Page 241 - Like me, or else... - Michelle Achterberg

Fronto-striatal connectivity predicts patience
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stable period in between. This finding corresponds to previous longitudinal studies demonstrating protracted maturation of large association fiber bundles (Bava et al., 2011; Lebel and Beaulieu, 2011). Studies testing for non-linear relations in white matter tracts are scarce (Olson et al. (2009) and reported similar cubic relations in white matter, with the strongest changes in FA and MD during pre-adolescence and young adulthood. Importantly, in a longitudinal study, Simmonds et al. (2014) recently reported -in white matter tracts connected to the PFC-, a period of rapid growth in childhood, followed by a slowdown of growth in mid-adolescence and acceleration of growth again in late adolescence/early adulthood. Our longitudinal results also suggest a ‘plateau’ in white matter development during mid-adolescence, but this seems to be the result of larger variation in white matter development during this period; some children show increases in white matter integrity, while others remain stable or even show decreases. Our results add to increasing evidence obtained from several neuroimaging modalities, showing large variability in brain activity, morphology and connectivity during mid-adolescence (Scherf et al., 2012) pointing towards a unique period of vulnerabilities and opportunities (Crone and Dahl, 2012). It must be noted however, that due to a relative smaller number of participants early adulthood (N=21), the increase in white matter integrity in this period must be interpreted with caution (Mills and Tamnes, 2014). Indeed, the analyses without these young adults hint towards highest values of white matter integrity during late adolescence, leveling off thereafter. Thus, replication of these results in a larger number of adults is warranted to typify the exact nature of fronto- striatal white matter development after adolescence.
Next, we tested whether white matter integrity of fronto-striatal connections was related to individual differences in the ability to delay gratification in adolescents, similar as to what has previously been reported in adults (Peper et al., 2013; van den Bos et al., 2014). Our results showed that white matter integrity of the fronto-striatal tract (specifically FA) mediated the relation between age and delay discounting, consistent with findings of a recent study on the relation between fronto-striatal connectivity and adolescent delay discounting (van den Bos et al., 2015). However, it is not clear whether the relationship between age and delay discounting is eliminated, or merely diminished, when connectivity is taken into account (Steinberg and Chein, 2015). Our results on T1 show a full mediation (the direct effect is no longer significant), while our results on T2 show a partly mediation.
Finally, we for the first time tested whether white matter frontostriatal connectivity predicted change in delay discounting across development. Predicting change is important for potential early identification of adolescents who are prone to impulsive choice (see also Ullman et al. (2014). The results showed that fronto-striatal white matter integrity was a significant predictor of the ability to delay gratification two years later, while taking into account delay of gratification performance at baseline. These findings indicate that brain
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