Key limitation of fMRI?

• A. It cannot measure any brain activity.
• B. It uses ionizing radiation.
• C. It measures neural activity directly.
• D. It has a time delay due to the hemodynamic response.

Answer: (D)

The main idea is that fMRI signals come from blood flow changes rather than direct neural firing. When neurons become active, the brain’s blood vessels respond with increased blood flow, and fMRI detects this hemodynamic change as the BOLD signal. That response isn’t immediate—it rises and peaks several seconds after the neural activity and can last for many seconds. This lag gives fMRI excellent spatial resolution but poor temporal resolution, making it hard to pinpoint exactly when events occur or to track rapid sequences of neural activity. That time delay is the fundamental limitation at play here. By contrast, the other statements don’t fit: fMRI does measure brain activity indirectly (not measuring neurons directly), it uses non-ionizing magnetic fields (not ionizing radiation), and it does not measure neural activity directly.