How Does Functional MRI Detect Brain Activity?

Functional magnetic resonance imaging (fMRI) exploits the fact that oxygenated and deoxygenated haemoglobin have different magnetic properties. When neurons in a brain region become active, local blood flow increases to deliver oxygen. The ratio of oxygenated to deoxygenated haemoglobin shifts, altering the MRI signal — this is called the blood-oxygen-level-dependent (BOLD) response. By scanning the brain every 1–2 seconds, fMRI creates a dynamic map of activity. Researchers use it to study everything from language processing to emotional responses. Clinical applications include pre-surgical brain mapping to identify critical areas like speech and motor centres before tumour removal.