tDCS Reduces False Memories Figure 2. Performance as indexed by number of false memories during sham, unilateral and bilateral stimulation. Columns represent the mean number of false memories and error bars indicate mean standard error. doi:10.1371/journal.pone.0004959.g002 Discussion Our results confirm the notion that modulating activity of the ATL with brain stimulation before or during a given cognitive task is an effective method to change memory processing [6]. In our study, we found evidence that anodal tDCS to the left ATL before the encoding and retrieval phase is effective in reducing false memories while maintaining veridical memory performance unchanged. Our findings support existing evidence that the left ATL is critical for semantic processing [14,15,16]. Interestingly, both unilateral and bilateral tDCS (as depicted in Fig. 1) were effective in reducing false memories (see Fig. 2). However, although not significant, the magnitude of effect after bilateral stimulation was larger than that of the unilateral stimulation. To explain this trend, two hypotheses need to be entertained: (1) bilateral tDCS increases the amount of current injected into the left anterior temporal lobe compared with unilateral stimulation because the larger contralateral reference electrode used in unilateral stimulation might increase electrical Table 1. Mean number of veridical memories during and after stimulation. Veridical Memories tDCS Bilateral Unilateral Sham doi:10.1371/journal.pone.0004959.t001 During 7.6 7.7 8.3 60.2 60.3 60.2 After 7.8 8.3 7.9 60.6 60.1 61.0 current shunt; or (2) bilateral tDCS (see Fig. 1) increases the excitatory effects on the left ATL due to the activity of transcallosal fibers. When compared with a previous 1 Hz rTMS study [6] using the same task and a similar methodology (except for the stimulation timing), our results might appear contradictory. This is because 1 Hz rTMS is associated with a decrease in cortical excitability [28] and anodal tDCS is associated with an increase in cortical excitability [26]. However, we believe that the two results are complementary as the mechanisms of action of these two techniques are quite different. In fact, 1 Hz rTMS induces a large and comparatively focal electrical current that is strong enough to inhibit the main circuits associated with semantic processing, which is hypothesized to increase literal skills [4]. The effects of tDCS are relatively more diffuse than rTMS – anodal tDCS increases activity in a large cortical area, which could unnaturally compete with the semantic centers associated with ATL. This defocusing effect could de-emphasize and reduce the efficiency of the main semantic processing circuits. However, because we did not use brain neuroimaging to study the mechanisms of action of DC stimulation in this study, we can only speculate on the mechanisms underlying the DC effects on false memories. Our hypothesis is based on the notion that individuals start off with more literal perception and memory systems, but that with maturity, conceptual (semantic) processing becomes more highly developed and efficient [17]. Although it seems adaptive to have a high-efficient circuit for semantic processing, it may also lead to false memories when the neural processing in this circuit becomes faster and more efficient than the other, more literal, memory-related neural circuits. In fact, we believe that the undeveloped brain (also perhaps in autism), produces fewer false memories, because the neural circuit related PLoS ONE | www.plosone.org 4 March 2009 | Volume 4 | Issue 3 | e4959