Magnetic resonance imaging (MRI) uses radio waves and a strong magnetic field to provide images of internal organs and tissues. It does not use X-rays. Functional MRI is a procedure that uses MRI to indirectly measure the neural activity occurring in the brain of humans (or other animals). When neurons are active, they consume oxygen. The local response to this oxygen consumption is an increase in blood flow, localized to the regions of increased neural activity.

Functional magnetic resonance imaging (fMRI)

This hemodynamic response occurs after a delay of 1 – 5 seconds, lasts several seconds, and leads to local changes in the relative concentration of oxygenated and deoxygenated haemoglobin. As haemoglobin is diamagnetic when oxygenated but paramagnetic when deoxygenated, the MRI signal of blood is slightly different depending on its level of oxygenation. Using an appropriate sequence (often referred to as Blood Oxygenation Level Dependent or BOLD contrast), MRI is capable of measuring that slight difference in magnetic property. Unlike event-related brain potentials, functional MRI has a poor temporal resolution (in the order of seconds), but a high spatial resolution (in the order of millimetres). Therefore, both acquisition methods are complementary.