The researchers wanted to investigate whether flies would learn to suppress innate responses to certain behaviors. This is common in the animal kingdom; it's called "learned helplessness," and it's a response characterized by sleep and eating disorders, reduced immune activity and increased serotonin levels. When animals feel harassed, stressed or otherwise helpless, they may respond with these symptoms that are characteristic of depression. Rats harassed with a robot exhibit these symptoms. In other studies, slugs, cockroaches, fish and many other fauna developed a helplessness response when they were tortured with electric shocks. In these setups, a "master" animal can control the shocks by its behavior, while a "yoked" one can't do anything about it.