FMRI is a comparative technique in nature. An fMRI experiment must include at least two conditions in contrast. A task (or stimulus) condition places specific demands to the brain, while a user-defined control condition may involve a “baseline” task or a resting state. The signal difference between the task and control conditions is evaluated, typically voxel by voxel, to identify the regions engaged in the task execution. Popular fMRI experimental designs define the alternation of conditions in either a block-design or event-related manner. In line with earlier PET paradigms, the block-design involves prolonged task and control periods, so as to observe sustained BOLD signal changes with a high contrast-to-noise ratio (CNR). The event-related design focuses on the averaged single-trial BOLD response in ways analogous to event-related potentials/fields (ERP/ERF) in EEG/MEG [108, 109].
