Leksell served as a volunteer medical doctor in Finland when it was attacked by the Soviet Union in November 1939. Later he told that during that war, he often speculated on the possibility of extracting bullets from the brain with minimal damage to the surrounding brain tissue using a mechanically guided instrument. In the early 1940s Leksell joined Ragnar Granit, Nobel Laureate 1967, for experimental studies in neurophysiology. In 1945 he presented a PhD dissertation, a monograph on the motor gamma system titled ‘‘The action potential and excitatory effects of the small ventral root fibers to skeletal muscle.’’ This was a major milestone in the understanding of muscle control and has now become part of basic neurophysiology. During these years he, together with Granit and Skoglund, made another major contribution by describing the phenomenon of ephapsis, ‘‘artificial synapses,’’ caused by local pressure on a nerve, as a possible mechanism involved in trigeminal neuralgia. After resuming clinical work, he started work on the development of a stereotactic instrument. The stereotactic method is the product of Robert Clarke, an engineer, physiologist and surgeon, who, together with neurosurgeon Victor Horsley, devised an instrument for simulating and making lesions at exact locations within the brains of experimental animals: the Horsley–Clarke apparatus. Clarke came up with the concept in 1895, and the original device was constructed in 1905 and first used in 1906. The first successful cranial application of stereotactic surgery in humans is credited to the team of Ernest Spiegel and Henry Wycis in the Department of Experimental Neurology at Temple University in Philadelphia (Spiegel et al. 1947). Their original frame, using a Cartesian coordinate systems and similar in design and operation to the Clarke-Horsley device, was fixed to a patient's head by means of a plaster cast. The frame and cast were removable, allowing separate imaging and surgery sessions. Contrast radiography, ventriculography and later pneumoencephalography permitted the visualization of intracranial reference points from which the location of target structures of interest could be determined. Initial applications were for psychosurgery. In 1947 Leksell visited Wycis in Philadelphia and then developed and described his instrument in a publication in 1949. This was the first example of a stereotactic system based on the principle of ‘‘center-of-arc’’. In contrast to the Cartesian coordinate system of the Spiegel-Wycis device, Leksell's frame utilized three polar coordinates (angle, depth and anterior–posterior location). This ‘‘arc-quadrant’’ device provided maximum flexibility in choosing