Overnight Noninvasive Brain Stimulation May Improve Memory

Damian McNamara

August 22, 2018

Noninvasive brain stimulation technology delivered during sleep may improve memory, new research suggests.

Investigators at the University of New Mexico found closed-loop transcranial alternating-current stimulation (tACS) delivered overnight to augment endogenous slow-wave (SW) oscillations in humans improves generalized memory.

"Our results show that it is possible to non invasively modulate neural oscillations relevant to memory consolidation during sleep," study investigator Praveen K. Pilly, PhD, senior research staff scientist at the Information and Systems Science Laboratory at the HRL Laboratories Center for Human Machine Collaboration, Malibu, California, told Medscape Medical News.

The study was published online July 23 in the Journal of Neuroscience.

Enhancing a Natural Process

The investigators note that the transfer of memories from the hippocampus to the neocortex for long-term storage is thought to be enabled by synchronization of these parts of the brain during sleep.  

In an attempt to enhance this natural process, the investigators used a closed-loop tACS system to match the phase and frequency of ongoing SW oscillations during sleep.

Previous research has shown that more traditional transcranial direct-current stimulation (tDCS) has a stable, dose-dependent effect in improving individuals' ability to find hidden targets in complex scenes. However, these studies have not examined changes in performance overnight.

"This raises an intriguing question," the investigators note. "If tDCS-enhanced performance was paired with tACS enhanced sleep-dependent consolidation, could behavioural improvements be further augmented?"

The current study included 21 volunteers from the University of New Mexico and surrounding community; 16 individuals completed the study. The mean age was 22 years.

Participants were trained and tested on a realistic visual discrimination task in which they had to detect potentially threatening hidden objects and people, such as explosive devices and enemy snipers.

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.

The volunteers could stop the task at any time if the stimuli were too uncomfortable or made them anxious. None chose to do so.

Improved Memory

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].

In contrast, 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.

"Because of this null effect, and previous studies showing no overnight change in performance related to tDCS in this target detection task, it is likely that all overnight behavioral effects can be attributed to SW tACS," the investigators write.

Clinical trials to validate the technology for the restoration of memory function in different patient populations, especially those with sleep deficits, are warranted, Pilly said.

In addition, "more studies need to be done to assess the efficacy of the intervention for various other kinds of learning tasks for longer-term retention — on the order of months and years — as well to ensure there are no unwanted side effects."

The closed-loop design of the new technology could be advantageous in these future applications, Pilly added, because it "would definitely enable faster transition to commercial products for healthy users."

Not Ready for Prime Time

"This study is very technical and of uncertain direct relevance to the clinical practice of memory care providers," Donn D. Dexter, MD, assistant professor of neurology at the Mayo Clinic College of Medicine and Science in Rochester, Minnesota, told Medscape Medical News when asked to comment on the research.

"It was quite small with only 16 of 21 paid volunteers completing the study," said Dexter, who is also a fellow of the American Academy of Neurology.

However, he added, "I do agree with the final statement that more work in this field could be valuable."

The Defense Advanced Research Projects Agency (DARPA) and the Army Research Office supported this study. Any opinions, findings, and conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of the DARPA or the Army Research Office. Pilly is an employee of HRL Laboratories. Dr Dexter has disclosed no relevant financial relationships.

J Neurosci.  Published online July 23, 2018. Abstract

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