Page 214 - Like me, or else... - Michelle Achterberg

Chapter 7
[23-46%], Table 3). Absolute displacement also showed influence of genetics, with a heritability estimate of 65% (95% CI: [54-73%]), see Table 3.
As is often the case with childhood samples, some participants displayed excessive head motion: up to 18 mm mean framewise displacement (Figure 5a) and 38 mm mean absolute displacement (Figure 5b). To prevent the genetic analyses from being biased by these extremes, we also investigated heritability of “moderate” head motion (Figure 5). For these analyses, we only included participants with mean framewise displacement <1 and <3 mm absolute displacement. Within-twin correlations for moderate framewise displacement were similar for MZ twins (rmz=.29, p=.005) and DZ twins (rdz =.28, p=.02, see Table 3). Similarly, within-twin correlation for moderate absolute displacement were similar for MZ twins (rmz=.29, p=.005) and DZ twins (rdz =.23, p=.06). Behavioral genetic analyses revealed low heritability estimates for moderate head motion (compared to overall head motion), and in addition showed influence of shared environment. That is to say, influence of genetics on moderate framewise displacement was estimated as 12% (95% CI: [0-51%]) and 22% of the variation was explained by shared environment (95% CI: [0-45%]). Influence of genetics on moderate absolute displacement was 14% (95% CI: [0-46]), and 15% of the variation was explained by shared environment (95% CI: [0-39%], Table 3).
As previous studies showed the tremendous effect of motion on fMRI signals in pediatric samples (Poldrack et al., 2002; Satterthwaite et al., 2013), and recent studies advise more stringent quality control (Power et al., 2014; Power et al., 2015) we performed additional analyses on “minimal” head motion (Figure 5). For these analyses, we only included participants with mean framewise displacement <0.3 mm and <1 mm absolute displacement. Within-twin correlations for minimal framewise displacement did not differ for MZ twins (rmz=.26, p=.183) and DZ twins (rdz =.49, p=.04; Z=-0.83, p=.406, see Table 4). Similarly, within-twin correlation for minimal absolute displacement were similar for MZ twins (rmz=.32, p=.123) and DZ twins (rdz =.28, p=.225; Z=0.14, p=.888). Behavioral genetic analyses revealed even lower heritability estimates for minimal head motion (compared to overall and moderate head motion, see Table 3). There was no influence of genetics on minimal framewise displacement (A=0.00, 95% CI: [0-47%]) and 33% of the variation was explained by shared environment (95% CI: [0-54%]). Influence of genetics on minimal absolute displacement was 6% (95% CI: [0-61]), and 29% of the variation was explained by shared environment (95% CI: [0-55%]). Note that the sample size for analyses on minimal head motion was considerably smaller (n=44 twin pairs, 55% MZ) than for analyses on moderate (n=159 twin pairs, 59% MZ) and excessive (n=237 twin pairs, 54%MZ) head motion. Figure 5 provides a visual representation of the within-twin correlation of extreme, moderate and minimal head displacement, split out by zygosity.
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