Deep structures within the brain further away from the electrodes will not contribute directly to an EEG these include the base of the cortical gyrus, mesial walls of the major lobes, hippocampus, thalamus, and brain stem. This means not all neurons will contribute equally to an EEG signal, with an EEG predominately reflecting the activity of cortical neurons near the electrodes on the scalp. Furthermore, the value recorded is distorted by intermediary tissues and bones, which act in a manner akin to resistors and capacitors in an electrical circuit. As the electrical activity monitored by EEG originates in neurons in the underlying brain tissue, the recordings made by the electrodes on the surface of the scalp vary in accordance with their orientation and distance to the source of the activity. Voltage fluctuations measured by the EEG bioamplifier and electrodes allow the evaluation of normal brain activity. Clinical interpretation of EEG recordings is most often performed by visual inspection of the tracing or quantitative EEG analysis. Electrocorticography, involving surgical placement of electrodes, is sometimes called ' intracranial EEG'. It is typically non-invasive, with the EEG electrodes placed along the scalp (commonly called 'scalp EEG') using the International 10–20 system, or variations of it. The biosignals detected by EEG have been shown to represent the postsynaptic potentials of pyramidal neurons in the neocortex and allocortex. Electroencephalography ( EEG) is a method to record an electrogram of the spontaneous electrical activity of the brain.

Epilepsy eeg vs normal eeg