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Chapter 7
Reassuringly, heritability estimates for subtle head motion (after exclusion based on excessive head motion) were considerably smaller, ranging from 0-14%. This is contrary to previous findings of Engelhardt et al. (2017) who reported similar within-twin correlations on framewise head displacement before and after scrubbing (i.e., exclusion of frames with excessive motion). Differences might be due to the smaller sample size (Nmz=12 and Ndz=22) and the differences in exclusion based on head motion, since Engelhardt et al. (2017) excluded volumes with excessive head motion, whereas we excluded complete runs of participants with excessive head motion. Thus, in line with previous studies (Van Dijk et al., 2012; Couvy-Duchesne et al., 2014; Engelhardt et al., 2017), we report that excessive head motion is heritable and systematic, but additionally show that, after careful motion correction and exclusion based on excessive head motion, subtle head motion shows little influence of genetics. Possibly, subtle head movement is more strongly dependent on participant instruction and scanner adjustments. Indeed, behavioral genetic analyses on quality controlled head motion not only revealed small heritability estimates (0-14%, compared to 29-65% in overall head motion), but also showed that a similar, or even larger, proportion of the variance was explained by shared environmental influences (15-33%).
Environmental influences on scan quantity and quality
An additional goal of this study was to examine how emotional state towards the scanner was related to scan quality and quantity. Consistent with findings for quality controlled head movement, reports of emotional states showed little to no influence of genetics, but a moderate to strong relation with shared environmental influences. These findings suggest that emotional states can be significantly influenced by preparation of the scanner experiences. It was interesting to note that children’s tension was on average rated higher by researchers and parents than by children themselves, which is in line with previous studies suggesting that children may underreport their anxiety (Tyc et al., 1995; Durston et al., 2009). Multi-informant estimates of children’s emotional state towards the MRI scan were significantly associated with MRI quantity, as we found that children with higher estimated excitement and lower estimated tension completed more runs during the MRI scan. However, the association between children’s emotional state towards the MRI scan and scan quality was less clear, as the correlations did not survive Bonferroni correction. These findings suggest that by decreasing scanner related distress researchers can increase scan quantity, but more detailed future studies are necessary to reveal whether this would also lead to an increase in scan quality.
One aspect that did show influence on scan quality was the length of the MRI scan session. Results showed that a protocol of >30 minutes resulted in less than 50% sufficient quality on all scans in this age range of 7-9-year-olds. This is
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