Episodic memory improvements due to noninvasive stimulation targeting the cortical–hippocampal network: A replication and extension experiment

Introduction: The distributed cortical network of the human hippocampus is important for episodic memory. In a previous experiment, noninvasive stimulation of the hippocampal-cortical network applied for five consecutive days improved paired-associate learning measured after the stimulation regimen via cued recall (Wang et al., Science, 2014, 345, 1054). This finding has not yet been directly replicated. Furthermore, evidence for long-lasting effects of stimulation on paired-associate learning was obtained by analyzing relatively small subsamples (Wang & Voss, Hippocampus, 2015, 25, 877) and requires further evaluation. Methods: Sixteen healthy young adults participated in this replication study using the same experimental design as the original study. Participants received 1 week of active stimulation and 1 week of sham stimulation, with memory assessments occurring at the beginning (pre) and end (post) of each week. Assessments included the paired-associate task used in the original study, as well as a long-term episodic memory retention task in order to test the hypothesis that increased paired-associate learning could come at the cost of accelerated long-term forgetting. Change in memory scores was evaluated within (pre vs. post) and across (active vs. sham) weeks. Results: Similar to Wang et al., paired-associate learning was significantly improved after 1 week of active stimulation but not after 1 week of sham stimulation. We found no evidence that stimulation increased long-term forgetting for either week. Conclusion: These findings confirm the beneficial effects of stimulation on episodic memory that were reported previously and indicate that stimulation-related gains in new learning ability do not come at the price of accelerated long-term forgetting.