For a few days the research team in Hannah Monyer’s lab tracks a particular mouse’s movements and neural activity in its cage. And here’s a curious thing: when the mouse rests or sleeps, they notice the same neurons fire in the same sequences, or in curious reversed sequences, as they did when the mouse was moving around—but faster. Ten to twenty times faster. Monyer shows me printouts where these firing sequences have been expressed as graphs. You can compare the wakeful versions with the sleep versions. The patterns are not quite the same, but clearly similar, or symmetrically inverted. It’s as if, in sleep, the mouse were reliving its experience, in fast forward and fast rewind, and committing it to memory in some way. So that then when the experience is repeated—when, that is, the mouse finds itself in the same position in the cage (which must be rather often)—perception and memory are simultaneous as the familiar pattern fires off. At which point memory could be understood not exactly as something stored, but as a tendency for neural patterns to repeat. A kind of habit. Perhaps. Though why the repetition would come with a conscious recollection that it is a repetition is not clear. In any event, these are the stories one tells oneself in response to the read– outs produced after inserting electrodes into a mouse’s hippocampus.