The Reference Problem and Re-Referencing

Scalp EEG measures a potential difference, so every recorded value is defined only relative to a reference electrode, and no point on the body is truly at zero potential. This means the reference choice (linked mastoids, a single vertex electrode, an average) is not a neutral formatting decision: it adds its own signal to every channel, reshapes topographies, and can create or destroy apparent connectivity because a shared reference injects a common, zero-lag component into all pairs of channels.

The reference electrode standardization technique (REST) attempts to recover the theoretical reference at infinity by using a head model and an equivalent source layer to transform any montage to a reference-independent one. More generally, awareness of the reference problem dictates that spatial filters and connectivity metrics be chosen to be reference-robust (surface Laplacian, average reference on dense arrays, or metrics that discard zero-lag interactions).