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Subcortical-PFC resting state connectivity
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excluded based on excessive head motion. An additional 11 participants were excluded due to registration problems. The final sample consisted of 220 children (41% boys, mean age 8.00±0.67, age range 7.02-9.08), of which 64 complete twin pairs (128 children, 58% MZ). There was no association between age and motion in the final sample (r=.06, p=.35). Moreover, there were no significant influences of heritability for head motion in the final sample (A=0%, 95% CI: 0-35%, see Table S2), implying that only more extreme motion is heritable, and this is not true of more subtle motion. For an overview of sample selection and dropout, see Table S1.
For the first set of analyses (examining replicability of childhood RS connectivity) we divided the sample into two subsamples of genetically independent individuals. Of the 64 complete twin pairs, we randomly chose either the youngest or oldest child within a twin pair. The other half of the twin pair was left out of the replication analyses. The replication sample therefore consisted of 156 (220-64) genetically independent children who were divided over two samples of N=78. Table 1 provides an overview of demographic characteristics, estimated IQ and motion in samples I and II. There were no significant differences in demographic characteristics between the samples (Table 1). Moreover, the distribution of gender did not significantly differ from chance (Sample I - 45% boys, t(77)=0.91, p=.37; Sample II - 44% boys, t(77)=1.13, p=.26).
For the second set of analyses (testing heritability of childhood RS connectivity), we estimated the contributions of genetic and environmental factors to subcortical-cortical and subcortical-subcortical functional brain connectivity using behavioral genetic modelling on seed-ROI connections. The complete twin pairs were therefore divided in monozygotic (N=37) and dizygotic (N=27) twin pairs. Table 2 provides an overview of demographic characteristics, estimated IQ and motion in MZ and DZ twins. There were no significant differences in demographic characteristics between the samples (Table 2). For the twin samples, the distribution of gender significantly differed from chance, with the inclusion of fewer boys than girls in both samples (MZ - 35% boys, t(73)=2.66, p=.01; DZ - 30% boys, t(53)=3.25, p=.002).
Data Acquisition
MRI scans were acquired with a standard 32 channel whole-head coil on a Philips Ingenia 3.0 Tesla MR system. Resting state data was acquired at the end of a fixed imaging protocol. Children were instructed to lie still with their eyes closed for 5 minutes. They were explicitly told not to fall asleep. To prevent head motion, foam inserts surrounded the children’s heads. A total of 142 T2 -weighted whole- brain echo planar images (EPIs) were acquired, including 2 dummy volumes preceding the scan to allow for equilibration of T1 saturation effects (scan duration 316.8 sec; repetition time (TR) = 2.2 sec; echo time (TE) = 30 ms; flip
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