The human brain consists of region cells that are responsible for sensory cues to actions and behaviors. They even are responsible for cataloging the link as a memory. These connections have been considered exceedingly steady and settled.
According to a new study by the Harvard Medical School (HMS), the neurons responsible for such tasks may be less stable, but yet more flexible than previously thought. The study doubts on the traditional notion that memory formation involves hardwiring information into the brain in a fixed and highly stable pattern.
The study mainly points on a critical plasticity in neuronal networks. This allows the neural network to easily incorporate new learning. Thus it eliminates the need of forming new links to separate neurons every time. In addition, if a memory is no longer needed, neurons can be more easily reassigned to other important tasks.
Study senior author Chris Harvey said, “Our experiments point to far less stability in neurons that link sensory cues to the action than we would have expected and suggest the presence of much more flexibility, and indeed a sort of neuronal efficiency.”
“We believe this trade-off ensures the delicate balance between the ability to incorporate new information while preserving old memories.”
Scientists did experiments with mice that were repeatedly running through a virtual maze over the course of a month. They then analyzed images of brain activity in a brain region that especially involved in navigational decision-making.
Scientists discovered that neurons forming the mice’s maze-running memories kept changing. They switching roles in the memory pattern or left it altogether.
Harvey said, “Individual neurons tended to have streaks where they’d do the same thing for a few days, then switch. Over the course of weeks, we began to see shifts in the overall pattern of neurons.”
The experiments also reveal how the brain captures external cues and behaviors to perform recurring tasks such as navigating a space using landmarks.
During the study, scientists trained the mice to run down a virtual passage. Researchers imaged hundreds of neurons in the part of the brain responsible for spatial decision-making as the mice were galloping down the virtual maze.
Once the navigational links were firmly established in the mice’s brains over the course of a few weeks, the researchers expected the activity of the neurons to look the same from day to day.
During maze runs that happened in 24 hours of each other, that was, indeed, the case. Mice turned right if they were given a black cue or left if they were given a white cue. Neurons that activated in response to the white cue could be distinguished from neurons that activated in response to the black cue.
Researchers observed that throughout a little while the line between signals in singular neurons obscured and the memory pattern began to drift across neurons. The neuron that associated with the black cue, lose its specialization and be replaced by another.
Study first author Laura Driscoll said, “We were so sure that the neurons would be doing the same thing everyday that we designed the study expecting to use the stable pattern as a baseline.”
“After we realized the neurons were changing roles, we had to rethink parts of the study.”
Scientists then tested how the pattern changed when they added shapes as a third cue while the mice were navigating the maze. After reassigning individual neurons as the mice learned the new cue, there was little change to the overall activity pattern.
Scientists hypothesize that brain flexibility may differ across various brain regions. It depends on how often the skill or memory they encode needs to be modified.
Driscoll said, “The results provide a fascinating early glimpse into the complexities of memory formation. To explain the big picture of memory formation and storage across brain regions, researchers say they hope to study other areas of the brain involved with different types of decision-making and memories.”
“I hope this research inspires people to think of memory as something that is not static. Memories are active and integrally connected to the process of learning.”