Targeted Electrical Brain Stimulation Improves Memory in Older Adults

Four 20-minute sessions of brain stimulation are sufficient to improve short-term and long-term memory function in older adults, suggests a new study published in Nature Neuroscience. The research, conducted by a team at Boston University, used a group of 150 healthy volunteers aged between 65 and 88.

Targeting brain activity

“The prevalence of Alzheimer’s is surpassed greatly by a larger segment of the general population that suffers from age-related cognitive decline,” said the study’s senior author Robert Reinhart, a professor at Boston University, in a press conference. Memory is one cognitive system that is particularly affected by aging and was the target of Reinhart’s new paper.

Our memory is thought to be divided into short- and long-term systems. Both are underpinned by neuronal circuits that signal through oscillatory electrical activity. Reinhart’s lab uses a technique called transcranial alternating current stimulation (tACS) to alter the rhythms of targeted brain activity patterns.

By varying the level of stimulation and the brain areas it is targeted at, the team hope to be able to use tACS for applications as diverse as understanding how attention works in the brain, relieving the impulses behind obsessive–compulsive disorder and – in this latest work – improving memory in older adults.

Putting memory to the test

Reinhart and his team recruited their subjects for four consecutive days of short tACS sessions, while measuring how they performed on a battery of memory tests. Volunteers were asked to listen to and then immediately recall from 5 lists of 20 words.

Their results showed that by applying weak, high-frequency stimulation to the prefrontal cortex, long-term memory could be improved selectively, meaning volunteers improved their ability to recall words from the start of the list.

Conversely, applying the same gentle stimulation to the parietal cortex, this time at a low frequency, improved volunteers’ ability to remember the words that they had heard most recently. This, said Reinhart, suggests that it selectively improved short-term memory. These two brain areas were chosen as they have been implicated in memory processes in previous research.

Over the four-day period, Reinhart said, “We watched the memory improvements accumulate over time with each passing day, so that improvements in short-term and long-term memory were observable at the one-month time point.” This shows that even a limited stimulation protocol can produce lasting effects on memory, say the authors.

The team believe that these enduring benefits are due to an effect called entrainment, which is where brain waves tend to naturally synchronize to external stimuli such as music or flickering lights. “The consequence of changing the timings at which brain cells activate,” said study first author Shrey Grover, a PhD student in Reinhart’s lab, “is that it induces this process of plasticity.” Neuroplasticity refers to the constant change in connection strength between our neurons in response to brain activity. The team believe that by inducing plasticity, the brain stimulation can lock-in changes that are more enduring than the alterations to brain oscillations that tACS initially produces.

Clinical studies planned

They also provided data suggesting that volunteers who had the poorest memory at baseline and showed the most rapid initial improvements in their memory had the most significant and durable memory enhancements one month later. “We think this bodes well,” said Reinhart, “for transferring our protocol to a full clinical study in people with Alzheimer’s disease who are suffering from more severe impairments.”

The researchers hope to next measure the technique’s effect on measures on everyday cognitive tasks to expand tACS’s potential real-world applications.

Reference: Grover S, Wen W, Viswanathan V, Gill CT and Reinhart RMG. Long lasting, dissociable improvements in working memory and long-term memory in older adults with repetitive neuromodulation. Nat. Neurosci. 2022. doi: 10.1038/s41593-022-01132-3