We studied 6 epilepsy patients who were evaluated for epilepsy surgery. All patients had partial seizures defined by EEG recordings and behavior during seizures. The seizures were medically intractable and severely interfering with the patients’ quality of lives. Non-invasive studies (scalp and sphenoidal-EEG telemetry, neuropsychological evaluation, functional imaging studies (PET), and structural imaging studies (CT, MRI)) did not allow a definitive localization of the seizure focus. Thus, the patients were evaluated with chronically indwelling subdural grid electrodes for definitive localization of the seizure focus. Table 4.1.1 (page 56) shows the patients’ clinical data and seizure types. All patients had given written informed consent to undergo invasive presurgical evaluation and to participate in the SEP studies.