Objective: The aim of our study was to identify the neural substrates of inhibitory processing on the stop signal task in patients with predominantly left hemibody parkinsonism.

Background: Imaging studies using the stop signal task in healthy individuals indicate the subthalamic nucleus (STN)  the pre-supplementary motor area (pre-SMA), and the inferior frontal gyrus (IFG) are key components of a right hemisphere “inhibitory network”. A causal role of the right STN in inhibitory control is also reported in both human and non-human studies. The pathophysiology of Parkinson’s disease (PD) is characterized by overactivity of the STN and underactivity of the SMA. We therefore investigated the neural substrates of motor inhibition in PD patients with predominant left-sided motor features, mainly influencing right hemispheric networks.

Method: Fourteen PD patients with left predominant Parkinsonism (LPD) and 23 age-matched healthy controls performed a stop signal task (staircase tracking procedure) with the right and left hands in two separate fMRI sessions.

Results: LPD patients showed delayed response inhibition with either hand compared to healthy controls. There were differences in patterns of brain activation during successful inhibition with the left more affected and right less affected hand. When performing with the more affected left hand, successful inhibition was associated with less activation of the IFG bilaterally, insula and posterior right putamen in PD relative to healthy controls. In LPD, functional connectivity analysis with the right STN engaged a network with the right globus pallidus internal (GPi) and bilateral cerebellar hemispheric visuomotor and sensorimotor regions (Crus I and Lobules IV,V,VI) during motor inhibition with the more affected left hand.

Conclusion: Altered inhibitory control in patients with predominantly left hemibody parkinsonism is associated with reduced activity in right hemispheric regions dedicated to inhibition in healthy controls, which then requires engagement of additional regions (GPi and cerebellum) not observed in healthy controls to successfully stop ongoing behaviours.

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MDS Abstracts - https://www.mdsabstracts.org/abstract/the-right-hemisphere-inhibitory-network-in-patients-with-asymmetrical-parkinsons-disease-an-fmri-study/