The scientists showed the participants images of celebrities and famous landmarks, both separately and combined, such as Clint Eastwood appearing in front of the Leaning Tower of Pisa.
Image credit: University of Leicester
Published in the journal Neuron, the findings are the result of a 5-year study led by Dr. Matias Ison and Prof. Rodrigo Quian Quiroga at the University of Leicester and Dr. Itzhak Fried at Ronald Reagan UCLA Medical Center.
"Almost a decade ago, when I obtained a PhD in Statistical Physics from the University of Buenos Aires in Argentina," says Dr. Ison, "I decided to join Quian Quiroga's team at the University of Leicester to work on Neuroscience. Back then it was hard to imagine that I was going to be able to contribute to the understanding of how the brain works at such [a] vital level. It really feels like a major achievement."
This study, which was conducted in humans, builds on findings from previous animal studies of the role the medial temporal lobe plays in the formation of episodic memories.
However, animal studies are limited in what they can tell us, because the experiments are reliant on extensive reward-based training with stimuli that are not natural. The authors behind the new study say that this approach left it unclear as to how individual neurons might encode episodic memories naturally.
The activity of more than 600 individual neurons was recorded in the new study using electrons implanted in the median temporal lobes of 14 participants. All of the participants had severe epilepsy, and the electrodes had been implanted to identify the focus of seizures for potential surgical cure.
The first phase of the experiment involved the participants viewing pictures of family members, famous actors, athletes and places, such as the Eiffel Tower and the White House. The scientists then showed the participants composite images that put the people and places the patients had viewed separately in combinations, such as Clint Eastwood appearing in front of the Leaning Tower of Pisa.
Composite images caused neurons previously activated by just one image to fire for both
After seeing the composite images just once, the patients learned to associate the people and places.
Analyzing the participants' neural activity, the researchers found that - after viewing the composite images - the same medial temporal lobe neurons that fired when the participants first viewed, for instance, Clint Eastwood, now also fired when the participants saw the Leaning Tower of Pisa on its own. And the same neurons that were activated when the patients first saw the Leaning Tower of Pisa image now also lit up when they saw Clint Eastwood on his own.
"It was impressive to see how individual neurons signaled the learning of new contextual associations between people and places and that the changes in firing could occur just after one instance," says Dr. Ison. "This was also compatible with basic mechanisms underlying episodic memory formation."
"The astonishing fact was that these changes were dramatic, occurring at the exact moment of learning, even after one trial. The emergence of associations between concepts after a single trial, linked to rapid neural activity changes, turned out to be ideal for the creation of new episodic memories."
Next, the researchers will investigate why some related concepts are consolidated into long-term memories while others are discarded. The team wants to see if it is possible to retrieve these forgotten memories by stimulating the brain.