Objective: To investigate neural mechanisms underlying motor imagination (MI) in essential tremor (ET) patients compared to healthy controls using high-density EEG.

Background: ET ranges among the most common movement disorders and is characterized by action and postural tremor. A growing body of literature suggests aberrant cerebello-thalamo-cortical coupling. MI tasks are associated with involvement of cortical and cerebellar pathways and specifically with modulation of the beta frequency band in the sensorimotor system^1,2. And although impairment of MI has been described in ET patients³ underlying neurophysiological processes remain to be elucidated and particularly with respect to MI of tremulous movements.

Method: 23 ET patients and 22 matched healthy controls were asked to imagine tremulous movements of the right hand during high-density EEG recordings. Of these, 32 subjects received 3T structural MRI. Following standard preprocessing, time-frequency analyses with multiple tapers were performed using the Fieldtrip toolbox. Time-frequency representations within the beta frequency band where compared between MI of tremor and resting-state using cluster-based permutation testing in an ANOVA-like (2 x 2) design. The resulting sensor-level clusters were finally subjected to source analysis using a Dynamical Imaging of Coherent Sources (DICS) beamformer approach in the frequency domain.

Results: Permutation tests between ET and healthy controls revealed significant differences in beta band power during MI of tremor (p=0.05). Resulting clusters projected mainly to bilateral frontal and to left parietotemporal regions. Beamforming then identified the source of the higher beta-band power in the bilateral left-dominant lateral part of the motor cortex and in parts of the left dorsolateral prefrontal cortex.

Conclusion: We could show differences within MI tasks of tremulous movements between ET-patients and healthy control subjects with the former showing stronger and earlier re-occurrence of beta activity. Interestingly, this finding was related to cortical dysfunction with a maximum in the contralateral frontal cortex, as fundamental part of the cerebello-thalamo-cortical pathway during tremor generation. Taken together, our findings support the notion of impaired neural coupling in cortical structures in ET that affects MI of tremor.

References: 1. Schnitzler A, Salenius S, Salmelin R, Jousmäki V, Hari R. Involvement of primary motor cortex in motor imagery: A neuromagnetic study. Neuroimage. 1997;6(3):201-208. doi:10.1006/nimg.1997.0286
2. Pfurtscheller G, Neuper C. Motor imagery activates primary sensorimotor area in humans. Neurosci Lett. 1997;239(2-3):65-68. doi:10.1016/S0304-3940(97)00889-6
3. Lo YN, Louis ED, Fook-Chong S, Tan EK. Impaired motor imagery in patients with essential tremor: A case control study. Mov Disord. 2007;22(4):504-508. doi:10.1002/mds.21356

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MDS Abstracts - https://www.mdsabstracts.org/abstract/impaired-imagery-of-tremor-in-patients-with-essential-tremor/