Surgeons rely on intraoperative monitoring to evade irreparable damage to the patient's sensory, auditory, and visual structures, as well as injury to the nerves that control movement and reflexes. Cortical mapping allows the surgeon to safely navigate the brain by identifying the central sulcus and localizing specific motor and sensory regions. Although pre-operative localization of both motor and sensory cortex can be done non-invasively with the use of MRI, PET, MEG, or SPECT technology, surgical resection of the brain often requires direct recording from the surface of the brain. Grid electrodes placed on the brain surface generate responses that set spatial limits for the surgeon by indicating which zones of the brain must be avoided so as to eliminate post-operative deficits. The objective of the cortical mapping procedure is to minimize operative risk and morbidity while maximizing the extent of resection.

Intraoperative monitoring (IOM) has become an essential tool during this procedure because of the invasive nature of a craniectomy and the specific location of the surgical exposure, putting structures at risk. The Auditory Brainstem Respnse (ABR) has shown to be very valuable in the assessment of the 8th cranial nerve. During the exposure stage of the surgery, retractors are placed in the surgical opening which puts pressure on the cerebellum and brainstem. When the retractors are placed, CN VIII can suffer either temporary or permanent damage that is observed immediately in the ABR. If this does occur the IOM staff is able to notify the surgeon to make an adjustment to the retractors to prevent permanent damage and preserve hearing. The cases invloving hemifacial spasms can be monitored with the use of spontaneous and evoked EMG's. The evoked EMG's are seen in the form of a lateral spread. When there is a normally functioning 7th nerve, no lateral spread will be seen. Therefore, during the decompression of CN VII the lateral spread should disappear. The use of IOM in this case is to verify that the nerve has been completely decompressed.