Our memory stores not only information about past events, but also the circumstances under which we experienced them. Researchers have now discovered how this works. Accordingly, our brain uses two different groups of neurons to store content and context separately. These groups of nerve cells then work together in a coordinated manner to form our memories. This division of labor makes our memory particularly efficient because it allows it to reuse the same concept in countless new situations without needing a specialized neuron for each combination, as the team explains.
Our episodic memory is remarkably comprehensive and precise: it stores learned information, remembers the appearance and behavior of countless people with whom we have and have had contact, and can also recall past emotions. However, our memory is not static: even when it is stored, feelings, expectations and internal filter mechanisms influence what our brain stores in the long term and what not. At the same time, the memories stored in our brain networks are dynamic – they can also be changed and falsified subsequently. The hippocampus is the center of our memory; it controls the storage and retrieval of memory content in the cortex. The stronger the connection between the “center” and the storage locations distributed throughout the brain, the better we can remember the respective information.
Neurons overheard firing
But for a memory to be complete, the brain must store and recall the memory contents with their context. Only then can we, for example, remember the same person or the same object in completely different situations. We easily distinguish a dinner with a friend from a business meeting with the same friend. “We already know that deep in the memory centers of the brain, specific cells, so-called concept neurons, react to this friend, regardless of the environment in which he appears,” says senior author Florian Mormann from the University of Bonn. In order to provide the memories of him with the appropriate context, these concept neurons have to work together with other brain cells and brain areas. “However, how memory content and context are combined at the neuron level in humans is still unclear,” write Mormann and his team. In mice, the neurons of the hippocampus store memories in a context-related manner, but this is not the case with human concept neurons.
“We therefore asked ourselves: Does the human brain function fundamentally differently here?” says lead author Marcel Bausch from the University of Bonn. “Does it represent content and context separately to allow for more flexible memory? And how does this separate information connect when we need to remember specific content according to context?” To answer these questions, Bausch and his colleagues analyzed the activity of individual neurons in the brains of 16 epilepsy patients. For better diagnosis and preparation for surgery, electrodes were implanted in the hippocampus and surrounding brain regions – and thus in regions that are also essential for memory. The researchers used this to analyze the brain reactions when storing and recalling episodic memory in more detail. To do this, the patients were shown pairs of images on a screen, which they had to compare based on different questions. For example, they had to decide whether an object appearing in both pictures differed in size.
Content and context separated
“This allowed us to observe how the brain processes exactly the same image in different task contexts,” says Mormann. When the data from more than 3,000 individual neurons was evaluated, it became apparent that two largely separate groups of neurons fired in the test subjects’ brains while viewing the images. A group of brain cells called content neurons fired in response to specific images regardless of the task or context. The second group of context neurons, on the other hand, became active in response to specific task contexts – regardless of the image or object shown. Also important: In contrast to mice, these neurons initially acted independently of each other. The experiment also showed that the cooperation between the groups of neurons increased over the course of the experiment. When a content neuron fired, the matching context neuron also responded a few dozen milliseconds later. This apparently ensures that only the relevant context that was previously active is retrieved. This process allows the brain to reconstruct the entire memory context from just a piece of information.
“This division of labor likely explains the flexibility of human memory. Because the brain can reuse the same concept in countless new situations without needing a specialized neuron for each individual combination, by storing content and context in separate “neural libraries,” says Bausch. Mormann adds: “The ability of these groups of nerve cells to spontaneously connect allows us to generalize information while preserving the specific details of individual events.” Next, the researchers want to investigate whether a targeted disruption of the interaction between these neurons prevents a person from recalling the correct memory in the context or making the right decision.
Source: Marcel Bausch (Bonn University Hospital) et al., Nature, doi: 10.1038/s41586-025-09910-2