Rats can recreate in their imagination the places they have visited
Using virtual reality, researchers have studied the ability of rats to "dream".Scientists fitted several rats with a device that monitored brain activity and watched the rodents mentally maneuver in a virtual reality environment. The results of the study show that the rats are able to think about places and objects that are not directly in front of them.
Previous research has shown that the rodents' hippocampus stores a "model" of each environment they explore. These detailed cognitive maps are created by spatial neurons, place cells that are activated when the animals reach a particular area in the environment. The researchers used the activity of these cells to learn what the animals were thinking.
The researchers developed a neural interface and implanted it in the rodents' brains. The device recorded electrical activity in the rats' hippocampus and quickly streamed it to a decoder. This allowed the researchers to track which spatial neurons were activated. "If an animal generated a pattern of neural activity associated with a corner of the room near a window, the decoder would output that position," said Albert Lee, a co-author of the study.
After surgery, the rats were placed on a spherical treadmill surrounded by screens. The researchers used a projector to broadcast a virtual reality environment onto the screens, which created the illusion that the rats were moving in space. In the first stage, the animals were allowed to explore the virtual space and build a "map" of it, with scientists recording spatial neurons whose activation corresponded to each location.
The scientists then secured the rats on a treadmill, not allowing the animals to move freely through the virtual environment. Instead, the rats' movement in the virtual reality simulation was controlled by which spatial neurons they activated in their minds. This forced them to mentally visualize the steps they would have to take to get the reward.
In the second task, the rats could not move through the virtual environment at all. To get the reward, they had to move an external object to the designated reward location, like a Jedi summoning a distant lightsaber. But instead of the Force, the rats had to use their spatial neurons to guide the object through virtual reality.
The researchers found that their rodent subjects were perfectly capable of doing both tasks. The rats voluntarily reactivated location-specific neurons in their hippocampus when they remembered the remote location of their reward. And in the "Jedi" pattern, by activating the neurons, the rodents moved their attention to the desired location and held it on the distant object for several seconds.