Functional MRI

Functional MRI (fMRI) is a variant of MRI that maps brain activity rather than just brain structure. Where a standard structural MRI captures a still picture of anatomy, fMRI captures changes in blood oxygenation over time — and uses those changes as a proxy for which brain regions are working hardest, moment to moment.

It is the most widely used method for visualising brain function non-invasively in humans.

How it works

When a brain region becomes more active, it consumes more oxygen. Local blood flow increases to compensate, and the ratio of oxygenated to deoxygenated blood shifts. Oxygenated and deoxygenated haemoglobin have slightly different magnetic properties, which an MRI scanner can detect.

This signal is called the BOLD response — Blood-Oxygen-Level-Dependent. Tracking it across the whole brain produces a time series — effectively a movie of brain activity.

What it's used for

Two broad classes of study:

• Task-based fMRI — measure brain activity while a participant performs a specific task (reading, decision-making, finger tapping). Identifies the brain regions involved.
• Resting-state fMRI — measure spontaneous brain activity when the participant is doing nothing in particular. Reveals stable functional networks like the default mode network.

Strengths and limits

• Strength — non-invasive, whole-brain coverage, millimetre spatial resolution
• Limit — the BOLD signal is indirect (blood flow, not neuron firing) and slow (seconds, not milliseconds)
• Limit — requires a scanner; not portable, expensive

For millisecond-scale activity, EEG or MEG are better suited.