INFOGRAPHIC: Effects of transcranial direct current stimulation on enhancing motor training gains in people with Parkinson's Disease

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In people with Parkinson's disease (PwPD) occurs the progressive degeneration of dopaminergic neurons in the substantia nigra pars compacta, which are responsible for producing dopamine—a neurotransmitter that, among other roles, regulates cortical activation. That degeneration leads to reduced excitatory activation of the thalamus over the primary motor cortex and increased inhibitory activation of the basal ganglia on the pedunculopontine nucleus and the reticular formation. The result is hypoactivation of regions involved in motor responses, particularly those responsible for balance and locomotion control in PwPD 1. Research has shown that physical interventions can positively affect the symptoms of PD 2. In recent years, evidence has accumulated that transcranial direct current stimulation (tDCS) can be used as a non-invasive procedure for treating PwPD, showing promising results in enhancing the effects of motor training3. The manuscript presents the theoretical background on the potential of anodal tDCS to increase motor training gains in PwPD. Transcranial direct current stimulation involves the application of a low-intensity electrical current (typically 2–4 mA) to the scalp, aiming to modulate the activity of cortical and subcortical neurons. The mechanism of tDCS operates through a subthreshold shift in neuronal resting potential, achieved via anodic or cathodic stimulation. In PwPD, anodal stimulation is more commonly employed, as it induces depolarization of neuronal membranes. Additionally, studies have observed increased dopamine release in the caudate nucleus and striatum 4. Further supporting its therapeutic potential in making the neural environment more conducive to activation when recruited. Previous studies conducted by our group demonstrated that combining tDCS with aerobic exercise amplifies the benefits in acute (single session) 5. Additionally, preliminary unpublished data indicate positive effects of chronic (multi-session) tDCS when combined with body balance training protocols. These studies highlighted the potential of tDCS to enhance outcomes in gait impairments when paired with aerobic exercise, as well as in postural instability when combined with balance training (see Beretta et al. 3, for a review). As tDCS is a technique still under development, the literature lacks a standardized protocol for stimulation parameters that optimize its effectiveness in treating PwPD. Consequently, it is usual to find studies with similar objectives employing varying stimulation intensities and target regions. Despite these differences, studies investigating the application of anodal tDCS during motor training in PwPD—regardless of intensity or location—suggest that electrical stimulation acts antagonistically to hypoactivation by facilitating the use of neural resources during the intervention. Thus, it seems to enhance the training outcome, leading to more effective results 5.