What does fMRI primarily measure?

• A. Brain activity through blood oxygen levels
• B. Brain structure
• C. Brain metabolism
• D. Neural connectivity

Answer: (A)

fMRI measures brain activity indirectly by tracking changes in blood oxygenation levels. When a brain area becomes active, neurons consume more oxygen, and the body responds by increasing blood flow to that region, delivering more oxygenated hemoglobin than needed. This shift in the balance between oxyhemoglobin and deoxyhemoglobin changes the magnetic properties of blood, which the MRI scanner detects as the BOLD signal. Because this signal reflects vascular responses to neural activity rather than firing directly, it provides a functional readout with a short lag of a few seconds.

It’s not about brain structure—that would be determined by anatomical MRI. It doesn’t measure metabolism directly (that’s typically done with PET). You can, however, use fMRI data to infer functional connectivity by looking at how activity in different regions co-varies over time, but the primary measurement remains the functional activity via blood oxygenation changes.