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                                Chapter 8
 Discussion
Development in risk-taking tendencies and impulsive control have been attributed to an imbalance between subcortical and cortical brain regions (Somerville et al., 2010), but very few studies examined the anatomical connections between these areas in relation to impulsive choice. One important dimension of impulsivity is the ability to delay gratification (Whelan et al., 2012). Next to examining developmental patterns in impulsive choice and fronto-striatal white matter integrity, the current study aimed to test if the integrity of fronto- striatal white matter connections mediated and predicted the ability to delay gratification across development. We were able to demonstrate that age-related increases in the preference for delayed rewards (i.e. less impulsive choice) was significantly dependent on a better quality of connections between the PFC and striatum. Moreover, the longitudinal analysis revealed that stronger connectivity between striatum and PFC predicted less impulsive choices two years later.
The first question addressed in this study was to test age related change in the ability to delay gratification between childhood and young adulthood. From our results it appears that delay of gratification is largest around late adolescence followed by a slight decline in young adults. This finding fits well with a recent study on age-related changes in discounting of real rewards (Scheres et al., 2006). It appears that there is a gradual increase in delay of gratification skills between childhood and late-adolescence, reaching a plateau in late adolescence/ early adulthood. Prior studies also suggested most reward oriented behavior in mid- adolescence and a steep increase in late-adolescence in self-control (Steinberg et al., 2008; Olson et al., 2009; de Water et al., 2014). A possible explanation that follows from these findings is that adolescents — more than children — flexibly apply self-control for the purpose of reward maximization, which levels off in early adulthood.
Next to developmental change, there was also evidence for consistency in behavior across sessions within individuals. That is to say, we found correlations between delay of gratification skills at T1 and T2, showing that participants who were better able to delay gratification at T1 were also better able to delay gratification at T2 which is consistent with prior studies (Audrain-McGovern et al., 2009; Anokhin et al., 2011). These results indicate a substantial level of trait- like, individual stability in delay of gratification skills in adolescence (Casey et al., 2011). These findings set the stage for examining the hypothesis in this study: how individual variation in behavior is mediated and predicted by striatum- prefrontal cortex connectivity.
In a set of longitudinal analyses we investigated the age-related change in fronto-striatal white matter connections. Results indicated that the integrity of fronto-striatal white matter increases with age, and seems to do so in a cubic fashion: the most pronounced increases in white matter integrity appear to take place in pre-adolescence and young adulthood, with — on average — a relatively
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