craniotomy. Once the dura mater is peeled back, an electrode is placed on the brain to test motor, sensory, language, or visual function at a specific brain site. The electrode delivers an electric current lasting from 2 to 10 seconds on the surface of the brain, causing a reversible lesion in a particular brain location. This lesion can prevent or produce a testable response, such as the movement of a limb or the ability to identify an object. The electric current from the electrode stimulates whatever function that site in the brain is responsible for, in essence telling the surgeon or examiner what a specific locale in the brain does. Electrodes are usually made of stainless steel or platinum-iridium embedded in a silastic material, and are usually circular with diameters of 2 to 3 mm. Electrode positioning varies from patient to patient, and electrodes can come in rows, in a grid array, or can be individually arranged. The number of electrodes necessary and their exact spatial arrangement is often determined in the operating room. Cortical stimulation mapping allows electrodes to be placed in exact locations to test brain function and identify if stimulation of the brain location causes a functional impairment in the patient. CSM can be completed using anesthetized patients or awake patients. Electrodes can either be placed directly on brain areas of interest or can be placed in the subdural space of the brain. Subdural electrodes can shift slightly and can be affected by cerebrospinal fluid in the subdural space, which could interfere with the current used to stimulate the brain from the electrodes and possibly cause shunting and dissipate the current, making the stimulation's effect less accurate. However, an advantage of subdural electrode grids is that they can be left in the brain for multiple days, and allow functional testing during stimulation outside the operating room. Current levels and density are an important consideration in all cortical stimulation mapping procedures. Current density, that is the amount of current applied to a defined area of the brain, must be sufficient to stimulate neurons effectively and not die off too quickly, yet low enough to protect brain tissue from damaging currents. Currents are kept at levels that have been determined safe and are only given as short bursts, typically bursts that slowly increase in intensity and duration until a response (such as a muscle movement) can be tested. Current