A brain system that helps us find our way to the supermarket may also help us navigate a lifetime of memories.
At least that's the implication of a study of rats published in the journal Neuron.
It found that special brain cells that track an animal's location also can track time and distance. This could explain how rat and human brains are able to organize memories according to where and when an event occurred.
The cells, called grid cells, appear to be "laying down the sequence of space and time that provide a framework for events that are unfolding," says Howard Eichenbaum, an author of the study and director of the Center for Memory and Brain at Boston University.
Grid cells were discovered in 2005 by two Norwegian researchers who won the Nobel Prize in 2014. Until now the cells have been known primarily for their GPS-like function, which helps the brain maintain an internal map of the environment.
"This is the first evidence that I'm aware of to directly show that grid-like representations are involved in representing time. And time is, of course, a critical aspect of memory," says Joshua Jacobs, an assistant professor of biomedical engineering at Columbia University who was not involved in the study.
The finding appears to unite several different lines of research involving navigation and memory.
Scientists have known for many years that areas of the brain involved in navigation are also involved in memory. And they've also known that these areas, the hippocampus and entorhinal cortex, are among the first to be affected by Alzheimer's disease.
"People with Alzheimer's disease often have navigational problems," Eichenbaum says. Without a working GPS system in their brains, he says, "they can't put themselves into the environment that they're in so they seem to be lost."
The goal of the new rat study was to learn more about the link between navigation and memory, Eichenbaum says. So his team monitored grid cells while each rat ran on a treadmill.
That meant the animal, though running, was no longer moving through space. As a result the brain's GPS couldn't rely on motion or changes in scenery to trigger activity in the grid cells.
Yet most grid cells continued to fire in a different, but still predictable, way. They weren't responding to visual information anymore, but instead to the distance the animal had run or the amount of time that had elapsed.
"So at the beginning the animal might run for, say, five seconds and all of a sudden the cell fires in a burst," he says. And the burst would occur at the same time during every treadmill run. This suggested that instead of representing space, the grid cells had begun representing time.
And time is one of the most important ways the human brain organizes and navigates episodic memories, the collection of personal experiences our brains acquire throughout life, Jacobs says.
You can get a glimpse of how we navigate these memories if you imagine what happens when you realize you've misplaced your keys. "You might think of where you were when you put your keys down," Jacobs says. "You might think of the time of day you put your keys down and you might kind of start rehearsing back and forth in time around that moment."
What you're doing, he says, is trying to figure out where you were in your day, much the way you try to figure out where you are in a city when you drive.
Jacobs says it makes sense that evolution would take a brain system with a relatively simple job, navigating, and apply it to a much more complex task, organizing memories. Even so, he's cautious about reading too much into what happens in a rat's brain during a few seconds on a treadmill.
If grid cells in the hippocampus and entorhinal cortex are truly helping us navigate our memories, they are "not only representing a few seconds of time, but thousands and thousands of seconds of time over the course of one's lifetime," he says. "And that's a significant jump."
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