The man who walks too close to you on the street. The child who breathes against your back on the empty subway. The woman who leans in too closely to speak to you. Everyone at some point has felt that his or her personal space has been violated. Scientists now believe they have found the mechanism in the brain responsible for the social behavior and why some people do not seem to have a sense of personal space. The discovery by California Institute of Technology neuroscientists, described in journal Nature Neuroscience, could offer insight into autism and other disorders where social distance is an issue.

 

The structure, the amygdala—a pair of almond-shaped regions located in the medial temporal lobes—was previously known to process strong negative emotions, such as anger and fear, and is considered the center of emotion in the brain. However, it had never been linked to real-life human social interaction. The scientists were able to make the link with the help of a 42-year-old woman known as SM, who has extensive damage to the amygdala on both sides of her brain.

 

“SM is unique, because she is one of only a handful of individuals in the world with such a clear bilateral lesion of the amygdala, which gives us an opportunity to study the role of the amygdala in humans,” says Daniel Kennedy, the lead author of the report. SM has difficulty recognizing fear in the faces of others, and in judging the trustworthiness of someone, two consequences of amygdala lesions that the researchers published in prior studies.

 

During his years of studying her, Ralph Adolphs, professor of biology, also noticed that the very outgoing SM is almost too friendly, to the point of “violating” what others might perceive as their own personal space. “She is extremely friendly, and she wants to approach people more than normal. It's something that immediately becomes apparent as you interact with her,” Kennedy says.

 

Adolphs, Kennedy, and their colleagues were interested in her personal behavior and devised an experiment to compare her sense of personal space with that of healthy volunteers. The experiment used the stop-distance technique, where the subject stands a predetermined distance from an experimenter, then walks toward the experimenter and stops at the point where they feel most comfortable. The chin-to-chin distance between the subject and the experimenter is determined with a digital laser measurer. Twenty subjects and SM were tested.

 

Among the 20 other subjects, the average preferred distance was .64 meters—about two feet. SM's preferred distance was .34 meters, or about one foot. Unlike the [MISSING WORD], who reported feelings of discomfort when the experimenter went closer than their preferred distance, there was no point at which SM became uncomfortable; even nose-to-nose, she was at ease. Furthermore, her preferred distance didn't change based on who the experimenter was and how well she knew them.

 

“Respecting someone's space is a critical aspect of human social interaction, and something we do automatically and effortlessly,” Kennedy says. “These findings suggest that the amygdala, because it is necessary for the strong feelings of discomfort that help to repel people from one another, plays a central role in this process. They also help to expand our understanding of the role of the amygdala in real-world social interactions.”

 

The researchers conducted another experiment to bolster their findings. Using a magnetic resonance imaging scanner to examine the activity of the amygdala in a different group of healthy subjects, the neuroscientists examined levels of comfort in close proximity. The subjects were told when an experimenter was either in close proximity or far away from them. When in the scanner, subjects could not see, feel, or hear the experimenter; nevertheless, their amygdalae lit up when they believed the experimenter to be close by. No activity was detected when subjects thought the experimenter was on the other side of the room.

 

The findings may shed light into autism, a neurodevelopmental disorder that affects an individual's ability to interact socially and communicate with others. “We are really interested in looking at personal space in people with autism, especially given findings of amygdala dysfunction in autism. We know that some people with autism do have problems with personal space and have to be taught what it is and why it's important,” Kennedy explains.

 

But the findings cannot fully explain the disorder, he adds. “It's clear that amygdala dysfunction cannot account for all the social impairments in autism, but likely contributes to some of them and is definitely something that needs to be studied further.”