The way brains form memories could be completely different than previously believed according to new research. It suggests we make two simultaneous memories: one for short-term and one for the long-term.
Until now the prevailing belief was that the hippocampus creates a memory that is then transferred to the cortex. That was partly derived from patients with conditions, similar to those depicted in the movie Memento, in which hippocampus damage means they are unable to form new memories but retain their long-term memories from before the damage.
The work at Riken-MIT’s Center for Neural Circuit Genetics suggests this is not the case and instead that the hippocampus and cortex simultaneously create two “copies” of a memory.
The findings came from a study on mice involving beaming light into the brain to activate or deactivate neurons while tracking the reactions to a shock. The researchers found that in normal conditions the mice didn’t refer to the relevant section of the cortex for several days after the shock, which fit the theory of the transferred memory.
When the researchers deactivated the relevant sections of the hippocampus and cortex the mice stopped reacting to the recent shock as if they had forgotten it. Switching the hippocampus section back on appeared to make the mice regain their memory of the event.
The surprise came when the hippocampus was kept deactivated but the cortex activated, with the mice reacting, again from remembering the incident. In other words, despite being only a short time after the shock, a separate memory of the event had already formed in the cortex.
The researchers did find a connection however. While both memories were in place from the outset, the long-term cortex “copy” strengthened (or rather became more “functionally mature”) as the short-term hippocampus “copy” faded away.