The pathophysiological mechanisms of primary dystonia have largely remained obscure. Yet there is one undeniable observation: lesioning or high-frequency stimulation of the internal segment of the globus pallidus (GP) ameliorates dystonic symptoms. The latter observation implicates abnormal pallidal activity in the genesis of primary dystonia. Recently, excessive oscillatory pallidal activity in the 3-10 Hz frequency range, synchronized with dystonic EMG, has been related to the occurrence of involuntary muscle activity in these patients. However, it is unclear whether this pathological synchronization is driven by GP, caused by re-afference from dystonic muscle, or due to a combination of these two processes. Here we used the Directed Transfer Function as a spectral measure to identify the degree and direction of coupling across time between GP and muscle in seven patients with primary dystonia. We show that pallidal local field potential activity