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INTRODUCTION
The cognition domain, measured by the National Institutes of Health (NIH) Toolbox Cognition Battery (CB) of the NIH Toolbox® for Assessment of Neurological and Behavioral Function was developed as a standard set of measures of cognitive function across the lifespan (aged 3–85 years), intended to address the needs for a brief assessment tool for large-scale epidemiologic and longitudinal studies and to allow for international cross-study comparisons (Gershon et al., 2010). Six subdomains are assessed by the NIH Toolbox CB: executive function (with tests of cognitive flexibility and inhibitory control/attention), episodic memory, language, reading, working memory, and processing speed (Weintraub, Bauer, et al., 2013).
In addition to the scores of the individual measures for these subdomains, the NIH Toolbox CB provides composite scores, aiming to allow for evaluation of overall and higher level cognitive functioning. The basis for defining the NIH Toolbox CB composite scores is to group those subtests together as a composite score that relate to each other and share theoretical and psychometric characteristics across the lifespan (Akshoomoff et al., 2013). Basis for this approach is one of the most popular models for the structure of human intelligence, the two-component theory of intellectual development (Cattell, 1971; Horn, 1970), which postulates the organisation of cognitive abilities as indicators of fluid and crystallised intelligence (Li et al., 2004). Fluid cognitive abilities are defined as problem-solving and information processing and play an important role in adapting to novel situations in everyday life. Fluid abilities are less dependent on past learning experiences and cultural biases but depend more on biological processes. Crystallised cognitive abilities represent verbal knowledge and skills, depend upon past learning experiences, and are strongly influenced by education and cultural exposure (Heaton et al., 2014; Li et al., 2004). As first discovered by Spearman (1961), all cognitive tests are positively correlated with each other, encompassing one common factor, the general factor of intelligence (Carroll, 1993).
Based on these assumptions, the NIH Toolbox CB provides three summary scores: (i) Fluid Cognition Composite, including all the NIH Toolbox CB tests that are fluid ability measures (cognitive flexibility, inhibitory control/attention, episodic memory, and processing speed). The Fluid Cognition Composite score is derived by averaging the standard scores of each of the measures and then deriving standard scores based...