This causes a change of the relative levels of oxyhemoglobin and deoxyhemoglobin (oxygenated or deoxygenated blood) that can be detected on the basis of their differential magnetic susceptibility.
Although most fMRI research uses BOLD contrast imaging as a method to determine which parts of the brain are most active, because the signals are relative, and not individually quantitative, some question its rigor.
The typical discarding of the low-frequency signals in BOLD-contrast imaging came into question in 1995, when it was observed that the "noise" in the area of the brain that controls right-hand movement fluctuated in unison with similar activity in the area on the opposite side of the brain associated with left-hand movement.
The proof of concept of BOLD-contrast imaging was provided by Seiji Ogawa and Colleagues in 1990, following an experiment which demonstrated that an in vivo change of blood oxygenation could be detected with MRI.
[7] Other notable pioneers of BOLD fMRI include Kenneth Kwong and colleagues, who first used the technique in human participants in 1992.