In the context of fMRI, brain activity means what?

• A. Changes in blood flow and oxygenation, not direct measurement of neuron firing.
• B. Direct measurement of neuron firing.
• C. Electrical currents in neurons.
• D. Neurotransmitter release rates.

Answer: (A)

Brain activity in fMRI is detected indirectly through changes in blood flow and oxygenation, not by directly measuring neuron firing. When a brain region becomes active, its neurons use more energy, triggering an increase in local blood flow that overcompensates for the oxygen use. This shifts the balance of oxyhemoglobin and deoxyhemoglobin, and because these forms affect the MRI signal differently, the scanner detects a BOLD (blood-oxygen-level dependent) signal change. So the fMRI readout reflects the hemodynamic response to neural activity rather than the actual electrical spikes of neurons. The signal lags behind the neural activity by a few seconds and has millimeter-scale spatial resolution, but it is inherently an indirect measure.