“This is an exciting finding,” says Sheena Josselyn, a neuroscientist at Toronto’s Hospital for Sick Children, who also studies the cellular underpinnings of memory formation but was not involved with the latest study. As a field, she says, neuroscience is just starting to understand how emotional memories are stored. The road to translating these basic findings to therapies that could one day help people with emotional memory deficits is even longer and more challenging, she adds. But Ramirez’s latest paper moves the field “one step closer to achieving this goal.”

Every memory begins as an altered set of connections in the brain, whether that brain belongs to a mouse or a human. If, a year from now, you happen to remember reading this article, it’s because a network of neurons knit itself together more tightly, both physically and electrically. And if you happened to be doing something particularly pleasurable while reading this article—like munching a delicious pastry or cuddling with a loved one, the warm fuzzy feelings associated with these sentences will be encoded in a particular set of neurons in your hippocampus—a cashew-shaped structure located in the inner folds of your brain. If something terrible were to happen to you, a different set of cells in the same organ would record the ensuing emotions. Forever after, when you thought of this story, you’d retrieve from them those feelings of disgust or sadness. But if you were to let Ramirez fire lasers into your brain, he might be able to turn off those bad feelings.

At least, that’s his hunch, based on what he’s seen in mice. In his new study, his team identified the hippocampal cells that encoded negative feelings and modified them to respond to light. They then surgically inserted two optic fibers into their brains. Each mouse was shooed into a special plexiglass box, where it received a mild electric shock to its foot, leading the mouse to form a fearful memory of the cage. (And, they figured, some not great feelings about that.) A few days later, they returned each mouse to the cage and shined a laser into its brain twice a day, in the morning and evening, for five days in a row, forcing those neurons to fire over and over again. “We were trying to see if we could permanently reprogram a memory from within the brain,” says Ramirez. “It was like putting the brain cells on a weight-lifting program.”

What they found was that by artificially activating negative memory cells in the top part of the hippocampus, they could strip the mice of their traumatic feelings about the box. They no longer froze up every time they were placed in one. But location mattered: Stimulating memory cells in the bottom part of the hippocampus made negative memories even more vivid.

To complicate things even further, when Ramirez’s group brought its memory-altered mice into cages with other mice, they showed a dramatic dampening in social behaviors. They were no longer interested in the opposite sex or their favorite foods (condensed milk and cheddar goldfish crackers). Those results are currently unpublished, but they suggest that there’s more to therapeutically modulating memories than a dial you can simply turn up and down. “What we’re finding out is that the brain isn’t like a waffle, where this square is emotion, and that square is spatial navigation, and that square is depression,” says Ramirez. “It’s really more like a bowl of spaghetti, and we’re trying to disentangle it one strand at a time. But every end we pull on winds up tugging on a handful of others.”