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Direct electrical stimulation of the brain is the gold standard technique used to define functional-anatomical relationships during neurosurgical procedures. Areas that respond to stimulation are considered “critical nodes” of circuits that must remain intact for the subject to maintain the ability to perform certain functions, like moving and speaking. Despite its routine use, the neurophysiology underlying downstream motor responses to electrical stimulation of the brain, such as muscle contraction or movement arrest, is poorly understood. Furthermore, varying and sometimes counterintuitive responses can be seen depending on how and where the stimulation is applied, even within the human primary motor cortex. Therefore, here we review relevant neuroanatomy of the human motor system, provide a brief historical perspective on electrical brain stimulation, explore mechanistic variations in stimulation applications, examine neurophysiological properties of different parts of the motor system, and suggest areas of future research that can promote a better understanding of the interaction between electrical stimulation of the brain and its function.

Background: Parasagittal and Parafalcine Meningiomas (PSPF) have a higher rate of recurrence, increased risk of postoperative morbidities, and less favorable outcomes after stereotactic radiosurgery compared to other ocations. Herein, we try to find factors associated with treatment failure after radiosurgery in patients with PSPF meningiomas. Methods: We retrospectively reviewed records of 104 patients with 130 Gamma Knife® Radiosurgery (GKRS) treatments for individual meningiomas at a single institution. 38 were PSPF compared to 92 in non-PSPF locations. Results: The PSPF group showed a significantly higher rate of surgical intervention after radiosurgery compared to the non-PSPF group (18.4% vs 4.4, p = 0.008 in univariate analysis). The relative risk for a PSPF tumor requiring surgery after GKRS was 4.24, with an odds ratio of 4.97 (95% CI: [1.32-13.63], p = 0.015). The average tumor size between the PSPF group and the non-PSPF group was 3.53 cm3 and 2.28 cm3, respectively; this difference was statistically significant (p = 0.035) on univariant analysis, but not multivariate (p = 0.125). In the whole sample, for tumors >5 cm3, the relative risk ratio for needing surgery after GKRS was 6.23 with an odds ratio of 8.32 (95% CI: [2.25-30.67], p = 0.0015). Both PSPF and non-PSPF meningiomas were similar in gender, follow-up length, prior surgical intervention, and WHO grades. Conclusion: Meningiomas’ outcome and response to radiosurgery depend on their location. We have two possible explanations. First, PSPF tumors are located near sensitive cerebral cortex areas. Second, the tumor’s mutational profile affects meningiomas’ location and prognosis.

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