TWO ARTICLES in this issue of the Archives1,2 use a common method for studying neurophysiological processes in the intact brains of human subjects. This method is based on the fact that slowly changing membrane potentials of dendrites and nerve cell bodies generate extracellular current flow. Excitatory and inhibitory postsynaptic potentials cause outflows of ionic charges into extracellular fluid, which are then pumped back into the cell. The movement of charges constitutes current flow and in a summated form results in volumeconducted potentials recorded at the scalp as the electroencephalogram (EEG).
WHAT ARE EVENT-RELATED POTENTIALS?
Event-related potentials (ERPs) are EEG changes that are time-locked to sensory, motor, or cognitive events and provide a neurophysiological reflection of the processing of these events. At the scalp, ERPs are relatively small compared with the background EEG. Therefore, signal averaging is used to enhance the signalto-noise ratio (SNR). This process assumes that the ERP