Our memories are annoyingly glitchy. Names, dates, birthdays, and the locations of car keys fall through the cracks, losses that accelerate at an alarming pace with age and in neurodegenerative diseases. Now, by applying electromagnetic pulses through the skull to carefully targeted brain regions, researchers have found a way to boost memory performance in healthy people. The new study sheds light on the neural networks that support memories and may lead to therapies for people with memory deficits, researchers say.
Transcranial magnetic stimulation, or TMS, is an increasingly popular therapy for psychiatric disorders that involves placing fist-sized coiled magnets on the scalp to stimulate different brain regions. Although researchers aren't sure why or how it works, it does appear to benefit some patients. Last year, for example, the U.S. Food and Drug Administration approved several TMS devices for treating migraines and depression. Studies have also shown that the technique can improve performance on different types of memory tests, but few researchers have investigated whether benefits persist after stimulation stops or looked at how stimulation affects the brain's memory circuits, says Joel Voss, a neuroscientist at Northwestern University’s Feinberg School of Medicine in Chicago, Illinois.
To begin answering those questions, Voss and colleagues recruited 16 healthy adults who were between the ages of 21 and 40. Using structural and functional MRI scanners, the researchers made detailed maps of the subjects’ brains, locating the hippocampus, a brain region key to memory, and its connections to another brain region called the parietal cortex. Functional MRI scans of brain activity show greater neuronal traffic between the two areas when people are performing memory-related tasks, and lesions between the areas can result in severe deficits in the ability to remember proper labels for things, such as matching names with faces, Voss says.
After administering a baseline memory test to the participants, the team began the brain stimulation sessions, focusing rapid-fire magnetic pulses on a fingertip-sized area toward the back of the skull for 20 minutes per day. The location of the stimulation differed slightly among individuals, based on brain scans showing their unique connections between the parietal cortex and hippocampus, Voss explains. After 5 days, the participants were given a 24-hour break from stimulation and asked to repeat the memory test. People who had received TMS improved their scores by roughly 20% to 25%, whereas controls who had not received the stimulation showed little to no improvement, Voss and his colleagues report online today in Science. Brain scans also showed increases in the amount of communication between the hippocampus and parietal cortex in subjects who received the stimulation. The more the two regions worked together, the better people performed on the memory test, Voss says.
The study is "very cool" because it supports scientists' growing understanding of the hippocampus as one vital node in a larger memory network spread throughout the brain, says Alvaro Pascual-Leone, a neurologist at Harvard Medical School in Boston who was not involved in the research. It also "elegantly shows" for the first time that stimulation on the surface of the skull can reach deep brain structures (such as the hippocampus) and increase communication and synchrony throughout the network, ultimately improving performance on a memory test, he says.
Whether TMS will someday be a cure for memory deficits is "a reasonable question to ask, but it's not answered yet," Pascual-Leone says. Scientists will need to conduct many studies in people with illnesses such as Alzheimer's disease to determine whether stimulation is effective for them—the disease might do so much damage that stimulation doesn’t work or even has deleterious effects, Pascual-Leone says.
The fact that the TMS stimulation used in the study had such a targeted effect on memory networks makes Voss optimistic that the technology could counteract memory loss. In an upcoming trial, Voss and colleagues will study the electromagnetic stimulation's effect on people with early-stage memory loss, he says.
Studies like this one raise the ethical issue of whether it's a good idea to use such technologies on healthy people to change a normal brain, Pascual-Leone notes. For one thing, it’s unclear how long the improved recall lasts, or if changes to the brain could be permanent. "How long does it take to go back?" he wonders. And although the prospect of memory enhancement may be enticing for those of us who are constantly losing our keys, it's possible that boosting function in one cognitive skill will take away from another, he says. "The brain may be a zero-sum game in that sense."