Scientists have puzzled for decades over how our brains convert experiences from short-term memories into stored recollections that can be retrieved years later. Now, MIT researchers say they have made a discovery that provides new insight into the circuitry behind how we remember.
A study published Thursday in the journal Science suggests that the seeds of both short-term and long-term memories are planted nearly simultaneously in different parts of the brains of mice.
The phenomenon, which researchers believe is similar to the function of human brains, could lead scientists to rethink some of the popular models of how memories are captured and stored. It could also lead toward a better understanding of memory diseases like Alzheimer’s.
Professor Susumu Tonegawa , the senior author on the paper, said he and his fellow researchers were able to watch how memories were stored in both the hippocampus, which captures recent experiences, and the prefrontal cortex, which handles older events.
“The hippocampus is very good at quickly forming memory about the various things that happen to you, but for the purpose of keeping the memory — evolution decided to use another part of the brain for the long-term storage,” he said in an interview.
“This paper tells us in more detail how this cooperation between the dual systems . . . actually takes place.”
Tonegawa, a 1987 Nobel Prize winner for his immunology work, has recently made several notable discoveries in the separate field of neuroscience. He led a 2012 study that showed that memories are stored in specific cells, called engrams, and demonstrated a way to activate them with light.
The technique has provided a new way to test theories about memory, leading to studies like the one published this week.
Many neuroscientists have long believed that memories were formed in the hippocampus, then transferred to the neocortex (which contains the prefrontal cortex). More recently, researchers have theorized that memory retention involves a more complex interaction between those parts of the brain.
In Tonegawa’s study, researchers applied a mild electric shock to mice, then used both natural means and the light stimulation to make them recall the unpleasant event.
They found that the mice could recall the memories stored in their hippocampus simply by going to the place where the shock happened. But scientists also found the memory embedded in the prefrontal cortex. Though the mice couldn’t recall it naturally at first, researchers were able to use light to activate a fearful response to the memory.
Over time, the cells in the area associated with long-term memory became more active, and those associated with short term memories became silent, though they could still be activated artificially.
The researchers also found elements of the memory in the basolateral amygdala, which stores emotional associations. That area did not change during the study.
The idea of silent memory cells is relatively new, Tonegawa said, and it has been discussed largely in the context of memory disorders. The discovery of how they can become dormant — and awaken — in normal function holds promise for research into how such diseases work.