than in adolescents. This difference implies that damage to the PFC during adolescence might be more detrimental to associated cognitive functions than damage that is suffered during adulthood. There is evidence that damage to the PFC (including the mPFC) in adulthood does not impair mentalizing. A comparison between people who have suffered mPFC damage at different ages would be interesting. Perhaps the mPFC is important for the early development of mentalizing, but becomes less important with age. On the other hand, as discussed above, this region is consistently activated when adults think about mental states. The question, then, is what is the role of the mPFC in social cognition in adulthood? Also, how and why does its role in social cognition change between adolescence and adulthood?

Adolescence is a period in which individuals undergo changes in their social behaviour, yet few empirical behavioural studies have reported significant behavioural development that is specific to social cognition and that cannot be explained by general improvements in attention, concentration, memory and so on. One possibility is that adolescents are more adept at completing complicated social cognition tasks in the laboratory than they are at dealing with situations that arise in everyday life. More naturalistic paradigms might be useful in addressing this question. Another possibility is that it is not mentalizing per se that changes, but rather the modulation of mentalizing by executive functions. An example of this kind of relationship was reported by Monk and colleagues in their study of face processing. In this study, whether participants directed their attention to emotional facial features (the eyes) or non-emotional facial features (the nose) affected neural activity associated with face processing differentially in adolescents and adults. However, it is important not to try to explain all of adolescent behaviour in terms of neuroanatomical changes, as this neglects other important factors such as hormonal and social changes (although these could in turn trigger neuroanatomical changes). There are presumably large differences between individuals in the development of the social brain, but these have so far been neglected in the literature. Furthermore, synaptic plasticity is a baseline property of the brain even in adulthood, and it occurs whenever something is learned. An unanswered question is whether and how plasticity that takes place during adolescence differs from plasticity in adulthood.

These are just some of the many questions that remain to be investigated in this new and rapidly expanding field. The study of neural development during adolescence is likely to have important implications for society in relation to education and the legal treatment of teenagers, as well as for various mental illnesses that often have their onset in adolescence.

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Introduction

Humans are inherently social beings, with a significant portion of the brain dedicated to social interaction and understanding. This complex network of brain regions, termed the "social brain," undergoes substantial structural and functional changes throughout adolescence, a period marked by significant physical, psychological, and social transition. This review explores the functional development of the social brain during adolescence, focusing on processes like recognizing individuals and comprehending emotions and mental states, and examines how these functional changes relate to concurrent structural brain development.

The Social Brain

The social brain encompasses a network of brain areas enabling us to recognize others and decipher their mental states, emotions, and actions. Key regions include the medial prefrontal cortex (mPFC), anterior cingulate cortex (ACC), inferior frontal gyrus, superior temporal sulcus (STS), amygdala, and anterior insula. Extensive research has illuminated how these regions contribute to social cognitive functions such as recognizing faces and body language, evaluating emotions, and predicting future actions. This review will primarily focus on the ability to understand others, from recognizing individuals to comprehending emotions and attributing mental states, as these processes have been specifically studied in the context of adolescent development.

Recognition of Conspecifics

Recognizing fellow humans is fundamental to social interaction. Newborns exhibit an innate preference for faces, suggesting a rudimentary, likely subcortical, recognition mechanism. In adults, this process engages cortical areas, notably the posterior STS (pSTS), identified through single-cell recordings in monkeys and various human studies as a specialized region for face and gaze detection.

Recognizing individuals also involves perceiving biological motion. Infants as young as three months old demonstrate sensitivity to biological motion patterns. Both monkeys and humans exhibit pSTS activation during biological motion perception, often studied using point-light displays that depict movement through illuminated joints.

Beyond simply recognizing individuals, we automatically assess their emotional states. A complex network encompassing the amygdala, anterior insula, STS, and PFC is implicated in recognizing basic emotions like disgust and fear, as well as more intricate emotions like trustworthiness, guilt, and embarrassment. The mPFC plays a specific role in understanding social emotions. Posterior regions of the IFG contribute to emotional judgment and may influence top-down aspects of emotion recognition, such as determining appropriate responses or predicting actions based on perceived emotions.

Attribution of Mental States

A crucial aspect of social cognition, enabling us to anticipate others' actions, is the ability to infer their mental states – their intentions, desires, and beliefs. This capacity, known as "theory of mind" or "mentalizing," has been extensively studied using functional imaging and diverse stimuli designed to elicit mental state attribution, such as stories, sentences, words, cartoons, and animations. These studies consistently implicate a network of brain regions, including the pSTS at the temporoparietal junction (TPJ), temporal poles, and mPFC, in mentalizing. Lesion studies corroborate the involvement of the superior temporal lobes and PFC, revealing impaired mentalizing abilities following damage to these areas. Notably, there is a case report of a patient with extensive PFC damage who retained intact mentalizing abilities, suggesting that this region might not be universally essential for this function. However, alternative explanations exist, including the possibility of neural plasticity allowing for compensatory mechanisms or that damage timing (e.g., early vs. late in life) differentially impacts mentalizing ability. This pattern has been observed with orbitofrontal cortex (OFC) damage, where adult-onset lesions spared social-moral reasoning abilities, while early childhood lesions resulted in significant impairments.

Meta-analyses of mPFC activation during mentalizing tasks pinpoint the dorsal mPFC as a key region, consistently activated when considering psychological states, whether attributed to oneself, others, imagined individuals, or even animals. Competitive games requiring mental state inference also activate the mPFC, leading to the proposition that it facilitates the decoupling of mental states from physical reality. Although the mPFC shows increased activity during mentalizing tasks compared to matched controls, activity in this region often peaks during low-demand baseline conditions. This apparent deactivation during mentalizing tasks relative to baseline is likely an artifact of using rest or low-demand tasks as a reference point, as these conditions allow for a high degree of spontaneous mentalizing.

Given the pSTS's role in perceiving faces and biological motion, it has been proposed that the nearby pSTS/TPJ region, implicated in mentalizing, contributes to predicting others' movements. However, an alternative view suggests the pSTS/TPJ specifically processes the understanding of others' mental states. A recent fMRI study demonstrated that the specific pSTS/TPJ region activated during mental state attribution is also recruited during a non-social attentional reorienting task, suggesting a more general role in representing beliefs about or directing attention towards stimuli, regardless of their social nature.

The consistent localization of activity to the pSTS/TPJ, mPFC, and temporal poles during mentalizing tasks is noteworthy because subtracting control conditions from a mentalizing condition isolates a high-level cognitive process rather than a low-level sensory one. This consistency suggests that these regions are crucial for mentalizing, despite the complex and multifaceted nature of this cognitive process.

The following section delves into the functional development of these brain regions during adolescence, focusing on their roles in mentalizing and other aspects of social cognition.

Functional Changes in the Adolescent Social Brain

While a vast literature documents the development of social cognition during infancy and childhood, research on social brain development beyond early childhood remains surprisingly limited. Recent studies, however, support the notion that adolescence represents a period of significant social development, echoing evidence from social psychology.

Most researchers define adolescence as beginning with the onset of puberty, with the end being less clearly defined and subject to cultural variations. However, the end of the teenage years serves as a general consensus in Western societies. Adolescence is characterized by psychological changes impacting individuals' sense of self, self-consciousness, and relationships with others. Compared to children, adolescents are more sociable, form more intricate and hierarchical peer relationships, and exhibit heightened sensitivity to peer acceptance and rejection. While these social changes likely stem from multifaceted factors, social brain development represents a plausible contributing factor. Neuroimaging studies have begun to investigate this development, focusing primarily on face processing and mentalizing.

Development of Recognition of Conspecifics During Adolescence

Early empirical studies on cognitive development during adolescence examined the impact of puberty on face recognition in girls, yielding unexpected results. While face recognition performance steadily improved during the first decade of life, it declined around age 12. Puberty, rather than age itself, was implicated in this decline, with mid-pubertal girls performing worse than age-matched prepubertal or postpubertal girls. A more recent study also found evidence of a pubertal "dip" in a match-to-sample task requiring participants aged 10 to 17 to match emotional faces to corresponding words. Children of pubertal age (10-11-year-old girls and 11-12-year-old boys) exhibited a 10-20% increase in reaction time compared to younger children, with performance subsequently improving and reaching previous levels by around 16-17 years of age. Further research is needed to replicate this finding, determine its specificity to face processing, and elucidate its underlying causes.

Several functional neuroimaging studies have explored the neural correlates of facial expression recognition across childhood and adulthood. One fMRI study reported increased activity in lateral and superior prefrontal regions (bilaterally in girls, right-lateralized in boys) in response to fearful faces in individuals aged 8 to 15, suggesting increased frontal activity from childhood to adolescence. Conversely, another study found that adolescents (aged 9-17) exhibited ACC and left OFC activation during passive viewing of fearful faces compared to neutral faces, while adults (aged 25-36) did not. When attention was directed towards a non-emotional aspect of fearful faces, adolescents displayed higher ACC activity than adults. These findings suggest that frontal activity related to face processing may decrease from adolescence to adulthood and highlight a developmental shift in attentional modulation. While adults modulate brain activity based on attentional demands, adolescents modulate activity based on a stimulus's emotional salience. This suggests continued maturation of the neural mechanisms underlying the ability to attend to non-salient stimuli in the presence of emotionally evocative, attention-grabbing stimuli.

In summary, despite potential sex differences, there is evidence to suggest that activity in certain prefrontal regions during face processing tasks increases from childhood to adolescence and subsequently decreases from adolescence to adulthood.

Fewer studies have investigated the developmental trajectory of the neural substrates underlying biological motion processing. However, one recent study indicated that activity in the STS, associated with biological motion perception, increased with age in children aged 7-10.

Development of Mentalizing Ability During Adolescence

While evidence for significant changes in mentalizing task performance during adolescence remains limited, fMRI studies on mental state attribution have revealed decreased frontal cortex activity from adolescence to adulthood. One study investigated the development of communicative intent perception using a task requiring participants to differentiate between sincere and ironic statements. Understanding irony necessitates separating a comment's literal meaning from its intended meaning. Children/young adolescents (aged 9-14) exhibited greater mPFC and left inferior frontal gyrus activation during this task compared to adults (aged 23-33). This heightened mPFC activity in younger individuals was interpreted as reflecting the increased cognitive effort required to resolve the discrepancy between the literal and intended meanings of ironic remarks. The specific mPFC region showing increased activation in younger individuals, as well as the region displaying a significant negative correlation between activity and age across all participants, falls within the dorsal mPFC, consistently implicated in adult mentalizing tasks.

A similar dorsal mPFC region exhibited greater activity in adolescents than adults during an fMRI study involving reflections on one's own intentions. Adolescents (aged 12-18) and adults (aged 22-38) were presented with scenarios depicting either intentional causality (involving intentions and their consequential actions) or physical causality (involving natural events and their consequences). Both groups showed activation of the classic mentalizing network (mPFC, temporal poles, and pSTS/TPJ) during intentional causality compared to physical causality. However, adolescents exhibited greater dorsal mPFC activation than adults during intentional causality relative to physical causality. Conversely, a region within the right STS displayed greater activation during intentional causality in adults compared to adolescents. These results suggest a developmental shift in the neural strategy for processing intentions. While the same network remains active, the relative contributions of different areas change with age, with activity shifting from anterior (mPFC) to posterior (STS) regions.

In the aforementioned intentional causality study, scenarios pertained to the self by prompting participants to consider their own hypothetical intentions. Another developmental study focused on processing self-related sentences, asking children (aged 9.5-10.8) and adults (aged 23-31.7) to read phrases describing academic skills and social competence. In the "self" condition, participants indicated whether phrases accurately described themselves, while in the "other" condition, they made the same judgment for a fictional, familiar individual (Harry Potter). Children exhibited greater mPFC and ACC activation than adults during self-knowledge retrieval compared to other-knowledge retrieval. This finding suggests that adolescents may rely more heavily on "on-line" self-reflective processing mediated by the mPFC compared to adults.

Another mentalizing study scanned participants aged 9 to 16 during an animation-based task. Age positively correlated with dorsal mPFC activity and negatively correlated with ventral mPFC activity. This pattern was interpreted as potentially reflecting a strategic shift from simulation in childhood (relying on the self and ventral mPFC) to a more objective, less self-referential strategy in adulthood (involving the dorsal mPFC). However, this study's findings seemingly contradict those of other mentalizing studies reporting decreased dorsal mPFC activity with age. This discrepancy may be attributable to task differences, as this study employed a non-verbal task with subjective, illusory mental state attributions. Additionally, the oldest participants were 16, leaving open the possibility of continued dorsal mPFC activity decline beyond this age.

In summary, despite the limited number of developmental neuroimaging studies on social cognition, a consistent trend emerges regarding frontal activity changes. Across studies, activity in various frontal regions generally decreases from adolescence to adulthood, despite comparable performance across age groups. This performance matching is crucial for interpreting functional neuroimaging data, as significant performance differences would confound the relationship between brain activity and age. However, ensuring matched performance may obscure important developmental differences in social cognition between adolescents and adults, as reported in numerous social psychology studies. If the neural underpinnings of social cognition indeed change during adolescence, what are the implications for social cognitive behavior? Most developmental studies on social cognition focus on early childhood, possibly because even complex mentalizing tasks are mastered by age five. Designing tasks that do not result in ceiling effects in older children and adolescents remains a challenge. One possibility is that adolescents' proficiency in navigating complex social cognition tasks in laboratory settings may not translate to real-world social situations. Employing more naturalistic paradigms could address this discrepancy. Alternatively, it may not be mentalizing itself that undergoes significant development, but rather the interplay between mentalizing and executive functions. For example, a study on face processing demonstrated age-related differences in how directing attention towards emotional versus non-emotional facial features modulated neural activity. Nevertheless, it is crucial to avoid attributing all adolescent behavior to neuroanatomical changes, as this overlooks the potential influence of hormonal and social factors, which themselves could trigger neuroanatomical alterations. Furthermore, individual variability in social brain development likely exists but remains largely unexplored in the current literature.

The following section examines structural brain development during adolescence, specifically focusing on synaptic pruning and changes in gray and white matter volume.

Structural Brain Development During Adolescence

Cellular Development

Research conducted in the 1970s and 1980s using post-mortem brain tissue revealed a prolonged period of synaptic development in the human PFC extending well into adolescence. This finding contrasted with earlier animal studies demonstrating early synaptogenesis and synaptic pruning in sensory and motor regions. For example, in the macaque monkey primary motor cortex, synaptogenesis begins prenatally, resulting in higher synaptic density in early infancy compared to adulthood. Excess synapses are subsequently pruned, reaching adult levels by approximately three years of age (corresponding to sexual maturity in monkeys). In contrast, human brain studies revealed that peak synaptic density in the primary auditory and visual cortices occurs before one year of age, while the PFC (specifically the middle frontal gyrus) reaches peak density around three and a half years of age. Furthermore, while synaptic elimination in the human auditory cortex concludes by age 12, pruning in the PFC continues until mid-adolescence.

Developmental MRI Studies of the Human Brain

Advancements in structural MRI have enabled researchers to investigate the developing human brain in vivo. Numerous large-scale studies, both longitudinal and cross-sectional, have primarily focused on developmental changes in gray matter (representing cell bodies, synapses, and neuropil) and white matter (representing myelinated axons). These studies consistently demonstrate substantial changes in white and gray matter volume within several cortical regions, including the PFC, temporal cortex, and parietal cortex, as well as various subcortical structures, throughout the first two (and in some cases, three) decades of life.

White matter volume appears to increase linearly during the first two decades of life, likely reflecting ongoing axonal myelination in certain cortical regions. Conversely, gray matter volume in several cortical areas decreases from childhood to adulthood. A longitudinal study of anatomical brain development in individuals aged 4 to 21 revealed an initial wave of gray matter loss in primary sensorimotor areas, followed by the PFC, parietal and occipital cortices, and finally, the temporal cortex. This finding has been replicated in other studies demonstrating continued gray matter volume decline throughout adolescence, particularly in the lateral and superior PFC. This decrease is thought to reflect, at least in part, synaptic elimination.

Some studies report a nonlinear, "inverted-U"-shaped pattern of gray matter change with age in frontal, parietal, and temporal regions. One early study found that the temporal cortex exhibited the latest peak in gray matter volume (around age 16), followed by a decline during late adolescence and early adulthood. A similar pattern was observed in the STS. The discrepancy between studies reporting an inverted-U-shaped pattern versus a steady decline in gray matter volume remains unclear. One possibility is that the initial increase in gray matter volume during childhood reported in some studies reflects prolonged synaptogenesis.

Relationship Between Neuroanatomical, Cognitive, and Functional Brain Development

The relatively late elimination of excess synapses in the human PFC and other cortical regions may contribute to less efficient information processing during adolescence. Excess, "untuned" synapses are thought to result in a lower signal-to-noise ratio. Input-dependent synaptic pruning eliminates these extraneous connections, effectively fine-tuning the remaining synapses into specialized functional networks. Post-pruning, fewer synapses may be needed to perform the same amount of work due to increased efficiency, leading to a higher signal-to-noise ratio and potentially more efficient cognitive processing and improved performance with age. The observed dip in performance on face processing tasks around puberty might be linked to increased gray matter volume in frontal and temporal regions around this age. However, other potential explanations for this dip exist, including hormonal fluctuations and social environmental changes associated with puberty, such as transitioning to new schools and encountering many unfamiliar faces. Further complicating matters, hormonal and environmental changes could trigger or influence alterations in gray and white matter volume within the social brain, which could, in turn, impact social cognition and behavior. Disentangling genetically programmed developmental changes from those influenced by environmental factors remains a challenge.

Developmental neuroimaging studies on social cognition generally report decreased prefrontal activity from adolescence to adulthood. In face processing tasks, activity in the lateral and superior PFC appears to increase from childhood to adolescence, followed by a decrease from early adolescence to adulthood. This nonlinear activity pattern might be related to synaptic reorganization. Excess synaptogenesis during childhood could lead to higher activity levels in relevant brain regions due to a lower signal-to-noise ratio. Subsequent synaptic pruning during adolescence could then result in a higher signal-to-noise ratio, potentially manifesting as decreased activity levels. This might explain the observed activity decrease in the lateral and superior PFC during face processing tasks and in the mPFC during mentalizing tasks from late childhood/early adolescence to adulthood.

This remains a speculative idea based on several untested assumptions. First, it assumes that a higher synapse density within a given brain tissue unit translates to an increased blood-oxygen-level-dependent (BOLD) signal when those synapses are active, implying a relatively linear relationship between synaptic density and the BOLD signal. The precise nature of this relationship, however, remains to be determined, and it likely varies across brain regions and exhibits some degree of nonlinearity. Second, it assumes a correlation between vascular changes and synaptic changes, a relationship that requires further investigation. Moreover, exploring potential correlations between structural (gray matter) and functional (BOLD signal) changes within individuals would be valuable. While one neuroimaging study has explored this correlation in the context of executive control, no study has yet attempted to correlate structural and functional development within the social brain.

Conclusion and Questions for Further Research

This review has presented evidence suggesting that specific areas of the social brain, particularly the pSTS and mPFC, undergo significant functional and structural development throughout adolescence. Recent functional neuroimaging studies suggest that activity in several prefrontal areas increases from childhood to adolescence, followed by a decrease from early adolescence to adulthood. This decrease may be linked to structural development in this area, specifically the elimination of unused synapses.

Numerous questions remain unanswered. One fundamental question is how synaptic reorganization, potentially triggered by hormonal changes at puberty, impacts both neural activity and cognitive function. The influence of environmental factors on brain development during adolescence represents another empirical question. Synaptic pruning during early development has been proposed to fine-tune neuronal circuitry in an input-dependent manner. Studies in rats have shown that those raised in enriched environments exhibit higher synaptic density in the visual cortex and perform better on spatial navigation tasks compared to rats raised in less stimulating environments. However, the relationship between environmental enrichment, synaptic density, and memory remains correlational, precluding causal inferences. In humans, synaptic pruning during early childhood has been implicated in sound categorization, with the ability to distinguish certain speech sounds depending on early exposure. This fine-tuning of sound categorization is thought to rely on synaptic pruning within sensory areas involved in auditory processing. Whether synaptic pruning within the PFC during adolescence is similarly susceptible to environmental influences remains unknown. Recent research suggests that cannabis use during adolescence might impact brain development, although the precise mechanisms by which cannabis might affect synaptogenesis or synaptic pruning are unclear.

Several neuroimaging studies report lower activity levels in the PFC during face processing, emotion recognition, and mentalizing tasks in adults compared to adolescents. This difference implies that PFC damage during adolescence might have more detrimental consequences for associated cognitive functions than damage sustained during adulthood. Studies suggest that PFC damage (including the mPFC) in adulthood does not necessarily impair mentalizing. Comparing individuals with mPFC damage acquired at different ages would be insightful. It is possible that the mPFC plays a critical role in the early development of mentalizing but becomes less essential with age. Conversely, as discussed earlier, this region consistently shows activation when adults engage in mental state reasoning. Therefore, what is the precise role of the mPFC in adult social cognition, and how and why does this role change throughout adolescence and into adulthood?

Adolescence represents a period of significant social behavioral changes, yet few empirical studies have reported specific and substantial developmental changes in social cognition that cannot be attributed to general improvements in attention, concentration, memory, or other cognitive functions. One possibility is that adolescents' ability to successfully navigate complex social cognition tasks in laboratory settings may not accurately reflect their social cognitive abilities in real-world situations. Utilizing more naturalistic paradigms could provide valuable insights into this discrepancy. Alternatively, it may not be mentalizing itself that undergoes significant development, but rather the interplay between mentalizing and executive functions. For example, studies have shown that adolescents and adults differ in how they modulate neural activity associated with face processing based on attentional focus (emotional vs. non-emotional features). Nevertheless, attributing all adolescent behavior to neuroanatomical changes would be an oversimplification, as it neglects the potential contributions of hormonal and social changes, which themselves could drive neuroanatomical alterations.

Furthermore, individual variability in social brain development likely exists but has been largely overlooked in existing research. Additionally, synaptic plasticity, a fundamental property of the brain, occurs throughout life, even in adulthood, whenever learning takes place. How plasticity during adolescence differs from plasticity in adulthood remains an open question.

These are just a few of the many intriguing questions that warrant further investigation within this rapidly evolving field. Understanding neural development during adolescence has significant implications for various aspects of society, including education, the legal treatment of teenagers, and mental health interventions, as many mental illnesses often emerge during this critical developmental period.

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The Adolescent Social Brain: A Work in Progress

Humans are undeniably social beings. A significant portion of our brains is dedicated to navigating social interactions and understanding others. This intricate network of brain regions, known as the "social brain," undergoes continuous development, particularly during adolescence - a period of significant physical, psychological, and social transformation. This review explores the social brain, its development during adolescence, and how these changes relate to observable behaviors.

The Social Brain: Decoding the Social World

The social brain encompasses a complex network of regions that enable us to recognize others and decipher their mental states (intentions, desires, beliefs), emotions, personality traits, and actions. Key players in this network include the medial prefrontal cortex (mPFC), anterior cingulate cortex (ACC), inferior frontal gyrus, superior temporal sulcus (STS), amygdala, and anterior insula. Through these regions, we can recognize faces and body language, interpret emotions, predict actions, and engage in communication.

This review will focus on understanding others, from recognizing fellow humans to deciphering emotions and mental states, as these processes have been studied in the context of adolescent development.

Recognizing Our Own Kind

A crucial aspect of social interaction is the ability to recognize other humans. Newborn infants already display a preference for faces, suggesting an innate ability to detect them, likely driven by subcortical brain structures. In adults, this ability relies on cortical areas, particularly the posterior STS (pSTS), which specializes in recognizing faces and interpreting eye gaze.

Beyond static features, we also perceive the movement of others. Research using point-light displays, which depict movement through illuminated dots on a dark background, shows that even infants can discern biological motion. The pSTS plays a vital role in this perception in both humans and monkeys.

Beyond recognizing someone as human, we instinctively assess their emotional state. A complex network, including the amygdala, anterior insula, STS, and PFC, enables us to recognize basic emotions like disgust and fear, as well as complex emotions like trustworthiness, guilt, and embarrassment. The mPFC, specifically, contributes to understanding these social emotions. The posterior inferior frontal gyrus (IFG) also plays a role in emotional judgment and using emotional cues to guide actions and predict behavior.

Unraveling Intentions: Theory of Mind

The ability to infer another person's mental state – their intentions, desires, and beliefs – is known as "theory of mind" or "mentalizing." This ability allows us to anticipate their actions. Neuroimaging studies consistently highlight a network of brain regions engaged in mentalizing, including the pSTS at the temporoparietal junction (TPJ), temporal poles, and mPFC. Damage to the superior temporal lobes and PFC impairs mentalizing abilities, highlighting their crucial role. However, the exact function of each region is still debated. Some propose the pSTS/TPJ predicts actions, while others argue it specifically decodes mental states. Yet, others suggest a broader role in representing beliefs and directing attention, whether social or not.

Interestingly, the mPFC, particularly the dorsal mPFC, exhibits a consistent pattern of activation during mentalizing tasks. It activates when we consider psychological states, whether our own, those of others (real or imagined), or even animals. However, the mPFC often shows decreased activity during mentalizing tasks compared to low-demand baseline conditions. This might be because during rest, individuals naturally engage in spontaneous mentalizing, leading to higher baseline activity.

The consistent activation of the pSTS/TPJ, mPFC, and temporal poles during mentalizing tasks is intriguing because it suggests a modular organization for a complex cognitive process that likely involves multiple, non-specific components.

The next section delves into how these brain regions, involved in mentalizing and other social cognitive processes, develop during adolescence.

A Time of Change: Functional Development of the Adolescent Social Brain

While extensive research exists on social cognitive development in infancy and childhood, research beyond childhood is limited. Recent studies, however, support the notion that adolescence represents a period of significant social development, echoing findings from social psychology.

Adolescence, often marked by the onset of puberty, brings about psychological shifts that impact an individual's sense of self, self-awareness, and relationships. Compared to children, adolescents display greater social engagement, form more intricate and hierarchical peer relationships, and become more sensitive to peer acceptance and rejection. While these changes are multifaceted, development within the social brain likely contributes to these shifts.

The Changing Landscape

Early studies on cognitive development during adolescence investigated the effect of puberty on face recognition in girls, revealing a surprising pattern. While face recognition steadily improved during childhood, it declined around age 12. This decline, linked to puberty rather than age itself, was more pronounced in mid-pubertal girls compared to pre- or post-pubertal girls of the same age. A more recent study also observed a pubertal dip in performance on a face-emotion matching task in children aged 10-12 years, with performance recovering by ages 16-17. The reasons behind this dip remain unclear, demanding further research.

Neuroimaging studies exploring the neural basis of facial expression recognition across development have shown increased activity in prefrontal regions in response to fearful faces between childhood and adolescence. However, other studies found decreased frontal activity, particularly in the ACC and OFC, between adolescence and adulthood when processing fearful faces. These findings suggest that the neural underpinnings of attending to non-salient stimuli in the presence of emotionally charged stimuli continue to mature throughout adolescence.

In summary, despite potential sex differences, activity in certain prefrontal regions during face processing appears to follow a non-linear pattern, increasing between childhood and adolescence, then decreasing towards adulthood.

Research on the development of biological motion processing is limited. However, one study found that activity in the STS, associated with biological motion perception, increased with age in children aged 7-10 years.

Navigating the Social World: Mentalizing in Adolescence

Although mentalizing performance remains relatively stable during adolescence, neuroimaging studies reveal changes in brain activity patterns. One study investigating the development of understanding communicative intent, specifically irony comprehension, found increased activity in the mPFC and left inferior frontal gyrus in children/young adolescents compared to adults. This heightened mPFC activity might reflect the effort required to reconcile the literal and intended meaning of ironic statements. The specific mPFC region showing increased activity in younger individuals falls within the dorsal mPFC, consistently implicated in adult mentalizing tasks.

A similar pattern emerged when studying how adolescents think about their own intentions. Adolescents showed higher dorsal mPFC activity than adults when reasoning about intentional actions versus physical causality. This suggests a shift in neural strategies for thinking about intentions, moving from anterior (mPFC) to posterior (STS) regions with age.

Further supporting this idea, a study investigating self-related processing found increased mPFC and ACC activity in children compared to adults when retrieving self-knowledge versus knowledge about others. This implies that adolescents might rely more on "online" self-reflection mediated by the mPFC.

Another study using a non-verbal, animation-based mentalizing task observed increased activity in the dorsal mPFC and decreased activity in the ventral mPFC with age, potentially reflecting a shift from self-based simulation in childhood to a more objective, less self-referential strategy in adulthood. However, this interpretation requires further investigation, as the study did not include an adult comparison group.

Overall, these findings suggest that prefrontal activity during social cognition tasks decreases between adolescence and adulthood, even when performance is comparable across age groups. This decrease in activity might be linked to synaptic reorganization within the PFC during adolescence.

Shaping the Social Brain: Structural Development during Adolescence

From Childhood to Adulthood: A Cellular Perspective

Research on post-mortem brain tissue revealed that the human PFC undergoes prolonged synaptic development, extending well into adolescence. Unlike sensory and motor regions, which experience early synaptogenesis and pruning, the PFC shows a different developmental trajectory. Synaptic density in the PFC peaks later than in sensory areas and undergoes pruning until mid-adolescence.

Insights from MRI: A Dynamic Picture of Brain Development

Structural MRI allows researchers to study the living human brain. These studies consistently show substantial changes in white and grey matter volume in various cortical regions, including the PFC, temporal cortex, and parietal cortex, throughout the first two decades of life. White matter volume, reflecting myelinated axons, increases linearly, indicating ongoing myelination. Conversely, grey matter volume, representing cell bodies, synapses, and neuropil, decreases in several cortical areas, particularly in the PFC, during adolescence, likely reflecting synaptic pruning.

Linking Structure, Function, and Cognition

The delayed elimination of excess synapses in the human PFC might explain the decreased efficiency of information processing in this region during adolescence. Excess synapses might lead to a lower signal-to-noise ratio. Synaptic pruning fine-tunes neural circuits, eliminating redundant connections, resulting in a higher signal-to-noise ratio and more efficient cognitive processing. This might explain the improved performance on some cognitive tasks with age.

The observed dip in face processing performance during puberty might relate to the increased grey matter volume in frontal and temporal regions around that age. However, other factors like hormonal fluctuations and changes in social environments cannot be ruled out.

The decrease in prefrontal activity observed in neuroimaging studies of social cognition during adolescence might reflect synaptic reorganization. Excess synapses in childhood could lead to higher activity levels due to a lower signal-to-noise ratio. Synaptic pruning during adolescence might then result in a higher signal-to-noise ratio, potentially leading to decreased activity levels. This could explain the observed decrease in activity in the lateral and superior PFC during face processing and in the mPFC during mentalizing tasks between late childhood/early adolescence and adulthood.

However, this interpretation remains speculative. Further research is needed to understand the relationship between synaptic density and the BOLD signal used in fMRI studies, as well as the relationship between vascular changes and synaptic changes. Investigating correlations between structural and functional changes within individuals would also be valuable.

The Future of the Adolescent Social Brain: Unraveling the Mysteries

This review highlights that specific areas of the social brain, such as the pSTS and mPFC, undergo significant functional and structural development during adolescence, likely contributing to the social and emotional changes observed during this period. Decreased prefrontal activity in response to social cognitive tasks might reflect synaptic pruning, leading to a more efficient system.

However, many questions remain unanswered. What triggers synaptic reorganization at puberty? How does the environment shape brain development during adolescence? Does damage to the PFC during this period have different consequences than damage occurring in adulthood? What is the role of the mPFC in social cognition in adulthood? And how does this role change during adolescence?

Behavioral studies specifically addressing social cognition development in adolescence are needed to understand the impact of these neurodevelopmental changes on real-life social interactions. Investigating individual differences in social brain development and exploring the potential differences between adolescent and adult plasticity are also crucial areas for future research.

The study of neural development during adolescence holds significant implications for understanding education, legal systems, and the onset of various mental illnesses that often emerge during this crucial period. Further research in this area is crucial to gain a comprehensive understanding of the developing social brain and its impact on adolescent behavior and well-being.

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The Teenage Brain: How Our Social Skills Develop

Introduction

We humans are social creatures. A big chunk of our brains is dedicated to interacting with and understanding other people. This network of brain regions is called the social brain. It changes a lot during our teenage years, which is a time of huge physical, mental, and social development. Research shows that during adolescence, our social brain continues to mature in areas like recognizing faces and understanding others' emotions, intentions, and beliefs. This lines up with what we know from psychology—teenagers become more self-aware, friendships become more important and complex, and understanding others gets better. Many things probably influence how the social brain develops during the teenage years, including hormones, our social surroundings, and major changes in the brain's structure.

Regions of the social brain. Many parts of the brain help us understand others. The medial prefrontal cortex (mPFC) and the temporoparietal junction (TPJ) help us figure out what others are thinking. The posterior superior temporal sulcus (pSTS) is active when we see faces and body language. Other important parts of the social brain include the inferior frontal gyrus (IFG), the interparietal sulcus (IPS), the amygdala, the anterior cingulate cortex (ACC), and the anterior insula (AI).

This article explores the social brain and how it develops during adolescence. We'll also look at how these changes in brain function are connected to structural changes in the brain.

Understanding the Social Brain

The social brain is like a team of brain areas that help us recognize other people and understand their thoughts (like intentions and beliefs), feelings, personality traits, and actions. Key players in this team include the mPFC, ACC, inferior frontal gyrus, STS, amygdala, and anterior insula.

Scientists have made progress in understanding how the brain allows us to understand and connect with each other. This includes recognizing faces and body language, figuring out what someone is thinking or feeling, predicting what they might do next, and communicating with them. This article focuses on how we understand others, from recognizing them as people to understanding their emotions and thoughts—because these abilities develop significantly during adolescence.

Recognizing People. Being able to recognize other humans is crucial for social interaction. Even newborn babies seem hardwired to spot human faces—they prefer looking at pictures and drawings of faces over other objects or upside-down faces. While this early face recognition probably relies on deeper brain structures, adults use additional areas in the brain's outer layer (the cortex) for this task. A part of the STS, called the pSTS, is particularly important for detecting faces and eye gaze in humans.

Recognizing other people also involves detecting how they move. Studies often use point-light displays (recordings of a moving person with lights attached to their joints) to show a simplified version of biological motion. Even three-month-old babies can tell the difference between upright and upside-down point-light displays, showing an early sensitivity to biological motion. In both humans and monkeys, the pSTS is involved in perceiving this type of movement.

We also automatically assess someone's emotional state when we see them. Recognizing basic emotions like disgust and fear, as well as more complex emotions like trustworthiness, guilt, and embarrassment, involves a network of brain regions, including the amygdala, anterior insula, STS, and PFC. The mPFC helps us understand social emotions, while areas at the back of the inferior frontal gyrus (IFG) help us make judgments about emotions, predict someone's actions, and decide how to react to their emotions.

Understanding What Others Are Thinking. Another part of social cognition, crucial for predicting how others might act, is figuring out their thoughts, intentions, desires, and beliefs. This ability is sometimes called "theory of mind" or "mentalizing." Brain imaging studies using stories, sentences, words, cartoons, and animations have consistently identified a network of brain regions involved in mentalizing, including the pSTS at the TPJ, the temporal poles, and the mPFC.

Interestingly, while the mPFC is more active when we think about mental states compared to other tasks, its activity can actually be highest during very simple tasks, like staring at a cross. This might seem counterintuitive, but it could be because our brains naturally wander to thoughts about mental states during downtime.

Research suggests that the pSTS/TPJ might help us predict someone's next move, while others believe it's specifically involved in understanding other people's mental states. However, recent findings suggest the pSTS/TPJ might have a broader role in representing beliefs about, or paying attention to, different things, whether they're social or not.

The consistent activation of the pSTS/TPJ and the mPFC during mentalizing tasks is remarkable because it reveals that these regions are essential for this complex cognitive process.

The next section explores how these brain regions involved in mentalizing and other social skills develop during adolescence.

Changes in the Adolescent Social Brain

A lot happens in our brains during the teenage years. Social skills, like making friends and understanding social cues, improve a lot during this time. Research shows that these changes are linked to how the social brain develops.

Most researchers believe adolescence begins with puberty, but it's harder to pinpoint when it ends. During adolescence, we experience big changes in how we see ourselves, how self-conscious we feel, and how we relate to others. We become more interested in spending time with friends, our relationships become more complex, and we're more sensitive to what our peers think of us. These changes are likely due, in part, to the ongoing development of the social brain.

Developing Facial Recognition During Adolescence. Research using fMRI scans shows that our brains process faces differently as we move from childhood into adulthood. For example, teenagers use different parts of their brains to recognize and understand facial expressions compared to adults. This suggests that the brain areas responsible for understanding social information are still maturing during the teenage years.

How Our Ability to "Read Minds" Develops During Adolescence. While we don't know for sure if our ability to understand other people's thoughts actually gets better during adolescence, brain scans show that teenagers use different brain areas for this task compared to adults. This suggests that the strategies we use to understand social information become more sophisticated as we get older.

One study found that teenagers show more activity in a brain area called the mPFC when thinking about intentions compared to adults. Another study found that a different part of the mPFC is more active in children than adults when thinking about themselves. These findings suggest that the way our brains process social information changes as we mature.

So, why do these changes in brain activity happen during adolescence? One possible explanation is the incredible rewiring that takes place in the teenage brain.

Brain Structure Changes During Adolescence

Cellular Development. The brain is constantly changing and rewiring itself. This process is especially important during the teenage years. One of the biggest changes happens in a part of the brain called the PFC, which is involved in planning, decision-making, and social cognition. During adolescence, the PFC undergoes a process called 'synaptic pruning', where unused connections between brain cells are eliminated, and important connections are strengthened. This is like trimming a plant to help it grow stronger and healthier.

MRI Studies of the Human Brain. Special imaging techniques, like MRI, allow scientists to see inside the living brain and track how it changes over time. These studies show that gray matter, which contains brain cells and connections, decreases in several brain areas, including the PFC, during adolescence. This decrease is thought to be due, at least in part, to synaptic pruning.

How Brain Structure, Function, and Thinking Skills are Connected. Scientists believe that synaptic pruning in the PFC and other brain areas might explain why our thinking skills improve so much during adolescence. As unnecessary connections are eliminated, the remaining connections become more efficient, leading to better communication between different parts of the brain. This is like upgrading from a dial-up internet connection to high-speed internet – everything runs faster and smoother!

Conclusion

The social brain undergoes important changes during adolescence, both in how it functions and how it's structured. These changes help explain why our social skills and understanding of others improve during this time. While there are still many unanswered questions, research in this area is helping us understand the complex interplay between brain development, social cognition, and behavior.

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Our brains are wired for connection

A large part of our brain helps us understand and interact with other people. This network of brain areas is called the 'social brain.' The social brain keeps developing throughout our teenage years, which explains why our social skills and understanding of others improve so much during this time.

Many areas work together as the social brain, like the medial prefrontal cortex (mPFC) and the temporoparietal junction (TPJ). Another part, the pSTS, helps us understand faces and how people move. Other areas, like the IFG and the amygdala, help with emotions.

Let's learn about the social brain and how it changes during teenage years.

What is the Social Brain?

The social brain is like a team of areas in the brain that help us get to know others, figure out thoughts and feelings, and understand why we act the way they do. Some of the key players on this team are the mPFC, the ACC, the inferior frontal gyrus, the STS, the amygdala, and the anterior insula. These areas work together, like a well-coordinated team, to help us navigate the social world.

Recognizing People and Emotions. Even as babies, we're drawn to human faces. As we grow, our brains get better at recognizing faces and understanding how people move. Areas like the pSTS help us tell the difference between a person walking and a random jumble of moving dots!

We're also really good at figuring out how someone is feeling just by looking at their face. A whole network of brain areas, including the amygdala, the anterior insula, the STS, and the PFC, helps us recognize emotions like happiness, sadness, anger, and fear.

Understanding What Others Are Thinking. Imagine you see your friend looking sad. You can probably guess that something might be bothering them, right? This is called "theory of mind," which means being able to understand what's going on in someone else's head.

Brain areas like the pSTS/TPJ, the temporal poles, and the mPFC are important for understanding other people's thoughts, beliefs, and intentions. These areas help us make sense of social situations and predict how others might act.

Next, we'll learn how the social brain changes during the teenage years and why those changes are important.

Functional Changes in the Adolescent Social Brain

A lot happens in our brains during the teenage years. Social skills, like making friends and understanding social cues, improve a lot during this time. Research shows that these changes are linked to how the social brain develops.

Most researchers believe adolescence begins with puberty, but it's harder to pinpoint when it ends. During adolescence, we experience big changes in how we see ourselves, how self-conscious we feel, and how we relate to others. We become more interested in spending time with friends, our relationships become more complex, and we're more sensitive to what our peers think of us. These changes are likely due, in part, to the ongoing development of the social brain.

Developing Facial Recognition During Adolescence. Research using fMRI scans shows that our brains process faces differently as we move from childhood into adulthood. For example, teenagers use different parts of their brains to recognize and understand facial expressions compared to adults. This suggests that the brain areas responsible for understanding social information are still maturing during the teenage years.

How Our Ability to "Read Minds" Develops During Adolescence. While we don't know for sure if our ability to understand other people's thoughts actually gets better during adolescence, brain scans show that teenagers use different brain areas for this task compared to adults. This suggests that the strategies we use to understand social information become more sophisticated as we get older.

One study found that teenagers show more activity in a brain area called the mPFC when thinking about intentions compared to adults. Another study found that a different part of the mPFC is more active in children than adults when thinking about themselves. These findings suggest that the way our brains process social information changes as we mature.

So, why do these changes in brain activity happen during adolescence? One possible explanation is the incredible rewiring that takes place in the teenage brain.

Structural Brain Development During Adolescence

Cellular Development. The brain is constantly changing and rewiring itself. This process is especially important during the teenage years. One of the biggest changes happens in a part of the brain called the PFC, which is involved in planning, decision-making, and social cognition. During adolescence, the PFC undergoes a process called 'synaptic pruning', where unused connections between brain cells are eliminated, and important connections are strengthened. This is like trimming a plant to help it grow stronger and healthier.

MRI Studies of the Human Brain. Special imaging techniques, like MRI, allow scientists to see inside the living brain and track how it changes over time. These studies show that gray matter, which contains brain cells and connections, decreases in several brain areas, including the PFC, during adolescence. This decrease is thought to be due, at least in part, to synaptic pruning.

How Brain Structure, Function, and Thinking Skills are Connected. Scientists believe that synaptic pruning in the PFC and other brain areas might explain why our thinking skills improve so much during adolescence. As unnecessary connections are eliminated, the remaining connections become more efficient, leading to better communication between different parts of the brain. This is like upgrading from a dial-up internet connection to high-speed internet – everything runs faster and smoother!

Conclusion

The social brain undergoes amazing changes during adolescence, both in how it functions and how it's structured. These changes help explain why our social skills and understanding of others improve so much during this time. While there are still many unanswered questions, research in this area is helping us understand the complex interplay between brain development, social cognition, and behavior.

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