In Harry Potter, the young wizard is given a piece of parchment called the Marauder’s map on which is a detailed layout of Hogwarts School of Witchcraft and Wizardry. The magical map reveals the movements of people (and ghosts) through the school with evanescing ink footsteps. In the hippocampus—a small horseshoe-shaped area of mammalian brains—there is a kind of Marauder’s map, which keeps track of other individuals’ movements.
Two new studies published last week in Science show the hippocampus is not only responsible for figuring out an animal’s own position in space—something previously known—but also that of others. This finding explains why soccer games do not always end up with a pile of humans in the middle of the field—players are really good at not bumping into one another.
In 1971 John O’Keefe, a neuroscientist at University College London (U.C.L.), and his student Jonathan Dostrovsky discovered “place cells”—neurons in the hippocampus that fire when an animal goes to a specific spot in space. This finding earned O’Keefe a Nobel Prize in 2014. But until recently research had only looked at how the brain maps an animal’s own position.
Being able to map where others are in space, “is important for any kind of social interactions [such as] courtship, coordinated hunting [and] monitoring the position of predators or prey,” says Nachum Ulanovsky, a neurobiologist at the Weizmann Institute of Science in Israel and senior author of one of the studies. “Here, in one bang, we had two studies in two different species of mammals showing [how the brain does this] for the first time.”
Ulanovsky and his team trained four pairs of Egyptian fruit bats to fly to and from a set of raised balls in a tiny room. Each pair began perched on a “start” ball. The alpha bat was trained to fly across the room to either ball and back whereas the other male was trained to stay still and observe. Soon the observer bat realized that if it flew the same path as the other bat, it would receive some delicious banana mush. Using tiny implanted electrodes, the researchers recorded the activity of neurons in the observing bat’s hippocampus as it stood still and watched the alpha as well as while it traced a similar path across the room.* They found that 18 percent of the 378 neurons they recorded fired when the bat was watching the alpha. They named these neurons “social place cells.” Whereas some of these only fired in response to perceiving the positions of the alpha bat, others also fired when the observer bat itself moved, meaning they also functioned as typical place cells.
The hippocampus has long been known to be involved in navigation and memory—and it is one of the first regions to be affected by Alzheimer’s disease. But recent research had suggested the hippocampus might also play a role in social interactions.
Ulanovsky wanted to test if these neurons were specifically responding to a social cue. His team conducted the same experiments, except they replaced the alpha bat with a similar size plastic football or cube. They manually moved the objects to and from the start position to the two balls across the room and again trained the bat to mimic the trajectory. They found the observer bat’s neurons fired at different times when it watched the two moving objects rather than when it watched the alpha bat.
“I think it’s an exciting finding, definitely, because it moves the focus for the hippocampus a little bit away from just representing self-location,” says Edvard Moser, a neuroscientist at the Norwegian University of Science and Technology. Moser and his wife at the time, May-Britt Moser, discovered another type of navigational neuron called a grid cell, for which they shared the Nobel Prize with O’Keefe. But, Edvard Moser adds, social place cells may not necessarily be confined to their role of firing in “social” situations. “I think social cognition is just one example of many things that can be represented in the hippocampus.”
Thousands of miles away, Shigeyoshi Fujisawa, a neuroscientist at the RIKEN Brain Science Institute in Japan independently performed a similar experiment with a different social creature. “People think place cells are like a map in the brain, but if this is actually a map, it should be used for not only [one’s own position] but for plotting other information,” Fujisawa says—for example, tracking other animals and having better social interactions.
His team similarly had one thirsty rat observe a demonstrator rat navigate a T-shaped maze. In two separate experiments they trained the observer to go to either the same or opposite side of the demonstrator in order to receive a water reward. But to know where the reward was located, the observer had to pay attention to where the demonstrator rat went. As the observer watched the other rat move through the maze, the researchers measured brain activity in its hippocampus via implanted electrodes.
Place cells can also fire based on where the animal intends to go. But their experiment helped differentiate those neurons from the ones that fired in response to observing another rat’s movements. If the observer rat was trained to know the water would be waiting for it at the opposite side of the demonstrator, its goal-oriented neurons would not be firing when observing the demonstrator scurry the other way.
The researchers found 85 percent of the more than 1,000 place cells they studied also fired in response to the other rat’s position. Fujisawa and his team proposed the hippocampus maps out space in different ways that encode the position of the animal itself, the locations of other animals or both together. “As long as the animal cares [about] other animals or objects, I think there is a representation,” Fujisawa says. He doesn’t necessarily think it has to be a response to a social situation.
Kate Jeffery, a neuroscientist at U.C.L. who was not part of either study, found the results interesting. She hopes subsequent experiments will be able to parse the subtle description between these neurons firing in response to a “social” cue or something else entirely. “It might just be something that is directing the attention of the animal to that point in space,” she suggests. “What we don’t know is, is it that this rat cares about where that rat is or does it just care about the place?”
There are other unknowns, such as what would happen if the rats and the bats were placed in the same room. Would each species care enough about the other to cause brain sparks to fly? And is this behavior unique to rats and bats or is it generalizable to all mammals? “At least to me,” Moser says, “it seems quite likely that if it is present both in rats and in bats, it’s very likely that it exists across most or many mammals.”
*Editor’s Note (1/18/18): This sentence was updated after posting to correct an error. The bat brain region the researchers recorded from was the hippocampus, not the hypothalamus.