The present review collects the most relevant empirical evidence available in the literature until date regarding the effects of transcranial direct current stimulation (tDCS) on the human motor function. tDCS in a non-invasive neurostimulation technique that delivers a weak current through the brain scalp altering the cortical excitability on the target brain area. The electrical current modulates the resting membrane potential of a variety of neuronal population (as pyramidal and gabaergic neurons); raising or dropping the firing rate up or down, depending on the nature of the electrode and the applied intensity. These local changes additionally have shown long-lasting effects, evidenced by its promotion of the brain-derived neurotrophic factor. Due to its easy and safe application and its neuromodulatory effects, tDCS has attracted a big attention in the motor neurorehabilitation field among the last years. Therefore, the present manuscript updates the knowledge available about the main concept of tDCS, its practical use, safety considerations, and its underlying mechanisms of action. Moreover, we will focus on the empirical data obtained by studies regarding the application of tDCS on the motor function of healthy and clinical population, comprising motor deficiencies of a variety of pathologies as Parkinson’s disease, stroke, multiple sclerosis and cerebral palsy, among others. Finally, we will discuss the main current issues and future directions of tDCS as a motor neurorehabilitation tool.
The central nervous system (CNS) works thanks to the communication between more than 100,000 millions of neurons, whose activity and networking is modulated by chemical and electrical processes . Across history, humans have been trying to alter the electrical brain processes to enhance human’s brain function, for the treatment of psychopathologies and for a better understanding of the brain physiology. For example, in the antiquity, modulation of the electrical processes of the brain started with the use of electrical impulses of torpedo fishes applied directly on the CNS, for therapeutic purposes . In 1746, Musschenbroek (1692–1761) used Leyde jars and electrostatic devices to treat neuralgia, contractures and paralysis. The discovery of biometallic electricity and the invention of the voltaic battery augmented the interest in the therapeutic effects of galvanism. Afterwards, Duchenne de Boulogne (1806–1875) upgraded the electrotherapy with volta and magnetofaradaic apparatuses. Fortunately, in the past Century, the technological advances and its integration in health sciences have let us go from uncontrolled and unsafe interventions with side effects to well-controlled, more effective and safe stimulation devices .
Currently, the most used stimulation devices can be divided into invasive techniques, such as deep brain stimulation (DBS), and non-invasive brain stimulation (NiBS) techniques, whose most representative methods are transcranial magnetic stimulation (TMS) and transcranial direct current stimulation (tDCS) .
Although results are variable , DBS has reported positive results over the motor function, especially on the motor symptoms of Parkinson’s disease. However, DBS is a technique that needs the implantation of the electrodes on the stimulated area, which is associated with the typical risk derived from surgery, as infections. Therefore, there is an increasing tendence on the search for non-invasive brain stimulation techniques, which can modulate the motor function avoiding those risks.
Hence, NiBS are characterized for its easy and safe use and relatively cheap price, demonstrating also successful results in the treatment of neurological and psychiatric alterations . In the last decades, TMS has been the most researched and developed neuromodulation technique. TMS generates fast changes in the magnetic field delivering electrical currents through the brain, allowing the specific modulation of the cortical excitability through the initiation of action potentials . Multiple studies have already shown its efficacy and safe use for the treatment of multiple pathologies , serving also as a useful tool for the functional location of brain areas, especially regarding the motor cortex [8, 9]. However, TMS requires the participation of the participant, and due to its functioning, it is difficult to perform a sham condition, which is highly desirable especially in the research field. In addition, TMS produces in most of the cases undesirable side-effects, as headache .
Therefore, the tDCS technique is attracting a strong interest in the neuroscience research field. tDCS has supposed a revolution in the last 15 years of research, solving most of the disadvantages of TMS . tDCS is a neuromodulation tool consisting on a battery connected to two electrodes, the anode and cathode, which are placed directly over the brain scalp and over extracephalic regions. The current flows between both electrodes and induces the depolarization or hyperpolarization of the membrane of the underlying neurons, which depends of the anodal or cathodal nature of the electrode , altering the neuronal excitability resulting in the modification of the brain activity . This device is completely portable, as it is provided by built-in rechargeable battery with duration of approximately 6 h stimulation time at 1 mA (0.5–1.5 W of power consumption), and needs approximately 7 h for complete recharging. In addition, including battery, it has a weight of 0.8 kg. Its portability is one of the biggest advantages of tDCS in the context of NiBS. Therefore, tDCS can be considered as a suitable complementary technique on motor rehabilitation therapy, allowing its application in different contexes, during the motor training and even combined with aerobic exercise [13, 14].
This non-invasive brain manipulation has opened the doors for a variety of potential treatments for the major neurological and psychiatry diseases , as depression , schizophrenia , Obsessive–Compulsive disorder  and addictions , among others.
However, motor functions are the major target for clinical and non-clinical studies regarding tDCS, serving mainly as a potential tool in post-stroke rehabilitation , but also in pathologies like Parkinson’s disease . In addition, numerous studies have shown that tDCS produces changes in the brain plasticity processes, generating long-lasting effects that enhances even further its applicability in the neurorehabilitation field [22, 23].
The purpose of this review is to assess the current and future stage of tDCS regarding its use on the human motor function, identifying the empirical cues that point out its benefits as well as its potential limitation, providing a comprehensive framework for designing future research in the field of brain stimulation with tDCS and human motor rehabilitation. The present review is divided in four parts. The first part is based on a detailed definition on what we know about tDCS, the protocols of montage and parameters of stimulation, comprising the mechanisms of action of tDCS, what differs tDCS from other non-invasive neuromodulation techniques, and the main need to-know safety standards. Given the conciseness of this first part, we will present the recent studies focusing exclusively on the empirical data obtained from the use of tDCS in the human motor function, regarding, in the second part, healthy humans; in the third part, its clinical application on deteriorated human motor functions across different pathologies as Parkinson disease, stroke and cerebral palsy. Finally, in the fourth part of this review, we will discuss the main current issues of tDCS applied on the human motor function.[…]