The oscillation occurs when a person or animal is traveling from one place to another. When the person or animal stops, so does the oscillation. But it's never been clear whether the signal is truly helping with navigation or just a related function, like movement or processing visual information.
"When you're walking around the world you have all this incoming information," Vass says. "You have your feet moving and stepping on the ground. You've got landmarks and buildings that you recognize and street signs."
So the UC Davis team designed an experiment that allowed them to eliminate all that information by having people travel in a virtual environment via teleporter. "With teleportation, you don't see anything," Vass says. "And nobody feels anything. You're not walking anywhere."
Yet you are still traveling. So your brain still needs to somehow navigate itself to a particular place.
The team figured that if the slow oscillations in the hippocampus were associated with movement or sensory information, they would stop during teleportation. But if they were involved in navigation, they would continue.
To find out, the team recruited three volunteers who were awaiting surgery for severe epilepsy. As part of the treatment, surgeons had temporarily placed wires in the patient's brains that monitored electrical activity. This let the UC Davis team measure low frequency oscillations coming from the hippocampus as they happened.
The volunteers were asked to navigate from place to place in a virtual town. One option was to walk. But in several spots, players could enter a teleportation port that would act as a short cut to a specific spot in town.
"They would pop out of the teleporter, and we would give them a new place to find," Vass says. "So we would say OK, now go find the pet store. And they have to know, 'OK, based on the direction I'm facing now, do I need to turn to the right or the left or is it behind me?' "
As expected, the brain oscillations in the hippocampus were present while players were moving about in the virtual environment and absent when they stopped.
"So the critical question for us was what happens as soon as you enter the teleporter," Vass says. "What happens to the oscillation? And what we found was it was still present."
What's more, the oscillation changed a bit depending on whether the person had been teleported a long distance or a short one.
And, when the volunteers arrived, they knew where they were. During teleportation, their brains had successfully navigated to the new destination.
"This is what we call mental navigation," says Gyorgy Buzsaki, a brain scientist at New York University who wasn't involved in the teleportation study. "Somehow the brain can disengage from the environment and we are still navigating mentally."
And the study's results suggest those low frequency oscillations are coordinating the activity of neurons involved in mental navigation, Buzsaki says.
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