An important aspect of human memory is our ability to evoke specific moments from the wide range of experiences that have occurred in a given context. For example, if you are asked to recommend a tourist itinerary for a city that you have visited several times, your brain allows you to sort of recall and selectively distinguish specific memories of your different trips to respond to them.
Studies have shown that declarative memory - the type of memory you consciously remember like your home address or your mother's name - is based on healthy structures in the medial temporal lobe in the brain, including the hippocampus and cortex. entorhinal (EC). These regions are also important for spatial cognition, demonstrated? through the Nobel Prize-winning discovery of "place cells" and "grid cells" in these regions - neurons that activate to represent specific locations in the environment during navigation (similar to GPS). However, it has not been clear whether or how this "spatial map" in the brain relates to a person's memory of events at these locations, and how neural activity in these regions allows us to target memory. particular for recovery among related experiences.
A team led by Columbia Engineering neuro-engineers found the first evidence that individual neurons in the human brain target specific memories during a recall. They studied the recordings of neurosurgical patients who had electrodes implanted in their brains and examined how the patients' cerebral signals corresponded to their behavior while performing an object location memory task in virtual reality (VR). The researchers identified "memory trace cells" whose activity was spatially adjusted to where the subjects remembered meeting specific objects. The study is published today in Nature Neuroscience.
"We found these neurons with memory traces mainly in the entorhinal cortex (EC), which is one of the first regions of the brain affected by the onset of Alzheimer's disease," says Joshua Jacobs, associate professor of engineering biomedical, who led the study. "Because the activity of these neurons is closely linked to what a person is trying to remember, it is possible that their activity is disrupted by diseases like Alzheimer's disease, leading to memory deficits. Our results should open up new avenues 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 was able to measure the activity of individual neurons by taking advantage of a rare opportunity: the invasive brain registration of 19 neurosurgical patients in several hospitals, including the Columbia University Irving Medical Center. The patients had drug-resistant epilepsy and therefore had already had recording electrodes implanted in their brains for their clinical treatment. The researchers designed experiments as captivating and immersive VR computer games, and the bedridden patients used laptops and portable controllers to move around in virtual environments. While performing the task, subjects first navigated the environment to find the location of four unique objects. Next, the researchers removed the objects and asked the patients to move around the environment and mark the location of a specific object on each trial.
The team measured neuron activity as patients moved through the environment and marked their memory targets. Initially, they identified purely spatially tuned neurons, similar to "place cells," which always activated when patients moved through specific locations, regardless of the subjects' memory targets. "These neurons only seemed to care about the spatial location of the person, like pure GPS," says Salman E. Qasim, doctoral student at Jacobs and lead author of the study.
But the researchers also noticed that other neurons only activate in memory-relevant locations that the patient remembered during this trial - whenever patients were asked to target a different memory for recall, these neurons changed their activity to match the memorized location of the new target. What particularly excited Jacobs and Qasim was that they could actually decode the specific memory that a patient targeted based on the activity of these neurons.
"Our study shows that neurons in the human brain follow experiences that we voluntarily remember and can change their activity patterns to differentiate memories. They are like the pins on your Google map that mark the locations you remember for them. important events, "said Qasim. "This discovery could
Salman E. Qasim, Jonathan Miller, Cory S. Inman, Robert E. Gross, Jon T. Willie, Bradley Lega, Jui-Jui Lin, Ashwini Sharan, Chengyuan Wu, Michael R. Sperling, Sameer A. Sheth, Guy M. McKhann, Elliot H. Smith, Catherine Schevon, Joel M. Stein, Joshua Jacobs. Memory retrieval modulates spatial tuning of single neurons in the human entorhinal cortex. Nature Neuroscience, 2019; DOI: 10.1038/s41593-019-0523-z