Tremor amplitude variability during Parkinson’s disease and essential tremor. (A) Schematic of resonance functions of the oscillators or systems of linked oscillators underlying Parkinson’s disease (PD) and essential tremor (ET). Parkinsonian tremor is shown in red with a platykurtic resonance function. Hence changes in instantaneous frequency caused by extrinsic driving forces lead to a relatively small change in instantaneous amplitude, measured from the tremor envelope. Essential tremor is shown in green with a more leptokurtic resonance function. Thus changes in instantaneous frequency caused by extrinsic driving lead to a relatively big change in instantaneous tremor amplitude. (B) Group data for change in tremor amplitude for a certain change in absolute phase. Only phase changes to which at least 70% of the patient cohort contributed for a given pathology are shown. Serial independent Mann Whitney tests at each phase change bin indicate that Parkinson’s and essential tremor response to phase changes differ (P = 0.007 at phase change bin 0.4–0.6 radians). Significance is indicated with a red plus symbol.