It is important during neurosurgical procedures to identify and preserve eloquent functional cortex adjacent to a resectable lesion. Resection of a lesion infiltrating vital cerebral cortex can be associated with postoperative neurological deficits if the surgeon cannot clearly distinguish between the infiltrating borders of a lesion and surrounding functionally eloquent tissue. Spatial relationships between a lesion and surrounding normal brain can change significantly from those determined by preoperative methods such as CT and MRI scans. Necessary intraoperative interventions such as cerebrospinal fluid drainage, osmotic diuresis and lesion debulking cause quantitatively unpredictable brain shift in three dimensions. Therefore functional localization in real time that can be performed in the operating room is desirable. However, intraoperative real-time functional mapping techniques now available cannot be used in many surgical situations and are not sufficiently reliable in all cases in which they are used.