The neuronal metabolism is dependent on blood oxygen supply, as the production of energy from glucose is mainly of the aerobic type. Neuronal activity provokes an increase in oxygen consumption and an even higher increase in local blood flow (neurovascular coupling).
As the increase in flow exceeds the increase in oxygen consumption, neuronal activity is expressed as a relative increase in oxyhemoglobin compared to deoxyhemoglobin in the activated zones. The relative decrease in deoxyhemoglobin concentration, which has a paramagnetic effect, can be detected by MRI as a weak transient rise in the T2* weighted signal. This is the BOLD contrast principle (Blood Oxygenation Level Dependent).