Reduced cortico-muscular beta coupling in Parkinson's disease predicts motor impairment.

Long-range communication through the motor system is thought to be facilitated by phase coupling between neural activity in the 15-30 Hz beta range. During periods of sustained muscle contraction (grip), such coupling is manifest between motor cortex and the contralateral forearm muscles-measured as the cortico-muscular coherence. We examined alterations in cortico-muscular coherence in individuals with Parkinson's disease, while equating grip strength between individuals with Parkinson's disease (off their medication) and healthy control participants. We show a marked reduction in beta cortico-muscular coherence in the Parkinson's disease group, even though the grip strength was comparable between the two groups. Moreover, the reduced cortico-muscular coherence was related to motor symptoms, so that individuals with lower cortico-muscular coherence also displayed worse motor symptoms. These findings highlight the cortico-muscular coherence as a simple, effective and clinically relevant neural marker of Parkinson's disease pathology, with the potential to aid monitoring of disease progression and the efficacy of novel treatments for Parkinson's disease.