Relationship between action potentials and EEG recordings
Most of the EEG waves are not directly generated by action potential, because it is too short and has a high frequency. They are generated by synaptic activity (overall propagation potential of axons).

The amplitude of this propagation potential depends on the impedance of the brain. Higher impedance means higher amplitude. Impedance is higher for low frequency signals.

The amplitude is also affected by the direction of the EEG vector so parallel axons which go in the same direction should produce a higher wave in amplitude.



Reference: Rudell AP, Fox SE. The propagation potential. An axonal response with implications for scalp-recorded EEG. Biophys. J. 1991 Sep;60(3):556-67. doi: 10.1016/S0006-3495(91)82085-9. PubMed PMID: 1932547.