Walking backward may seem a simple task, but researchers don’t know how the mind controls this behavior. A study published online today in Science provides the first glimpse of the brain circuit responsible —at least in fruit flies. Geneticists created 3500 strains of the insects, each with a temperature-controlled switch that turned random networks of neurons on when the flies entered an incubator. One mutant batch of fruit flies started strolling in reverse when exposed to warmth (video, right panel), which the team dubbed “moonwalkers,” in honor of Michael Jackson’s famous dance. Two neurons were responsible for the behavior. One lived in the brain and extended its connections to the end of the ventral nerve cord—the fly’s version of a spine, which runs along its belly. The other neuron had the opposite orientation—it started at the bottom of the nerve cord and sent its messaging cables—or axons—into the brain. The neuron in the brain acted like a reverse gear in a car; when turned on, it triggered reverse walking. The researchers say this neuron is possibly a command center that responds to environmental cues, such as, “Hey! I see a wall in front of me.” The second neuron functioned as the brakes for forward motion, but it couldn’t compel the fly to moonwalk. It may serve as a fail-safe that reflexively prevents moving ahead, such as when the fly accidentally steps onto a very cold floor. Using the two neurons as a starting point, the team will trace their links to sensory neurons for touch, sight, and smell, which feed into and control the moonwalking network. No word yet on the neurons responsible for the Macarena.
Video: How to Make Flies Do the Moonwalk