She'll stand abnormally, uncomfortably close to you without realizing she's doing it.

By Melissa Dahl, Science of Us
May 25, 2015

Even if she only met you moments ago, S.M. wastes little time in getting close. She'll chat with her new acquaintances while touching them on the arm, which is not so bad in itself, but she also is just as likely to poke them in the stomach, all while standing abnormally, uncomfortably close. In short, S.M. essentially has no sense of personal space. Her case was reported in a 2009 edition of Nature Neuroscience (and highlighted in Science, which revealed the stomach-poking detail).

The patient: S.M. has Urbach-Wiethe disease, an inherited condition that damages the amygdala — the almond-shaped cluster of neurons located deep within the brain — causing it to harden and cease functioning properly. The amygdala has been shown to be associated with emotions, particularly fear, and, as it happens, not much scares S.M. Her lack of fear has been documented in the scientific literature and the popular media — she's the "woman without fear," impossible to startle in a haunted house and unafraid of live, writhing snakes or creepy, crawling spiders. But something about her condition, and the damage it's done to this part of her brain, also appears to have eroded away her sense of personal space.

The problem: The researchers, led by Daniel P. Kennedy of Indiana University in Bloomington, asked S.M. to do a series of tasks demonstrating her apparent disinterest in personal space. In one of those tests, an experimenter approached S.M. from across a room, asking her to indicate when she felt most comfortable. The experimenter started 15 feet away and stepped closer and closer until she was just about one foot away, at which point S.M. finally told her to stop. In contrast, when the researchers ran this test with 20 individuals who didn't have brain damage, their answers ranged from about five to one and a half feet, and averaged two feet.

They then tried measuring her sense of personal space by rephrasing the question a bit, asking her to rate how uncomfortable she was:

And they also tested her without her exactly knowing she was being tested (though she knew something was up):

And S.M. did indicate that she understood, at a cognitive level at least, the general concept of personal space. "She spontaneously stated that she did not want to make the experimenter uncomfortable by standing too close," Kennedy and the rest of the study authors write, "and also stated that she believed her personal space was smaller than most."

The conclusion: Kennedy and his team next tested the relationship between personal space and amygdala response, using brain scans from eight healthy individuals. When these people knew the experimenter was standing immediately next to the scanner, the researchers were able to observe more activity in the amygdala than when they knew the experimenter was standing farther away.

Weirdly, they didn't have S.M. undergo a brain scan, so there's no comparison between the responses of a healthy amygdala and a damaged one. But the researchers argue that amygdala activation might play a role in the strong emotional response most people have to someone invading their personal space; what's more, this finding could point toward research that could help people with autism or William's syndrome, two conditions on opposite ends of the personal space respecting spectrum. "The amygdala may be required to trigger the strong emotional reactions normally following personal space violations, thus regulating interpersonal distance in humans," the study authors write, giving all of us a reason to appreciate a healthy and functioning amygdala in ourselves and those around us, maybe especially on the train.

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