Brain tumour is a malformation of intracranial cells (neurons, glia or meninx) causing functional deficits due to directly destroying neurons or causing pressure on eloquent cortex, i.e. on functionally/behaviourally important cortex. Tumours can be either benign or malignant. In 2006 there were 4,532 new cases of brain or central nervous system cancers diagnosed. Despite the fact that this accounts only for 2% of all cancer cases, brain tumours are also responsible for 7% of deaths under age 70 (Holman 1987). Life expectancy after brain tumour diagnosis is very low – only 14% of patients survive more than 5 years. Therefore the treatment is targeted at balancing between extending patients life expectancy and maintaining its quality. The main treatment is maximal surgical tumour extraction. The procedure almost always fails in extracting all of the malignant cells (because of the extensive infiltration of the neighbourhood areas). That is where the aforementioned balance comes into play. Extracting as much of the tumour as possible slows down its growth and decreases pressure on neighbouring areas. At the same time lesioning eloquent areas might cause cognitive deficits decreasing patients quality of life. Therefore during the operation surgeon maps the cortex using Extra- Cortical Stimulation (applying small current temporally disturbing local activity and behaviourally testing for deficits). Resection is limited by the areas that are crucial for motor, language and cognitive skills and sensory inputs.