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Fronto-striatal connectivity predicts patience
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Introduction
Between childhood and adulthood, vigorous advancements in the ability to sustain goal-directed cognition in the face of immediate rewards are observed (Eigsti et al., 2006; Olson et al., 2007; de Water et al., 2014). This ability to delay gratification can be captured in delay discounting tasks, estimating an individual’s preference for a smaller immediate reward over larger, delayed rewards. A crucial element of these tasks is that the subjective value of a reward decreases when the delay to that reward increases (Critchfield and Kollins, 2001). This capacity has been interpreted as an index of impulse regulation, which changes considerably during adolescence (van den Bos et al., 2015).
A leading hypothesis suggests that maturation of this type of impulse regulation capacity is driven by increased regulatory control of the prefrontal cortex (PFC) over reward-related striatal areas (Figner et al., 2010; Christakou et al., 2011). Several studies showed that the striatum is more activated by decisions involving immediately available rewards, whereas prefrontal and parietal cortices are activated when individuals control the temptation to choose immediate rewards (McClure et al., 2004; Peters and Buchel, 2011). These results lead to the question whether maturation of prefrontal-striatal white matter connections concurs with, and predicts future-oriented choices across development.
The integrity of connections between the striatum and prefrontal cortex can be assessed by using diffusion tensor imaging (DTI). DTI measures the diffusion profile of water molecules in vivo allowing us to probe microstructural properties of the connecting white matter (WM) fiber bundles (Jones, 2008). The measurements most commonly derived from DTI are fractional anisotropy (FA), measuring the directional variation of diffusion, and mean diffusivity (MD), measuring the amount of diffusion (Basser and Pierpaoli, 1996). Several DTI studies revealed higher WM integrity across adolescence (Olson et al., 2009; Bava et al., 2010; Simmonds et al., 2014; Peper et al., 2015), although the shape of the trajectory is not yet well understood, some reporting linear and others non-linear changes (for an overview see Schmithorst and Yuan (2010)). Moreover, recent studies in adults (Peper et al., 2013) and adolescents (Van den Bos et al., 2015) reported an association between higher fronto-striatal WM integrity and increased preference for delayed rewards. From these studies, two important issues remain unresolved: 1) whether the relationship between age and discounting is eliminated—or merely diminished—when brain connectivity is taken into account (Steinberg and Chein, 2015) and 2) whether maturation of fronto-striatal white matter connections across development is an important predictor of individual development of delay of gratification skills
To address these questions, the current study followed participants with ages ranging from childhood throughout early adulthood (age 8-26) over a two- year period. This longitudinal design enabled us to (1) test whether the association between age and discounting behavior is mediated by WM integrity
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