For the first time, neuroscientists at Indiana University (IU) have demonstrated that non-human animals can mentally replay past memories.
That's a pretty spectacular finding - but it could also potentially help out researchers looking to develop new treatments for memory loss that happens in Alzheimer's disease and similar afflictions.
"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," said lead researcher, neuroscientist Jonathon Crystal from IU.
Although scientists already knew that rats could remember multiple unique episodes, until now it was unknown whether they could replay a string of events in order.
As a result, the discovery could shift the current paradigm for Alzheimer's memory loss research.
For years, most preclinical studies on Alzheimer's drugs have been based around how these drugs affect spatial memory tests - mostly because spatial memory is easy to measure in animals.
This has been quite limiting for the trials, however, because the most debilitating effects of Alzheimer's are not caused by spatial memory.
"If your grandmother is suffering from Alzheimer's, one of the most heartbreaking aspects of the disease is that she can't remember what you told her about what's happening in your life the last time you saw her," said first author, Danielle Panoz-Brown.
"We're interested in episodic memory - and episodic memory replay - because it declines in Alzheimer's disease, and in ageing in general."
Episodic memory is distinct from spatial memory; it is essentially our ability to recall specific events in the correct order.
The classic example for episodic memory is when you lose something, and this is one of the reasons why people with memory impairment are always misplacing stuff.
In order to figure out where you lost your car keys, for instance, you need to mentally retrace your steps from "episode" to "episode".
But if you can't recall each episode or place the episodes in chronological order, you're bound to end up terribly confused and still without your keys.
To figure out how exactly we can measure episodic memory in animals, Crystal's lab has spent nearly a year working with 13 rats.
The researchers first trained the rats to memorise a list of 12 unique smells in order. Then, the rats were placed inside several "arenas", each with a slightly different pattern of these 12 smells.
In the arenas, the rats were rewarded if they identified the second odor on the list and the fourth odor on the list.
The researchers wanted to make sure, however, that the rats were recalling not just stagnant events, but a stream of events in the correct order.
As such, the team mixed up the order of the list before each test to ensure that these rats were using their episodic memory to recall the actual list and not just using their sense of smell to identify a familiar odor.
Overall, the rats were able to complete the task about 87 percent of the time, well above random chance. Crystal says this is strong evidence that they were using episodic memory replay.
Furthermore, some follow-up experiments confirmed that these memories were long-lasting (even after an hour delay) and resistant to "interference" from other memory tasks – two aspects which are both representative of episodic memory.
Still, the researchers had not ticked all the boxes for episodic memory just yet.
By temporarily inhibiting the hippocampus – the main site for episodic memory activity in the brain – the researchers were able to confirm that the rats were using this part of their brain to identify the smells.
Now, the researchers were confident that they had indeed demonstrated for the first time that animals can use episodic memories when trying to remember a sequence of events.
The findings suggest that episodic memories go far back in the evolutionary timescale, and that rats can be used as models for investigating the workings of human episodic memory.
The next step is to come up with new ways of testing and measuring episodic memory in rats. That way, future preclinical trials for Alzheimer's drugs can begin drawing on episodic memory as well as spatial memory.
"We're really trying push the boundaries of animal models of memory to something that's increasingly similar to how these memories work in people," said Crystal.
The study has been published in Current Biology.