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Like humans, rats can voluntarily reactivate mental maps of familiar places.ESVETLEISHAYA/ISTOCK
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Close your eyes and picture yourself running an errand across town. You can probably imagine the turns you’d need to take and the landmarks you’d encounter. This ability to conjure such scenarios in our minds is thought to be crucial to humans’ capacity to plan ahead.
But it may not be uniquely human: Rats also seem to be able to “imagine” moving through mental environments, researchers report today in Science. Rodents trained to navigate within a virtual arena could, in return for a reward, activate the same neural patterns they’d shown while navigating—even when they were standing still. That suggests rodents can voluntarily access mental maps of places they’ve previously visited.
“We know humans carry around inside their heads representations of all kinds of spaces: rooms in your house, your friends’ houses, shops, libraries, neighborhoods,” says Sean Polyn, a psychologist at Vanderbilt University who was not involved in the research. “Just by the simple act of reminiscing, we can place ourselves in these spaces—to think that we’ve got an animal analog of that very human imaginative act is very impressive.”
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Researchers think humans’ mental maps are encoded in the hippocampus, a brain region involved in memory. As we move through an environment, cells in this region fire in particular patterns depending on our location. When we later revisit—or simply think about visiting—those locations, the same hippocampal signatures are activated.
Rats also encode spatial information in the hippocampus. But it’s been impossible to establish whether they have a similar capacity for voluntary mental navigation because of the practical challenges of getting a rodent to think about a particular place on cue, says study author Chongxi Lai, who conducted the work while a graduate student and later a postdoc at the Howard Hughes Medical Institute’s Janelia Research Campus. In their new study, Lai, along with Janelia neuroscientist Albert Lee and colleagues, found a way around this problem by developing a brain-machine interface that rewarded rats for navigating their surroundings using only their thoughts.
First, the team designed a miniature virtual reality arena and displayed it on a screen surrounding a spherical treadmill, a bit like the trackball of a computer mouse. Rats could scamper through this arena by running on the treadmill, and if the animals navigated to particular objects in the arena, they’d receive a sugary reward. Throughout the rats’ wanderings, the team measured brain activity in the animals’ hippocampus.
Next, Lai and colleagues disconnected the treadmill so that the rats could still see the virtual reality arena, but their running had no impact. Instead, the team hooked up the display to real-time readings of the rats’ brain activity. By reproducing the brain activity they’d showed during the previous training sessions, the rats could navigate to reward locations using just their thoughts. Some scampered futilely on the treadmill as they did so, but others remained still.
In another version of the setup, the rats’ brain activity was used to control the location of a box on the screen rather than the animals’ own movement in the arena. (The researchers call this the “Jedi” experiment, after the telekinetic powers in Star Wars.) Once again, the team found that the rats could reactivate neural patterns from the training sessions to steer the box toward a goal and earn a reward.
Shayna Rosenbaum, a cognitive neuroscientist at York University, says the researchers took a clever approach “to gain insight into the inner workings of the rodent mind.” Although it’s not clear what the rat is experiencing while it’s navigating or how closely it mimics what happens in humans, the team has done a convincing job of showing rodents can think “about something that’s not in their immediate vicinity, and [re-engage] a pattern of activity when they’re not actually moving to a location,” she says. “I think it does constitute imagining.”
The work could help researchers learn much more about hippocampus function, Rosenbaum says, adding it would be interesting to see what other regions might be involved in mental navigation. The team is working on such questions now, says Lee, who has since moved his lab to the Beth Israel Deaconess Medical Center. He and colleagues also want to test whether rats can access hippocampal representations of distant remembered locations, as if “you’re on your couch, and you’re thinking about Spain.”
