During neurosurgical operations for aneurysms, tumors, or other lesions located in the skull base, the surgeon must employ retracting devices in order to displace one or more lobes of the brain enough to gain adequate surgical exposure. These retractors are adjusted by hand to optimize exposure. It is often difficult for the surgeon to gauge the amount of pressure actually applied to the brain during such placement of the retractor. Moreover, it is also possible to position the blade of the retractor inadvertently such that a focal pressure point occurs at the tip of the retractor blade against the brain. Thus, injury to the brain can occur as a result of brain retraction when either the force applied is excessive or when the pressure is not adequately distributed to a large enough area of brain. This injury is thought to be the result of ischemia (inadequate blood flow) caused by the retraction, local trauma, or a combination of both. It has been estimated that this type of brain retraction injury occurs in approximately 10% of major cranial base tumor procedures or 5% of intracranial aneurysm surgeries. The specific aim of this research is to identify changes in electrical activity of brain tissue subjected to necessary retraction during neurosurgical procedures that may give forewarning of imminent brain retraction injury. It is anticipated that this information will permit development of guidelines that will enable the neurosurgeon to take steps to minimize such injury, i.e., by temporarily releasing or otherwise modifying the brain retraction. Cerebral electrical activity, together with the amount of retraction pressure being applied, will be recorded directly from the tissue at risk by means of a silastic electrode grid containing a pressure monitor placed on the surface of the cerebral cortex underneath the retractor blade.