Brain Stimulation Leads to Long-Lasting Memory Improvement

Noninvasive electrical brain stimulation led to selective improvements in working and long-term memory in older adults that lasted for at least a month, a randomized double-blind study showed.

Among 150 people ages 65 to 88, investigational transcranial alternating current stimulation for 20 minutes over 4 consecutive days produced selective boosts in auditory-verbal working memory and long-term memory, reported Robert Reinhart, PhD, of Boston University, and colleagues in Nature Neuroscience.

Low-frequency modulation in the parietal cortex improved working memory on days 3 and 4, and at 1 month after the intervention. High-frequency activity in the prefrontal cortex bettered long-term memory on days 2 to 4 and 1 month later.

"We found that by applying extremely weak electrical current safely and noninvasively to the prefrontal cortex at a high frequency, we could selectively improve long-term memory in older people aged 65 to 88 years old without changing short-term memory," Reinhart said in a press briefing.

"And conversely, we found that we can apply the same kind of specialized alternating current, but this time now to the parietal cortex farther back in the brain at a low frequency and by doing this, we could selectively improve short-term memory in older people without changing long-term memory," he added.

"That is, based on the spatial location and the frequency of the electrical stimulation, we can improve either short-term memory or long-term memory separately," he noted.

The rate of memory improvement over 4 days predicted the size of memory benefits 1 month later, the researchers found. Moreover, people with lower baseline cognitive function experienced larger, more enduring memory improvements.

"Older people with poor general cognitive functioning at baseline coming into the experiment were the individuals who showed the largest improvements during both the intervention and the 1-month time point, which we think bodes well for transferring this intervention over to a proper clinical study of people with Alzheimer's disease who are suffering from more severe memory impairments," Reinhart observed.

The study involved 150 older adults who received electrical currents through a cap embedded with electrodes. Mean baseline scores of participants ranged from 25.45 to 27.4 on the 30-point Montreal Cognitive Assessment (MoCA) test. Scores of 18-25 on the MoCA suggest mild cognitive impairment, while scores of 26 or higher are considered normal.

Participants heard and were asked to recall five lists of 20 words. Targeting the inferior parietal lobule at 4 Hz boosted participants' ability to recall words from the end of the list, indicating working memory. Targeting the dorsolateral prefrontal cortex at 60 Hz improved recall from the beginning of the list, reflecting long-term memory storage.

"Statistically, the effects were moderate to large" and most participants experienced memory benefits, Reinhart said. "Specifically, 85% to 90% experienced the memory improvements during the intervention that were then predictive of the benefits we saw at the 1-month timepoint after the intervention."

The findings build on research from Reinhart's lab in 2019 that showed stimulating temporal and prefrontal brain areas simultaneously improved memory for at least 50 minutes.

That work reported short-lasting improvements in visuospatial working memory only, Reinhardt noted. In the new study, the researchers aimed to target additional memory systems and induce more long-lasting memory benefits.

"The advance of this work over and above that of our 2019 research was that we constructed a repetitive intervention, one where we're stimulating people 20 minutes each day for 4 consecutive days, unlike in 2019 where it was just a one-shot stimulation," he said.

While promising, the intervention still is in early stages, the researchers acknowledged. What's not clear is whether it has benefits for people with neurodegenerative diseases, especially those at risk for dementia.

But as people age, noninvasive brain stimulation may help improve daily activities and could be personalized based on functional or anatomical characteristics, possibly leading to more lasting effects, they suggested.

"There's been remarkable progress in the neurosciences over the decades on characterizing the brain circuits and networks that underpin our memory capacity, both short-term and long-term," Reinhart pointed out.

"There's also evidence showing that rhythmic or oscillatory brain activity patterns in certain regions are important for organizing and retrieving memory," he added. "What we need now are really innovative technologies that allow us to act on these large-scale oscillatory brain networks and determine whether it's possible to protect or even enhance memory for older people."

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