It's something that immediately becomes apparent as you interact with her,' says Kennedy.

Previous studies of humans never had revealed an association between the amygdala and personal space.

From their knowledge of the literature, however, the researchers knew that monkeys with amygdala lesions preferred to stay closer to other monkeys and humans than did healthy monkeys.

Intrigued by SM's unusual social behaviour, Adolphs, Kennedy, and their colleagues devised a simple experiment to quantify and compare her sense of personal space with that of healthy volunteers.

The experiment used what is known as the stop-distance technique. Among the other subjects, the average preferred distance was .64 metres-roughly two feet.

SM's preferred distance was just .34 meters, or about one foot. Unlike other subjects, 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.

The discovery appeared in the Sunday issue of Nature Neuroscience.