Method: 62 patients have posterior fossa tumour will include the study. Patients will divide to 2 groups according to the surgical position, the prone (n=31) or the sitting (n=31). Patients heart rate, mean blood pressure (MAP), cerebral oxygen saturation (SctO2), peripheral oxygen saturation (SpO2), BIS values will record before the induction of anesthesia. Five minutes after the standard anesthesia induction all values will record and it will accept as a baseline. After that all these parameters will record in each 3 minutes until the beginning of surgery. Mean while, more than 5 % reduction in SctO2 and more than 20 % reduction in SctO2 and/or MAP will record. As well as, if the SctO2 reduces than 55 and 60 %, it will record.