The Uncertain Outcome of Prefrontal tDCS

Abstract

Background Transcranial direct current stimulation (tDCS) is increasingly used in research and clinical settings, and the dorsolateral prefrontal cortex (DLPFC) is often chosen as a target for stimulation. While numerous studies report modulation of cognitive abilities following DLPFC stimulation, the wide array of cognitive functions that can be modulated makes it difficult to predict its precise outcome. Objective The present review aims at identifying and characterizing the various cognitive domains affected by tDCS over DLPFC. Methods Articles using tDCS over DLPFC indexed in PubMed and published between 2000 and January 2014 were included in the present review. Results tDCS over DLPFC affects a wide array of cognitive functions, with sometimes apparent conflicting results. Conclusion Prefrontal tDCS has the potential to modulate numerous cognitive functions simultaneously, but to properly interpret the results, a clear a priori hypothesis is necessary, careful technical consideration are mandatory, further insights into the neurobiological impact of tDCS are needed, and consideration should be given to the possibility that some behavioral effects may be partly explained by parallel modulation of related functions.

Does transcranial direct current stimulation enhance cognitive and motor functions in the ageing brain? A systematic review and meta- analysis

“…In many motor studies, TMS is used to precisely locate target areas in the motor cortex and directly measure tDCS-induced changes in cortical excitability through the amplitude of motor-evoked potentials (MEPs). Localization of nonmotor targets, in contrast, is much less precise and any cognitive task is likely to involve a number of cortical areas making it difficult to attribute performance changes to stimulation of a specific cortical region (Tremblay et al, 2014). These factors may have contributed to the lower effect size associated with studies of cognitive function.…”

“…However, a caveat to this conclusion is warranted. A recent review of 61 studies applying tDCS to the DLPFC found a lack of consistency in outcomes making causal conclusions between DLFPC stimulation and a particular cognitive function difficult (Tremblay et al, 2014). One reason for the inconsistency is that tDCS appears to simultaneously modulate activity in a number of brain areas subserving a variety of cognitive functions with the maximum field strength being located some distance (20-40 mm) from the target area (e.g., DLPFC) (Rampersad et al, 2014).…”

“…Such a review is important and timely because despite the claims being 9 made about the capacity of tDCS to enhance performance in young healthy adults, some recent reviews have seriously questioned the ability of tDCS to produce reliable physiological and cognitive performance effects Horvath et al, 2015;Tremblay et al, 2014). Further, our systematic review and meta-analysis compared the effectiveness of tDCS in enhancing cognitive versus motor task performance.…”

Effects of acute transcranial direct current stimulation in hot and cold working memory tasks in healthy and depressed subjects

“…The effects of tDCS over the prefrontal cortex in nonneuropsychiatric samples have yielded mixed results [25]. The lack of effects in the hot working memory task might also be explained by a "ceiling" effect in healthy samples that already adequately process emotional content, leaving little room for improvement for the task.…”

Stimulating minds to wander

“…Viewing the literature as a whole, there are concerns about the replicability of tDCS findings. For example, tDCS of left DLPFC, using the same electrode montage as the authors, is sometimes associated with increases in working memory capacity and other times with decreases (18). Moreover, Horvath et al (19) suggested that the cognitive effects of single-session tDCS may have been overstated and that neurophysiological effects may not extend beyond enhancement of motor-evoked potentials (20).…”