Page 208 - Like me, or else... - Michelle Achterberg

Chapter 7
Statistical Analyses
Statistical analyses were performed in the Statistical Package for Social Sciences (SPSS version 24) and in R version 3.5.0 (R Core Team, 2015). Scanner related distress over time was examined with repeated measures ANOVAs in SPSS. Associations between emotional state, scan quantity, and scan quality were investigated using Pearson’s correlations (in SPSS). To estimate familial influences on our outcome measures we calculated Pearson within-twin correlations for monozygotic (MZ) and dizygotic (DZ) twin pairs. Similarities among twin pairs are divided into similarities due to shared genetic factors (A) and shared environmental factors (C), while dissimilarities are ascribed to unique environmental influences and measurement error (E). Behavioral genetic modeling with the OpenMX package (Neale et al., 2016) in R (R Core Team, 2015) was used to provide estimates of these A, C, and E components. The correlation of the shared environment (factor C) was set to 1 for both MZ and DZ twins, while the correlation of the genetic factor (A) was set to 1 for MZ twins and to 0.5 for DZ twins (see Figure S1). The last factor, unique environmental influences and measurement error, was freely estimated. We calculated the ACE models for emotional state towards the MRI scan, scan quantity, and scan quality. High estimates of A indicate that genetic factors play an important role, whilst C estimates indicate influences of the shared environment. If the E estimate is the highest, variance in motion is mostly accounted for by unique environmental factors and measurement error. We first examined genetic influences on mean FD and mean AD for all scanned participants. Next, we examined the influence of genetics on moderate head motion, by excluding participants with excessive head motion (>1 mm mean FD, >3 mm mean AD). To investigate the effects of minimal head motion we only included participants with little head motion (<0.3 mean FD, < 1 mm mean AD).
Results
Scanner Related Distress
Scanner related distress over time
To investigate scanner related distress preceding and following the MRI scan, we measured the emotional state towards the scanner using the visual analogue scales. Over time, children reported more excitement and less tension, see Figure 1b. That is to say, children reported being significantly more excited before the MRI scan (M= 5.10, SD= .93), and after the MRI scan (M= 4.95, SD= 1.23), compared to before the MRI simulation (M= 4.72, SD= .94; F (491) = 23.25, p<.001, all Bonferroni corrected pair-wise comparisons p<.05). Furthermore, children reported significantly less tension before the MRI scan (M= 3.28, SD= 1.44), and
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