 The purpose of this study is to investigate the effect of sensory afferent input on surround inhibition in the motor cortex in dystonia. Surround inhibition is a cortical phenomenon, the function of which may be to suppress unwanted movements in surrounding muscles during voluntary actions. In support of this, a recent study showed that motor output to the little finger was reduced during self-paced, voluntary movements of the index finger, despite an increase in spinal excitability. Work has also shown that in relaxed muscles, homotopic and heterotopic peripheral stimulation results in time dependent modulation of motor cortical excitability. However, no studies have examined the relationship between volitional movement and sensory input on cortical surround inhibition, the phenomenon which we hypothesize to be defective in dystonia. The aim of this study then is to examine the changes in surround inhibition when electrical stimulation is applied to the finger being actively moved, leading to an inhibition of a surrounding finger heterotopic inhibition, at different time intervals prior to the initiation of movement, to assess the effect of heterotopic peripheral stimulation on surround inhibition in dystonia patients compared to normal subjects. 