Inter and intra-individual behavioral variability predicted by neural activity in the multiple demand network

A wide range of cognitive tasks consistently identify a Multiple Demand (MD) network in frontal and parietal brain regions. Its activity is closely linked to executive functions (EFs) such as attention, task switching, solving novel problems and manipulating information in the working memory. We here investigate the relation between MD neural activity and EFs using a large fMRI dataset (n=120). We examine this relation through two approaches (1) inter-individual variability, addressing several methodological challenges: We find that MD activity - which varies substantially across individuals, but is consistent within individuals across time - can explain a substantial proportion of variance in individual performance on a spatial working memory task such that individuals who find the task challenging, increase their MD activity substantially to improve their performance. This suggests that MD activity tightly reflects the executive demand of an individual. In the second approach we examine (2) trial-by-trial variability by employing three different models to fuse reaction time (RT) data with the BOLD time-series. We find that BOLD amplitude increases with longer RTs. This is consistent with the findings from the first approach showing increased MD activity for slower individuals. Together both findings support the view that within and between individual differences are manifested in the same brain regions. These results have implications for (1) understanding brain processes of EFs through ID studies (2) given that ID in EF are largely genetically determined, genetic variability can be linked to the neural activity of the MD network as an intermediate stage to link genetics with behavior (3) using ID in fMRI responses as clinical biomarkers.