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Focused high intensity ultrasound can be used, under accurately controlled dosage conditions, to produce either temporary or permanent changes in practically any desired brain structure. Volumes of tissue smaller than one tenth of a cubic millimeter can be affected in deep brain structures of experimental animals (cats and monkeys), and regions as large as desired can be changed by moving the focal spot of the ultrasonic beams through an appropriately chosen path. The changes can be induced without adversely affecting intervening brain structure and without interrupting the vascular system even within the site in which irreversible or permanent changes in the neural components are produced. The selectivity and absence of effects on the intervening tissue make focused ultrasound a tool of considerable power for investigating basic brain mechanisms. It is now being used in an extensive experimental animal program involving neuroanatomical, behavioral and physiological studies. It is also being used to study and modify the symptoms of various neurological disorders in humans. The signs and symptoms which have been and are under investigation in human patients at the present time include abnormal movements (tremor and nonpatterned), muscular rigidity), intractable pain (following amputations, cerebral vascular accidents, the acute phase of herpes zoster) and hypersensitivity to stimulation of the body surface.