For the first time, a non-human animal has demonstrated the ability to recall events from their past and use them to answer an unexpected question. The breakthrough comes after the possibility was first explored six years ago and could potentially unlock new treatment avenues for patients with memory loss.
Episodic memory is the string of past experiences in our lives that we can recall as specific events. From your 16th birthday party to your best friend’s wedding, and even where you parked your car this morning, that’s all captured as episodic memory. We can access details about these events much later, even if they didn’t seem so important at the time.
We know from studies that various animals also possess this type of memory, and a 2018 study from a team at Indiana University was the first to report that rats can mentally replay a series of events from their past, just as humans can. The same team has now delved into this further and discovered that the rodents can access these memories and use them to their advantage.
Rats may not appear to be the most obvious model organism to test this on – primates might seem like a more human-like starting point – but first author Cassandra Sheridan disagrees.
“Using rats can help further the field of memory by providing a way to identify and measure behavioral patterns and changes that are more sophisticated and complex than mouse models, as well as model diseases like Alzheimer's to test treatments before they reach the clinical stages,” Sheridan explained in a statement. “This is what I am most excited about.”
Working with Professor Jonathon Crystal, who also supervised the 2018 study, Sheridan and colleagues created an experiment that would require the rats to use not only their episodic memories, but also their impressive noses.
First, the nine rats were presented with a selection of common spices one by one, including cinnamon and paprika. They then underwent a memory assessment, whereby they were shown two of the scents and then had to choose which scent was the third-to-last that they had smelled.
The rats were then put into a radial maze that contained spice containers with scented lids. After sniffing around the maze, they were once again given the chance to identify the third-to-last scent from the original list.
The researchers only had to run the experiment once: every single rat answered the question correctly after their time in the maze, giving a success rate of an astonishing 100 percent.
“What we wanted to test is a property of what people do in everyday life that has never been shown in a non-human animal,” said Crystal. “We remember information even though it was seemingly unimportant when it was encountered. When we happen to need that information, we replay the stream of events to identify the information needed to solve our current problem.”
The findings not only tell us more about the impressive cognitive abilities of rats, but they could also have important implications for research into human disease. Many drugs for Alzheimer’s disease that are tested in animals ultimately do not succeed in human trials. Crystal believes that this could be due to a lack of specific focus on episodic memory, and hopes that the team’s findings could help inform the next generation of treatments.
“What you really want to know when you test Alzheimer’s patients is that there is relief in remembering that their granddaughter visited last week and talked about interesting things going on in her life. Those are the things that are going to produce huge societal changes when the drugs are effective, not just at general aspects of memory but specifically targeting episodic memory.”
The study is published in the journal Current Biology.