Too close? Because of a damaged amygdala, patient SM has a smaller personal space than normal.

Daniel Kennedy et al. Nature Neuroscience (2009)

Don't Stand So Close to Me

In a famous episode of the TV show Seinfeld, a "close talker" makes others uncomfortable by standing mere centimeters from their faces while speaking. What makes this invasion of our personal space so uncomfortable? A new study fingers the amygdala, a region of the brain that acts like a warning bell when someone gets too close for comfort.

Psychologists have studied personal space since the 1960s. They've found that Americans and northern Europeans prefer a larger personal space than southern Europeans, for example, whereas people with autism tend to unknowingly invade others' personal space. Studies in monkeys have hinted that the amygdala, an almond-shaped region in the middle of the brain that helps us recognize threats, plays a role in personal space. But the theory proved hard to test in humans.

Then, about 15 years ago, neuroscientists at the California Institute of Technology in Pasadena met a 42-year-old woman with a rare genetic disorder that destroyed both sides of her amygdala. In early experiments, the scientists discovered that the woman, referred to as SM, couldn't spot fear in other people's faces; she also rated people as more trustworthy than an average person did. And she was extremely outgoing, "almost to the point where it isn't normal," says team member Daniel Kennedy. Even if she's only just met someone, he says, SM will invade their personal space--touching their arm as she talks or poking their stomach.

In the new study, Kennedy and his colleagues more rigorously tested SM's sense of personal space. They compared her with 20 healthy subjects in a series of experiments. In one test, an experimenter slowly walked toward a subject until the subject felt uncomfortable and told the experimenter to stop. SM let experimenters get about twice as close as other subjects did, 0.34 meters versus 0.64 meters, the team reports online this week in Nature Neuroscience. She even felt fine standing nose to nose with an experimenter.

Further experiments revealed why. Kennedy and his colleagues placed eight healthy subjects, one at a time, inside a functional magnetic resonance imaging scanner, which measures brain activity. Then an experimenter stood either about 4.5 meters away from the machine or right next to the machine's opening. The subjects' amygdalas lit up with significantly more activity when the stranger stood close by. "Our findings support the idea that the amygdala functions as the brakes in social interactions," Kennedy says. "If you take away the amygdala, it seems like you are less tuned to ... social [behaviors] that can cause discomfort."

The study is "a novel piece of research" that is the first to identify a neural source of personal space in people, says Richard Davidson, a neuroscientist at the University of Wisconsin, Madison. "It's also part of a growing series of studies that underscore the importance of the amygdala in human social interactions," he says.