Before a patient undergoes surgery to treat epilepsy, pre-surgical tests-which may be invasive-are vital to assess where in the brain seizures start. All surgical candidates must undergo several non-invasive tests, described in the prior video, during their pre-surgical assessment to find the area where seizures start, called the seizure focus. If these tests suggest that surgery is likely to help control seizures, the doctor often recommends additional tests that are more invasive and do come with some risk.A commonly used invasive test is the intracarotid hemispheric dominance, or Wada, test. It involves a catheterization, in which a thin plastic tube is put into an artery where the upper thigh and pelvis meet. anesthetic is injected. This is performed sequentially on both sides, putting the left and right sides of the brain to sleep, in turn allowing speech and memory to be tested. In most people, language is on the left side of the brain and memory is on both sides. However, if memory is impaired on one side, then it suggests that the temporal lobe on that side is not working well and may be the source of seizures. The Wada test helps a neurosurgeon plan safe boundaries for epilepsy surgery. For example, if memory is impaired in the right half of the brain but excellent on the left side, a right temporal lobectomy can likely be done without risk of harming memory. Meanwhile, because some seizures originate deep in the brain, their onset is not always seen clearly by scalp EEG recordings. In these cases, electrodes are placed under the skull to help a doctor get a close-up look at the seizure focus. Two different kinds of electrodes are used, A. subdural electrode grids, or strips, which are put under the skull on top of the brain, B. and depth electrodes which go directly into the brain tissue. The grid electrodes placed on the surface of the brain can get a close-up recording of seizures. These grid electrodes can help map the functions, such as language or movement, of the underlying region of brain. This is done by sending a small electrical current through an electrode, which can inactivate a part of the brain for a few seconds. If an activity, like counting to ten for example, is interrupted by this, then the part of the brain which was stimulated can be marked as a control area for speech. Similar analysis can be applied to movement, reading, or other brain functions. Grid mapping is useful when seizures are close to a critically important region of the brain. Subdural or depth electrodes can be used in surgical procedures intended only to diagnose seizure focus, but they can also be used as part of a two-stage epilepsy surgery. In the latter case, the first stage involves recording the seizures and possibly mapping them... while the second stage involves removing the electrodes, followed by the removal of the brain tissue from where the seizures arise. If these tests show a person to be a good surgical candidate, then he should engage in discussion about the risks and benefits of the recommended procedure with an epilepsy team.