Brain stimulation technology improves memory overnight

A new research suggests that a noninvasive brain stimulation technology delivered overnight during sleep may help in the improvement of memory.

Findings, published in the Journal of Neuroscience demonstrate that closed-loop transcranial alternating-current stimulation (tACS) delivered overnight for the augmentation of endogenous slow-wave (SW) oscillations in humans improved generalized memory.

Nicholas Ketz, Information and Systems Sciences Laboratory, Center for Human-Machine Collaboration, HRL Laboratories, Malibu, California, and colleagues investigated the potential for augmenting endogenous slow-wave (SW) oscillations in humans with closed-loop transcranial alternating current stimulation (tACS) with an aim toward enhancing the consolidation of recent experiences into long-term memory.

The transfer of memories from the hippocampus to the neocortex for long-term storage is thought to be enabled by synchronization of these brain parts during sleep, note the researchers. A closed-loop tACS system was used to the phase and frequency of ongoing SW oscillations during sleep, in an attempt to enhance this natural process.

For the study, Sixteen (three female) participants were trained presleep on a target detection task identifying targets hidden in complex visual scenes. During post-training sleep, closed-loop SW detection and stimulation were used to deliver tACS matching the phase and frequency of the dominant oscillation in the range of 0.5–1.2 Hz. Changes in performance were assessed the following day using test images that were identical to the training (“repeated”), and images generated from training scenes but with novel viewpoints (“generalized”).

The 6-day protocol included an initial orientation session, three nights spent in a sleep laboratory, and two-afternoon follow-up testing sessions. When a participant correctly indicated "target present," they saw a short video depicting the mission progressing as planned with a voiceover praising the participant for choosing correctly. If the participant incorrectly indicated that a target was present, a voiceover chastised them for delaying the mission or insulted them by indicating they were cowardly. If the participant correctly indicated no target present, feedback was given that the mission was progressing as planned. If participants incorrectly indicated no target present when there was one, they saw a video showing the consequence of missing the target.

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Key Results:

• Daytime, active tDCS stimulation demonstrated no significant difference in performance compared with the sham condition [t(32.9) = 0.881; P = .385],, and there was no difference in performance for the repeated vs generalized images [t(29.18) = −1.067; P = .295].


• when participants receive tACS stimulation overnight they showed improved performance in detecting targets the next day vs when they did not receive overnight stimulation. This finding suggests an integration of recent experience into a more robust and general memory.


• With the overnight tACS treatments, a t-test comparing the post- vs presleep change in F1 score on generalized images for active vs sham conditions showed a significant difference [t(15) = 2.79; P = .014].


• The researchers found spatiotemporal biomarkers in sleep EEG spectral power that correlate with overnight performance changes. The biomarkers can be used to control the number of stimulations that must be applied for each user in future applications.


• Results showed no significant effect of tDCS with a 1.0-mA current dose, even though a nonsignificant difference in the appropriate direction was observed.

"This human study demonstrates the use of a closed-loop noninvasive brain stimulation method to enhance endogenous neural oscillations during sleep with the effect of improving consolidation of recent experiences into long-term memory," concluded the authors.

For further reference log on to https://doi.org/10.1523/JNEUROSCI.0273-18.2018