The cerebral cortex contains numbers of neurons. Activity of these neurons is to some extent synchronized in regular firing rhythms (brain waves). Electrodes placed on the scalp can pick up variations on electrical potential that originate from this underlying cortical activity. The EEG represents the sum of a large amount of underlying neural activity.
Single of dual channel EEG recordings may be useful for the identification of alpha activity in conscious subjects or for the recording of other specific waveforms (beta, Delta, Sigma, Theta) that are present in the EEGs of sleeping human adults.
In some cases (such as epileptic studies), deeper brain activity cannot be recorded accurately (if at all) by scalp EEG instruments. Therefore invasive forms of EEG such as intracranial EEG (icEEG) where electrodes are placed directly inside the skull, subdural EEG (sdEEG) and electrocorticography (ECoG) where electrodes are placed directly on the exposed surface of the brain, are used to record electrical activity from the cerebral cortex.