## 3 Brain Stimulation Techniques  
Noninvasive brain stimulation (tDCS) has been studied extensively in recent years, aiming to improve neural plasticity, and is, therefore, a technique used in the treatment of depression. Along with transcranial magnetic stimulation (rTMS), it is being studied and used as a treatment for major depression, especially for patients resistant to conventional treatment methods [24]. Noninvasive electrical brain stimulation therapy is increasingly considered to be safe, tolerable, and practical, either as monotherapy or in combination with other interventions, such as medication and psychotherapy. Moreover, because stimulation parameters can be targeted to specific brain areas, the noninvasive neurostimulation technology is also in the process of shifting the paradigm towards a precision-oriented framework that takes into account \"knowledge of the underlying brain circuits. at the base of complex cognitive, emotional and self-reflexive functions\", to guide individualized patient-oriented treatments [6]. HD-tDC consists of four small disk electrodes arranged around a central electrode that helps to determine the direction of the unifocal modulation. Thus, greater individualization proves helpful in adapting to the possible perturbations offered by the assembly, and by using MRI techniques, there can be pre-existing mapping for the optimal placement of the electrodes. In this review, we are particularly interested in the use of tDCS for depression. Regulatory authorities have already approved other therapies, such as rtMS treatment, for clinical use in many countries, including the United States, Israel, Canada, and Brazil [25]. Still, access to such equipment is limited, resulting in several limitations for patients who can benefit from it. On the other hand, tDCS devices are more accessible to the general public due to their low cost and portability. In principle, tDCS is a noninvasive form of transcranial stimulation (Figure 1) that alters the excitability of cortical tissue by applying a weak direct current (0.5-2 mA) through electrodes placed on the scalp. Several tDCS protocols have been developed and tested in clinics, varying the number of sessions, the current intensity, the stimulating electrode size, or the electrodes' localization. In the treatment of depression, the most common tDCS protocols [26; 27; 28] imply the positioning of the anodal electrode on F3 (left dorsolateral prefrontal cortex), while the reference electrode is placed on F4 (right dorsolateral prefrontal cortex), Fp2 (right orbitofrontal cortex), or F8 (right inferior frontal gyrus) (Figure 1). The therapeutic application of electrical stimulation dates to the first century, when placing a live electric fish on the scalp was used to alleviate headaches through its strong electric current. [PERSON] et al. [29] documented that applying electric currents to the scalp could alleviate symptoms of melancholy. The beneficial effects of tDCS have been reported in the treatment of psychiatric (especially depression and schizophrenia) and neurological disorders, as well as in the rehabilitation of cognitive, motor, and sensory functions after a stroke [27]. In 1804, [PERSON] first documented that applying electric currents to the scalp could help alleviate symptoms of melancholy [30]. Noninvasive brain stimulation (NIBS) refers to techniques that stimulate the brain without the need for any implants. One such method, tDCS, involves applying a weak electric current through electrodes placed on the scalp. This modulates the resting potential of neuronal membranes but does not directly induce neuronal firing [31]. Similarly, transcranial alternating current stimulation (tACS) uses a sinusoidal alternating current that is delivered at a specific frequency to influence the brain's natural oscillatory patterns [32]. Another form of NIBS, transcranial random noise stimulation (tRNS), involves an alternating current with random frequencies and amplitudes [33].