“For a long time, people have speculated that we use the same mechanisms to reason about other people as about our hypothetical selves,” says Rebecca Saxe from MIT. “So this new study fits really well.”
Saxe should know. She was one of the first scientists to link the rTPJ to theory of mind—the ability to understand the mental states of other people. In 2005, she and Nancy Kanwisher scanned people’s brains while they listened to stories in which protagonists made poor choices based on false beliefs. This experiment showed that the TPJ is active specifically when people are “reasoning about the contents of another person’s minds”—the essence of theory of mind. This region, the duo wrote, helps people to think about thinking people.
At the same time, many other neuroscientists were doing similar experiments and getting the same answers. The consensus was striking, Saxe later wrote. “Because there was almost no pre-existing neuroscience of theory of mind, researchers came to the topic with unusually few preconceptions about where to look in the brain. In those circumstances, neuroimaging is notoriously fickle, producing many false positives and false negatives. Yet every group that sought to identify brain regions implicated in ToM got essentially the same answer; and in study after study, we still do.”
Many other studies have since expanded on those early results. If the rTPJ is bigger, people are more likely to behave altruistically. If the neurons within it are better-connected (and well-linked to other parts of the brain), people show less bias towards their own in-groups. If the area is stimulated by electric currents, people become better at taking someone else’s perspective.
And if the region is disrupted, it changes our ability to reason about morality. Consider a woman who poisons her friend’s coffee—if she does so deliberately, we’d judge her more harshly than if she acted accidentally. Intent matters, and we need the rTPJ to judge intent. When Liane Young, one of Saxe’s former students, disrupted the rTPJ using magnetic fields, she found that people were more lenient towards the deliberate poisoner, as long as her friend survived. With their ability to gauge intent disrupted, they started looking to outcomes instead.
Not everything fits with the idea of the rTPJ as a nexus for theory of mind. For example, many studies suggest that it affects our ability to shift our attention from one part of space to another, like a technician moving a spotlight around. “Even in my own small lab, people disagree about the function of the rTPJ,” says Young, now a professor at Boston College.
If you look at the debate and relax your eyes, you can probably merge the two viewpoints into one. Maybe the rTPJ is a region that redirects our attention from one thing to another—whether between objects in the world around us, or between our minds and other people’s. Alternatively, it’s likely that what we call the rTPJ is not actually a singular bit of the brain. “There’s a lot of work suggesting that there are different sub-regions—one of which does spatial reorienting, and the other does perspective-taking,” says Young.