Researchers have achieved some insight into how fleeting experiences become memories in the brain in a new study.
Their experimental system could be a way to test or refine treatments aimed at enhancing learning and memory, or interfering with troubling memories.
For the study, researchers at Yerkes National Primate Research Center, Emory University, set up a system where rats were exposed to a light followed by a mild shock.
A single light-shock event isn't enough to make the rat afraid of the light, but a repeat of the pairing of the light and shock is, even a few days later.
"I describe this effect as 'priming'," Ryan Parsons, first author of the paper, said.
"The animal experiences all sorts of things, and has to sort out what's important. If something happens just once, it doesn't register. But twice, and the animal remembers," he said.
Even though a robust fear memory was not formed after the first priming event, at that point Parsons could already detect chemical changes in the amygdala, part of the brain critical for fear responses.
Long term memory formation could be blocked by infusing a drug into the amygdala. The drug inhibits protein kinase A, which is involved in the chemical changes Parsons observed.
It is possible to train rats to become afraid of something like a sound or a smell after one event, Parsons said. However, rats are less sensitive to light as compared to sounds or smells, and a relatively mild shock was used.
Fear memories only formed when shocks were paired with light, instead of noise or nothing at all, for both the priming and the confirmation event.
Parsons measured how afraid the rats were by gauging their "acoustic startle response" (how jittery they were in response to a loud noise) in the presence of the light, compared to before training began.
Scientists have been able to study the chemical changes connected with the priming process extensively in neurons in culture dishes, but not as much in live animals.
The process is referred to as "metaplasticity", or how the history of the brain's experiences affects its readiness to change and learn.
"This could be a good model for dissecting the mechanisms involved in learning and memory," Parsons said.
"We're going to be able to look at what's going on in that first priming event, as well as when the long-term memory is triggered," he said.
"We believe our findings might help explain how events are selected out for long-term storage from what is essentially a torrent of information encountered during conscious experience," Parsons and Davis write in their paper.
The study has been published in the Journal of Neuroscience.