Declarative memory is what enables humans to consciously recall details like someone’s mother’s name or home address. This very significant memory involves brain areas called the medial temporal lobes, which again include important parts involved in memory formation—the hippocampus and the entorhinal cortex (EC). These brain regions also play crucial roles for spatial navigation. A Nobel winning discovery has already shown that special kinds of brain cells known as place cells and grid cells are present in these regions. The place and grid cells function by getting activated to represent specific locations in the environment during navigation. This can be thought of as a GPS system structured inside the brain.
But, till now it has not been clear how the spatial map in the brain is related to memories of events at those locations. For example, when a person navigates an area, the place and grid cells come into play and help in memorising the spatial details of the area. But suppose, a person has some kind of a specific experience at a particular location that falls under the area; in that case how is that specific experience recalled when the location is visited sometime later and how neuronal activities in these regions enable to target a particular memory?
A research published in Nature Neuroscience has obtained some facts that could address those questions. The research, done by a team of Columbia University School of Engineering and Applied Sciences, reveals that individual neurons in the human brain target specific memories when recalled.
Joshua Jacob, the corresponding author of the study was quoted to have said,“We found these memory-trace neurons primarily in the entorhinal cortex (EC), which is one of the first regions of the brain affected by the onset of Alzheimer 's disease. Because the activity of these neurons is closely related to what a person is trying to remember, it is possible that their activity is disrupted by diseases like Alzheimer's, leading to memory deficits. Our findings should open up new lines of investigation into how neural activity in the entorhinal cortex and medial temporal lobe helps us target past events for recall, and more generally how space and memory overlap in the brain."
The team studied recordings done on neurosurgical patients’ brain signals while they were subjected to perform a virtual-reality (VR) object location memory task. The brain recordings could identify memory-trace-cells. These brain cells or special neurons, get activated when one recalls memories involved in specific locations.
The neurosurgical patients had drug resistant epilepsy and they already got electrodes implanted in their brains for clinical treatment. The researchers designed experiments very similar to video games where they could navigate through virtual environments. In the experiment, the patients first navigated through the virtual environment to learn the locations of four unique objects. The second part of the experiment involved removal of the objects and the patients were asked to move through the environment and mark the location of one specific object on each trial.
What the research team did was they measured the activity of neurons as the patients moved through the environment and marked their memory targets. At the initial stage, they observed that purely spatially tuned brain cells very similar to place cells were always activated when the patients moved through specific locations, regardless of the subjects’ memory target.
"These neurons seemed only to care about the person's spatial location, like a pure GPS," says Salman E. Qasim, lead author of the study.
They also found that other neurons were only activated in locations which were related to the memories the patient was recalling on that trial. Whenever the patients were asked to target a different memory to recall, these neurons changed their activity to match the new target’s remembered locations.
“Our study demonstrates that neurons in the human brain track the experiences we are willingly recalling, and can change their activity patterns to differentiate between memories. They're just like the pins on your Google map that mark the locations you remember for important events. This discovery might provide a potential mechanism for our ability to selectively call upon different experiences from the past and highlights how these memories may influence our brain's spatial map,” says Qasim.
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