EEG recordings from the scalp as well as intracranial recordings indicate that cortical communication is the result of neuronal oscillations (Akam & Kullmann, 2014; Cohen, 2017; Stam, 2014; Uhlhaas et al., 2010; Uhlhaas & Singer, 2010). Both intracranial and scalp recordings show clear sinusoidal activity patterns, produced by the oscillations that are caused by the concerted changes in voltages across the postsynaptic membranes in the dendritic trees of pyramidal neurons. These voltage changes either inhibit or sensitize neurons to create action potentials. The unique orientation of the pyramidal neurons—systematically perpendicular to the cortical sheet—and the often strongly correlated coactivations across many neurons result in a large collective dipole: the summed activity of the local field potentials that is enough to propagate through the surrounding tissues to reach externally attached electrodes (Buzsáki et al., 2012). Although the activity on the microscopic scale of a single neuron cannot be detected, there is enough information for recording synchronous activity of patches of neurons, at the scale of a few cm2 of cortical tissue.
