Final answer:
In magnetoencephalography (MEG), small magnetic field gradients entering and exiting the scalp are measured to assess brain activity, which is different from the methods outlined in the provided answer options.
Step-by-step explanation:
The brain activity is assessed by measuring small magnetic field gradients entering and exiting the scalp in magnetoencephalography (MEG). This technique is distinct from the methods mentioned in the options provided, which include Electroencephalogram (EEG), Magnetic Resonance Imaging (MRI), Positron Emission Tomography (PET), and Functional Magnetic Resonance Imaging (fMRI). Among the given options, none explicitly measure the magnetic fields produced by neural activity. Instead, EEG measures electrical activity, MRI and fMRI are based on magnetic properties of blood flow and tissue response in the presence of a magnetic field, and PET scans involve tracking a radioactive tracer within the bloodstream.