We know the hemodynamic signal measured by fMRI (or NIRS) is delayed respective to the onset of stimuli. But how long is the delay? Many people think it takes about 2s for the signal to arise. Actually the delay varies from region to region. As shown in this figure, signal in motor cortex (including SMA) arises very quickly (no delay); but the signal in visual cortex arises slow (2s delay).
The figure above is adapted from Cui et al, PLoS Biology (2009) Ready…Go: Amplitude of the fMRI Signal Encodes Expectation of Cue Arrival Time