Introduction: Whether learning a new movement or refining a pre-existing movement pattern, changes typically occur in the speed and/or accuracy at which the task is performed. For example, learning to play a song on a musical instrument involves learning the correct hand positions to create specifics notes or sounds and then these hand positions must be performed in the correct order and with the correct timing or rhythm to for these notes to become a song. However, simply observing the movements and hearing the notes isn’t enough to learn how to play the song. Practice is required to induce neuroplastic changes in the strength of connectivity between task-associated neurons within and between various regions of the brain (Andersen et al., 2017) and these changes improve the rate and clarity at which neural signals, or motor commands, reach the muscles involved in performing the specific task. This ultimately leads to motor skill learning. In motor skill learning, changes in performance begin to occur immediately upon beginning to practice a skill and these changes continue to take place between practice sessions in both fast/early as well as slow/late stages (Dayan & Cohen, 2011). During the early stages of learning, relatively large (compared to baseline) and rapid changes in skill performance occur in as little as a single practice session and the primary motor cortex (M1) and cerebellum (CB) both demonstrate learning-related neuroplastic changes (Dayan & Cohen, 2011; Kleim et al., 1998; Penhune & Steele, 2012). The motor cortex (M1) is the brain region where processed sensory information converges and is transformed into descending motor commands responsible for activating skeletal muscles and creating movement (Hamano et al., 2021). It has been well established that, in response to practice, task-related neurons within M1 undergo rapid reorganization and demonstrate long-term potentiation (LTP) mediated changes in synaptic strength (Classen et al., 1998; Hess et al., 1996; Karni et al., 1995; Pascual-Leone et al., 1995; Sanes & Donoghue, 2000). While various brain regions contribute to the generation, storage, and refinement of a motor command (Hamano et al., 2021; Huda et al., 2019), M1 appears to be crucially involved in the performance of movements which rely on fast and precise motion (Krakauer & Mazzoni, 2011; Penhune & Steele, 2012). The cerebellum serves a role in error-dependent learning and has been suggested to be particularly important during the early stages of skill learning when error-rates are highest (Cantarero et al., 2015; Penhune & Steele, 2012). When performing a motor action, sensory information regarding the motion is relayed back to the brain and filtered through the cerebellum where, through multiple closed-loop circuits, it refines the activity of M1 and improves upon the intent of the movement (Shadmehr et al., 2010; Spampinato et al., 2020). This movement refinement is largely facilitated by the cerebellar Purkinje cells which provide inhibitory input to the thalamocortical circuits responsible for exciting M1 (Hansel & Linden, 2000; Hirano, 2018). When learning a new task, there are high levels of discoordinated activity and asynchronous firing within task-specific circuitry of the cerebellum, thalamus, and M1 which leads to neuroplastic changes and long-term depression (LTD) within the cerebellar-thalamic synapses (Collingridge et al., 2010; Hanley, 2018; Hansel & Linden, 2000; Hirano, 2018; Huganir & Nicoll, 2013). LTD of the inhibitory cerebellar projections to the thalamus results in reduced inhibition of the thalamus which ultimately increases excitability of M1. This process is believed to contribute to the early stages of cerebellar-based error correction and improved motor task performance (Schlerf et al., 2012). Transcranial direct current stimulation (tDCS) is a form of non-invasive, subthreshold electrical brain stimulation that has been shown to modulate excitability within the underlying cortical structures and facilitate motor learning (Bhattacharya et al., 2022; Ehsani et al., 2016; He et al., 2020; Knotkova et al., 2019; Nitsche & Paulus, 2000, 2001; Nitsche et al., 2005; Paulus, 2011). Typically, tDCS is performed by passing a weak electrical current (~1-2 mA) between two or more electrodes positioned on the scalp. The direction of current flow between the electrodes determines the stimulation polarity (positive, Anodal; negative, Cathodal) (Rawji et al., 2018). Anodal stimulation (a-tDCS) is usually associated with subthreshold depolarization that increases neuronal excitability, whereas cathodal stimulation (c-tDCS) is associated with hyperpolarization that reduces neuronal excitability (Galea et al., 2009). The a-tDCS and c-tDCS excitability shifts are attributed to LTP and LTD-like mechanisms and are thought to alter spontaneous firing rates within affected neurons (Stagg et al., 2018; Wang et al., 2023). This makes tDCS an interesting tool for studying motor learning since the mechanisms underpinning these neuroplastic changes are attributable to spike-timing dependent activity between neurons (Kronberg et al., 2017; Stagg et al., 2018).
