BOLD effect

Blood Oxygenation Level Dependent (BOLD) effect is the source of contrast in fMRI images.

When neurons are activated, the resulting increased need for oxygen is overcompensated by a large increase in perfusion. As a result, the venous oxyhemoglobin concentration increases and the deoxyhemoglobin concentration decreases. As the latter has paramagnetic properties, the intensity of the fMRI images increases in the activated areas. As the conditions are alternated, the signal in the activated voxels increases and decreases according to the paradigm.

BOLD effect: balloon model