The simplicity of the technique of transcranial direct current stimulation (tDCS)
can be observed as it consists of a current generator and two electrodes that are
placed over the scalp and can deliver weak direct currents. Despite its simplicity,
the field of non-invasive brain stimulation has had a rapid and exponential increase
in the past 10 years. It is in fact an “old, new” technique – as external brain electric
stimulation with electric currents has been recurrently described in medical literature
since ancient times (Brunoni et al., 2011b), although the technique was reappraised
only recently after the seminal studies of Priori et al. (1998) and Nitsche and Paulus
(2000), which showed that it could modify cortical excitability in a polarity-dependent
manner, i.e., while anode induces neuronal depolarization and thus activation of neural
networks beneath the electrode, the cathode induces the opposite effects (i.e., hyperpolarization
and consequent inhibition). From 1998 onward, several studies showed that tDCS modulates
a plethora of behavioral, sensorial, or motor effects according to parameters of stimulation
and subjects’ characteristics. Two important characteristics of tDCS – the duration
of its effects and its safety – have attracted the attention of a large number of
scientists and clinicians. Indeed tDCS effects can last for several hours beyond the
period of stimulation in some cases (Fregni and Pascual-Leone, 2007) and induce changes
in brain biochemistry (Rango et al., 2008). In addition, studies in experimental animals
show that tDCS is safe (Liebetanz et al., 2009), and a systematic review found that
adverse effects are mild and transient (Brunoni et al., 2011a).
Another important characteristic of tDCS is that it can potentially be adapted for
home-use, which would bring about an important advance to the therapeutic field of
brain stimulation (Priori et al., 2009). From a methodological perspective, it has
a reliable sham method as compared with, for instance, rTMS. Such characteristics
(ease of use, low cost, portability, safe, potent effects) render tDCS a sound device
for further clinical research, either as a substitutive therapy or a complementary
treatment for other interventions (drug therapy, physical therapy, psychotherapy,
and so forth) (Brunoni et al., 2011c), especially considering patients that are unable
or unwilling to receive standard treatments.
Nonetheless, tDCS clinical trials are still in their infancy. One possible reason
is that tDCS use requires basic knowledge on a neural basis of electrical current
fields and neuroscience. In fact, an incorrect electrode montage or stimulation of
the “wrong” area might generate non-specific or even negative effects (Datta et al.,
2010; Mahmoudi et al., 2010; Mendonca et al., 2011). Therefore, it is more difficult
to observe positive clinical effects by serendipity – also because tDCS has presently
no standard clinical use, all effects can only be observed through research. Further,
tDCS trials are methodologically complicated due to attrition, since the protocols
demand daily stimulation for 1–4 weeks. A possible solution would be to use portable
devices – specific tailored caps could be assembled in for targeting only the desired
scalp areas. Furthermore, tDCS may be a device with little commercial interest compared
to other medicines or even rTMS – in fact, by being too affordable and with a limited
possibility of patenting, more robust business ventures are easily discouraged to
develop tDCS commercially. Not surprisingly, at the present time tDCS research is
mainly conducted in academic settings, usually with public grants. Nevertheless, this
scenario could rapidly change depending on whether effective parameters of stimulation
and findings are shown in clinical research. Finally, a simple reason to explain the
current stage of development of tDCS is timing. Clinical trials, as well as the reporting
and dissemination of results, usually has a significant time span.
Considering such challenges, we proposed a Research Topic in Frontiers in Psychiatry,
named The frontiers of clinical research on tDCS in neuropsychiatry. The results were
surprisingly positive, with 22 articles from new and experienced research groups that,
considered together, represent a robust contribution to the advancement of the field.
We are also grateful to all the reviewers – many of them productive researchers in
the field – for their invaluable help in making suggestions that ultimately improved
the manuscripts significantly. The articles hereby presented are divided in five main
sections – in the first one, the neurobiological effects of tDCS are reviewed (Medeiros
et al., 2012) and original articles on the electrophysiological effects of tDCS on
visuo-spatial working memory (Heimrath et al., 2012), human color discrimination (Costa
et al., 2012), and motor cortical excitability (Chaieb et al., 2012) are presented.
The second section contains original articles exploring the behavioral effects of
tDCS such as on the saccade task (Kanai et al., 2012), automatic verbal processes
(Vannorsdall et al., 2012), working memory (Jones and Berryhill, 2012), emotional
processing (Nitsche et al., 2012) and production of untruthful responses (Fecteau
et al., 2012), and one review by Brasil-Neto (2012) on tDCS’ effects in learning and
memory. The third section shows original articles on the clinical effects of tDCS
on tinnitus (De Ridder and Vanneste, 2012), major depressive disorder (Blumberger
et al., 2012; Knotkova et al., 2012) and pain (DosSantos et al., 2012), and reviews
its effects on Alzheimer's disease (Hansen, 2012), stroke (Adeyemo et al., 2012; Madhavan
and Shah, 2012), and smoking addiction (Fraser and Rosen, 2012). The fourth section
presents computational theoretical models of tDCS for further application in clinical
practice (Datta et al., 2012; Neuling et al., 2012; Sadleir et al., 2012). The last
section reviews the application of spinal tDCS (Cogiamanian et al., 2012).
Moving tDCS research from bench to bedside has significant challenges. Nevertheless,
there are opportunities for tDCS development as pharmacotherapy is reaching an efficacy
and safety plateau and there are still unmet demands for the treatment of several
disorders. tDCS therefore represents an interesting alternative that can offer additional
therapeutic gains with a minimum of or no side effects. Whether the obstacles of clinical
trials are solved or not, this collection of articles presented in this Research Topic
provides promising evidence that tDCS could rise in the near future as a novel therapeutic
tool and have a significant impact n psychiatry and neurorehabilitation.