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Chapter 5
Supplementary Materials
Participants and sample selection
Of the initial 256 families, 10 families (3.8%) dropped out of the study directly after W1, whereas one family (n=2) was included in the L-CID study after W1. An additional 19 families (7.4%) dropped out before W2, after randomization of the parental intervention (see Figure S1). The remaining 456 children participated in a second lab visit at W2 (time between waves 2.06±0.10, time range: 1.86-2.53). Participants underwent an MRI scan as part of the lab visits. All anatomical MRI scans were reviewed and cleared by a radiologist from the radiology department of the Leiden University Medical Center (LUMC). Four anomalous findings were reported. To prevent registration errors due to anomalous brain anatomy, these participants were excluded. At W1, 27 participants did not start the scan due to anxiety (n=13), contraindications (n=6), or lack of parental consent for MRI participation (n=4), or technical issues with the MR system (n=4) (Achterberg and van der Meulen, 2019). Eighty-nine participants were excluded at W1 due to excessive head motion, which was defined as >3 mm motion (1 voxel) in any direction (x, y, z) in more than 2 runs of the SNAT task (3 runs in total). An additional seven participants were excluded due to data export failures. At W1, 385 participants were included in the MRI analyses (mean age 7.99 ± 0.68, 47% boys, see also Achterberg et al. (2018b)). At W2 48 participants did not start the scan due to anxiety (n=26), contraindications (n=10), or due to lack of parental consent for MRI participation (n=10). 46 participants were excluded at W2 due to excessive head motion and two participants were excluded due to data export failures. At W2 360 participants were included in the MRI analyses (mean age 10.01 ± 0.67, 48% boys).
Of the initial sample that participated at W1, 246 families were contacted 1.5 year after W1 to inform them on a parenting support program for parents of twins (VIPP-Twins (Euser et al., 2016)). 91 families (37%) were assigned to the parental intervention group and received the VIPP-Twins, of which 9 families (9.9%) dropped out before the second MRI visit (final VIPP-Twins group: n=164, of which n=133 with sufficient quality MRI (Figure S1)). 129 families (52%) were assigned to the control group and received the dummy intervention, of which 7 families (5.5%) dropped out before the second MRI visit (final control group: n=244, of which n=186 with sufficient quality MRI (Figure S1)). Twenty-seven (11%) families did not want to be randomly assigned to one of the conditions. These families received the (non-randomly assigned) dummy intervention in order to keep this group comparable to the control group for future analyses within the longitudinal L-CID study. Given that the participants in the non- randomly assigned control group could not be included in the analyses, these participants were used as a reference group for regions of interest (ROI) selection (see section 2.4.4). Of the 27 families in the reference group, 3 dropped out before
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