Fig. 2
Joint models of brain structural (b) and functional (a) mechanisms for the explanation of transfer capability in healthy aging. a Fractional anisotropy (FA) of the genu and corpus of the corpus callosum decreases with age in cognitively healthy elderly; higher FA values indicate better structural integrity. Structural integrity of the corpus callosum predicts transfer capability determined by stable success (ST) versus non-transfer (NT) [11]. Categorial transfer was defined as an increase of fluid intelligence performance (transfer task) beyond the retest effect of untrained healthy elderly after successful training of logical reasoning skills [11]. Taken the corpus callosum structural integrity (FA) as surrogate of transfer capability, the model delineates a threshold of structural integrity (− − −) dividing ST and NT. Moreover, the model suggests that (e.g., z-standardized) FA values could be taken as dimensional predictors of the transfer amount in single subjects. b Increased hemispheric cooperation/HAROLD as measured by BOLD lateralization index [70] may mediate transfer capabilities in older adults since both are associated with the structural integrity of the corpus callosum. NT subjects may show less hemispheric cooperation compared to ST subjects at baseline thereby predicting less transfer success while both groups show the general pattern of lateralized to bilateral to disengagement of activity with increasing task demand [17]