Is the synchronization between pallidal and muscle activity in primary dystonia due to peripheral afferance or a motor drive?

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 310 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 <= 10 Hz is coherent with dystonic movements, and that although the coupling between GP and activity in the sternocleidomastoid muscle is bidirectional, the drive from GP to muscle significantly outweighs that from muscle to GP. In addition, the net GP drive to muscle is not stable but fluctuates across time, in keeping with the dynamic nature of dystonic muscle activity.