Neuroscientists discover that animals replay accidentally encoded episodic memories

Indiana University neuroscientists have discovered the first evidence that rodents can draw on their episodic memory (the ability to remember streams of seemingly unimportant events from the past) and use accidentally encoded information to respond to unexpected questions.

This breakthrough in neuroscience research, which builds on a 2018 study that first reported evidence that animals can replay past events, could potentially lead to advances in the development of therapies specifically targeting Episodic memory loss in patients with Alzheimer’s disease.

The study, led by IU graduate student Cassandra Sheridan, is published in Current biology.

Sheridan, working with Jonathon Crystal, senior professor of psychology and brain sciences in the College of Arts and Sciences at IU Bloomington, sought to observe whether rats could rely on a stream of past memories to respond to unexpected questions.

It is widely accepted that the use of primates in cognition research will produce the most valuable and accurate results. However, Crystal and her lab believe that precursors to cognition may be more widespread in the animal kingdom than simply an enhancement that evolved with primates. Rats also exhibit remarkable olfactory specialization, which is a key aspect of this study.

“Using rats can help further the field of memory by providing a means of identifying and measuring more sophisticated and complex patterns of behavior and changes than mouse models, as well as modeling diseases like Alzheimer’s disease to test treatments before they reach clinical stages.” » said Sheridan. “That’s what I’m most passionate about.”

Crystal’s lab created an experimental scenario that would require rats to use episodic memory to solve a series of tests. The researchers gave nine rats a list of odors using many common household spices, like cinnamon and paprika. Next, the rats underwent a memory assessment in which they were presented with two odors from the previous list. The rats then have to choose which odor is the penultimate odor presented.

To assess whether the rats could combine their ability to replay episodic memories with the ability to answer unexpected questions, the researchers placed the rats in a radial maze where they faced scented lids covering the food. After the rats had run through the maze, they were given the opportunity to report the penultimate odor, having to draw from memory the previously presented odors.

The rats remembered several supposedly unimportant pieces of information and then replayed a stream of episodic memories when that information was needed to solve an unexpected problem. The first and only trial ended with a 100 percent success rate.

Until this study, there was no scientific evidence that non-human animals were capable of replaying incidentally encoded streams of episodic memories.

“What we wanted to test is a property of what people do in everyday life that has never been demonstrated in a non-human animal,” Crystal said. “We remember information even if it seemed unimportant at the time it was encountered. When we need this information, we replay the stream of events to identify the information needed to solve our current problem.”

This development in episodic memory research could have an incredible impact on future Alzheimer’s disease research, Crystal said. The development of drugs and therapies against Alzheimer’s disease experiences a high failure rate due to the low transferability of animal models to human treatments. According to Crystal, most Alzheimer’s research and clinical trials that use animal models typically focus on cognition broadly, not episodic memory.

“This could be part of the puzzle of why translation failure occurs,” Crystal said. “What you really want to know when testing Alzheimer’s patients is that there is relief in remembering that their granddaughter visited them last week and talked about things interesting things happening in one’s life. These are the things that will produce huge societal changes when the drugs are effective, not only on general aspects of memory, but in specifically targeting episodic memory.