The brain consists of a vast number of individual cells called neurons. Individual neurons form highly complex patterns of interconnections with many other neurons. Each of these connections is called a synapse and a collection of interconnected neurons is called a neural network. It is within these networks of neurons and synapses that memories are formed and stored. The term memory trace, also called the engram, broadly refers to the change(s) in the brain that serves to store a memory. To fully understand the nature of a memory trace, at least three different but interrelated properties must be elucidated. First, the precise region within the brain where the memory dependent changes occur must be localized. This entails identifying the specific neural network (or neural circuit) that subserves the formation, storage, and retrieval of the particular memory and then localizing the site...