Researchers who study memory have had a thrilling couple of years. Some have erased memories in people with electroshock therapy, for example. Others have figured out, in mice, how to create false memories and even turn bad memories into good ones.
Today, another “No way, really?!” study gets added to the list. Scientists have boosted memory skills in healthy volunteers by zapping their brains with weak electromagnetic pulses.
The memory gain was fairly small — not enough for most of us to notice in our everyday lives, the researchers say. But even a modest improvement could be meaningful for people with conditions that damage memory, such as a stroke, heart attack, traumatic brain injury or Alzheimer’s disease.
“This memory network that we targeted has been shown to be impaired in a variety of disorders,” says lead investigator Joel Voss, a neuroscientist at Northwestern University.
If people with these disorders show similar memory gains in future experiments, the technology could be easily translated into the clinic, Voss adds. “It’s definitely the kind of thing that could be turned into an intervention that could be implemented in the hospital, and eventually maybe a doctor’s office.”
The technology, called transcranial magnetic stimulation or TMS, involves a wand emitting a changing magnetic field. When pressed against the skull, it induces changes in the electrical patterns of nearby neurons.
In the new study, published today in Science, the researchers used TMS to stimulate a spot in the outer layers of the brain called the lateral parietal cortex. Activity in this region is known to be strongly synchronized with the hippocampus, a region crucial for memory. (The hippocampus itself is too deep to be directly affected by TMS.)
To the person being stimulated, “it feels like somebody flicking the outside of your head with their finger”, says Voss, who says he has experienced it hundreds of times. “You can’t feel anything in terms of your thinking,” he says. “You don’t feel souped up afterwards.”
The researchers stimulated this area in 16 adult volunteers for five consecutive days. Each stimulation session lasted 20 minutes, during which volunteers would feel 2 seconds of pulsing, then 28 seconds of nothing, then 2 seconds of pulsing, and so on.
Brain scans of the volunteers before and after their week of stimulation showed that the treatment significantly increased connectivity between the hippocampus and four other areas, including the lateral parietal cortex. So it seems that stimulating one part of the hippocampal memory network (the lateral parietal cortex) led to more robust connections in other parts of the same network.
After stimulation the volunteers also performed better on a difficult memory test. They saw a series of 20 photographs of faces and heard a random word, such as ‘chair’ or ‘hat’, paired with each. A few minutes later they were shown the same pictures and asked what word had been paired with it. After five days of brain stimulation, the volunteers got roughly 13 pictures right, on average, compared with 10 before the treatment.
The researchers carried out some important control experiments. For example, they repeated the same procedure except zapped the motor cortex, which doesn’t have much synchrony with the hippocampus. This stimulation led to no noticeable differences in memory network connectivity or in memory test scores. (It did, however, lead to some noticeable changes in the volunteers’ behaviors. “It makes the hand and arm twitch 20 times a second, causing the arm to lift right up off the table,” Voss says. “It’s a little weird.”)
The work underscores the idea, long supported by animal studies, that memory is not just about the hippocampus, but the connections between the hippocampus and the brain’s outer layers.
“This study is exciting because it shows that the hippocampus doesn’t act alone — its connections with other brain regions are important for memory,” says Maureen Ritchey, a neuroscientist at the University of California, Davis, who was not involved with the work.
Ritchey and others say the study raises a host of intriguing questions to be tackled in future work. For example, Ritchey asks, would the same memory boost happen if a different part of the memory network is stimulated?
Bernhard Staresina, a neuroscientist at the University of Birmingham in the U.K, muses on a few more: How long do the effects last? And would the stimulation affect other kinds of memories, such as where you parked your car or whether you recognize a face? And perhaps most provocatively: Can the same approach be used not only to boost memories, but to weaken them?
“Sometimes negative experiences can exert lasting and debilitating effects, evidenced for example in post-traumatic memory disorder,” Staresina says. “Can [TMS] be used to disturb the memory network and thereby – perhaps complementary to psychotherapy – help alleviate detrimental effects resulting from unwanted memories?”
Voss and his colleagues are taking a step toward clinical translation by doing a similar study on elderly adults with mild cognitive impairment, a pre-symptomatic form of Alzheimer’s. “They all have reduced connectivity of this network so hopefully this will be something that works,” he says.
When it comes to people with healthy brains, however, Voss says we should forget about TMS. “I don’t think this is the kind of thing you’d want to do as a study aid,” he says. That’s because memory skills are determined in large part by what we can’t control: our genes.
“The number one way to improve memory abilities,” he jokes, “is to find two people with really good memories and get them to have children.”