When neurons fire or increase their firing rate, they draw on oxygen and various nutrients.
The circulatory system of the brain reacts by sending the region that just fired more highly-oxygenated blood than is needed. This results in an increased blood oxygen level in the activated region.
With right pulse sequence, an MRI scanner is able to detect this difference in blood oxygen level
Factors such as drugs, substances cite_note-3 and excitation cite_note-4 have been shown to increase BOLD response. Conversely, age and brain pathology cite_note-5 have been shown to decrease BOLD response