For memory to be useful, the brain needs to make connections between what happened and the circumstances in which it happened. Researchers at the University of Bonn have now revealed how the human brain handles this task. Their findings show that two distinct groups of neurons store content and context separately and coordinate their activity to form complete memories. Instead of mixing both types of information within the same cell, the brain separates them and links them as needed. The results were published in the journal Nature.
Humans have an excellent ability to recognize the same person or object in completely different situations. For example, it’s easy to see the difference between having dinner with a friend and attending a business meeting with the same person. “We already know that deep within the memory centers of the brain, specific cells called conceptual neurons respond to this friend, regardless of the environment in which it appears,” says Professor Florian Moorman of the UKB Epilepsy Clinic and member of the Interdisciplinary Research Area (TRA) Life and Health at the University of Bonn.
At the same time, the brain must connect this stored content with the surrounding context to create meaningful memories. In rodents, individual neurons often combine both types of information. “We asked ourselves: Is the human brain functioning fundamentally differently here? Does it map content and context differently to allow for more flexible memory? And how are these separate pieces of information connected when specific content needs to be remembered according to context?” says Dr Marcel Bausch, working group leader in the Department of Epilepsy at the University of Bonn and member of TRA ‘Life & Health’.
Observe brain activity in real time
To explore these questions, the research team recorded electrical signals from individual neurons in patients with drug-resistant epilepsy. As part of the clinical evaluation, electrodes had already been placed in the hippocampus and nearby areas important for memory. Patients also independently participated in computer-based tasks while doctors monitored seizures to evaluate treatment options.
During these experiments, participants viewed pairs of images and answered different types of questions about them. For example, the question “Is it large?” might ask you if an object is “large.” “This allowed us to observe how the brain processes the exact same image in the context of different tasks,” Morman says.
Two different neuronal systems for memory
The researchers examined the activity of more than 3,000 neurons and identified two main groups. One group, known as content neurons, responded to specific images, such as biscuits, regardless of the task being performed. The other group, called context neurons, responded to different types of questions, such as “Is it bigger?”, regardless of the image shown. In contrast to findings in rodents, only a small number of neurons could handle both roles simultaneously.
“The key finding was that these two independent groups of neurons most reliably encode content and context together when patients solve a task correctly,” Bausch says.
How the brain reconstructs memories from cues
As the experiment progressed, the interaction between these two groups of neurons became stronger. The activity of content neurons began to predict the responses of context neurons after only tens of milliseconds. “It was as if the ‘biscuit’ neurons were learning to stimulate the ‘bigger than?’ neurons,” Morman said.
This interaction acts like a control system that ensures that only relevant context is returned during recall. This process, known as pattern completion, allows the brain to reconstruct a complete memory even when only part of the information is available. The researchers say this separation of roles helps explain why human memory is so adaptive. By storing content and context in separate “neural libraries,” the brain can apply the same knowledge to many different situations without requiring unique neurons for every possible combination.
“This division of labor probably explains the flexibility of human memory. By storing content and context in separate ‘neural libraries,’ the brain can reuse the same concepts in countless new situations without the need for specialized neurons for each combination,” Bausch says. Professor Morman added: “The ability of these groups of neurons to spontaneously link together allows them to generalize information while preserving the specific details of individual events.”
What comes next for memory research?
In this study, the context was defined by the questions displayed on the screen. However, real-world contexts can also be as passive as your environment. Future research should determine whether the brain processes these everyday contexts in the same way. Scientists also plan to test these mechanisms outside of clinical settings.
Another important next step is to examine what happens when the interactions between these groups of neurons are intentionally disrupted. This could reveal whether such interference affects a person’s ability to recall the correct memories in the appropriate context or make accurate decisions.
This research was funded by the joint project “iBehave” between the DFG, the Volkswagen Foundation and the state of NRW.
