Objective: To investigate the effect of sustained exposure to negative emotional stimuli on isometric grip-force control in Parkinson’s disease (PD).

Background: Recent animal studies have shown that stress may contribute to the onset and worsening of motor symptoms associated with PD and that negative emotions can profoundly affect motor behaviour by acting on the dopaminergic system¹. However, the influence of emotional stress on motor function in PD remains to be elucidated.

Methods: Eighteen patients with idiopathic PD (tested off-medication) and 18 healthy control volunteers completed an emotional-force control task. Participants produced an isometric precision-grip contraction at 15% of maximum force in four conditions: while viewing visual feedback of force output, or while viewing unpleasant, pleasant or neutral emotional images (without feedback; blocked presentation). Force output was continuously recorded together with change in forearm muscle activity using electromyography (EMG).

Results: In controls, the decay in force amplitude during force maintenance was attenuated while viewing unpleasant images compared with pleasant and neutral images, consistent with our previous findings². PD participants however, showed a pronounced decay in force amplitude following feedback occlusion that was not modulated by emotion. Importantly, unlike controls, PD participants exhibited increased force variability while viewing unpleasant images. This effect was enhanced with repeated exposure to the negative stimuli, and associated with increased power at 4-8 Hz in the force spectrum, greater EMG amplitude of flexor digitorum superficialis and co-contraction with extensor digitorum communis.

Conclusions: These data show that sustained exposure to negative affect in PD exacerbates impairments in isometric force control. Greater fluctuations and 4-8 Hz oscillations of force as well as increased flexor and extensor forearm muscle activity may reflect acute emotional stress-induced enhancement in gain of the motor neuron pool and reduction in the ability to use sensory feedback to modulate the descending drive to the muscles in PD. ¹Metz, G. A. (2007) Stress as a modulator of motor system function and pathology. Reviews in the Neurosciences 18, 209-222. ²Blakemore, R.L., Rieger, S.W., Vuilleumier, P. (2016) Negative emotions facilitate isometric force through activation of prefrontal cortex and periaqueductal gray. Neuroimage 124, 627-640.

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MDS Abstracts - https://www.mdsabstracts.org/abstract/negative-emotional-signals-impair-isometric-force-control-in-parkinsons-disease/