Different techniques used to measure the electrical activity of the brain include, electroencephalograph (EEG), ERP (evoked response potential), MEG (magnetoencephalography) and single cell recording.
Neural activity produces the magnetic field as it generates electrical field. A single neuron produces a small magnetic field, while multiple neurons produce a large magnetic field. Magnetoencephalography can measure the electrical activity of many neurons.
MEG also provides the three-dimensional (3-D) localization of the cell groups (i.e. of neurons) generating the measured field.
Thus, the blanks can be filled with electrical activity of many neurons; 3-D localization of the nerve cell groups generating the measured field.