Final answer:
MEGs detect the synchronized magnetic fields produced by coordinated nerve impulses in the brain, reflecting the organized electrical activity of neurons.
Step-by-step explanation:
Measurements of the weak and fluctuating magnetic fields associated with brain activity, called magnetoencephalograms (MEGs), reflect the electrical activity of neurons. These magnetic fields arise from ionic currents as neurons transmit information.
Coordinated nerve impulses in the brain produce synchronized magnetic fields that MEGs can detect. MEGs have been instrumental in studying brain function and disorders such as Alzheimer's disease and epilepsy, often using superconducting quantum interference devices (SQUIDs) for their exceptional sensitivity.