Study shows short- and long-term memories form simultaneously
A new study could revolutionize ourunderstanding of how memories form in the brain. The research revealsthat short-term memories and long-term memories are formedsimultaneously in different parts of the brain, a discovery thatchallenges our current models of memory formation.
For much of the 20th centuryneuroscientists assumed that memories form in the hippocampus and are then slowly transferred to the neocortex for long-term storagevia a process called "consolidation."
Known as the standard model, thistheory was inspired by the landmark study of a patient in the 1950snamed Henry Molaison. This particular patient's hippocampus wasdamaged in an operation, and soon after he was unable to store newlong-term memories, yet he was still able to recall memories formed beforethe surgery.
This led scientists to begin understanding thevital role the hippocampus plays in processing and storing memories.It could almost be seen as acting like a temporary storage devicebefore the brain files the memories into its long-term bank.
More recently, another theory of memoryformation arose dubbed the multiple-trace model, which argued there was adifference in the processes in forming semantic andepisodic memories – semantic memories being those founded on factualinformation and episodic memories being more related to specificlived experiences.
This model argued that semanticknowledge is stored more swiftly than previously thought into areas of the neocortex, whileepisodic traces of the memories could still be found in the hippocampus for weeks, months, or even years, following the experience. Thistheory still assumed a process of consolidation from the hippocampusto the neocortex in the process of memory formation.
The new research follows on from a 2012study that developed a process allowing scientists to trace thecircuits involved in memory storage and retrieval. This process alsoestablished a way to artificially reactivate memories usingoptogenetics, using light to stimulate memory cells.
Using this approach the researchersgenerated a fear-conditioning event in mice, creating a memory wherea mouse received an electric shock when entering a specific chamber.
The study revealed that just one-dayafter the conditioning event the mice had developed new memories inboth the hippocampus and the neocortex. Most revealingly, theydiscovered the memories in the neocortex were initially "silent" orinactive. These neocortex memories could bestimulated through the artificial optogenetic process, but they werenot immediately active during normal memory recall processes.
"This is contrary to the standardtheory of memory consolidation, which says that you graduallytransfer the memories," says lead scientist Takashi Kitamura. "The memory is already there."
Even more compellingly, the researcherswatched over the following two weeks as the "silent memory cells"in the neocortex matured and slowly became active, while the relatedpathways in the hippocampus became silent.
Traces of the memories still remainedin the hippocampus after the memories became inactive and scientistswere able to artificially activate those memories, signalling thepossibility that some form of trace could still remain in that regioneven after the pathways have gone quiet.
The new mystery the researchers face isin understanding how this maturation process of memory cells in theneocortex occurs. Early indications still indicate an importantrelationship between the hippocampus and the neocortex in thematuration of these long-term memory cortical cells. When thepathways between the two regions were blocked the cortical maturationprocess was hindered.
This new research points to an entirelynew understanding of how we process and store memory.
The research was published in thejournal Science.