Scientists Taught Mice to Smell an Odor That Doesn’t Exist

When neuroscientists David Hubel and Torsten Wiesel wanted to figure out how the brain parses its visual environment, they went as simple as they could go. Last week, Chong and his colleagues published a study in the journal Science showing that they’d worked out some of the details of just how the olfactory bulb represents odors—by making mice smell scents that don’t actually exist in the real world. “They’ve taken advantage of advanced methods trick the animal into thinking it’s smelling a particular smell.” By avoiding the issues of manipulating odor molecules entirely and instead going directly to the brain, Chong and his colleagues were able to investigate in detail the aspects of brain activity that matter most for our sense of smell. Though making mice sense impossible odors might sound like something out of science fiction, Chong’s general approach—stimulating a part of the brain to figure out its logic—has been around since before Hubel and Wiesel did their cat experiments. So to make the mice smell odors that weren’t actually present, Chong used a technique called optogenetics, which allows scientists to stimulate groups of neurons using only light.