Transcranial Direct Current Stimulation and Post-COVID-19-Fatigue

Abstract

frequency (~300 Hz) oscillations in GP and STN. This evoked activity resembled a sinusoid, growing in the shape of a sigmoid throughout the burst, and decaying exponentially after the last burst stimulation. This novel sigmoid-exponential-sinusoid (SES) model successfully fit the data with R-squared values of up to 0.9. Theories of the origin of this phenomenon involve local patterns of inhibition and excitation in STN, or interactions between STN and connected structures, such as the pallidum. Our detected high-frequency oscillations in STN and GP from pallidal stimulation, complementing those previously observed from STN stimulation, support the hypothesis that the phenomenon is generated by a loop involving both structures. No oscillatory response was detected when stimulating or recording in thalamus. This implies that the phenomenon is likely unique to BG, making it a promising biomarker. It has also been suggested as a possible feedback signal for closed-loop DBS, which our SES model parameters provide convenient quantitative measurements for. Finally, our findings in a dystonic patient imply that this phenomenon is not unique to Parkinson's disease, but likely generalizable to other movement disorders and perhaps even healthy subjects. Abstract Attention at encoding shapes long-term memory: Attended stimuli are better remembered than unattended stimuli. This memory selectivity effect is smaller when attention is switched between different tasks and stimuli, suggesting a tight connection between selective attention and selective memory. In order to investigate the neural substrate of this connection, we stimulated brain areas associated with selective attention during a study phase and tested incidental recognition memory in a subsequent offline test phase. Specifically, we stimulated the superior parietal cortex (associated with the dorsal attentional network), the inferior pari-etal cortex (associated with the ventral attentional network) and the dorsolateral prefrontal cortex (associated with top-down cognitive control) with transcranial direct current stimulation (tDCS). The study phase consisted of a task switching procedure where pictures and words were presented simultaneously and participants had to switch between a picture and a word categorization task. Memory for attended and unattended stimuli was assessed in a subsequent recognition test. We hypothesized that a relative increase in activity in the dorsal network would boost selective attention while increased activity in the ventral network would impair selective attention, and in consequence, we expected to find corresponding effects on memory: Enhanced selective attention should lead to higher memory selectivity, while impaired selective attention should lead to lower memory selectivity. Our results replicated that task switching reduced memory selectivity. However, we found no significant effects of tDCS. This result is surprising because our study was based on established stimulation protocols. However, they are in line with a growing number of studies that failed to find tDCS effects. Abstract Background: The cortical response to transcranial magnetic stimulation (TMS) has notable inter-trial variability. One source of variability is the influence of the phase of neural oscillations on TMS-induced potentials (TIPs). Here, we investigate the effect of brain oscillatory activity on TIPs in both healthy controls (HC) and patients with major depressive disorder (MDD). Methods: We analyzed data from 33 MDD and 35 HC participants, who had received single pulse (SP) TMS over the left DLPFC. After pre-processing and artifact removal, the Laplacian transformation was applied to the TMS-EEG signal, and then the data were filtered into delta, theta, alpha, and beta frequency bands. Each trial was classified by the pre-TMS phase (3 ms before onset) which was separated into 5 categories: 0 , 90 , 180 , 270 , and random phase. The effect of SP TMS on each trial was measured by the amplitude of TIP components, represented by cortical evoked activity (CEA). Results: In both the HC and MDD group, in all frequency bands, CEA was significantly larger for 270 and/or 90 compared with positive and/or negative peaks pre-TMS phase (corrected p-value <0.001). The random phase resulted in higher activity compared with positive and/or negative peaks, but not 270 and 90. In HC 270 and 90 in the beta band and 270 in the theta band caused larger cortical reactivity than random phase (p-value <0.001). In MDD, 90 in the beta band and 270 in the alpha band resulted in a greater CEA than the random phase (p-value <0.001). Conclusion: Our results suggest the phase of brain oscillations can modulate the TIP in both HC and MDD. Aligning the TMS pulse with a specific phase of EEG might reduce variability in the neurophysiological measurement, and subsequently can produce more robust long-lasting plastic changes and increase the efficacy of neuro-modulatory treatment.