Interaction between task-evoked and spontaneous brain activities: Insights from two event-related fMRI studies

Human brain activities measured by fMRI can be categorized into at least two distinct categories; 1) spontaneous: slow (< 0.1 Hz) but inter-regionally coherent fluctuations in MR signal, and 2) task-evoked: change in the MR signal due to external stimulation. To date, there is no consensus in the field about the roles of spontaneous brain activity and whether it interacts with task-evoked activity. This is mainly because direct measurements of spontaneous activity during task performance is challenging, if not impossible. Here for the first time, we used a data-driven approach to obtain the time course of the spontaneous brain activity in two of the well-studied brain networks; 1) the Default mode network (DMN), where the strongest task-evoked negative BOLD response is also observed and 2) the dorsal attention network (DAN), where some of the strongest task-evoked positive BOLD response is observed. Voxel-wise multiple regression analysis is used to model fMRI timeseries with anticipated task-evoked BOLD response, the obtain time course of the spontaneous activity, and their interaction at each participant. Using 46 young and cognitively normal participants performing an event-related perceptual speed task inside the fMRI scanner, we showed that there is no significant interaction between the spontaneous and the task-evoked activity either as negative BOLD response in the DMN or as positive BOLD response in the DAN. We replicate our findings in a separate dataset with 24 young and cognitively normal participants performing a different event-related task inside another fMRI scanner. Our results suggest that the presence or absence of coherent spontaneous activity in the DMN, or DAN has negligible effect on the magnitude of the BOLD response and vice versa, which consequently means that the fMRI signal is a linear superposition of spontaneous and task evoked activity.