Comparison of phase-coupling indices for tremor

Tremor can be a severely disabling symptom. It occurs in patients with many movement disorders including Parkinson's disease (PD) and essential tremor (ET). Physical tremor can be measured with a variety of instruments including electromyograms (EMG) and accelerometers. Neural tremor can be measured by microelectrodes placed near nerve cells or obtained from brain cells during stereotactic neurosurgery. Recent studies have demonstrated that tremor activity measured from two distinct signals, possibly from different types of sensors, may be uncoupled even when the dominant frequency of the tremor is the same in both signals. The coupling between signals may also change over time. Several measures of phase coupling have been proposed in previous studies. We describe the results of a simulation study to assess the reliability and inverse variability of three popular phase-coupling indices and an index based on the correlation coefficient of the instantaneous frequency (IF) signals. The results of our study demonstrate that the common-mean correlation coefficient of the IF signals is the most accurate measure of phase coupling and has many desirable properties not shared by the other indices.