What does functional magnetic resonance imaging primarily measure to infer brain activity?

• A. Changes in blood oxygenation linked to neural activity
• B. Electrical impulses from neurons directly
• C. Brain temperature changes
• D. Structural density of gray matter

Answer: (A)

Functional MRI is based on the brain’s blood flow changes that accompany neural activity. When a region becomes more active, it uses more oxygen, and blood flow to that area increases, shifting the balance of oxygenated versus deoxygenated hemoglobin. This change alters the MR signal, producing the BOLD signal that fMRI tracks to infer activity in that region. It does not measure electrical impulses directly (that’s EEG/MEG), nor brain temperature changes, nor static gray matter density, which are captured by other imaging methods. The signal is an indirect proxy of neural activity, with good spatial detail but slower temporal information due to the vascular response.