Cortical Beta and Gamma Power Changes During Working Memory Encoding Reflect Neurophysiological Signatures of Cognitive Impairment Specific to Parkinson’s Disease

Introduction: While many Parkinson’s disease (PD) and essential tremor (ET) patients receive deep brain stimulation (DBS) for motor dysfunction, cognitive impairment (CI) is a common feature of both disorders that remains largely unexplored as a neuromodulatory target. In PD, decreased dopamine elevates beta power in cortico-striato-thalamo-cortical circuits, and we have previously shown that cognitively impaired PD patients exhibit a smaller decrease in corticostriatal beta power during working memory encoding. ET motor and cognitive symptoms may stem from similar parallel deficits in fronto-cerebellar circuits, but whether ET and PD CI exhibit distinctive neurophysiological signatures remains unknown.

Methods: Thirteen PD patients and fourteen ET patients undergoing awake DBS surgery with electrode trajectories traversing the dorsolateral prefrontal cortex (DLPFC) participated in this study. Subjects completed a 2-back working memory task where they identified whether a presented word matched the word presented two trials prior. Using preoperative neuropsychological assessments, we categorized patients as cognitively impaired if they scored < 9th percentile on at least two task measures. Through cluster-based permutation analyses, we identified frequencies where cognitively impaired and unimpaired subjects exhibited significantly different spectral power during working memory encoding.

Results: Compared to cognitively normal PD patients, impaired PD patients exhibited a smaller decrease in beta power and a smaller increase in gamma power in the DLPFC during working memory encoding. No time-frequency clusters of significant difference between impaired and unimpaired ET patients were identified. A two-way ANOVA found a significant effect of disorder on beta power changes and a significant interaction effect of CI and disorder on gamma power.

Conclusion : DLPFC beta power decreases and gamma power increases during working memory encoding. Power changes were attenuated in cognitively impaired PD patients but not in impaired ET patients. These findings support distinctive circuitry involved in cognitive deficits in these disorders and could inform neuromodulation strategies for their respective cognitive symptoms.