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Proposed generator sites for the N18 component of the somatosensory evoked potential (SEP) range in location from the medulla to the thalamus. Additional knowledge regarding the generators of the N18 will be important in interpreting the results of intra-operative monitoring during skull base surgery and providing the surgeon more specific information. The goal of this study was to use both intracranial electrical recording and the effects of acute brainstem ischemia in humans to further define the generators of N18. Monopolar electrodes were used to record SEP (after median nerve stimulation) from the brainstem surface in eight patients undergoing posterior fossa surgical procedures. Recordings were made from various locations, from the cervico-medullary junction to the level of the aqueduct of Sylvius. As the electrode moved rostrally on the brainstem surface, the difference in latencies between the scalp N18 potential and the electrode potential approached zero, suggesting an upper pontine-lower midbrain origin of the N18 potential. These findings were supported by the lack of change in the N18 potentials of ten patients with basilar tip aneurysms who experienced marked changes of their N20/P22 potentials during temporary occlusion of the distal basilar artery.