It’s the end of the day: you’re going home, but when you’re about to open the front door, you can't find your keys! Don’t worry, thanks to your episodic memory, you’ll be able to recall all of the specific events you experienced today. And by going back in time, you'll eventually find your keys and return home. We humans are indeed able to replay past episodes from our lives. But what about animals? Research has revealed that rats also share this ability.

As you know, memory comes in many forms, and our episodic memory allows us to mentally travel in time; our episodic memories are characterized by the repetition of several unique events in sequential order. And the hippocampus plays a crucial role in these episodic memories. While studies have shown that rats can remember several events, none have demonstrated their capacity to replay a series of episodes in a sequence. Published in Current Biology, this research conducted at Indiana University (USA) has just offered some answers to this question.

In order to assess the ability of rodents to replay past events, the team of scientists, led by Jonathon D. Crystal, initially trained 13 rats to memorize a list of up to 12 smells in a pre-defined order. In a second phase, the rats were placed inside several “arenas” with different odors from the list. When they correctly identified the second to last odor, the rats were given a reward (food). But how can we be certain that the rodents remembered not only the unique events but a sequence of events occurring in a specific order? Before each test, the research team modified the number of odors used in the list. Thus, they made sure that the rats weren’t just using their sense of smell to identify a familiar odor, but also their episodic memory, which is crucial for remembering the entire list in the original order and ultimately identifying the position of the target odor (always the second to last).

In nearly 87% of the trials, the animals succeeded at the task. Additional experiments even proved that the rats’ memories were long lasting (still present on hour later) and resistant to “interferences" from other memories coming from other memory tasks in which the rodents participated in the meantime. To confirm the major role of episodic memory, the experimenters also temporarily inhibited the rats’ hippocampi.

The results of this study suggest that rats’ ability to replay a sequence of episodic memories developed early on from an evolutionary standpoint. The authors also suggest that these animals could be used as models to study the functioning of episodic memory in humans. Indeed, according to J.D. Crystal: "The reason we’re interested in animal memory isn’t only to understand animals, but rather to develop new models of memory that match up with the types of memory impaired in human diseases.”

Keep this study about the discovery of episodic memory in mind because, in a few years, it could be the source of other promising research on memory dysfunction in humans.