Why is it so uncomfortable to stand really close to a stranger? Sure, there are the potentially icky things. Sometimes an elevator car is so crowded that you can smell a fellow rider's shampoo or chewing gum (or worse). But even when a stranger is perfectly groomed, it's usually a bit revolting to be pressed against him in public. Why?
Evolution seems to have programmed this discomfort via a brain structure called the amygdalae, a pair of almond-shaped brain regions deep within each temporal lobe that control fear and the processing of emotion. It's your amygdalae that keep you from getting so close to another person that he could easily reach out, gouge an eye, and then drag your woman off by her hair.
So what happens if you disable the amygdalae? This is not something you could (ethically) do to a research subject, but scientists have been studying a 42-year-old woman who has such severe damage to her amygdalae — due to a rare genetic condition called Urbach-Wiethe disease, which causes calcification in the temporal lobes — that they have stopped functioning. The patient's identity isn't public, but neuroscientists call her SM, and a new paper in the journal Nature Neuroscience reports the results of experiments judging her conception of personal space.
A team of scientists from Caltech put SM through a series of tests in which they asked her to indicate the position at which she became uncomfortable as another woman, a researcher, approached her. SM's preferred personal distance was 1.1 ft. (0.34 m), about half the preferred distance (2 ft., or 0.64 m) of a group of comparison subjects. At 1 ft., you can easily discern whether someone showered after the gym — although in the lab experiment, the Caltech researchers made sure the experimenter was well-scrubbed and had just chewed gum before interacting with SM.
In another trial, SM was asked to walk toward an experimenter and stop at the point at which she felt the distance was comfortable. SM walked until her nose was virtually touching the experimenter's, all the while saying she felt perfectly at ease.
The researchers then put eight subjects with healthy amygdalae into a functional magnetic-resonance-imaging device. They found that the amygdalae in those individuals lit up when the participants were told that an experimenter was standing close to them, even if the participants couldn't actually see, hear, smell or in any way sense the experimenter. In short, that suggests that we are wired to repel close human contact — except, of course, when sex is a possibility. Which explains why so many introductions in bars go wrong. One party's amygdalae gets primed by proximity even as the other party's amygdalae submit to a more primal force: the need to procreate. (Past research has shown that the brain's limbic system, which includes the amygdalae, lights up in response to sexually arousing stimuli — not surprisingly, more vigorously in men than in women.)
Because the new paper is mostly based on one unusual subject, it shouldn't be overinterpreted. But the findings may have relevance for research into autism, whose sufferers sometimes have trouble understanding personal space and are thought to have amygdalae impairment. Previous studies of SM show that her brain impairment makes it difficult for her to recognize expressions of fear or judge a person's trustworthiness — problems that are also common among people with autism. Researchers think people who suffer from extreme shyness may turn out to have a problem in their temporal lobes as well. There's no known way of repairing amygdalae, so such conditions can't be reversed. But at least it's now possible to understand why it can be so unbearable to be in the middle of a packed line for the Hulk roller coaster: your amygdalae is going nuts.