Targeted Right Medial Temporal Lobe tDCS and Associative Spatial and Non-Spatial Memory

Advances in finite element modeling afford targeting of cortical and subcortical brain regions with transcranial electrical stimulation, presenting the possibility for neuromodulating medial temporal brain regions. We examined transcranial direct current stimulation (tDCS) targeting the hippocampus and parahippocampus, brain structures implicated in flexibly forming novel memory associations. We expected active (versus sham) stimulation to alter aspects of relational memory, specifically the ability to develop flexible relations in a paired-associates task, and develop flexible spatial associations during a virtual navigation task. Fifty participants received either active or sham tDCS, delivered via five electrodes positioned at scalp sites Fp1, Fp2, AF3, F4, and P8. Participants completed two tasks shown to selectively modulate hippocampal activation in previous research: first, a paired-associates task testing the learning of simple and complex conjoined associations, and second, a virtual navigation task involving the encoding and subsequent retrieval of novel large-scale spatial memory. The study phase of the paired-associates task was done online during stimulation, and the virtual navigation and test phase of the paired-associates task were done offline following stimulation. No statistically significant influences of active versus sham tDCS were found on the paired-associates task, during navigation, or during spatial memory retrieval. Moreover, Bayes factors consistently demonstrated anecdotal to moderate support of the null hypothesis across all tasks and dependent measures. The present results suggest potential challenges in selectively targeting subcortical structures with tDCS, prompting continuing research in this domain. © 2018, Springer International Publishing AG, part of Springer Nature (outside the USA).