fMRI ususally measures BOLD, changes in blood oxygenation (well, deoxygenation). The point of the paper is that you can get relative changes like that in lots of ways: you could have more or less blood, or take out more/less oxygen from the same blood.

These can be measured themselves separately (that's exactly what they did here!) and if there's a spatial component, which the figures sort of suggest, you can also look at what a particular spot tends to do. It may also be interesting/important to understand why different parts of the brain seem to use different strategies to meet that demand.