BOLD

An MRI technique which records changes associated to brain function in successive images is termed functional MRI (fMRI). The most important kind  of fMRI is blood  oxygen level detection (BOLD) that provides a very sensitive measure of cerebral cortical activity with  a voxel  (volumetric pixel,  the  3-D analog of a pixel  in a 2-D image) size of 2 mm on  each side,  following changes in activity  with  a time  resolution of a few seconds. This technique depends on the ratio of oxygenated to deoxygenated hemoglobin and this varies with blood flow and metabolism.

Precisely how brain physiology generates the BOLD signal is not completely understood. There is no simple relationship between the rise in cerebral blood flow that accompanies neural activity and the local metabolic demands, neither is it clear how any of these variables correlates with electrical activity in the brain. Despite that BOLD studies have shown that most of the energy expenditure of the brain is related to synaptic events rather than the generation and propagation of action potentials. Indeed it seems that action potentials are produced using only 30 percent more energy than the calculated theoretical minimum.