Researchers clarify function of highly specialized nerve cells in memory

Specialized nerve cells in the temporal lobe respond very selectively to images and names of a single person or specific objects. Researchers at the University Hospital Bonn (UKB) and the University of Bonn have now directly demonstrated for the first time that concept neurons are indeed the building blocks of our memory for experiences. Their results are now published in Nature Communications.

Certain nerve cells in the brain fire when confronted with different images, the name of a specific person, or the identity of an object. They are highly selective and do not react to other people or objects. These conceptual neurons have so far only been discovered in humans, and only in the medial temporal lobe, which is essential for memory formation.

An international research team led by Professor Florian Mormann from the Department of Epileptology at the UKB, who is also a member of the Transdisciplinary Research Area (TRA) “Life and Health” at the University of Bonn, has already confirmed their important function for working memory in a 2017 study, in which individual concept neurons representing specific people or objects keep the memory content available for a short time.

These neurons remain active until a new image is shown and another neuron is stimulated. In addition, the research team was even able to use the activation of conceptual neurons during the working memory phase to predict whether the test subjects would later correctly remember the image they had already been shown.

But until now, it was unclear exactly how successful transfer of experiences into episodic memory, which stores autobiographical events and experiences, including place and time, works.

“We therefore pursued the hypothesis that these conceptual neurons provide the building blocks that are assembled to form a memory of an experience,” says first author Sina Mackay, a doctoral student at the University of Bonn in Professor Mormann’s research group at UKB.

Neural activity provides the “what” and “where” in memory formation

For its scientific work, the Bonn research team uses a special feature of the UKB epileptology clinic, one of the largest epilepsy centres in Europe. Here, people with severe forms of epilepsy can benefit from surgical and other treatments.

In some cases, electrodes are first implanted in the brain to locate the source of the seizure. The research team can then measure the electrical activity of individual neurons while epilepsy patients perform tasks. In the current study, this was an associative memory paradigm, in which people or objects were assigned a specific position on the screen.

The researchers took measurements in the medial temporal lobe and in the parahippocampal cortex, where the place cells are located. This time, they focused not only on the behavior of the conceptual neurons, but also on that of the place cells, which respond to certain positions on the screen on which the images are displayed, regardless of the images displayed at that position.

While Professor Mormann’s research team had already discovered a prediction effect, the Bonn researchers have now been able to show that the activity of concept neurons in the medial temporal lobe and place neurons in the parahippocampal cortex predicts the correct memorization of concept-location pairs.

“In both the object-selective and location-selective populations of neurons, firing rates were significantly higher if memories were subsequently successfully stored,” Mackay says. In contrast, the remaining neurons, which make up more than 90 percent of these regions, did not predict successful memory formation, highlighting the very specific role of concept and location neurons.

“We assume that mediotemporal conceptual neurons and perhaps also parahippocampal place cells, which are involved in our everyday experiences, are reactivated during memory consolidation, for example during deep sleep,” says Professor Mormann, who believes that future studies are needed to investigate this hypothesis.