Rats in augmented reality help show how the brain determines location
Before the age of GPS, humans had to orient themselves without on-screen arrows pointing down an exact street, but rather, by memorizing landmarks and using learned relationships among time, speed, and distance. They had to know, for instance, that 10 minutes of brisk walking might equate to half a mile traveled.
A new Johns Hopkins study found that rats’ ability to recalibrate these learned relationships is ever-evolving, moment-by-moment. The findings, published in Nature, provide insight on how the brain creates a map inside one’s head.
“The hippocampus and neighboring regions in the brain help us figure out where we are in the world,” says Manu Madhav, a postdoctoral associate in the Johns Hopkins Zanvyl Krieger Mind/Brain Institute and one of the study’s primary authors. “By studying the firing patterns of neurons in these areas, we can better understand how we map our location.”
The brain receives two types of cues that aid in this mapping; the first is external landmarks, like the pink house at the end of the street or a discolored floor tile that a person remembers to mark a certain location or distance.
“The second type of cue is from one’s self-motion through the world, like having an internal speedometer or a step-counter,” says Ravi Jayakumar, a primary author on the paper and PhD candidate in the Department of Mechanical Engineering. “By calculating distance over time, based on your speed or by adding up your steps, your brain can estimate how far you’ve gone even when you don’t have landmarks to rely on.” This process is called path integration.
But if you walk for 10 minutes, is your estimate of how far you’ve traveled always the same or is it molded by your recent experience of the world? To investigate this, the research team studied rats running laps around a circular track. They projected various shapes to act as landmarks onto a planetarium-like dome over the track and moved the shapes either in the same direction as the rats or the opposite way. As in a computer game, the landmark speed depended on how fast the animal was running at each moment, creating an augmented reality environment where rats perceived themselves as running slower or faster than they actually were.
During these experiments, the research team studied the rats’ ‘place cells,’ or hippocampal neurons that fire when an animal visits a specific area in a familiar environment. When the rat thinks that it has run one lap and has returned to the same location, a place cell would fire again. By looking at these neurons’ firing pattern, the researchers determined how fast the rat thought it was running through the world.