What does fMRI measure to infer brain activity?

• A. Electrical potentials
• B. Magnetic fields only
• C. Metabolic rate of glucose consumption
• D. Blood flow and oxygenation

Answer: (D)

The key idea is that fMRI tracks brain activity indirectly through vascular responses rather than measuring neural signals directly. When a region becomes active, neurons need more oxygen, and blood flow to that area increases. This surge overcompensates for the oxygen use, changing the balance of oxygenated versus deoxygenated blood. Deoxyhemoglobin is paramagnetic and would dampen MRI signals, but its local concentration decreases, producing a detectable change in the MR signal known as the BOLD (blood-oxygenation level dependent) signal. This BOLD signal reveals where and when brain activity occurs.

In short, fMRI is about blood flow and oxygenation changes as a proxy for neural activity, not direct electrical activity or direct measurement of glucose metabolism.