Human memory—the encoding of information into our very brains—is a process at once fascinating and still pretty mysterious. In a study published Wednesday in study published Wednesday in Neuron, researchers offer new insight into how new memories are created, with a little help from Jennifer Aniston, Clint Eastwood, and Tiger Woods.
In the study, researchers from the University of Leicester in the UK and UCLA in the US looked at how neurons changed as people formed new episodic memories after the researchers showed them images of people including family members and celebrities in different places.
They found that neurons that were initially responsive to a specific person began to respond to a specific place too, after the two had been linked in the subject's memory.
"If you want to remember something from your past, basically what you remember is a few concepts that you relate together. For example, 'I remember meeting a friend in the cinema one or two months ago,'" explained Rodrigo Quian Quiroga, an author on the paper from the University of Leicester, in a phone call. "So basically you make a link between a few concepts; this is, let's call it the 'skeleton' of what we call episodic memories." Episodic memories are memories related to your personal experience.
The new study suggests how neuronal activity in a part of the brain called the medial temporal lobe changes as these concepts—people and places, for instance—are linked.
In one example, the researchers showed subjects a picture of Clint Eastwood and a picture of the Leaning Tower of Pisa, and then a picture of Clint Eastwood photoshopped at the Leaning Tower of Pisa. They found that the neurons that originally fired in response to the actor would then start firing in response to the landmark, and vice versa, even though they didn't before. A new association between the two formerly unrelated things—Eastwood and the tower—was encoded.
Showing the pictures of people in places was intended to mimic the experience of meeting someone somewhere.
Quiroga gave another example of Jennifer Aniston placed in the Eiffel Tower. "The moment the subject remembered the association, the neurons [that originally fired in response to Aniston] started firing to the Eiffel Tower," he said. "So basically we showed that these neurons representing concepts, they can also represent very quickly associations—and this is the basis of memory formation."
The study involved 14 patients with epilepsy who had electrodes implanted in their brains, which gave the researchers the opportunity to record the activity of single neurons. The electrodes were not implanted specifically for this study but for clinical purposes (to identify the focus of their seizures for potential surgery); however, it meant they were in a unique position to volunteer for the study.
But Quiroga said that the findings should hold for people who don't have epilepsy, as the patients had various types of condition within the broad umbrella of epilepsy yet all of them showed the same responses.
He said he was not surprised to see that associations were encoded somehow in the neurons, but he wasn't expecting it to be so fast. "The surprising fact is how quickly it happened," he said. "From the exact moment the subject remembered the association—the exact moment the subject remembered, 'Oh, Jennifer Aniston was in the Eiffel Tower'—the neuron would start firing to the Eiffel Tower."
This would happen after the subject had seen the image that associated the two separate images just once. That's telling, because we often form memories in real life after just a single event.
Of course, the brain is a complex beast, and the researchers note in the paper that the process of forming episodic memories "goes beyond the formation of contextual associations," but add that their study "suggests a fundamental mechanism of neuronal plasticity that may support episodic memory formation."
The findings help shed light on a fundamental neuroscience problem, and in the future it could also have clinical applications. "In the long term it can give us insight also into what happens in pathologies that involve deficits in exactly the same areas we studied—the most common one is Alzheimer's," said Quiroga.
He added that making one finding inevitably brings up a whole load more questions, like how stable the associations are, how many can be encoded, and what happens when you forget. "Once I tell you we've found a mechanism of how memories are formed, you have one question after the other," he said. "Basically we're trying to do experiments to answer these follow-up questions."