Memory Linked To Specific Brain Cells, Happy or frightful memories like the first kiss or a bump in the night leave memory traces or engrams that we may stimulate when we remember things in the past, complete with time, place and all the sensations we experienced.
The online journal Nature reveals the answer to the question of whether these engrams are conceptual or whether they consist of a physical network of neurons within the brain. Researchers at MIT decided to demonstrate that memories do, in fact, reside in specific brain cells and that by merely activating a tiny fraction of these cells a person can call an entire memory event. This would explain, for instance, how Marcel Proust was able to recall childhood memories from the smell of a Madeleine cookie he loved as a child.
The first step of their experiment involved identifying a specific set of brain cells within the hippocampus that was only active when a mouse was exploring a new environment. Once they established which genes were activated in the brain cells, they coupled them with the gene for channelrhodopsin-2 (ChR2), which is a light-activated protein used in optogenetics.
They then implanted the coupled gene into the mice's dentate gyrus, a part within the hippocampus thought to contribute to the formation of new memories, so they could study the mice by using tiny optical fibers to deliver pulses of light to the neurons.
They found an ingenious way to label the physical network of neurons linked to a specific memory engram for a particular experience by ensuring that the light-activated protein would only be expressed in the neurons involved in experiential learning.
The mice were then allowed to enter a new environment and received a small shock to the foot after a few minutes of exploration, which taught them to fear the particular environment in which they experienced the shock.
The researchers then tagged the brain cells with ChR2, which were activated during the moment the mice experienced fear and observed that these neurons involved in the fear memory were switched on when the mice were later exposed to triggering pulses of light in a completely different environment, and that the mice quickly adopted a defensive, immobile crouch.
