Raphael Thys
Many of us will struggle to remember things as we get older. A gentle form of brain stimulation might help, according to new research. The approach appears to boost the memories of older people and help them remember lists of words.
The technique can be adapted to improve either long-term or short-term memory, and the benefits appear to last for at least a month. It’s the first time this type of brain stimulation has been shown to have such long-lasting effects on human memory, say the researchers behind the work.
“This was a very short intervention which produced both an immediate effect and a very durable one,” says Marom Bikson, a neural engineer at the City College of New York, who was not involved in the study. “More research is needed, but if this works out it could be in every doctor’s office … and it could eventually be something that people use at home.”
Electric brains
“It’s an uncomfortable fact of life that as we age, we all become a little bit forgetful,” says Rob Reinhart, a neuroscientist at Boston University, who led the work. Reinhart studies brain networks for functions like cognition, attention, and memory—and how they seem to decline with age and as a result of some disorders.
Brain cells communicate with each other using electrical pulses, and brain networks and regions have their own pulses of electrical activity. Growing evidence suggests that applying electrical stimulation to these networks can change the way they work, potentially strengthening connections between brain regions.
To find out if this approach could improve memory, Reinhart and his colleagues turned to a form of brain stimulation called transcranial alternating current stimulation. tACS, as it’s known, allows gentle pulses of electricity to the skull via electrodes embedded in what is essentially a swimming cap.
Although the approach delivers electricity to regions of the brain, the doses are too low to trigger brain cells to fire. Instead, tACS modulates the way cells fire, says Reinhart. He prefers to describe his use of tACS as brain modulation rather than brain stimulation. “They’re noninvasive, safe, extremely weak levels of alternating current,” he says.
The team used a modern, high-definition form of tACS, which allows researchers to target small regions of the brain. The group chose to focus on two regions of the brain that are known to be involved in memory: part of the prefrontal cortex at the front of the brain, which is involved in long-term memory, and the inferior parietal lobe, a region toward the back of the brain thought to be involved in short-term memory.
Each of these two brain regions has its own characteristic pattern of electrical pulses of activity, or brain waves. In the first experiment, Reinhart and his colleagues delivered pulses of activity to match the natural rhythms of each region—high frequencies for the prefrontal cortex and low frequencies for the parietal lobe.
Tingling, itching, and warming
The team recruited 60 volunteers, all between the ages of 65 and 88, who were divided into three groups. In a task, each person was read a list of 20 words and had to recall them later. While they were performing this task, a third of the group had the prefrontal cortex of their brain modulated, and another third had their parietal lobes targeted. The remaining third wore a cap of electrodes but didn’t receive any stimulation.
Those who did receive brain stimulation did not feel anything dramatic, says Reinhart. “When the current is running, you feel like a mild tingling or itching or poking or warming sensation,” he says.
The 20-minute session was repeated for four consecutive days. Over those four days, people who received brain stimulation improved in their ability to remember words. There was no such improvement among those who weren’t stimulated.
And the type of memory improvement depended on which brain regions had been stimulated. Those who had the front of their brains stimulated were better at remembering the first words in the list, suggesting their long-term memory had improved. Those who had their parietal lobes stimulated saw improvements in their short-term memory.
By the end of the four days, those who’d had their brains stimulated improved their performance by around 50 to 65% and remembered around four to six extra words from the list of 20, on average, says Reinhart. “It’s very impressive,” says Roi Cohen Kadosh, a cognitive neuroscientist at the University of Surrey, who was not involved in the study.
“We can watch the memory improvements accumulate … with each passing day,” says Reinhart, who, along his colleagues, published the findings in the journal Nature Neuroscience on Monday.
The greatest improvements were among those who had the worst cognitive function at the start of the study. This suggests that the technique might one day be helpful for people with memory disorders such as Alzheimer’s disease or other dementias, says Reinhart.
When Reinhart’s team swapped the frequencies, targeting the front of the brain with low frequencies and the back of the brain with high ones, there was no improvement in either short- or long-term memory. This suggests that the type of stimulation must match the natural brain waves in order to work.
Reinhart and his colleagues only checked in on their volunteers a month after they did the experiment, and they don’t know if the improvements lasted beyond that point. And while the study found that the volunteers were better at remembering words from a list, Reinhart doesn’t know if their memories improved more generally, or if the stimulation improved their lives in any way.
“The effects are really specific, and not something that would benefit someone who would want to improve their memory [more generally],” says Cohen Kadosh. He points out that people who want to remember things for an exam, for example, don’t just want to remember the first and last things they read—they need to remember everything. “We need to see if there is really an effect … in everyday life functions,” he says. Bikson agrees this is a valid concern—some “brain training” games promise to boost a player’s cognition, but research suggests that in fact players only get better at playing the game, and don’t see wider benefits. Reinhart’s approach, though, is different, he points out. “If you are stimulating brain networks that are generally involved in some aspect of cognition … that gives credence to the [idea that the benefits] could generalize,” he says.