The hippocampus of mammals, including mice and humans, contains neurons called place cells that represent experiences in the brain. The team recorded the activity of these memory-related neurons while also tracking the animals’ internal brain state. “The internal brain state can be estimated from pupil size,” says CIN researcher and co-author of the study, Nicola Sartorato. Brain state and plasticityThe researchers then asked whether fluctuations in internal state reflect moments when the brain becomes more plastic and capable of change. To form a memory, neurons must rapidly adjust their connections and reorganize their activity patterns so that new information can be encoded.