Neuroscientists Watch Live How the Brain Recalls Memories
Neuroscientists at the Johns Hopkins University School of Medicine have tracked nerve cells firing in the brains of rats as they plan where to go next, and obtained new insights on how the brain reconstructs memories. The study suggests that memories are more like separate but connected “snapshots” in the brain than uninterrupted video reels.
The researchers studied how rats reconstruct spatial memories by using electrode implants to monitor the activity of “place cells” – specialized nerve cells in the brain that remember specific places and fire when the animal encounters those places.
The results, published in Science with the title “Autoassociative dynamics in the generation of sequences of hippocampal place cells,” support theoretical notions of neural network function and reveal a fundamental discretization in the retrieval of memory in the hippocampus.
Street View in the Brain
“[The] hippocampal neurons express sequenced reactivations, which we show are composed of discrete attractors,” note the scientists. “Each attractor corresponds to a single location, the representation of which sharpens over the course of several milliseconds, as the reactivation focuses at that location. Subsequently, the reactivation transitions rapidly to a spatially discontiguous location.”
“My own introspective experience of memory tends to be one of discrete snapshots strung together, as opposed to a continuous video recording,” says neuroscience professor David Foster. “Our data from rats suggest that our memories are actually organized that way, with one network of neurons responsible for the snapshots and another responsible for the string that connects them.”
It appears that, when rats want to go from one place to another, they use a mental map with only a few sparse memory points between origin and destination.
“The trajectories that the rats reconstructed weren’t smooth,” said Foster. “We were able to see that neural activity ‘hovers’ in one place for about 20 milliseconds before ‘jumping’ to another place, where it hovers again before moving on to the next point.”
We think that there is a whole network of cells dedicated to this process of fine-tuning and jumping. Without it, memory retrieval would be even messier than it is.
Each of the five panels in the image shows a memory snapshot created by hundreds of place cells.
According to these results, the process of memory retrieval seems similar to Google Street View, sort of. On a good computer with a fast Internet connection, Street View gives the impression of a smooth walk, but it consists of discrete snapshots and software that does the rest.
Understanding how the brain encodes and recalls memories is very important because it would permit “repairing” memory problems in humans and also devising efficient memory management methods for bio-inspired computer systems.
Images from Johns Hopkins University School of Medicine and Shutterstock.