What does fMRI measure to assess brain activity?

Functional Magnetic Resonance Imaging (fMRI) is a technique specifically designed to measure brain activity by detecting changes in blood flow. When a particular area of the brain is more active, it requires more oxygen, leading to an increase in blood flow to that region. This change in blood flow is used as an indirect indicator of neuronal activity since active neurons consume more oxygen than inactive ones. fMRI relies on the BOLD (Blood Oxygen Level Dependent) contrast, which reflects the differences in magnetic properties between oxygenated and deoxygenated blood.

The other options do not accurately describe what fMRI measures. While nerve impulse frequency relates to neuronal firing, fMRI does not provide direct measurements of action potentials or the frequency of these impulses. Electrical activity is typically assessed through techniques like electroencephalography (EEG), which records the electrical activity of the brain directly. Lastly, chemical neurotransmitter levels are studied through other biochemical techniques, rather than through fMRI, which focuses on the hemodynamic response associated with blood flow. Thus, blood flow is the correct measure that fMRI uses to assess brain activity.