Objective: To reveal how the pattern of evoked responses differs between deep brain stimulation (DBS) of the ventral intermediate nucleus of the thalamus (VIM) and DBS of the subthalamic nucleus (STN).

Background: Although DBS has become a routine therapy, its mechanism of action remains elusive. Recent research suggests that modulation of distant brain areas could be crucial to the clinical effect. Little is known, however, about the areas affected by DBS and about the differences between DBS targets.

Methods: We performed magnetoencephalographic (MEG) recordings in 9 patients with Essential Tremor (ET) and in 11 patients with Parkinson’s disease (PD) who were implanted with DBS electrodes targeting the VIM and the STN, respectively. Recordings were obtained the day after electrode implantation. DBS at 5 Hz was applied via an external stimulator and we assessed magnetic fields evoked by DBS. Seven datasets from each patient group were of sufficient quality for source analysis, which was performed using dipole fitting software (XFit, Elekta Neuromag) and individual boundary-element head models.

Results: VIM and STN DBS led to short latency cortical responses around 1 ms, probably representing antidromic activation of corticofugal fibers. The pattern of medium latency cortical responses following VIM DBS consisted of peaks at 13, 40, 77, and 116 ms on average. The associated dipoles were localized within the central sulcus in all cases, and the direction of current flow alternated between anterior and posterior flow. Three patients showed an additional response in the cerebellum at ~56 ms. Responses evoked by STN DBS peaked at 4, 11, and 27 ms on average. While most dipoles localized to pre- or postcentral gyrus, the spatial distribution was less homogenous compared to VIM stimulation and included additional sites, such as cingulate cortex.

Conclusions: Responses evoked by 5 Hz DBS are highly consistent within but clearly distinct across DBS targets. The response to VIM stimulation is reminiscent of a damped beta oscillation and iterative interaction between primary motor and primary somatosensory cortex. Cortical modulation mediated by STN DBS appears less strong and less homogeneous than for thalamic stimulation, possibly due to the lower number of subcortico-cortical fibers. The modulation of cingulate cortex might underlie some of the non-motor effects of STN DBS.

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MDS Abstracts - https://www.mdsabstracts.org/abstract/brain-responses-evoked-by-thalamic-and-subthalamic-deep-brain-stimulation/