ABSTRACT
Electroencephalogram (EEG) is a record of the electric signal generated by the cooperative action of brain cells, or more precisely, the time course of neuronal extracellular field potentials generated by their synchronous action. EEG recorded in the absence of stimuli is called spontaneous EEG; a brain electric field generated as a response to external or internal stimulus is called an event-related potential (ERP). EEG can be measured by means of electrodes placed on the scalp or directly on the cortex. In the latter case it is called an electrocorticogram (ECoG); lately also iEEG (intracranial EEG) abbreviation is used. Electric fields measured intracortically with electrodes implanted in the brain structures were named local fields potentials (LFP). The same electrodes (if small enough) can also record action potentials (spikes). The amplitude of EEG of a normal subject in the awake state, recorded with the scalp electrodes, is 10-100 μV. In case of epilepsy the EEG amplitudes may increase by almost an order of magnitude. Scalp potentials are largely independent of electrode size which is due to severe space averaging by volume conduction between brain and scalp. Intracranial potentials amplitude depend on electrode size (smaller electrodehigher potential) and may vary in size over four orders of magnitude.
