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Chapter 6
To address the second question, concerning the heritability of limbic connectivity, we compared MZ and DZ twin pairs using ACE modeling. This decomposition model provides an estimate of the proportions of the variance in the data that are attributed to heritable, shared environmental, and unshared/unique environmental factors. Previous studies have shown both influences of genetics (Richmond et al., 2016) and environmental contributions (Tottenham and Galvan, 2016), indicating that there could be an interplay between genetics and environment (Yang et al., 2016).
Methods
Participants
Participants were part of the Leiden Consortium on Individual Development (L- CID) twin study. Families with a same-sex twin pair born between 2006 – 2009, living within two hours travel time from Leiden, were recruited through the Dutch municipal registry and received an invitation by mail to participate. 256 families with a twin pair (512 children) were included in the L-CID study, of which 443 children underwent the RS scan (Table S1). The Dutch Central Committee on Human Research (CCMO) approved the study and its procedures (NL50277.058.14). Written informed consent was obtained from both parents. Families received financial compensation (€80.00) for their participation in the L- CID study. All participants were fluent in Dutch, had normal or corrected-to- normal vision, and were screened for MRI contra indications. All anatomical MRI scans were reviewed and cleared by a radiologist from the radiology department of the Leiden University Medical Center (LUMC). Three anomalous findings were reported and these participants were excluded. Participants’ intelligence (IQ) was estimated with a verbal intelligence subtest (Similarities) and a performance intelligence subtest (Block Design) of the Wechsler Intelligence Scale for Children, third edition (WISC-III, Wechsler (1991)).
Since head motion can result in substantial changes in RS-fMRI connectivity (Van Dijk et al., 2010; Power et al., 2012), we investigated micro- movement using the motion outlier tool in FSL version 5.0.9 (FMRIB’s Software Library, Smith et al. (2004)). Volumes with more than 0.5 mm framewise displacement (FD) were flagged as outliers. In line with recent studies (Couvy- Duchesne et al., 2014; Engelhardt et al., 2017), our twin analyses indicated that motion (amount of FD) was heritable. That is to say, there was a stronger correlation within MZ than DZ twins (rmz=.44, p<.001; rdz=.25, p=.02). Behavioral genetic modeling of the amount of motion in the initial sample pointed towards genetic influences (A=38%, 95 confidence interval (CI): 26-56%, see Table S2). Children with more than 20% of their volumes flagged were excluded from further analyses (Power et al., 2012). In total, 209 participants (47.5%) were
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