The purpose of this study is to investigate the mechanism of sensory tricks on surround inhibition in the motor cortex in focal dystonia. Sensory tricks or Geste antagoniste, are various manoeuvres used by dystonic patients to transiently diminish their spasms. Many of these manoeuvres involve motor as well as sensory input and here we use the term 'tricks' to refer to both phenomena. Little is known of this unique feature that is an evidence for the abnormality of sensorimotor integration in focal dystonia. The relief of dystonic symptoms by the application of tricks suggests its potential as a clue to the understanding of the yet unclear mechanism of dystonia. There is evidence that surround inhibition, a cortical phenomenon that functions to suppress unwanted movements in surrounding muscles during voluntary actions, is impaired in dystonia. Since tricks transiently relieve the dystonic symptoms, we hypothesize that the mechanism of tricks is the normalization of surround inhibition which is defective in dystonia.