All mice groom themselves to keep their fur clean, but some in a lab in Columbia University, New York,  have started grooming to an unusual and excessive degree. This isn’t vanity. Instead, it’s the rodent equivalent of the repetitive rituals that many people with obsessive-compulsive disorder (OCD) go through, like an irresistible urge to wash their hands or clean themselves.

The mice didn’t start off with their compulsions. Over five days, psychiatrist Susanne Ahmari used flashes of light to activate neurons that run between two regions at the front of their brains—the orbitofrontal cortex (OFC) and the striatum. She didn’t do it for long—just five minutes a day—but slowly, the mice groomed themselves more and more.

Meanwhile, a few states