Index and concept neurons - the brain's meticoules librarians

The hippocampus - our brain-main-library
When you meet a friend in a café, certain neurons in your hippocampus activate to recognize key details, such as your friend’s face and the setting. 

The hippocampus is the main structure in the brain responsible for organizing and retrieving memories. It binds together different elements of an experience—such as people, locations, and sensory details—into a unified memory trace. This process allows for the later recall of specific events, even after a single exposure.

The hippocampus also expedites memory integration, linking related experiences across time. When similar events occur, overlapping neural activity strengthens connections, supporting the gradual transformation of more short-term memory into long-term memory, your knowledge. 

Beyond its role in memory, the hippocampus is critical for navigation - basically, to help us find our way around. 

When navigating, it generates cognitive maps of the environment, enabling orientation and wayfinding. Specialized "place-memory" cells within the hippocampus activate in response to specific locations.

But how on earth does the hippocampus remember and steer around all this stuff? There are several theories - here is one of them.

Index neurons store the full episode
Some scientists believe that the hippocampus stores entire experiences as a whole. This is done via brain cells that work like an index in a book.

Index neurons act as pointers to the different components of an event, binding them together into a unified memory. 

The theory is that when an episode occurs - such as meeting your friend at the cafe from before - the various sensory and emotional inputs activate a specific pattern of neurons. The hippocampus then creates an index, linking these elements together. 

When you recall this memory later, the index neurons reactivate, bringing back the entire experience rather than just isolated details. The specific neurons are activated only when encoding the specific episode, and the same neurons fire again when successfully recalling it. 

Research in humans and animals supports this theory. In birds that hide food for later retrieval, unique neuron patterns are activated during caching and reactivated when searching for stored food. This barcode-like activity pattern strengthens the idea that index neurons store memories in detail so that they can be reliably accessed later.

Concept neurons detect familiar things
Concept neurons are specialized brain cells that respond to specific details, people, objects, or ideas. They act as building blocks for memories, storing the essential elements of an experience.

Unlike “ordinary” neurons that detect raw stimuli, concept neurons fire when you see an image of your friend or hear their name, regardless of location.  They respond to the concept of your friend.

Another difference from other neurons is that concept neurons do not follow a strict arrangement, meaning that two neurons coding for different concepts can be located close together. 

This - a little bit messy - but also flexible organization supports memory formation by allowing different concepts to be associated with each other without requiring long neural connections. That's pretty smart.

Studies show that some concept neurons form quickly in emotionally significant situations. This suggests that emotional intensity accelerates the formation of a concept in your mind.

How index neurons evolve into concept neurons
Over time, repeated experiences strengthen connections between neurons, allowing index neurons that initially stored entire episodes to evolve into concept neurons that encode common or more abstract elements.

Let's say you repeatedly meet the same friend in different locations and settings: the gym, at a bar, and at work. With enough encounters, some of the index neurons stop coding the individual episodes and instead start to respond to the shared element, which is your friend or, more precisely, the concept of your friend.

This transformation process allows the brain to build stable representations of familiar people, places, and ideas while still preserving the ability to recall distinct events. 

Our memories stabilize
Not all neurons are equally likely to participate in memory storage. Research shows that neurons with higher excitability have a greater chance of being included in a memory network.

This competition among neurons determines which ones will be part of an engram, the physical trace of a memory.

Neurons that are recently active remain more excitable for a while, making them more likely to be recruited into new memories. This mechanism explains why memories of similar events, like repeated visits to the same café, become linked.

As memory neurons mature, their excitability decreases, meaning they become less active. That reduces their ability to encode new, unrelated events.

It sounds like a bad thing, but it is really quite good for us. This natural decline helps stabilize long-term memories by sticking to neural circuits for frequently encountered concepts.

About the scientific paper:

First author: Luca D. Kolibius, USA
Published: Trends in Cognitive Sciences, 2025
Link to paper: https://www.cell.com/trends/cognitive-sciences/fulltext/S1364-6613(25)00031-2