Transcranial Alternating Current Stimulation Boosts Consolidation of Long-term Memory

August 2, 2018

By Will Boggs MD 

NEW YORK—Closed-loop slow-wave transcranial alternating current stimulation (tACS) enhances consolidation of long-term memory, researchers report.

"By measuring the electric fields on the scalp, we can detect changes in the underlying brain state that indicate when recent experiences are being reactivated during sleep," said Dr. Praveen K. Pilly from HRL Laboratories, in Malibu, California.

"We can enhance this process by prolonging this state through electrical stimulation, which results in a better integration of recent experiences into a coherent and robust memory," he told Reuters Health by email.

Research suggests that recent experiences are reactivated during slow-wave sleep and that this reactivation allows for the integration of these experiences into long-term memory. tACS has been shown to enhance this sleep-dependent consolidation process.

Dr. Pilly and colleagues used a novel closed-loop tACS protocol to target endogenous slow-wave oscillations during sleep and investigated whether enhancing these oscillations would influence overnight memory performance in 16 healthy volunteers.

Transcranial direct current stimulation (tDCS) had no reliable effect on performance, the team reports in the Journal of Neuroscience, online July 23.

In contrast, long-term memory consolidation improved after closed-loop tACS, and this improvement correlated with increases and subsequent decreases in endogenous slow-wave oscillation power and spindle power.

The longer the stimulation-induced power increase was maintained, the greater was the improvement in post-sleep memory performance.

"There are recent reports in the literature questioning the efficacy and reliability of transcranial alternating current stimulation (tACS) in entraining neural oscillations and improving overnight memory performance," Dr. Pilly said. "As opposed to open-loop stimulation protocols, our novel closed-loop method provides evidence for modulating endogenous oscillations to improve long-term memory generalization. Due to the closed-loop aspect, we can take advantage of the endogenous oscillations to increase the efficacy of the otherwise difficult task of stimulating through the scalp."

"Our results provide evidence for the role of slow-wave oscillations during stages 2 and 3 of non-rapid eye movement (NREM) sleep in sleep-dependent memory consolidation for long-term retention and generalization," he said. "The method could benefit patients suffering from sleep deficits, especially related to slow-wave sleep, who show concomitant impairments in learning and memory."

"There is a lot of scope for optimizing learning and memory, and possibly even restoring memory function, by intervening during sleep and other offline periods with a completely non-invasive stimulation system that is driven by endogenous neural oscillations," Dr. Pilly said. "More work needs to be done to understand the limits of this technology and the implications for other experiences that may not have received the benefits of stimulation."

Thomas Reed, who recently reviewed the mechanisms of transcranial electrical stimulation, is currently completing his PhD in closed-loop brain stimulation at the University of Oxford, U.K. He told Reuters Health by email, "The idea of using tDCS delivered during a behavioral task to improve memory encoding, combined with tACS during sleep to improve memory consolidation, is an interesting idea, and the results observed on the second day of testing do lead to the conclusion that this stimulation protocol is able to improve memory consolidation."

"However, as participants either received sham stimulation or combined tDCS and tACS, it is difficult to distinguish whether the behavioral changes would not have been observed if participants had only received tDCS whilst completing the task on the first day or only received tACS during sleep," he said.

"The fact that tACS is not delivered continuously and is instead only delivered when slow waves are detected is a big positive for this closed-loop design, as it allows the overall duration of tACS to be increased," Reed said. "When combined with the realistically styled training, this study demonstrates how non-invasive brain stimulation techniques could be used in a real-world setting."

He noted, "This paper used tACS that was in phase with the brain's endogenous oscillations; it would have been nice to see an extra condition in which stimulation was administered out of phase with endogenous oscillations - possibly disrupting consolidation and further proving the link between slow-wave oscillations and consolidation."

"Closed-loop stimulation such as this is going to become a more common practice as the technology develops," Reed added. "We already see closed-loop stimulation in deep brain stimulation for treatment for Parkinson's disease, and that technology is now being used with noninvasive brain stimulation in examples such as this one. The next step for this type of technology is building a more intelligent stimulation protocol that is able to adapt to what the individual is doing (probably using machine learning) by changing stimulation parameters and giving them a 'mental assist.'"

Dr. Pilly has a pending patent application on the closed-loop tACS method. DARPA (Defense Advanced Research Projects Agency) and the Army Research Office supported this research.

SOURCE: https://bit.ly/2AzuWCV

J Neurosci 2018.

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