Environment-by-brain development interactions as predictors of adolescent depressive symptoms and psychological well-being: structural brain development as a marker of responsivity to maternal parenting and socioeconomic status

Abstract

Background Adolescence is widely reported to be a time of increased risk for depression and lower well-being. Importantly, however, outcomes are heterogeneous, and most adolescents do not develop mental health problems. In order to understand how these differences emerge, both environmental and biological factors have been examined in the literature. Evidence indicates that parenting behaviour, socioeconomic status and neurobiology may contribute and, further, that individual differences in brain development may moderate the extent to which contextual factors influence adolescents. That is, individual differences in brain development may confer ‘responsivity’ to context. Several developmental and evolutionary-developmental models provide frameworks with which to interpret such brain-by-environment interactions, and to describe biological responsivity – these are broadly associated with diathesis-stress and differential susceptibility/biological sensitivity frameworks.

Broad PhD aim This thesis aims to investigate whether structural brain development moderates adolescent sensitivity to maternal parenting behaviour in the prediction of adolescent depressive symptoms and psychological well-being. Two empirical studies were completed to address this aim. Study 1 examined whether longitudinal change in brain structure modified adolescent vulnerability or susceptibility to aggressive and positive maternal parenting. Effects were assessed to infer evidence in support of either diathesis-stress or differential susceptibility frameworks. Study 2, in light of evidence that positive parenting protects against adversity, considered whether brain development moderated adolescent sensitivity to positive parenting, and whether this association was more pronounced for adolescents with low-socioeconomic status (SES).

Methodology During early adolescence (age 13 years), participants completed observed interactions with their mothers, and the frequency of positive maternal behaviour was coded. At three time points (mean ages 13, 17 and 19 years), participants completed structural magnetic resonance imaging (MRI) scans. During late adolescence (age 19 years), participants completed self-report measures of depressive symptoms and psychological well-being. Two separate analyses (studies) were conducted in predicting late adolescent (age 19) outcomes (depressive symptoms and psychological well-being). For both studies, longitudinal brain development was indexed by changes in cortical thickness of structures within the frontal lobe, and volumetric changes of subcortical structures, from early to late adolescence. Study 1: Regression models analysed interactions between maternal behaviour and longitudinal brain development in the prediction of adolescent outcomes. Indices designed to distinguish between diathesis-stress and differential susceptibility effects were employed. Study 2: Regression models were used to investigate interactions between SES (parental occupation), positive maternal behaviour, and longitudinal brain development in the prediction of adolescent outcomes.

Results Study 1: Results supported differential susceptibility, whereby less thinning of frontal regions (the left medial orbitofrontal, rostral middle frontal and superior frontal cortices, and the right pars opercularis) was associated with higher well-being in the context of low levels of aggressive maternal behaviour, and lower well-being in the context of high levels of aggressive maternal behaviour. Study 2: Results indicated that individual differences in structural brain development moderated the extent to which positive parenting impacted adolescents dependent on SES. High levels of positive parenting were associated with reduced depressive symptoms for low-SES adolescents with greater volumetric reduction of the right putamen. Further, low positive parenting was associated with reduced psychological well-being for individuals with greater neurobiological sensitivity, however, patterns of brain development that were associated with sensitivity differed by SES. Specifically, low positive parenting was associated with reduced psychological well-being for individuals with more thinning in the context of low-SES, but for individuals with less thinning in the context of high-SES.

Significance Results across studies suggested that structural brain development may be associated with individual differences in how sensitive adolescents are to context. Study 1 indicated that reduced frontal cortical thinning during adolescence increased susceptibility to maternal aggressive behaviour in the prediction of well-being, for better and for worse. This finding is significant because it suggests that neither more or less cortical thinning is consistently good or bad for mental health. Results from Study 2 indicated that, although brain change was associated with responsivity to parenting behaviour, patterns of brain development associated with heightened responsivity to parenting were different for high- and low-SES. These results suggested that responsivity functions in a context dependent fashion and highlights the complex interactions that may occur across biological and multilevel environment factors. Results from these studies suggest that structural brain development may be a marker of responsivity to environmental influence. They also emphasise the importance of examining how brain development moderates the impact of multilevel environmental factors on mental health outcomes. Such study designs may better reflect the social settings in which adolescents develop.