Supercharging the brain: enhancing motor learning through transcranial direct current stimulation and elucidating its neural effects

Transcranial direct current stimulation (tDCS)—a form of electrical brain stimulation, consisting of an excitatory anode and inhibitory cathode, which modulates neural excitability—has become a popular tool in neuroscience, especially in the context of accelerating learning. Despite considerable behavioural evidence of facilitation, little is understood about the neural processes that tDCS mediates. In the motor domain, tDCS implemented through the less explored bihemispheric (Bi-tDCS) montage (with anode over contralateral and cathode over ipsilateral cortex) has shown increased efficacy relative to unihemispheric tDCS—which has been conventionally attributed to inhibition of the ipsilateral hemisphere. The overall purpose of this thesis was to better understand the neural effects of Bi-tDCS in the context of motor learning, in particular the level at which it exerts its effects in the motor hierarchy. In Chapters 2 and 3, I demonstrate that Bi-tDCS affects both finger sequence and hand muscle synergy learning at a largely effector-independent level, such that behavioural advantages transfer to the untrained hand. This observation led me to hypothesise that the efficacy of Bi-tDCS may not be due to ipsilateral inhibition but, rather, the facilitation of plasticity in both hemispheres. Indeed, in Chapter 4, I observed that polarity reversal leads to similar behavioural facilitation as conventional Bi-tDCS, suggesting that the supposition that its efficacy is due to ipsilateral inhibition is an oversimplification. In Chapter 5, using multivariate pattern analysis of functional magnetic resonance imaging data, I demonstrate the existence of effector-independent neural representations in the normal brain and in Chapter 6, I show that tDCS increases effector-independent encoding bilaterally. Finally, in Chapter 7, I explore whether tDCS affects the structural brain. Collectively, these findings suggest that the conventional model of Bi-tDCS should be abandoned in favour of a framework that acknowledges the roles that both hemispheres play in fine motor learning