Rats can use imagination to mentally recreate places they've visited

Just like someone may look back on a treasured holiday, wishing they were still there, rats can also imagine being in places they’ve visited before.

A region of the brain called the hippocampus, which plays a big role in memory, is very consistent across mammals. This led scientists to suspect that non-human animals are similarly capable of imagining places they have been to before, but it is difficult to prove that such a brain process occurs. “You can’t talk to animals and ask them to imagine something,” says Albert Lee at the Beth Israel Deaconess Medical Center in Boston, Massachusetts.

He and his colleagues hoped that advances in technology could overcome this by creating an interface between the animals’ brains and a machine, allowing them to communicate.

To put the idea to the test, the researchers devised a 360-degree virtual reality (VR) arena. They had three rats walk on a treadmill within this, but the VR made it look to the animal like it was moving through a space resembling a dark tunnel.

The rats were trained to find certain shapes within the VR for a reward of sugary water. While they looked for these shapes, electrical signals were recorded from their hippocampus. The researchers used this data to produce a brain-machine interface that reverse engineered the signals produced by the rats into images that were then displayed in the VR arena.

They also wondered if a rat imagining a certain location within the VR, which involves the activation of certain hippocampal cells, would similarly cause it to appear in the arena.

In a second experiment, the researchers tested this idea by getting the rats to do two tasks. These both involved the rats being placed on a treadmill, but, unlike before, moving didn’t affect what they saw.

In the first task, the rats had to imagine navigating to a particular shape they had already seen, for instance a circle, to receive sugary water. By envisaging locations in the VR that they had been through, the rats activated certain hippocampal cells, which was translated by the brain-machine interface into images in the VR.

The rats quickly learned that moving on the treadmill didn’t get them closer to the shape, but imagining how they would reach it led to what they saw in the VR changing, bringing them closer to the circle, says Lee. “They grasp it really fast,” says team member Chongxi Lai at the Howard Hughes Medical Institute in Maryland.

In the second task, the rats learned to move a shape in the VR to a specific location using their imagination. Unlike in the first task, the world around them didn’t move, which caused the animals to move less themselves. “We wanted them to sit still and imagine, just like humans,” says Lai.

The rats did both tasks successfully, the researchers write in their paper. “This is the first time it’s been shown that animals can control this internal model of the world in their hippocampus,” says Lee. “It’s a critical step that underlies imagination.” Lee expects that the similarity of the hippocampus across mammals means they are all capable of such thoughts.

But how similar a rat’s imagination is to a person’s remains an open question, says Lee, who adds that the researchers plan to examine the extensiveness of rats’ imagination in future studies.

“The study shows that rats can imagine doing things in a way very similar to us, without actually doing them,” says Frank Sengpiel at Cardiff University, UK. “The rats were able to wilfully control the activity of the nerve cells in their hippocampus, very much like what we do when we imagine things. To my mind, this shows not only that rats are capable of mental time travel, but also that they are conscious.”