Graduation Date

Fall 12-16-2022

Document Type

Dissertation

Degree Name

Doctor of Philosophy (PhD)

Programs

Neuroscience

First Advisor

Dr. Tony W. Wilson

Abstract

An exponential rise in the use of transcranial direct current stimulation (tDCS) to modify the human brain and behavior has been observed over the last 20 years, with studies often yielding mixed outcomes. Such heterogeneity is partially attributed to the lack of a concise understanding of the underlying mechanisms. Though considered to exert modulatory effects by changing neuronal membrane polarization and synaptic efficacy, the interactive effects of offline tDCS and the neural underpinnings of cognition, both at the regional and network-level, are yet to be fully understood. Using High-Definition tDCS (HD-tDCS), Magnetoencephalography (MEG), a range of well-established cognitive paradigms, and advanced analytical techniques, this work attempts to bridge critical gaps in the field. Specifically, by employing unique stimulation protocols such as varying combinations of stimulated regions, polarity, and tasks that differed considerably in their cognitive demands, we examined variable possible outcomes of HD-tDCS in healthy young adults. First, we identified the lateralized modulatory effects of anodal stimulation of higher-order regions, dorsolateral prefrontal cortices (DLPFC), on basic visuospatial processing (Chapter 1). Next, with the same stimulation protocols, we observed changes in fronto-parietal integration which provided important insights into the endogenous network-level dynamics, during a logical reasoning task (Chapter 2). Finally, we extended our approach with a polarity-based montage and probed the differential effects of anodal/cathodal occipital HD-tDCS on neural responses serving attentional reorientation which accorded well with the anodal excitation and cathodal inhibition dichotomy, in the visual cortices (Chapter 3). Overall, the findings of these studies provide important mechanistic insights into how the neuromodulatory effects of HD-tDCS are strongly influenced by the stimulation protocols and show a clear division between lower and higher-order cognitive constructs. Amidst several claims of tDCS-induced cognitive enhancement in the literature, our work cautions against those and further sets the ground for new studies aiming for standardization and optimization of the stimulation protocols. The resulting enhanced comprehension of tDCS-driven frequency-specific modulations of neural responses underlying different cognitive processes can lead to frequency targeted stimulation such as transcranial alternating current stimulation (tACS) and facilitate the development of therapeutic interventions for clinical indications.

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