Page 228 - Like me, or else... - Michelle Achterberg

Chapter 8
between striatum and PFC, and (2) move beyond a descriptive relation between age, WM integrity and behavior, by testing how brain maturation predicts change in behavior over time.
In line with the existing literature, we hypothesized that (i) the ability to delay gratification improves with increasing age (Green et al., 1994; Olson et al., 2007; de Water et al., 2014) and (ii) the integrity of fronto-striatal WM matures with increasing age (Olson et al., 2009; Bava et al., 2010; Schmithorst and Yuan, 2010; Simmonds et al., 2014; Peper et al., 2015). The longitudinal design allowed us to test in more detail the shape of change (Braams et al., 2015). In addition, we hypothesized that (iii) the increasing effect of age on the ability to delay gratification is further increased in individuals with relatively high fronto- striatal WM integrity (Liston et al., 2006) (positive mediation). Ultimately, we hypothesized that (iv) fronto-striatal WM integrity predicts the improvement of delay gratification over time. That is to say, we expect that fronto-striatal WM integrity at timepoint 1 can predict delay of gratification at timepoint 2, and that thereby brain maturation precedes and predicts behavioral change .
Methods
Participants
The current study was part of a large longitudinal study, referred to as Braintime, conducted at Leiden University, the Netherlands. A total number of 299 participants (ages 8-25) were recruited through local schools and advertisements at timepoint 1 (T1). All participants were fluent in Dutch, right-handed, had normal or corrected-to-normal vision, and an absence of neurological or psychiatric impairments. Two years later, at timepoint 2 (T2), 254 participants were included. From the 254 participants that had measurements on both time points, 14 participants had missing delay discounting data at one of the two time points and 13 participants had missing DTI data at one of the two time points. 34 participants were excluded due to erratic discounting behavior at one of the two time points. Consistent discounting behavior was defined as having at least two decreases in subjective value (indifference points) and not more than one increase in subjective value as time increased (Dixon et al. 2003). The excluded participants had similar demographic characteristics as the included participants (excluded participants: 50% male; age range 8.21-24.44; age at T2 M = 16.05, SD= 3.66). Results with the excluded participants remained unchanged.
There were no outliers in delay discounting data (Z-value < -3.29 or > 3.29). Outliers in DTI data were winsorized (Tabachnick and Fidell, 2013).The final longitudinal sample (participants included at T1 and T2) consisted of 192 participants (48.4% males; age range = 8.01 - 26.62; age at T2 M = 16.31, SD=3.61), see Table 1 for demographic characteristics.Written informed consent was obtained from all participants, or participant’s parents in the case of minors. All
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