The study of memory is currently at the forefront of neuroscience- how does the brain store memories across our entire lifetime, and recall specific memories based on environment, mood, or attention? Memory is believed to be stored, partially, in an area of the brain called the medial temporal lobe (MTL). Within this area, brain cells have been found in a smaller region, called the entorhinal cortex, which are more electrically active in specific regions of the physical environment, forming a grid-like pattern of activity. They were therefore named “grid cells.” The 2014 Nobel Prize in Physiology or Medicine was awarded to the scientists who discovered these, and other similar cells. Still, the role of these grid cells in memory is unclear. A new class of cell has recently been discovered in the entorhinal cortex by scientists at Columbia University, with other collaborators, that may help neuroscientists understand the link between navigation and memory.
While most neuroscience studies are performed in model organisms, like mice or monkeys, in rare instances scientists can use electrical sensing devices that have been implanted in human epileptic patients (to identify the starting locations of seizures) to obtain activity from individual brain cells. In this case, electrodes in the MTL recorded activity from 299 cells across 19 patients. The patients were placed on a virtual reality track and a cue was presented at the beginning of each trial corresponding to one of four objects that would be present along the track. During the initial “encoding” trials, the participant would be shown one of four cues and experience a short “holding period” before being moved through the virtual environment. Each cue has an associated object that was located in a specific position in the environment. The participants had to learn the location of the object corresponding to the initial cue, linking memory to physical location. During the retrieval trials, the participants had to press a button when they reached the location where the object corresponding to the provided cue would have been located, though the objects themselves were no longer there for these trials. 43 of the 299 cells were found to respond in retrieval trials in different locations of the track depending on which cue was presented in the beginning of the trial. Some cells only responded in a location if one specific cue was presented, while others responded in different locations for multiple cues. Comparing the activity of these cells during the initial holding period to that when the participant arrived at the chosen location revealed that these cells showed similar patterns of activity even before the movement began. A computer model could be trained on the activity patterns just from this holding period to predict the location chosen by the participant with near double accuracy from a random chance guess. Therefore, these cells were named “memory-trace cells” and are believed to be involved in “choosing” the correct memory for retrieval based on the context of the task.
The subject of memory is of great interest in neuroscience and has potential to impact medicine, bioengineering, and computer science. Of course, degradation of memory systems with age and in diseases like Alzheimer’s disease are one such application for medicine, and the identification of cells in this region of the brain- which has been shown to be affected early on in Alzheimer’s patients- suggests that this could be an important therapeutic target. In artificial intelligence and machine learning, storing of multiple “memories” has often been problematic- adding a new memory tends to erase the old ones, or prevent them from being properly accessed. Understanding how the brain successfully achieves such a task could help computer scientists build more creative architectures for which this is no longer a problem. There is still much to be learned about the memory system in our brains, but this discovery brings us one step closer in learning how we remember.
Managing Correspondent: Andrew T. Sullivan
Press Articles: Specific neurons that map memories now identified in the human brain, Science Daily
Discovered: the brain’s map pins, Science Focus
GPS System in Human Brain and Memory Recall, NewsClick
Original Journal Article: “Memory retrieval modulates spatial tuning of single neurons in the human entorhinal cortex,” Nature Neuroscience
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