A significant portion of the head cannot be practically surveyed, and important brain areas such as the basomesial temporal cortex and other deep sources are only indirectly accessible with standard scalp electrodes. Additional electrodes inferior to the 10–20 system must be employed to provide a better view. In certain circumstances, the information obtained from a combination of closely spaced scalp electrodes such as the international 10–10 system and sphenoidal electrodes can obviate the need for more invasive recordings .