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EXPLICIT MEMORY—THE CONSCIOUS recall of information about people, places, objects, and events—is what people commonly think of as memory. Sometimes called declarative memory, it binds our mental life together by allowing us to recall at will what we ate for breakfast, where we ate it, and with whom. It allows us to join what we did today with what we did yesterday or the week or month before that.

Two structures in the mammalian brain are particularly critical for encoding and storing explicit memory: the prefrontal cortex and the hippocampus (Chapter 52). The prefrontal cortex mediates working memory, which can be actively maintained for only very short periods and is then rapidly forgotten, such as a password that is remembered only until it is entered. Information in working memory can be stored elsewhere in the brain as long-term memory for periods ranging from days to weeks to years, and throughout a lifetime. Although long-term storage of explicit memory requires the hippocampus, the ultimate storage site for most declarative memory is thought to be the cerebral cortex.

In this chapter, we focus on the cellular, molecular, and network mechanisms of the hippocampus that underlie the long-term storage of explicit memory. Because the hippocampus receives its major input from a region of the cerebral cortex called the entorhinal cortex, an area that processes many forms of sensory input, we also consider how information from the entorhinal cortex is transformed by the hippocampus. In particular, we examine how neural activity in the entorhinal cortex and hippocampus contributes to spatial memory by encoding a representation of an animal’s location in its environment.

Explicit Memory in Mammals Involves Synaptic Plasticity in the Hippocampus

Unlike working memory, which is thought to be maintained by ongoing neural activity in the prefrontal cortex (Chapter 52), the long-term storage of information is thought to depend on long-lasting changes in the strength of connections among specific ensembles of neurons (neural assemblies) in the hippocampus that encode particular elements of memory.

The idea that memory storage involves long-lasting structural changes in the brain, first referred to as an “engram” by the German biologist Richard Semon in the early 20th century, dates back to the French philosopher Rene Descartes. In an attempt to locate an engram, the American psychologist Karl Lashley examined the effects of lesions in different regions of the neocortex on the ability of a rat to learn to navigate a maze. Since the performance in the maze seemed to be directly proportional to the size of the lesion, rather than its precise location, Lashley concluded that any memory trace must be distributed throughout the brain. Although it is now generally accepted that storage of an explicit memory is distributed throughout the neocortex, it is also clear that the process of storing memory requires the hippocampus, as demonstrated ...

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