Using a retrovirus that targets dividing, or reproducing, cells, the team tracked new neurons in the hippocampus (a midbrain structure involved with learning and memory) from their births to their deaths. The scientists could determine the behavior of cells by measuring their electrophysiological activity during different phases. “In young animals, cells are very active, very plastic, and they can change their properties readily,” he says. “This whole process [also] happens in the environment of adult circuitry.”

The team found that there is a two-week window, or critical period, about a month after these new cells hatch during which they act like the neurons of a newborn baby. The researchers cued the new cells with a pattern of electrical activation that mimics the sequence that takes place in the brain of a mouse as it learns about a special spot (such as a corner in its cage where it may receive food or a shock). During this time, the cell synapses (connections that allow neurons to communicate with each other) that are artificially stimulated become stronger.