Abstract
Objective: To examine the role emotions may play in modulating the characteristics of parkinsonian resting tremor.
Background: There is ample evidence showing cognitive stress can worsen tremor in Parkinson's disease. Patients often report, however, that both positive emotions (e.g., watching a sporting finale) as well as negative emotional stressors exacerbate their tremor. It is unclear whether the effects of stress on tremor are generalised to contexts with high emotional arousal or whether the emotional valence (pleasant or unpleasant) is also important.
Methods: Changes in tremor characteristics were examined in 34 PD patients presenting with rest tremor in their upper limb in a carefully matched affective paradigm in which patients were repeatedly exposed to either high or low arousing, pleasant or unpleasant emotional images. Tremor oscillations were measured using a tri-axis accelerometer on the index finger of the most-affected hand and surface electromyography from the corresponding flexor digitorum superficialis (FDS) muscle. Participants completed four 30-second trials in each emotional condition.
Results: Bayesian multilevel modeling revealed an increase in tremor amplitude (peak power of the acceleration signal) of 0.4 log((m/s2)2/Hz) (95% credible interval (CI) [0.1, 0.8]) during sustained viewing of unpleasant, high arousing (UH; stress condition) images compared to the pleasant, low arousing (PL) non-stress condition. Tremor during emotional stress exposure was also associated with an increase in FDS peak power of 0.6 log((mV)2/Hz) [0.1, 1.1] and an increase in FDS mean amplitude of 0.2 log(mV) [0.1, 0.4] compared to the non-stress (PL) emotional condition. The frequency of tremor oscillation was similar across emotion condition. Importantly, tremor characteristics in the pleasant-low condition were similar to those in the unpleasant-low and pleasant-high conditions.
Conclusions: In line with other tremor provocation studies, tremor amplitude was exacerbated by sustained exposure to emotional stress-evoking images, indicating a context-specific effect4; that the combination of negative affect and high arousal is necessary to increase tremor severity. Whether modulation of parkinsonian tremor results from acute negative stress-induced amplification of dopamine depletion and/or enhancement of sympathetic activity4 remains to be explored.