fMRI stands for functional magnetic resonance imaging, which measures brain activity via changes in blood oxygen levels in the brain: neural activity is measured by the degree of cerebral blood flow and energy demand. "It is currently believed that when a cognitive task is performed, the area of neural activation becomes more perfused as a result of an increased need for oxygen. This, in turn, increases oxyhemoglobin concentration in the local tissue while the deoxyhemoglobin (hemoglobin without any bound oxygen) found in red blood cells decreases. Deoxy and oxyhemoglobin have different magnetic properties. Deoxyhemoglobin is paramagnetic and introduces an inhomogeneity in the local magnetic field of the hydrogen atoms and reduces the MR signal (MR signal comes from water molecules, hydrogen atoms in the water molecule to be precise). Oxyhemoglobin, on the other hand, is diamagnetic and has little effect. So a decrease in deoxyhemoglobin (i.e. an increase in oxyhemoglobin) would result in an increase in the received signal" (Ozdemir 2012).

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