BOLD Effect

The Blood Oxygen Level Dependent (BOLD) effect is based on the magnetic properties of hemoglobin.

Oxygenated hemoglobin is diamagnetic while deoxygenated hemoglobin is paramagnetic. When neurons become active, they consume oxygen, leading to increased blood flow that overcompensates for the oxygen consumption, resulting in a net increase in oxygenated hemoglobin and a detectable change in the magnetic resonance signal.

This effect forms the basis of functional MRI (fMRI) and allows non-invasive measurement of brain activity through changes in blood oxygenation.