An interesting perspective on the role of the arMPFC in social cognition has recently been raised in the adult lesion and neuroimaging literature. Although the arMPFC is robustly activated by mentalising tasks, and adults presenting with MPFC lesions usually show mentalising deficits (Frith & Frith, 2003; see Frith, 2007), there is one report of an individual who suffered extensive bilateral MPFC damage during adulthood but who was unimpaired on mentalising tasks (Bird et al., 2004). The arMPFC is not recruited when adults make semantic discriminations among abstract social concepts, such as ‘brave’ or ‘stingy’ (Zahn et al., 2007). Rather, this process is associated with activity within superior ATC. In addition, a small number of social emotion studies in adults fail to find arMPFC activation, but do find activity within other regions of the mentalising network such as the STS and the ATC, as well as in more ventral and/or anterior prefrontal regions such as the fronto-polar and orbito- frontal cortex (Moll, de Oliveira-Souza, et al., 2005; Moll, Zahn, et al., 2005; Takahashi et al., 2004). The current study, as well as previous studies, found greater arMPFC activity in adolescents than in adults for mentalising relative to control tasks (Blakemore et al., 2007; Pfeifer et al., 2007; Wang et al., 2006). One interpretation of this collection of findings is that the role of arMPFC in mentalising
tasks becomes less crucial across age. Studies investigating mentalising ability in individuals who have sustained lesions to arMPFC during childhood, adolescence or adulthood would be important in evaluating this hypothesis.
Further work is needed on the development during adolescence of the cognitive strategies for understanding people. For example, it is not known whether the type of mentalising needed for social emotion understanding changes with development. It may be the case that a more explicit mentalising process is needed to learn about social emotions initially, but that more scripted, heuristic, or intuitive strategies are employed later on (Haidt, 2001).
Another factor which may contribute to a change in the role of the arMPFC with age is anatomical brain development. Volumetric MRI studies show that gray matter volume in social brain regions such as the MPFC and the TPJ decreases during adolescence, whereas white matter volume increases (Shaw et al., 2008; Gogtay et al., 2004; Giedd et al., 1999; Paus et al., 1999; Sowell et al., 1999). These changes are thought to be due to synaptic pruning and axonal myelination/increases in axonal calibre (see Introduction, sections 1.4.2 and 1.4.3.2.1.1/.2.2), and may result in increased coordination between components of the social brain network and greater processing efficiency within brain regions mediating social cognition.
5.5 Conclusion
This fMRI study showed evidence that the neural processing of social emotion develops between adolescence and adulthood. Although components of the mentalising system including the arMPFC were active in both groups, adolescents activated the lateral rostral MPFC more for social vs. basic emotion whereas adults did not. Adults activated the left ATC more for social vs. basic emotion than did adolescents. These results indicate that the neural processing of social emotion continues to develop between adolescence and adulthood, such that the predominant activity moves from anterior (arMPFC) to more posterior (temporal) regions with age. Further work is needed to ascertain how this is related to neuroanatomical development within social brain regions, and to possible changes in cognitive
strategies. Only female participants were included in the current study. It would be interesting to conduct a study with male participants.
The current study shows evidence that adolescents and adults use a similar neural network for social emotion processing, but with relative differences. However, the functional relevance of this developmental shift across adolescence is not clear. The current experiment did not look for behavioural differences between the age groups. Indeed, it was important that performance between groups was matched because, had performance differed between groups, it would have been impossible to interpret any differences in neural activity, which might have been a cause or a consequence of differences in task performance. Very few empirical studies have reported significant behavioural development during adolescence that is specific to social cognition and which cannot be explained by general improvement in attention, concentration, memory, and so on. The reasons for this are probably multifactorial. One possibility is that, in the lab, adolescents are able to pass complicated social cognitive tasks, which in everyday life they do not accomplish successfully. More naturalistic paradigms might be useful in addressing this issue.
5.5.1 The next chapter
In the current chapter, evidence was presented that the fMRI correlates of social emotion differ between adolescent and adult groups. Further studies are needed to establish the extent to which this age group difference is due to a shift in cognitive strategies, and/or changes in brain activity as a result of neuroanatomical development. However, an outstanding issue that should be addressed as a priority is the extent to which the collection of brain regions referred to as the mentalising system, or “mentalising network” (e.g. Frith & Frith, 2003), indeed function as a network during mentalising tasks relative to control tasks. A network is defined as a set of brain regions that demonstrate functional integration despite spatial separation, but there is currently limited evidence that the so-called mentalising network does indeed show functional integration.
The study described in the next chapter was conducted to address this outstanding issue. In Chapter 6, fMRI data from Chapter 5 were reanalysed to investigate functional integration between brain regions involved in mentalising. The technique which was chosen to address this research question is known as psycho-
physiological interaction (PPI) analysis. This method was used to test the hypothesis that there would be greater functional integration, i.e. connectivity, as assessed using PPI analysis, between the arMPFC and the pSTS/TPJ and ATC during mentalising (social emotion) relative to a control condition (basic emotion). In addition, it was hypothesised that functional integration within the mentalising system would differ between adult and adolescent age groups, in line with evidence from developmental connectivity studies using non-mentalising tasks (see section 6.1.3), and given the evidence that white matter tracts connecting spatially distant brain regions undergo continuing maturation across adolescence (e.g. Paus et al., 1999; Giedd et al., 1999a; Giorgio et al., 2008; Barnea-Goraly et al., 2005).