fMRI measures brain blood flow changes (hemodynamic response) to indicate neuronal activity.

“and they're putting them into a functional magnetic resonance imaging machine so where they can look at it's really blood flow it's really hemodynamic response for those who want to know it's the oxygen it's the ratio of the oxygenated to deoxygenated blood because when blood blood will flow to neurons that are active to give it oxygen and then it's deoxygenated and then there's a change in what's called The Bold signal so fmri when you see these like hot spots in the brain is really just looking at blood flow”