Page 259 - Like me, or else... - Michelle Achterberg

Summary and general discussion
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et al., 2018). Test-retest reliability is the extent to which a measure produces stable outcomes across different time points under comparable conditions (Dubois and Adolphs, 2016). Prior longitudinal developmental studies, including chapter 5 of this thesis, reported low intra-subject stability across different scan session (for an overview see Herting et al. (2018)). These could either reflect individual variability over time or might reflect unaccounted-for noise in the fMRI measurement (Dubois and Adolphs, 2016). The behavioral genetic analyses on fMRI in chapter 4, 5 and 6 showed that a large proportion of variance was explained by the E-factor, which includes both unique environmental influences and measurement error. An important objective for future research is to disentangle between the influence of unique environment and measurement error, for example by accounting for intra-subject fluctuations using repeated measures (Ge et al., 2017). Using such a repeated measures approach, one can tease apart the stable effects (which are due to unique environment) from the transient effects (which might arise from measurement error) (Ge et al., 2017).
Heritability estimates for fMRI are often lower than for structural MRI (sMRI) (Jansen et al., 2015). Similar to the difference between questionnaire data and experimental data, sMRI can be seen as a trait-like measure of the brain, whereas fMRI provides a state-like measure (Greene et al., 2018a). Indeed, questionnaire data often shows higher heritability and test-retest stability than experimental studies (Tuvblad and Baker, 2011), that are aimed to induce a specific state. A state can be defined as “the particular condition that someone is in at a specific time”, and by this definition it seems reasonable that there is more intra-individual variability across time for experimental (fMRI) studies. An important benefit of the state-inducing ability of fMRI is that it can isolate specific aspects of complex behaviors. A broad range of literature - including chapter 3, 4 and 5 of this thesis- have shown that experimental fMRI is meaningful in relation to behavior and can provide valuable information about the underlying mechanisms of specific behaviors. It should be noted that the field of developmental neuroscience, and specifically the use of longitudinal experimental fMRI studies, is still young (Crone and Elzinga, 2015; Herting et al., 2018). Perhaps the strength of fMRI lies in the combination of different MRI modalities (Dubois and Adolphs, 2016). That is, experimental fMRI might be used to detect meaningful associations between behavior and brain regions, which can be further examined by studying the stability or heritability within this region using additional MRI metrics (Greene et al., 2018a; Elliott et al., 2019a). This would provide an in-depth examination of both trait-like and state-dependent features of brain-behavior relations.
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