Scalp EEG Signals: Generators and Dipoles

SUMMARY: Accurate interpretation of scalp electroencephalogram (EEG) signals relies on understanding the biophysical principles underlying their generation. Although traditional EEG analysis often focuses on identifying negative phase reversals, this approach oversimplifies the complex nature of cortical current flow. Electroencephalogram signals are generated by extracellular currents resulting from postsynaptic potentials in pyramidal neurons, forming dipoles aligned along the apical dendrites. The orientation and location of these dipoles, whether radial, tangential, or oblique, depend on the spatial orientation of the active cortical surface. Orientation critically influences the scalp voltage topography. Therefore, the spatial distribution of both positive and negative poles on the scalp provides valuable insight into source localization. This review highlights the importance of including both polarities when interpreting EEG, particularly in distinguishing tangential and oblique from radial sources. It also emphasizes the role of electrode placement, montage selection, and digital filtering. Simultaneous MEG-EEG recordings further validate the physiologic significance of scalp positivities. Through practical examples and voltage maps, the authors advocate for a shift in clinical EEG interpretation, from relying solely on negativity and phase reversals to a more informed, topographic approach. Such an approach enhances diagnostic accuracy and aligns EEG interpretation with a more complete neurophysiologic understanding.