“There has to be some sort of triage to remember what’s relevant and forget the rest,” Zugaro said. “The understanding of how specific memories were selected for storage was missing … Now we have a good idea.”
Last December, a research team led by Bendor at University College London published related results in Nature Communications that predicted those of Yang and Buzsáki. They too found that sharp wave ripples fired when rats were awake and asleep appeared to tag experiences for memory. However, their analysis averaged several different trials together, an approach that was less accurate than what Yang and Buzsáki achieved.
The NYU team’s key innovation was to introduce the element of time, which distinguishes similar memories, into their analysis. The mice ran around the same maze patterns, and yet these researchers were able to distinguish between blocks of trials at the neuronal level—a feat that had never been achieved before.
The brain patterns mark “something that’s a little bit closer to an event, and a little bit less like common knowledge,” said Loren Frank, a neuroscientist at UC San Francisco who was not involved in the study. “That strikes me as a really interesting finding.”
“They show that the brain might create some kind of temporal code to distinguish between different memories that take place in the same place,” said Freyja Ólafsdóttir, a neuroscientist at Radboud University who was not involved in the study.
Shantanu Jadhav, a neuroscientist at Brandeis University, praised the study. “This is a good start,” he said. But he hopes for a follow-up experiment using a behavioral test. Showing that an animal forgot or remembered certain blocks of trials would be “the real proof that this is a tagging mechanism.”
The research leaves a burning question unanswered: Why is one experience chosen over another? The new work suggests how the brain tags a particular experience for memory. But it can’t tell us how the brain decides what is worth remembering.
Sometimes the things we remember seem random or irrelevant, and certainly different from what we would select if we had a choice. “There’s a sense that the brain is prioritizing things based on ‘importance,’” Frank said. Because studies have suggested that emotional or novel experiences tend to be better remembered, it’s possible that internal fluctuations in arousal or the levels of neuromodulators like dopamine or adrenaline and other chemicals that influence neurons ultimately select experiences, he suggested.
Jadhav echoed that idea, saying, “The internal state of the organism can influence experiences to be more effectively encoded and stored.” But it’s not known what makes one experience more likely to be stored than another, he added. And in the case of Yang and Buzsáki’s study, it’s not clear why a mouse would remember one trial better than another.
Buzsáki remains committed to investigating the role that sharp-wave ripples play in the hippocampus, though he and his team are also interested in potential applications that might arise from these observations. For example, it’s possible that scientists could disrupt the ripples as part of a treatment for conditions such as post-traumatic stress disorder, in which people remember certain experiences too vividly, he said. “The low-hanging fruit here is to erase the sharp waves and forget what you’ve experienced.”
But for now, Buzsáki will continue to focus on these powerful brain waves to learn more about why we remember what we do.
Original story reprinted with permission from Quanta Magazine, an editorially independent publication of the Simons Foundation whose mission is to enhance public understanding of science by reporting on research developments and trends in mathematics, the physical sciences, and the life sciences.