Objective: To better understand the mechanisms of how repetitive transcranial magnetic stimulation (rTMS) may improve cognitive function in Parkinson’s disease (PD).

Background: Cognitive dysfunction and dementia are major issues facing PD patients and current therapies are minimally effective. Studies in healthy adults and individuals with PD and normal cognition suggest that rTMS may improve cognitive function but the mechanisms of this effect are unknown. Gamma (40-80 Hz) oscillatory activity is thought to be a key neurophysiological component of normal cognitive functioning. Notably, frontal gamma activity has been demonstrated to increase following rTMS in younger adults performing a working memory task (N-back). Our objectives for the present study were to determine whether this may be a mechanism of cognitive enhancement in PD patients and thus serve as a potential therapeutic biomarker.

Methods: Participants were 11 healthy older adults and 26 PD patients with normal cognition. Whole-head MEG recordings were acquired during an N-Back task before and after bilateral high-frequency rTMS applied to the dorsolateral prefrontal cortex. Oscillatory activity in the gamma-band was analyzed in frontal areas using wavelet-based time-frequency methods and evoked power was derived for each subject. Mass univariate, non-parametric statistical analyses were performed across the entire time-frequency space and corrected for multiple comparisons using cluster size metrics, p < .05.

Results: 1. At baseline, no group difference was found in frontal evoked gamma. 2. In contrast to published results in younger adults, frontal gamma increases with higher cognitive demand at baseline were absent in PD and control groups. 3. In healthy adults, rTMS increases evoked gamma power proportionally to cognitive demands. However, no significant effect of active rTMS compared to sham stimulation was found on gamma power in PD.

Conclusions: Our results suggest that (i) gamma power modulation may not be related to therapeutic effects in PD and (ii) rTMS may be less effective in older adults and PD patients due to limitations in gamma modulation. Differences from published studies in young adults suggest both aging and PD pathology contribute to this effect.

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MDS Abstracts - https://www.mdsabstracts.org/abstract/does-repetitive-transcranial-magnetic-stimulation-rtms-affect-cognitive-function-in-parkinsons-disease-by-modulating-high-frequency-brain-oscillations-a-magnetoencephalography-meg-study/