A team of Indiana University researchers has reported the first evidence that non-human animals (rats) can replay a stream of multiple episodic memories. The study was published in the journal Current Biology.

Episodic memory is the ability to remember specific events. For example, if a person loses their car keys, they might try to recall every single step — or ‘episode’ — in their trip from the car to their current location.

The ability to replay these events in order is known as ‘episodic memory replay.’

“People wouldn’t be able to make sense of most scenarios if they couldn’t remember the order in which they occurred,” said Professor Jonathon Crystal, senior author on the study.

To assess animals’ ability to replay past events from memory, Professor Crystal and colleagues spent nearly a year working with 13 rats, which they trained to memorize a list of up to 12 different odors.

The rats were placed inside an ‘arena’ with different odors and rewarded when they identified the second-to-last odor or fourth-to-last odor in the list.

The scientists changed the number of odors in the list before each test to confirm the odors were identified based upon their position in the list, not by scent alone, proving the animals were relying on their ability to recall the whole list in order.

Arenas with different patterns were used to communicate to the rats which of the two options was sought.

“After their training, the animals successfully completed their task about 87% of the time across all trials,” Professor Crystal said.

“The results are strong evidence the animals were employing episodic memory replay.”

Additional experiments confirmed the rats’ memories were long-lasting and resistant to ‘interference’ from other memories, both hallmarks of episodic memory.

The team also ran tests that temporarily suppressed activity in the hippocampus — the site of episodic memory — to confirm the rats were using this part of their brain to perform their tasks.

“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 such as Alzheimer’s disease,” Professor Crystal said.

“Under the current paradigm, most preclinical studies on potential new Alzheimer’s drugs examine how these compounds affect spatial memory, one of the easiest types of memory to assess in animals.”

“But spatial memory is not the type of memory whose loss causes the most debilitating effects of Alzheimer’s disease.”

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Danielle Panoz-Brown et al. Replay of Episodic Memories in the Rat. Current Biology, published online May 10, 2018; doi: 10.1016/j.cub.2018.04.006