What you observe in fMRI are always changes, not absolute measurements. So you end up comparing differences between changes. Suppose you measure the height of

What you observe in fMRI are always changes, not absolute measurements. So you end up comparing differences between changes. Suppose you measure the height of two mountains their bases, but you want to make inferences about which mountain has the greatest elevation. To do that you have to compare the relative heights of the bases where the measurements were made.