How does the brain index memories?

The Episodic Memory System

The brain contains specialized systems that are designed to store and utilize the different kinds of information contained in our experiences

The episodic memory system automatically captures information as a consequence of our exploring and experiencing the environment.

Every episode of our lives is unique, even if it contains highly overlapping information. Thus, we can remember many different instances of practicing the piano or driving our car to the same parking lot.

Episodic memory is formed rapidly, in one shot. It is the type of memory used to store individual events in a snapshot-like way. It involves associating together many aspects of the situation, such as the spatial context and the objects present. An example of the type of event or episodic memory is the memory of seeing a particular person at a particular place on a particular occasion.

The Memory Indexing Theory provides an account of the storage and retrieval of episodic memory, based on the intrinsic organization of the hippocampus (the part of the brain which is responsible for the indexing of new memories) and its anatomical relationship to other regions of the brain such as the neocortex and the entorhinal cortex. The hippocampus is made up of four regions CA1, CA2, CA3, and CA4, each has its own role in the formation of new memories.

At the highest level of abstraction, the hippocampus enables arbitrary sets of concurrent activity, involving for example the spatial context where an episode occurred, the people present during the episode, and what was seen during the episode, to be associated together and stored as one event. It provides a quick and inexpensive way to access the content of an episode.

Memory Indexing

The content of our daily experiences is registered in the neocortex. Sensory information (from which episodic memories is derived) occurs in the primary sensory areas of the neocortex.

Each sensory area extracts different aspects and features of sensation. The entorhinal cortex receives highly processed input from every sensory modality, as well as input relating to ongoing cognitive processes and brings it to the hippocampus complex. Hence, the entorhinal cortex is functioning as a hub in a the brain’s widespread memory network and is the main interface between the hippocampus and neocortex.

As a result, the hippocampus receives from the entorhinal cortex highly elaborated information, which has already been processed extensively along different sensory pathways.

This information converges in the hippocampus into an auto-associative network in the CA3 region. This network involves arbitrary associations between any of the inputs to the hippocampus, e.g., spatial and visual object.

The outcome of this neocortical-hippocampus interaction is the memory trace. The experience is represented as the set of strengthened synapses and co-activated neurons in the hippocampus. The memory trace can be simply described as a hippocampus representation of co-occurring patterns of activity in the neocortex.

Memory Retrieval

A partial cue that activates the index can activate the neocortical patterns and thus retrieve the memory of the episode. The process goes as follows:

A subset of the original input pattern is received by the neocortex. The projections from these input patterns activate the connected neurons in the hippocampus representing the original experience. The activation of this representation then projects back to the neocortex to activate the pattern representing the entire experience.

It is this projection back to the neocortex that conveys the indexing property to the hippocampus representation.

Three Main Concepts

Binding and Projections. The hippocampus allows to rapidly and economically bind neocortical inputs into an index that can access those sites. Rather than strengthening connections among the neocortical modules that represent the content of an experience, projections from neocortical sites are rapidly and economically bound into an index in the hippocampus. There are no modifications among the neocortical activity patterns.

Pattern completion. A subset or portion of the experience that originally established the memory trace can activate or replay the entire experience. When neocortical activity patterns are produced by a subset of the elements of a prior episode, their projections to the hippocampus will activate the index and it will project back to neocortical units to activate the entire pattern of activity generated by the original episode.

Pattern separation. The episodic memory system has the capacity to maintain distinct representations of similar but separately occurring episodes. Provided the inputs do not extensively overlap, the index provides a way of separating overlapping episodes. This allows the appropriated patterns of neocortical activity to be activated during retrieval.


  1. The Hippocampal Memory Indexing Theory.
  2. The Hippocampal Indexing Theory and Episodic Memory: Updating the Index.
  3. A Theory of Hippocampal Function in Memory.