Altered activation and connectivity of the supplementary motor cortex at motor initiation in Parkinson's disease patients with freezing.

OBJECTIVE: Motor initiation failure is a key feature of freezing of gait (FOG) due to Parkinson's disease (PD). The supplementary motor cortex (SMC) plays a central role in its pathophysiology. We aimed at investigating SMC activation, connectivity and plasticity with regard to motor initiation in FOG. METHODS: Twelve patients with FOG and eleven without FOG underwent a multimodal electrophysiological evaluation of SMC functioning including the Bereitschaftspotential and movement-related desynchronisation of cortical beta oscillations. SMC plasticity was modulated by intermittent theta burst stimulation (iTBS) and its impact on gait initiation was assessed by a three-dimensional gait analysis. RESULTS: Prior to volitional movements the Bereitschaftspotential was smaller and beta power was less strongly attenuated over the SMC in patients with FOG compared to those without. Pre-motor coherence between the SMC and the primary motor cortex in the beta frequency range was also stronger in patients with FOG. iTBS resulted in a relative deterioration of gait initiation. CONCLUSIONS: Reduced activation of the SMC along with increased SMC connectivity in the beta frequency range hinder a flexible shift of the motor set as it is required for gait initiation. SIGNIFICANCE: Altered SMC functioning plays an important role for motor initiation failure in PD-related FOG.