However, traces of the memory remained: Reactivating those cells with light still prompted the animals to freeze.In the basolateral amygdala, once memories were formed, the engram cells remained unchanged throughout the course of the experiment.Just one day after the fear-conditioning event, the researchers found that memories of the event were being stored in engram cells in both the hippocampus and the prefrontal cortex.However, the engram cells in the prefrontal cortex were "silent" -- they could stimulate freezing behavior when artificially activated by light, but they did not fire during natural memory recall.
A new MIT study of the neural circuits that underlie this process reveals, for the first time, that memories are actually formed simultaneously in the hippocampus and the long-term storage location in the brain's cortex.
study, the researchers used this approach to label memory cells in mice during a fear-conditioning event -- that is, a mild electric shock delivered when the mouse is in a particular chamber.
Then, they could use light to artificially reactivate these memory cells at different times and see if that reactivation provoked a behavioral response from the mice (freezing in place).
This image shows memory engram cells (green and red) which are crucial for permanent memory storage in the prefrontal cortex.
When we visit a friend or go to the beach, our brain stores a short-term memory of the experience in a part of the brain called the hippocampus.
Most previous studies of memory were based on analyzing how damage to certain brain areas affects memories.