Intracranial EEG (I-EEG), sometimes called a "sub-dural EEG" (SD-EEG), is where the electrodes for an electroencephalograph (EEG) are placed near the surface of the brain. In some cases, such as epileptic studies, deeper brain activity cannot be recorded accurately or not at all by EEG. This is when a deeper EEG is required, due to the shielding effect of the scalp and skull. The EEG electrodes are placed under the surface of the scalp and skull directly on the brain surface. This is also called "electrocorticography" (ECoG). Electrodes can even be placed into specific brain areas, such as the hippocampus. The EEG signal then processes in the same manner as with a surface EEG, but with higher recording rates because of higher frequencies and little to no interference from outside sources -