When someone encounters a new experience — say, a terrifying rabbit — groups of neurons in their brain fire together, the connections between them become stronger, and molecules accumulate at the places where neurons meet. Many scientists believe that these preserved patterns of strengthened connections are the literal stuff of memories — the physical representations of the things we remember. These connected neuron groups are called engrams. When people bring up old memories, the engram neurons fire up again. They also enter a brief period of instability, when the molecules that preserved the connections between them disappear and must be remade. This process, known as reconsolidation, means that humans are partly reconstructing our memories every time they bring them to mind. And it means that the act of recollection creates a window of time in which memories can be updated, and fears can be unlearned. - medium.com