Abstracts

Rationale: Awake intraoperative cortical mapping using cortical stimulation while patients perform language tasks has been used to locate and preserve cortical areas that contribute to language functions during cortical resection in the neurosurgical treatment of both epilepsy and brain tumors. To date, this methodology has focused on isolating cortical areas involved in language function, but not other cognitive functions, such as memory. It has been well established that the hippocampal complex subserves memory processing. Recent research has shown that the fornix, the major efferent pathway from the hippocampal complex, also plays a critical role in learning new information (Dumont J, Petrides M, & Sziklas V. Functional dissociation between fornix and hippocampus in spatial conditional learning. Hippocampus 2007; 17: 1170-1179). The current case study describes the development and successful use of a memory-testing paradigm during awake intraoperative white matter stimulation of the fornices. Methods: The patient presented with a third ventricle tumor that displaced both fornices, making it difficult to identify them using preoperative imaging. A preoperative Wada procedure demonstrated that the patient had adequate memory function in bilateral mesial temporal structures. Thus, the intraoperative memory-testing paradigm was conducted to accurately locate the forniceal fibers involved in memory functions and then tailor the tumor resection to preserve those fibers. The memory-testing procedure involved showing the patient four pictures, which she was asked to name and remember, while a specified area of the brain was stimulated during the presentation of the third and fourth pictures. The patient was then shown an index card with three simple mathematical problems to verbally solve (distractor task). The patient was then shown eight successive individual pictures, four of which were the four that she was previously asked to name and remember and four of which were novel. The patient’s response (yes/no) to each recognition picture was monitored. Any region for which the patient did not recognize either of the two pictures that had been presented during stimulation was designated a positive memory area. Any region for which she was able to recognize both of the two pictures that had been presented during stimulation was designated a negative memory area. Results: Stimulation of several areas of both anterior left and anterior right fornix at 6mA caused a positive memory disruption, as the patient remembered the two pictures shown to her prior to stimulation but not the two pictures shown to her during stimulation. Using this awake intraoperative memory-testing paradigm, it was possible to isolate the fibers of the fornix and avoid those fibers critical to memory functioning during the tumor resection. Conclusions: This paradigm provides a unique methodology for assessing memory functioning intraoperatively and can be used with intraoperative cortical and white matter stimulation to preserve areas involved in memory functions during neurosurgical resection in the treatment of both epilepsy and brain tumors.