Slow oscillatory transcranial direct current stimulation (so-tDCS) during slow wave sleep has no effects on declarative memory in healthy young subjects

Background: The manipulation of specific brain oscillations by applying transcranial electrical stimulation techniques in order to enhance memory processes during sleep has become an intriguing field of research. A seminal study found a positive effect of slow-oscillatory transcranial direct current stimulation (so-tDCS) on sleep-dependent consolidation of declarative memories. Since then several studies have tried to replicate this result with inconsistent findings. Objective/Hypothesis: This study aimed to reexamine effects of so-tDCS on declarative memory observed in young participants based on a previously described stimulation protocol used in elderly subjects. Methods: 23 healthy participants (mean +/- SD: 23.2 +/- 1.9 years; 13 women) completed a word-pair test and a sequential finger tapping test before and after sleep. Participants received anodal so-tDCS bifrontaly at a frequency of 0.75 Hz or sham stimulation during NREM sleep N2, following a double-blind, placebo controlled, counterbalanced, randomized crossover design. Data were analyzed with respect to possible effects of stimulation on memory performances, sleep staging, spindle densities and EEG power in eight frequency bands. Results: Stimulation had no significant effect on sleep dependent memory consolidation or on sleep macro- and microstructure. Independent of stimulation, procedural memory performances increased and declarative memory performances decreased overnight. This decline was less pronounced when participants had more than one learning opportunity. Fast parietal but not slow frontal spindle densities diminished from baseline to stimulation-free intervals under both stimulation conditions. Conclusion: The present study could not reproduce the results of the seminal study in young subjects, but it is consistent with results observed in elderly subjects using the same protocol. Irrespective of stimulation, re-encoding opportunities in the word-pair test had an impact on memory strength and retrieval performance. (C) 2019 Elsevier Inc. All rights reserved.