Which measure is used to detect neural activity in functional MRI?

• A. Electroencephalographic signal
• B. Global brain activity
• C. Hemodynamic response
• D. Blood oxygen level-dependent signal

Answer: (D)

In functional MRI, neural activity is inferred from the blood oxygen level-dependent signal. When a brain region becomes active, neurons consume more oxygen, and blood flow increases to that area enough to oversupply oxygen, changing the ratio of oxyhemoglobin to deoxyhemoglobin. This change alters the magnetic properties of blood in a way that is detected by MRI, producing the BOLD signal. The BOLD signal is the specific MRI measure that tracks these hemodynamic changes and serves as an indirect index of neural activity. The broader hemodynamic response describes the process itself, but the actual quantity measured by fMRI is the BOLD signal. The other options don’t fit because EEG reflects electrical activity, not MRI-derived signals; a generic measure of global brain activity isn’t specific enough; and the hemodynamic response is the process, not the measurement used.