Implications of electrolyte dispersion for high resolution EEG methods.

The effective recording area of an EEG electrode is its electrical contact area with the scalp. With techniques that employ wet electrolyte, this area is primarily determined by the extent of electrolyte dispersion rather than by the size of the electrode. The effective recording areas of 10 widely distributed EEG electrodes embedded in an elasticized stretch hat were measured on 7 subjects using a digital multimeter. On average, conventionally prepared electrodes were associated with an electrolyte (standard gel) spread of approximately 1 cm in each of four directions (above, below, right and left of the electrode's center). This implies that EEG electrodes prepared with wet electrolyte should not be spaced less than 2 cm apart unless special precautions are taken to prevent the spread of electrolyte, and that in most circumstances there is little advantage to methods for designating the 3-D coordinates of an electrode that have a measurement error of less than 1 cm.