Which technique measures brain activity by detecting tiny magnetic fields?

The correct choice is MEG, which stands for magnetoencephalography. This technique is specifically designed to measure brain activity by detecting the tiny magnetic fields produced by electrical currents that flow when neurons in the brain are activated. The ability of MEG to provide precise timing and localization of neuronal activity makes it a valuable tool in both research and clinical settings.

Unlike fMRI, which measures changes in blood flow and oxygenation related to neural activity, MEG directly measures the electromagnetic signals generated by neuronal firing. PET involves the use of radioactive tracers to visualize metabolic processes, while EEG records electrical activity through electrodes placed on the scalp. While EEG also monitors neuronal activity, it relies on electrical signals rather than magnetic fields. Thus, MEG's unique focus on magnetic field detection directly correlates it to the measurement of brain activity.