The researchers looked into the memories of three different types of mice that had been genetically altered to display early signs of Alzheimer’s—accordingly, they failed tests of their short- and long-term memory. To try to retrieve the mice’s memories, the researchers looked at the dentate gyrus, a part of the hippocampus thought to be important to the formation of episodic memory. They used lasers to stimulate cells called engram cells, thought to store memories. After a few rounds of “light activation,” the mice performed better in the memory tests. The lasers, it seemed, helped the cells re-activate so that the mice could access their memories. That shows the memories had formed, after all—Alzheimer’s had interfered with the mice’s ability to access it. But it's important to note that as the disease becomes more advanced, it could also interfere with long-term memory storage.