Functional MRI

Haemoglobin is diamagnetic when carrying oxygen and paramagnetic when not. That difference distorts the local magnetic field enough to change the MR signal, so a sequence sensitive to field inhomogeneity reports on blood oxygenation — the BOLD contrast. Ogawa’s observation was that this happens without any injected agent: the contrast is already in the blood.

Why it took over

It requires no surgery, no radiation, and no tracer, so it can be used on healthy volunteers repeatedly. Combined with millimetre spatial resolution across the entire brain, that made human cognitive neuroscience into an experimental discipline rather than a mostly lesion-based one.

What the measurement actually is

Worth being precise about, because the gap between signal and inference is unusually wide here.

BOLD is a haemodynamic signal, not an electrical one. Neural activity increases local metabolic demand, blood flow over-compensates, oxygenation rises, and the MR signal changes — several steps of physiology between spiking and measurement. The response is slow, peaking some seconds after the activity that caused it and smearing anything faster, which caps effective time resolution at roughly the second scale no matter how quickly you sample.

It is also relative. There is no absolute unit of BOLD, so results are contrasts between conditions, and the choice of baseline is part of the result.

This is the exact complement of EEG: where EEG measures the electrical signal directly but cannot localise it, fMRI localises well but measures something several causal steps removed. Neither limitation is an engineering deficiency to be fixed; both are properties of what is being sensed.