Stereotactic neurosurgery originated from the pioneering work of Horsley and Clarke, who developed a stereotactic apparatus to study the monkey brain in 1908. Spiegel and Wycis applied this technology to the human brain in 1947, which ultimately lead to the development of multiple stereotactic neurosurgical devices during the 1950s. It was Lars Leksell of Sweden, however, who envisioned stereotactic radiosurgery. Leksell developed the gamma knife to treat intracranial lesions in a noninvasive fashion. His work stimulated worldwide interest and created the field of stereotactic radiosurgery.