“There has to be some kind of triage to remember what is relevant and forget the rest,” Zugaro said. “Understanding how specific memories were selected for storage was still lacking … Now we have a good clue.”
Last December, a research team led by Bendor at University College London published related results in Nature Communications that anticipated those of Yang and Buzsáki. They too found that sharp wave ripples that fired when rats were awake and asleep seemed to tag experiences for memory. However, their analysis averaged a number of different trials together—an approach less precise than what Yang and Buzsáki accomplished.
The NYU team’s key innovation was to bring the element of time, which distinguishes similar memories from one another, into their analysis. The mice were running around in the same maze patterns, and yet these researchers could distinguish between blocks of trials at the neuronal level—a resolution never reached before.
The brain patterns are marking “something a little bit closer to an event, and a little bit less like a general knowledge,” said Loren Frank, a neuroscientist at UC San Francisco who was not involved in the research. “That strikes me as a really interesting finding.”
“They’re showing that the brain is maybe creating some kind of temporal code to distinguish between different memories occurring in the same place,” said Freyja Ólafsdóttir, a neuroscientist at Radboud University who was not involved with the work.
Shantanu Jadhav, a neuroscientist at Brandeis University, praised the study. “This is a good start,” he said. However, he hopes to see a follow-up experiment that includes a behavioral test. Demonstrating that an animal forgot or remembered particular trial blocks 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 certain experience to remember. But it can’t tell us how the brain decides what’s worth remembering.
Sometimes the things we remember seem random or irrelevant, and surely different from what we’d select if given the choice. “There is a sense that the brain prioritizes based on ‘importance,’” Frank said. Because studies have suggested that emotional or novel experiences tend to be remembered better, it’s possible that internal fluctuations in arousal or the levels of neuromodulators such as dopamine or adrenaline and other chemicals that affect neurons end up selecting experiences, he suggested.
Jadhav echoed that thought, saying, “The internal state of the organism can bias experiences to be encoded and stored more effectively.” But it’s not known what makes one experience more prone to being stored than others, 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 exploring the roles that sharp wave ripples play in the hippocampus, although he and his team are also interested in potential applications that might arise from these observations. It’s possible, for example, that scientists could disrupt the ripples as part of a treatment for conditions like post-traumatic stress disorder, in which people remember certain experiences too vividly, he said. “The low-hanging fruit here is to erase sharp waves and forget what you experienced.”
But for the time being, Buzsáki will continue to tune in to these powerful brain waves to uncover 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 covering research developments and trends in mathematics and the physical and life sciences.