Our Most Important Memories Live in Just a Few Neurons

That favorite childhood memory of yours—you know, the one that still seems like just yesterday, the one that you can still smell and taste—may actually be the result of a select few neurons firing deep within your brain.

According to a recent MIT study, strong memory fragments—known as engrams—may actually be the result of a small population of neurons residing in the hippocampus, the brain's memory center. The team employed a technique known as optogenetics—one that electrically stimulates genetically-modified neurons that are designed to light up when active "that allows scientists to shine a laser on individual or groups of neurons and make them trigger action potentials using a protein found in algae" [thanks: Zeeshan O]-to identify in mice the neurons responsible for recently-learned memories. "We demonstrate that behavior based on high-level cognition, such as the expression of a specific memory, can be generated in a mammal by highly specific physical activation of a specific small subpopulation of brain cells, in this case by light," says Susumu Tonegawa, a member of the MIT research team.


The researchers tested their hypothesis by first identifying the subset of brain cells within the Hippocampus that were active only when a lab mouse learned a new maze route. They then combined the genes for these memory-making cells with a gene that produced a light-activated protein. This way, neurons responsible for new memory formation would literally light up whenever they activated. The team then exposed a group of mice to a new maze environment with certain areas deemed "off limits" and enforced with a mild electrical shock. The mice quickly learned to solve the maze without venturing into the off limits areas while activating a certain subset of hippocampus neurons.

When the mice were dropped into a new maze with different electrified areas, every time they made a misstep, the same neurons that fired when the mouse was shocked in the previous maze, did so again in the new one. Not only that, in the new maze, the mouse entered into a defensive crouch faster and more readily than in the first. This shows that the mouse not only remembered the pain from the previous maze, it also learned from the experience and changed its behavior accordingly. Basically, the mouse remembered how much the initial shock sucked due to these neurons firing and therefore reacted faster.

"Showing that the reactivation of those nerve cells that were active during learning can reproduce the learned behavior is surely a milestone." Charles Stevens, a professor in the 
Molecular Neurobiology Laboratory at the Salk Institute, told R&D Magazine. The ability to know which specific bits of our brains activate when used could be applied to a variety of neurodegenerative disorder studies such as Alzheimer's by identifying which specific regions of the brain aren't working as they should. [MIT via R&D Mag - Image: Sebastian Kaulitzki / Shutterstock]

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