Anodal tDCS Enhances Verbal Episodic Memory in Initially Low Performers
2017
The left
dorsolateral prefrontal cortex(DLPFC) is involved in encoding and retrieval of
episodic memories, and thus, is frequently targeted in non-invasive brain stimulation paradigms, aiming for its functional modulation. Anodal
transcranial direct current stimulation(tDCS), that boosts neuronal excitability in stimulated cortical areas, has been found to increase
cognitive skillsdifferentially, depending on the initial performance. We hypothesize that the benefit of tDCS on verbal
episodic memorycan be extrapolated from the participants' baseline performance. In the present randomized, double-blind, parallel group study, healthy young adults (n = 43) received either real anodal or sham tDCS over their left DLPFC during the encoding phase of a verbal
episodic memorytask. Forty words were presented visually thrice with immediate vocal retrieval after each block and an additional delayed recall. We conducted a moderation analysis to test the modulating effect of initial
episodic memoryretrieval, adjusted for primacy and recency effects, on delayed recall under real or sham stimulation. Despite the absence of a significantly beneficial tDCS effect at the group level, we found that the number of remembered midlist words in the first retrieval significantly moderated the stimulation effect in such a way that initially low performers experienced the highest gain from real stimulation. These results suggest that anodal tDCS to the left DLPFC improves memory functions only so far. While only marginal stimulation-induced gains occur in cognitively unimpaired populations, greater stimulation benefits might be expected in individuals with clinically relevant deficiencies in the verbal
episodic memorydomain.
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