Despite the amazing progress made by the stereotactic technique, particularly regarding the localization of the target, despite the extreme caution, which stereotactic neurosurgeons apply at every step of the procedures, despite the routine roentgenologic and physiologic controls (depth EEG, electric stimulation) preceding the production of a lesion, there remains a certain degree of uncertainty regarding the position, shape and extent of the lesion as well as of the electrode track and also regarding unintended lesions in the vicinity. The final answer to these questions depends on the...