Despite that deep brain stimulation (DBS) of the globus pallidus internus (GPi) 
is emerging as the favored intervention for patients with medically intractable 
dystonia, the pathophysiological mechanisms of dystonia are largely unclear. In 
eight patients with primary dystonia who were treated with bilateral chronic 
pallidal stimulation, we correlated symptom-related electromyogram (EMG) 
activity of the most affected muscles with the local field potentials (LFPs) 
recorded from the globus pallidus electrodes. In 5 dystonic patients with mobile 
involuntary movements, rhythmic EMG bursts in the contralateral muscles were 
coherent with the oscillations in the pallidal LFPs at the burst frequency. In 
contrast, no significant coherence was seen between EMG and LFPs either for the 
sustained activity separated out from the compound EMGs in those 5 cases, or in 
the EMGs in 3 other cases without mobile involuntary movements and rhythmic EMG 
bursts. In comparison with the resting condition, in both active and passive 
movements, significant modulation in the GPi LFPs was seen in the range of 8-16 
Hz. The finding of significant coherence between GPi oscillations and rhythmic 
EMG bursts but not sustained tonic EMG activity suggests that the synchronized 
pallidal activity may be directly related to the rhythmic involuntary movements. 
In contrast, the sustained hypertonic muscle activity may be represented by less 
synchronized activity in the pallidum. Thus, the pallidum may play different 
roles in generating different components of the dystonic symptom complex.