The rsearchers say that their findings may 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 seat of emotion in the brain.
However, this is the first time that it has been linked rigorously to real-life human social interaction.
Ralph Adolphs, Bren Professor of Psychology and Neuroscience and professor of biology, and postdoctoral scholar Daniel P. Kennedy were able to make this link with the help of a unique patient, 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," Nature magazine quoted Kennedy, the lead author of the new report, as saying.
The researchers have revealed that SM has difficulty recognizing fear in the faces of others, and in judging the trustworthiness of someone, two consequences of amygdala lesions.
Adolphs, who has studied the patient for years, has 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," says Kennedy.
Though previous studies on human subjects never revealed an association between the amygdala and personal space, the researchers knew from their knowledge of the lieterature that monkeys with amygdala lesions preferred to stay in closer proximity to other monkeys and humans than did healthy monkeys.
As part of their study, the researchers conducted an experiment in which 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.
Among the 20 other subjects, the researchers observed that the average preferred distance was roughly two feet, while SM's preferred distance was about one foot.
The team even observed that SM was at ease even at a nose-to-nose distance, and that 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. 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," Kennedy said.
The researchers later used a functional magnetic resonance imaging (fMRI) scanner to examine the activation of the amygdala in a separate group of healthy subjects who were told when an experimenter was either in close proximity or far away from them.
The subjects could not see, feel, or hear the experimenter while in the scanner, but still their amygdalae lit up when they believed the experimenter to be close by.
No such activity was detected when subjects thought the experimenter was on the other side of the room.
"It was just the idea of another person being there, or not, that triggered the amygdala," Kennedy said.
He added: "(The Study shows that) the amygdala is involved in regulating social distance, independent of the specific sensory cues that are typically present when someone is standing close, like sounds, sights, and smells."
He further said that the new findings might have relevance to studies of autism, a complex 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," he said.
Adding a word of caution, Kennedy also said: "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."