The role of inhibition from the left dorsal premotor cortex in right-sided focal hand dystonia.

BACKGROUND: The left dorsal premotor cortex (PMd) plays an important role in movement selection and is abnormally activated in imaging studies in patients with right-sided focal hand dystonia (FHD).

OBJECTIVE: The aims of this study were to assess the role of left PMd in patients with FHD and in the genesis of surround inhibition, which is deficient in FHD.

METHODS: Single- and paired-pulse transcranial magnetic stimulation (TMS) was applied during different phases of an index finger movement using the abductor pollicis brevis muscle (APB), a surrounding, nonsynergistic muscle, as target muscle. To look at the effect of PMd on the primary motor cortex (M1), a subthreshold conditioning pulse was applied to PMd 6 milliseconds before stimulation over M1.

RESULTS: There was surround inhibition during movement initiation in controls, but not in FHD patients. In contrast, FHD patients, but not controls, showed premotor-motor inhibition (PMI) at rest. During movement, PMI was absent in both groups.

CONCLUSIONS: We conclude that PMI does not appear to play a key role in the formation of surround inhibition in normal subjects, because it was not enhanced during movement initiation. However, in FHD, inhibition from PMd on M1 was abnormally increased at rest and declined during movement initiation. The behavior of PMd can therefore partly explain the loss of surround inhibition in the FHD patients. The functional significance of increased PMI at rest is not clear, but might be an attempt of compensation for losses of inhibition from other brain areas.