Summary: Researchers have created a new model that may help explain how the brain stores memories of tangible events. The new model explains how neural activity in the hippocampus can help map space, time and context in episodic memories.
Source: University of Freiburg.
Dr. Thomas Hainmüller and Dr. Marlene Bartos of the Institute of Psychology of the University of Freiburg have established a new model to explain how the brain stores memories of tangible events. The model is based on an experiment that involved mice seeking a place where they received rewards in a virtual environment. The scientific journal Nature has published the study.
Take a few steps forward, stop, and look around. In the world of the video game, the walls that depict a corridor four meters long are made up of green and blue patterned blocks. The floor is marked with turquoise dots. A short distance away, there’s a brown disc on the floor that looks like a cookie. That’s the symbol for the reward location. The mouse heads for it, gets there, and the symbol disappears. The next cookie promptly appears a bit further down the corridor. The mouse is surrounded by monitors and is standing on a styrofoam ball that is floating on compressed air and turns beneath the mouse when it runs. The ball makes it possible to transfer of the mouse’s movements to the virtual environment. If the mouse reaches the reward symbol, a straw is used to give it a drop of soy milk and stimulate it to form memories of its experiences in the virtual world. The mouse learns when, and at which location, it will receive a reward. It also learns how to locate itself and discriminate between different corridors in the video game.
Viewing the brain with a special microscope
“As the mouse is getting to know its environment, we use a special microscope to look from the outside into its brain and we record the activities of its nerve cells on video,” explains Thomas Hainmüller, a physician and doctoral candidate in the MD/PhD program of the Spemann Graduate School of Biology and Medicine (SGBM) of the University of Freiburg. He says that works because, in reality, the head of the mouse remains relatively still under the microscope as it runs through the virtual world of the video game. On the recordings, the mice’s genetically-manipulated nerve cells flash as soon as they become active. Hainmüller and Marlene Bartos, a Professor of Systemic and Cellular Neurobiology are using this method to investigate how memories are sorted and retrieved. “We repeatedly place the mouse in the virtual world on consecutive days,” says Hainmüller. “In that way, we can observe and compare the activity of the nerve cells in different stages of memory formation,” he explains.
Nerve cells encode places
The region of the brain called the hippocampus plays a decisive role in the formation of memory episodes – or memories of tangible experiences. Hainmüller and Bartos have published a study in the scientific journal “Nature.” In their article they demonstrate that the nerve cells in the hippocampus create a map of the virtual world in which single neurons code for actual places in the video game. Earlier studies done at the Freiburg University Medical Center showed that nerve cells in the human hippocampus code video games in the same way. The cells become activated and flash when the mouse is at the respective place, otherwise they remain dark. “To our surprise, we found very different maps inside the hippocampus,” reports Hainmüller. In part, they provide an approximate overview of the position of the mouse in the corridor, yet they also consider time and context factors, and above all, information about in which of the corridors the mouse is located. The maps are also updated during the days of the experiment and as a result can be recognized as a learning process.
Better understanding of memory formation
The research team summarizes, saying that their observations provide a model that explains how activity of the nerve cells in the hippocampus can map the space, time and and context of memory episodes. The findings allow for better understanding of the biological processes that effect the formation of memory in the brain. Hainmüller says, “In the long term, we would like to use our results to contribute to the development of treatments to help people with neurological and psychiatric illnesses.”
Source: Marlene Bartos – University of Freiburg
Publisher: Organized by NeuroscienceNews.com.
Image Source: NeuroscienceNews.com image is credited to Thomas Hainmüller, Marlene Bartos.
Original Research: Abstract for “Parallel emergence of stable and dynamic memory engrams in the hippocampus” by Thomas Hainmueller & Marlene Bartos in Nature. Published June 6 2018.
Parallel emergence of stable and dynamic memory engrams in the hippocampus
During our daily life, we depend on memories of past experiences to plan future behaviour. These memories are represented by the activity of specific neuronal groups or ‘engrams’. Neuronal engrams are assembled during learning by synaptic modification, and engram reactivation represents the memorized experience1. Engrams of conscious memories are initially stored in the hippocampus for several days and then transferred to cortical areas. In the dentate gyrus of the hippocampus, granule cells transform rich inputs from the entorhinal cortex into a sparse output, which is forwarded to the highly interconnected pyramidal cell network in hippocampal area CA3. This process is thought to support pattern separation. CA3 pyramidal neurons project to CA1, the hippocampal output region. Consistent with the idea of transient memory storage in the hippocampus, engrams in CA1 and CA2 do not stabilize over time. Nevertheless, reactivation of engrams in the dentate gyrus can induce recall of artificial memories even after weeks2. Reconciliation of this apparent paradox will require recordings from dentate gyrus granule cells throughout learning, which has so far not been performed for more than a single day. Here, we use chronic two-photon calcium imaging in head-fixed mice performing a multiple-day spatial memory task in a virtual environment to record neuronal activity in all major hippocampal subfields. Whereas pyramidal neurons in CA1–CA3 show precise and highly context-specific, but continuously changing, representations of the learned spatial sceneries in our behavioural paradigm, granule cells in the dentate gyrus have a spatial code that is stable over many days, with low place- or context-specificity. Our results suggest that synaptic weights along the hippocampal trisynaptic loop are constantly reassigned to support the formation of dynamic representations in downstream hippocampal areas based on a stable code provided by the dentate gyrus.