A defining feature of fMRI is that it relies on ...

• A. Ionizing radiation
• B. Blood-oxygen-level-dependent contrast
• C. Direct electrical measurement
• D. Invasive brain stimulation

Answer: (B)

fMRI relies on blood-oxygen-level-dependent (BOLD) contrast to map brain activity. When neurons in a region become more active, blood flow to that area increases, delivering more oxygenated blood than is immediately needed. This leads to a relative drop in deoxyhemoglobin, which has different magnetic properties than oxyhemoglobin. The MRI signal is affected by this change, producing a detectable contrast that indicates which areas are active. This approach is noninvasive and does not measure electrical activity directly or involve radiation or brain stimulation. The other options describe techniques that fMRI does not use: ionizing radiation, direct electrical measurement, or invasive stimulation.