Rats can imagine ‘virtual maps’.

We can imagine the route to work or the supermarket without actually walking it; we can, as it were, conjure up a virtual map in our minds and walk along it. This ability to mentally imagine virtual scenarios, such as commuting, is considered crucial to people’s ability to plan in advance.

When we imagine the path to a place, brain cells become active in a specific brain region, the hippocampus. This stores mental maps of the world that are used to remember past events and imagine future scenarios. Memory retrieval is associated with the generation of specific activity patterns in the hippocampus associated with locations and events.

Neuroscientists led by Chongxi Lai of the Howard Hughes Medical Institute Janelia Research Campus in Virginia, US, investigated whether rats can also mentally imagine places they have been before. This is not an easy task, because you can’t just talk to animals and ask them to imagine something. The results of the research have been published in the scientific journal “Science”.

The researchers tried to create a direct connection between brain activity and an external device using a brain-machine interface (BMI) and thus detect the processes in the rats’ brains. In the experimental setup developed over nine years, BMI linked the electrical activity in the rat’s hippocampus to its position in a 360-degree virtual reality area.

The rodents perceived this so-called virtual reality (VR) arena through a miniature screen in front of their eyes as they moved on a ball that responded to the haptic feedback from their paws; similar to the trackball of a computer mouse. The animals had the impression that they were moving through a dark tunnel and navigating a virtual maze. When they visited certain objects there, they received a sweet reward. As the rats moved through the VR arena, the researchers measured brain activity in the rodents’ hippocampus.

The scientists then created a kind of “thought dictionary” that they could use to decode the rat’s brain signals – that is, a compilation of the activity patterns in the hippocampus when the rat experiences something specific. The researchers were able to track which neurons were activated as the rat navigated the VR arena to reach the location with its reward. These signals formed the basis for the real-time hippocampal BMI in the next step, which involved translating the brain’s hippocampal activity into on-screen actions.

In this next step, the researchers turned off the spherical treadmill on which the rat moved. The animal could still see the VR arena, but its movements had no influence on it. To do this, the researchers connected the VR arena to the activity patterns in the rat’s hippocampus in real time. They rewarded the rodents if they reproduced the activity pattern in the hippocampus associated with a specific target location. The BMI translated the animal’s brain activity into movement on the screen of the VR arena.

The rat then navigated to its reward by thinking about where to go to get it. Some test animals moved on the disconnected treadmill, but others remained calm.

In a next step — the researchers called this experimental setup the “Jedi experiment,” referring to telekinetic powers in “Star Wars” — the rat’s brain activity was used to direct an object on the screen to a specific location. Even better, the rats managed to reactivate neural patterns from the training sessions to move a box to a target in the VR arena and thereby receive a reward.

The animals moved the virtual object using their thoughts alone – just as someone sitting in their office chair might imagine picking up a cup next to the coffee maker and filling it with coffee. The scientists then changed the location of the target and the test animals actually produced new activity patterns associated with the new location.

The rodents could control their hippocampal activity precisely and flexibly, just as humans probably do. The rats were also able to sustain this activity in their hippocampus, i.e. focus their thoughts on a specific location for many seconds. This is similar to the time frame in which people relive past events or imagine new scenarios. Study co-author Tim Harris noted:

The results of the study also show that BMI can be successfully used to investigate hippocampal activity. According to the authors, as the technology is increasingly used in prosthetics, the current work also opens the possibility of developing new prosthetic devices based on the same principles. (mr)