Sending weak electrical current into the brain for 20 minutes a day for four days in a row reversed declines in working and long-term memory that come with aging, scientists reported Monday in Nature Neuroscience.

The researchers found that the effects lingered even after the electricity was turned off. When they tested subjects a month later, many of the improvements from the brief sessions of brain stimulation remained.

By zapping the brain in precise regions with unique frequencies of alternating current “we could improve either short-term or long-term memory separately,’’ psychology researcher Robert Reinhart of Boston University, the study’s lead author, told reporters. “And with this intervention across four consecutive days, we could change memory and watch the benefits accumulate over those days, which is striking.’’

The findings provide some of the strongest support yet for a method called transcranial alternating current stimulation, or tACS, as a potential means for boosting mental functions essential to navigating the world and understanding one’s own place in it — functions that tend to deteriorate the older people get.

“This is a really elegantly designed study,’’ said Katharina Klink, a brain scientist at the University of Bern in Switzerland, who was not involved in the research. “These are such small currents that are being used, so to see effects on memory function after one month of not having any stimulation done to the brain, that’s quite impressive.’’

Many hurdles remain, however, for tACS to become a potential therapy. Larger studies performed over longer periods of time will be required to prove it’s safe, to understand how durable the effects are, and to see whether the artificial word-recall task used in the experiment translates into real-world benefits.

Unlike more invasive forms of brain stimulation that require brain surgery to install chips and implants, tACS involves little more than wearing a modified swimming cap studded with dozens of electrodes. The technology emerged from research into how neuronal networks strung widely throughout the brain coordinate to form, store, and retrieve memories through rhythmic oscillations in neuronal firing known as brain waves. Different parts of the brain pulse and thrum at different frequencies.

And over the last two decades, neuroscientists across many labs have found evidence that synchronizing these vibrations across brain regions, similar to how the wind, brass, and string sections of an orchestra tune up before a performance, is a critical component of transferring information from one part to another — the building block of memory.

But as people age, researchers have found, those oscillations tend to slow down, putting them out of sync with other parts of the brain. While still physically connected, functionally, brain circuits become uncoupled. The result is more difficulty remembering the name of someone you just met (short-term memory) or recalling the location where you last placed your keys (long-term memory).

Researchers like Reinhart hope to use tACS to bring those oscillations back up to the right speed so that far-flung regions of the brain can talk to each other again, like truck drivers hundreds of miles apart hopping on the same ham radio frequency.

In 2019, Reinhart and colleagues demonstrated that tACS could boost memory in older adults, but only for about 50 minutes. And they only attempted to improve working, or short-term, memory with slow-frequency theta waves.

In the new study, Reinhart and Shrey Grover, a PhD student in his lab, aimed to improve both working memory and long-term memory by delivering two different types of electrical currents to two distinct locations in the brain.

For their experiments, the BU scientists tested the working memories of 120 older adults (aged 65 to 88). People were read a list of 20 words at the rate of one word per second, and then were asked to recall as many of the words as they could. They did that five times with five different lists of words, in each case for a total of about 20 minutes.

Then the participants were divided into three groups. As they performed the same memory test, one group got sham stimulation — they wore a cap but no current was flowing through it — while the other two received either a high frequency aimed at the prefrontal cortex or a low frequency focused farther back in the brain.

Individuals who received the lower frequency (4 Hz) were better able to recall words from the end of the list — the last words the researchers read — indicating enhancements to storage in working memory. Participants who received the higher frequency (60 Hz) improved in their ability to remember words from the beginning of the list, reflecting boosts to long-term memory. All together, people who received electrical stimulation recalled four to six more words, compared to the placebo group — a 50 percent to 65 percent boost in recall.

Those changes were still significant — three to four more recalled words versus placebo — when the study participants returned a month later for another round of testing. Notably, the worse the memory of the older people at the start of Reinhart’s study, the more tACS improved it.

“That bodes well for transferring this intervention over to a proper clinical study with people with Alzheimer’s disease who are suffering from more severe memory impairments,’’ Reinhart said.

This latest work is especially appealing because it suggests it might be possible to treat different kinds of memory deficits just by tuning different brain frequencies. Parkinson’s patients, for example, have issues with short-term memory, while people who suffer from medial temporal lobe epilepsy have trouble with long-term memory.

Whether or not the findings might one day yield practical applications, they add compelling new evidence to one of neuroscience’s most enduring and contentious debates.

Is there just one system that the brain uses to make and house memories, or are there two distinct systems — one of short-term, or working, memory, and one for long-term memory? Behavioral data from psychological studies as well as experiments in non-human primates conducted in the ‘80s and ‘90s have long supported the latter idea. But more recent work has called the two-system models of memory into question. The Boston University team’s latest experiments throw more weight behind the two-system model.

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