Composite fMRI of a healthy brain showing a motor area

BOLD thinking

What does fMRI really measure?

The most common form of fMRI is blood-oxygenation-level-dependent (BOLD) fMRI. 

It is based on the idea that neurons require more energy when they fire and that increased blood flow to active parts of the brain supplies the oxygen required. This form of MRI measures changes in the concentration of oxygen in red blood cells.

The main limitation of fMRI is that it measures brain activity indirectly, using blood flow as an indication that neurons are active. In 2009, researchers in the USA published important work showing that parts of the brain that receive more oxygenated blood do not necessarily become more active. The researchers used fMRI to scan the brains of monkeys while they looked at pictures, and they found that the brain anticipates which of its parts will be activated over the next few seconds and pre-empts itself by sending more blood to them. Most areas that receive more blood are more active, but some areas that receive more blood do not become more active.

Lead image:

This is a composite image made up of functional and structural MRI scans. Functional MRI is a form of MRI used to show which parts of the brain become active during different mental processes. The area highlighted in red is the ‘pre-supplementary motor area’, which is heavily involved in the control of voluntary movement. In this case, the highlighted region was active when the subject was moving their eyes.

Parashkev Nachev/Wellcome Collection CC BY


About this resource

This resource was first published in ‘Inside the Brain’ in January 2013 and reviewed and updated in November 2017.

Inside the Brain
Education levels:
16–19, Continuing professional development