Corticomuscular coherence is increased in the small postural tremor of Parkinson's disease

The mechanisms and electrophysiological characteristics of the postural tremor in Parkinson's disease (PD) have not been defined. We hypothesized that PD subjects with small amplitude postural tremor would show increased corticomuscular coherence at certain frequencies compared to PD subjects without visible tremor. Four groups of participants were studied: (1) Control without postural tremor, (2) Control with small amplitude postural tremor, (3) PD without postural tremor, and (4) PD with small amplitude postural tremor. Accelerometry and electroencephalography-electromyography fast-fourier transform and corticomuscular coherence spectra were generated. Findings showed (1) elevated corticomuscular coherence centered at 12-18 Hz in PD with small amplitude postural tremor; (2) 5-12 Hz accelerometer frequency peaks that did not shift with increasing weight loads in some individuals; and (3) 5-8 Hz accelerometer peaks that shifted frequency with increasing weight loads, consistent with a peripheral-mechanical oscillator in all groups. The small amplitude postural tremor in PD arises from heterogeneous oscillator mechanisms. The discovery of increased corticomuscular coupling shows cortical involvement in the small amplitude postural tremor of PD.