The first came in the 1970s, when John O'Keefe, a neuroscientist at University College London, was recording the brain activity of rats freely moving about in a space. He observed that different subsets of neurons in the hippocampus, the part of the brain central to learning and memory, fired in response to the rats' specific locations. For example, if the rats were in the northeastern part of their space, one subset would fire; another subset was active when the rats were in the southwestern part. He concluded that the mammalian brain uses these so-called place cells to create a spatial map. Then, in the early 2000s, May-Britt Moser and Edvard Moser, a then-married couple working together at the Norwegian University of Science and Technology, discovered "grid cells" in the entorhinal cortex, a region right next to the hippocampus. These cells form an internal positioning system. Grid cells don't fire for one specific location, but rather encode a coordinate system represe