Recruitment and stabilization of brain activation within a working memory task; an FMRI study

Seventeen subjects underwent functional magnetic resonance imaging (fMRI) performing a 2-Back verbal working memory (VWM) task alternating with a control task to characterize the temporal dynamics of the specific brain regions involved in VWM. Serial sampling of 2-Back sub-blocks revealed many small areas of activation that grew and merged over time. Significant temporal effects for volume recruitment were seen in specific brain regions known to be involved in VWM, including the bilateral dorsolateral prefrontal (DLPFC), medial frontal (MFC), posterior parietal (PPC) cortices and also some extra-cortical and subcortical regions of interest (ROIs). Signal intensity increased over time in most ROIs recruited early in the task, including the DLPFC, MFC, and PPC but excluding dorsal premotor areas. MFC intensity increased rapidly then stabilized with time. The uniqueness of the MFC response raises the possibility that it drives the recruitment process. Increases in intensity and volume were associated with worsening VWM performance over time, suggesting that recruitment of brain resources is necessary in attempting to sustain difficult tasks. Worsening of performance over sub-blocks despite stable task demands reinforces this temporal "load effect".