ReviewBattery powered thought: Enhancement of attention, learning, and memory in healthy adults using transcranial direct current stimulation
Introduction
The advent of transcranial brain stimulation has led to a proliferation of research on neurocognitive enhancement. The primary rationale for such studies is the development of new and potentially more effective tools for the treatment and rehabilitation of patients with neurologic and psychiatric diseases (Flöel, 2014, Kuo et al., 2014). Control groups of healthy adults are often included in many investigations in order to establish the potential clinical efficacy of brain stimulation. At the same time, there is growing interest in developing methods of neurocognitive enhancement for healthy adults, for example to accelerate learning and skill acquisition in complex tasks that would otherwise take very long to master (Clark et al., 2012). Although there are many neuroenhancement techniques, as discussed in other papers in this special issue, transcranial direct current stimulation (tDCS) is a low-cost, portable method that is particularly well-suited for practical applications in both healthy and clinical populations, and well-tolerated by participants.
tDCS involves the passage of a small current (typically in the range of 0.5–2.0 mA) through the scalp and skull to modulate brain activity. Evidence for the use of electrical stimulation on the nervous system, employing animals such as electric fish and eels that produce electricity, dates back more than 2000 years. Later, other sources of electricity were used and electrical stimulation of the brain was sporadically studied in the middle of the 20th century under the term “brain polarization” (Kellaway, 1946). However, tDCS research in its current form is a fairly recent endeavor, being revived at the turn of the millennium by Paulus, Nitsche, Antal, and other researchers based in Gottingen, Germany. Since then, a growing number of researchers have pursued cognitive tDCS studies, examining the effects of tDCS on learning and memory, attention and perception, language production and acquisition, problem solving and decision making, and even deception (Karim et al., 2010).
In this paper, we first review the methodological history and currently known physiological effects of tDCS, followed by a discussion of peer-reviewed research examining the effects of tDCS on cognitive processes in healthy adults. We review tDCS effects on three fundamental cognitive processes: attention, learning, and memory. Although tDCS studies have also been reported for other cognitive processes, we focus on these because they may mediate higher-order cognitive processes such as decision-making and problem solving. For example, enhanced executive attention (or reduced distractibility) leads to more accurate and less biased decision making (Clark et al., 2004, De Martino et al., 2006, Parasuraman and Manzey, 2010). Furthermore, individual differences in working memory and executive attention are known to mediate inter-individual variation in complex decision-making and problem solving (Engle, 2002). Research into the use of tDCS for enhancement of attention and memory may therefore inform efforts to improve the efficiency of higher-order cognitive processes. Such research may also lead to the development of new treatments for clinical populations and highlight avenues for advancement of tDCS into a viable clinical tool. The use of tDCS in healthy adults is also of relevance to efforts to accelerate learning and enhance performance in such domains as education (Wlodkowski, 2003), the military (Clark et al., 2012, Nelson et al., 2014), and other work and everyday settings (Parasuraman, 2011, Parasuraman et al., 2012).
Section snippets
Methodological history of tDCS
The utility of electric current as a therapeutic technique dates back to antiquity. As early as 43 CE, Roman physician Scribonius Largus reported the application of electrical discharge from the Mediterranean torpedo fish for the treatment of headache, gout, and hemorrhoids (Kellaway, 1946). Not surprisingly, this strange treatment did not catch on at a large scale and was soon forgotten. The use of electric sea creatures did briefly resume more than 17 centuries later, however, when Dutch
Physiological effects of tDCS
For both intracranial and transcranial electrical stimulation of the brain, the polarity of the current is an important factor that influences its effects on neural activity, with positive polarization (anodal) at the scalp inducing excitation of the underlying cortex (Fig. 1) and negative polarization (cathodal) at the scalp resulting in the opposite effect (Bindman et al., 1964, Landau et al., 1964, Purpura and McMurtry, 1965). In a compelling demonstration of the influence of polarity on the
tDCS for enhancement of learning and memory
A major goal of educational efforts is to develop techniques for enhancement of learning and to promote better retention. tDCS has the potential to help in such efforts. As is well known, learning and memory involve both explicit (e.g. declarative) and implicit (e.g. procedural) processes, and enhancement of either or both may contribute to improved learning and retention (Squire, 1982). Accordingly, we describe tDCS studies of explicit and implicit learning separately. We begin with an
tDCS for enhancement of working memory, attention, and perception
We have focused this review of tDCS effects on learning and memory thus far because many high-order cognitive processes involve these more basic cognitive functions. Furthermore, learning itself depends on the more fundamental processes of working memory, attention, and perception. Accordingly, we review tDCS effects on these cognitive functions. Methodological parameters for the research reviewed in this section can be found in Table 2.
Discussion
Research examining the modulation of cognition using tDCS is one of the most rapidly advancing fields in cognitive neuroscience today. Recent studies have demonstrated significant, often strong, effects of tDCS on cognitive processes that, in many cases, are relevant to both clinical and non-clinical populations. Implicit memory, explicit memory, working memory, attention, and perception are all affected by tDCS, and some of these effects are profound.
The literature reviewed in this article has
Acknowledgments
This research was supported in part by AFOSR/AFRL grant FA9550-10-1-0385 to R.P. and V.C. was supported by R21MH097201, R21AA021201, P20GM103472, R01DA026505, and R01MH085010.
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