A new study could revolutionize our understanding of how memories form in the brain. The research reveals that short-term memories and long-term memories are formed simultaneously in different parts of the brain, a discovery that challenges our current models of memory formation.
For much of the 20th century neuroscientists assumed that memories form in the hippocampus and are then slowly transferred to the neocortex for long-term storage via a process called "consolidation."
Known as the standard model, this theory was inspired by the landmark study of a patient in the 1950s named Henry Molaison. This particular patient's hippocampus was damaged in an operation, and soon after he was unable to store new long-term memories, yet he was still able to recall memories formed before the surgery.
This led scientists to begin understanding the vital role the hippocampus plays in processing and storing memories. It could almost be seen as acting like a temporary storage device before the brain files the memories into its long-term bank.
More recently, another theory of memory formation arose dubbed the multiple-trace model, which argued there was a difference in the processes in forming semantic and episodic memories – semantic memories being those founded on factual information and episodic memories being more related to specific lived experiences.
This model argued that semantic knowledge is stored more swiftly than previously thought into areas of the neocortex, while episodic traces of the memories could still be found in the hippocampus for weeks, months, or even years, following the experience. This theory still assumed a process of consolidation from the hippocampus to the neocortex in the process of memory formation.
The new research follows on from a 2012 study that developed a process allowing scientists to trace the circuits involved in memory storage and retrieval. This process also established a way to artificially reactivate memories using optogenetics, using light to stimulate memory cells.
Using this approach the researchers generated a fear-conditioning event in mice, creating a memory where a mouse received an electric shock when entering a specific chamber.
The study revealed that just one-day after the conditioning event the mice had developed new memories in both the hippocampus and the neocortex. Most revealingly, they discovered the memories in the neocortex were initially "silent" or inactive. These neocortex memories could be stimulated through the artificial optogenetic process, but they were not immediately active during normal memory recall processes.
"This is contrary to the standard theory of memory consolidation, which says that you gradually transfer the memories," says lead scientist Takashi Kitamura. "The memory is already there."
Even more compellingly, the researchers watched over the following two weeks as the "silent memory cells" in the neocortex matured and slowly became active, while the related pathways in the hippocampus became silent.
Traces of the memories still remained in the hippocampus after the memories became inactive and scientists were able to artificially activate those memories, signalling the possibility that some form of trace could still remain in that region even after the pathways have gone quiet.
The new mystery the researchers face is in understanding how this maturation process of memory cells in the neocortex occurs. Early indications still indicate an important relationship between the hippocampus and the neocortex in the maturation of these long-term memory cortical cells. When the pathways between the two regions were blocked the cortical maturation process was hindered.
This new research points to an entirely new understanding of how we process and store memory.
The research was published in the journal Science.