Volume conduction
Volume conduction is the passive spread of ionic current through the conductive tissues of the head, from a neural source to a recording electrode. Because neural current sources are embedded in a large conductive medium (brain, cerebrospinal fluid, skull, scalp), the potential measured at any scalp site is a weighted sum of contributions from all active sources, not a local readout. The low-conductivity skull in particular smears and attenuates the field, so a focal cortical patch produces a broad, blurred scalp topography with spatial spread on the order of several centimetres.
The consequence is that scalp EEG channels are heavily mixed and correlated: neighbouring electrodes see overlapping projections of the same sources, and no single channel corresponds to a single brain region. At physiological frequencies volume conduction is effectively instantaneous (the quasi-static approximation), so it introduces zero-lag mixing that can masquerade as genuine neural synchrony — a central confound for connectivity analysis.
A radial dipole in a gyral crown produces a scalp maximum roughly above it, but a tangential dipole in a sulcal wall produces a dipolar map whose two extrema straddle the source, so the scalp peak is not over the generator.
Source orientation, not just position, determines where the scalp signal peaks.
A common misconception is that a strong signal at an electrode means the source lies directly beneath it; volume conduction and source orientation can shift the scalp maximum far from the true generator.