Why does EEG record synaptic currents of pyramidal cells and miss their action potentials? · Grey Matter

To reach the scalp, currents from thousands of cells have to add up.

Why does EEG record synaptic currents of pyramidal cells and miss their action potentials?

To reach the scalp, currents from thousands of cells have to add up. Pyramidal cells stand in parallel with their dendrites toward the surface, and their synaptic currents last tens of milliseconds, so inputs that arrive roughly together overlap in time and sum into one large dipole. An action potential lasts about a millisecond and its currents flow in opposite directions over a short stretch of axon, so spikes from many cells rarely overlap and largely cancel at a distance. Even so, a visible deflection needs something like 6 to 10 square centimetres of cortex active in synchrony.