Recent animal studies have shown that stress can profoundly affect motor behaviour and worsen motor deficits associated with Parkinson's disease (PD) by acting on the dopaminergic system, possibly due to stress-associated emotional changes. However, systematic investigation of the influence of acute emotional stressors on motor function in PD is scarce. Here we examined the effect of repeated exposure to negative emotional stimuli on grip-force control in PD. Eighteen patients with idiopathic PD (tested off-medication) and 18 healthy controls produced an isometric precision grip contraction at 15% of maximum force while viewing a series of unpleasant, pleasant, or neutral emotional images (blocked presentation; without visual feedback of force output). Force output was continuously recorded together with change in forearm muscle activity using electromyography. While viewing unpleasant images, PD participants exhibited increased variability and 4-8 Hz oscillations of force output, and greater flexor muscle activity. With feedback occluded, the decay in force amplitude was pronounced, but not modulated by emotion. In contrast, in controls, the decay in force amplitude was attenuated while viewing unpleasant images compared with pleasant and neutral images. The findings in PD may reflect an increased number of motor units discharging and reduced ability to use sensory feedback to alter the descending drive. Modulation of synaptic input to the motoneuron pool could result from acute stress-induced enhancement of sympathetic activity and/or amplification of dopamine depletion. Corroborating previous findings in animal models of PD, exposure to stress-evoking emotional stimuli can exacerbate impairments in fine motor control in individuals with PD.
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