Slow Oscillatory Transcranial Direct Current Stimulation Improves Memory Consolidation During Restricted Nighttime Sleep

Samantha Riedy, PhD

In a recent study on restricted nighttime sleep presented at the 2023 SLEEP Annual Meeting, held June 3-7, in Indianapolis, Indiana, patients given slow oscillatory transcranial direct current stimulation (SO-tDCS) had an increased word recall after 2 hours of sleep compared with those on sham, who recalled fewer words after the sleep period.¹ These results demonstrate critical implications for the development of strategies during restricted nighttime sleep schedules, such as on military operations, when sustaining performance with insufficient sleep.

At baseline, immediately after learning the words (T0), patients correctly recalled 35.0(±9.2) words (mean ± SD). Post-awakening, patients in the sham group correctly recalled significantly fewer words compared with those in the SO-tDCS group (sham: -0.8 [±1.6] words; SO-tDCS: 0.3 [±1.3] words; F = 6.20, P = .02). Similarly, fewer words were recalled at 20 minutes after learning (T20) than 120 minutes post-awakening (T120) in the sham group compared with the SO-tDCS group (T20: -0.6 [±1.5] words; T120: -0.1 [±1.6] words; F = 4.67, P = .04).

In previous research, SO-tDCS has shown to improve memory consolidation of paired words encoded immediately prior to sleep onset during the first 30 minutes of nighttime sleep.¹ Thus, lead author Samantha Riedy, PhD, principal investigator, Behavioral Biology Branch, Walter Reed Army Institute of Research, and colleagues investigated whether SO-tDCS given during the second half of a restricted nighttime sleep period improved memory consolidation for previously encoded paired words learned and mitigated subsequent deficits in performance.

The study enrolled healthy patients (ages, 18-33; n = 27; women, n = 17) after 3 nights of baseline sleep in a laboratory. The patients were administered SO-tDCS (n = 12) or sham (n = 15) during the 2nd hour of a 2-hour sleep period, between the hours of 11PM and 3AM. Prior to the sleep period, the patients were shown a series of 55 word-pairs. They were then asked to recall only 46 of the pairs immediately after learning, and again at 20 minutes and 120 minutes post-awakening. A mixed-effects ANOVA analyzed the number of pairs that were recalled at T20 and T120 relative to T0. Any negative values indicated the patients forgot pairs post-awakening relative to the initial learning recall.

Patients in the sham group, on average, forgot words upon awakening relative to T0 (T20: -1.0 [±1.4]; T120: -0.6 [±1.7] words), compared with those in the SO-tDCS group who forgot few to no words at T20 (0.0 [±1.5] words). Notably, patients in the SO-tDCS group recalled more words at T120 than T0 (0.6 [±1.1] words) in comparison with the sham group.

In contrast, a study that reexamined effects of SO-tDCS on declarative memory in young participants showed that stimulation had no significant effect on sleep dependent memory consolidation or on sleep macro- and microstructure.² The study enrolled 23 healthy patients (mean years, 23.2 [±1.9]; women, 13) who completed a word-pair test before and after sleep and received SO-tDCS or sham stimulation. Notably overnight, procedural memory performances increased, memory performances decreased.

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REFERENCES
1. Riedy S, Doty TJ, Balkin T, et al. Slow Oscillatory Transcranial Direct Current Stimulation During Restricted Nighttime Sleep Enhances Memory Consolidation. Presented at: 2023 SLEEP Annual Meeting; June 3-7; Indianapolis, Indiana. Abstract 0062.
2. Bueno-Lopez A, Eggert T, Dorn H, Danker-Hopfe H. Slow oscillatory transcranial direct current stimulation (so-tDCS) during slow wave sleep has no effects on declarative memory in healthy young subjects. Brain Stimul. 2019;12(4):948-958. doi:10.1016/j.brs.2019.02.012