Transcranial direct current stimulation (tDCS) enhances reconsolidation of long-term memory

A new and weak memory trace undergoes consolidation to gain resistance against interfering stimuli. When an encoded memory is recalled, it becomes labile and another round of consolidation, or reconsolidation, is required to restore its stability. Transcranial direct current stimulation (tDCS) is a non-invasive method of altering cortical excitability. The aim of this study was to examine the effects of tDCS on the reconsolidation of long-term verbal memory. Participants (n = 15) memorized words in the encoding session, then reactivated the memory of the words 3 h later using an old-new recognition task under anodal, cathodal and sham stimulation to the left dorsolateral prefrontal cortex (DLPFC). Finally, after another 5 h, they performed another round of the old-new recognition task and rated their confidence. Anodal tDCS during the second session resulted in significantly more words recognized in the third session as compared to cathodal and sham stimulation. Cathodal tDCS did not affect the recognition performance compared to sham stimulation. These results cannot be attributed to differences in response times and confidence ratings, as they were comparable in all conditions. In order to study whether the activation of a memory was crucial for the enhancing effects of anodal tDCS, a group of controls (n = 15) did not perform the recognition task in the second session but still underwent stimulation. Contrary to the main group, anodal stimulation did not enhance the memory performance for the control group. This result suggests that anodal tDCS over the left DLPFC can enhance the reconsolidation of long-term memory only when the memory has been reactivated.