Seeing a memory being made.

In Brain Science, Memory and Learning by Brainy Days Ahead

The power of imaging allows researchers to see both active and silent cells and map them to the structure of the brain. In this study, mice ran and 50 or so neurons in their hippocampi fired in sequence, possibly to help the animals measure the distance traveled. Later, when the mice were resting, subsets of those neurons turned on again. Researchers suspect this reactivation may reveal the encoding and retrieving of a memory — as if the mouse is recalling its earlier exercise.