System-researchers at the University of California, Los Angeles have shown what memories are really made of based on a study on sea slugs-organisms known for a relatively simple nervous system.
It is already known that memory formation involves the strengthening of synaptic connections between nerve cells.
Last year, a team led by Kelsey Martin, became the first to watch memories being made in sea slugs, in the form of new proteins appearing at the synapses.
They wanted to find where is knowledge stored in the complex brains of mammals.
Short-term memories, such as a telephone number about to be used, seem to be stored in two small curled-up structures called the hippocampi, buried deep in the brain's two hemispheres.
In 2008 Courtney Miller and David Sweatt at the University of Alabama in Tuscaloosa showed in mice that during the first hour after a memorable event there were chemical changes to the DNA of neurons in this area, altering the proteins produced.
Over the subsequent week, there were similar changes to the genes of neurons in the cortex, reports New Scientist.
These changes seemed to be permanent, indicating that long-term memories are stored there.
The pair believes that they watched short-term memories form in the hippocampus, which then became long-term memories in the cortex.
The brain pays extra attention to things that frighten us, as remembering them could mean the difference between life and death.
A structure next to the hippocampus called the amygdala is known to play a role in stamping this indelible mark.
Last year, a team led by Sheena Josselyn at the Hospital for Sick Children in Toronto, Canada, found that in mice they could erase a frightening memory of a noise by killing amygdala neurons whose synapses had recently been strengthened after exposure to the noise.
It was the first time a specific memory had been traced to the nerve cells that encoded it.